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MANUAL SOLUTION SI UNIT PROBLEMS CHAPTER 5
SONNTAG BORGNAKKE VAN WYLEN
FUNDAMENTALS
of
Thermodynamics
Sixth Edition
Sonntag, Borgnakke and van Wylen
CONTENT
SUBSECTION Correspondence table Concept-Study Guide Problems Kinetic and potential energy Properties (u,h) from general tables Energy equation: simple process Energy eqaution: multistep process Energy equation: solids and liquids Properties (u, h, Cv, Cp), ideal gas Energy equation: ideal gas Energy equation: polytropic process Energy equation in rate form Review Problems PROB NO.
1-19 20-27 28-34 35-60 61-73 74-81 82-88 89-102 103-115 116-125 126-138
Sonntag, Borgnakke and van Wylen
CHAPTER 5 CORRESPONDENCE TABLE
The correspondence between this problem set and 5th edition chapter 5 problem set. Study guide problems 5.1-5.19 are all new New 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 5th 1 4 2mod 3 new 5 new new 6 mod new 7 mod new 8 mod 9 mod new 10 mod new 12 14 11 new 13 15 21 new new new 26 41 new New 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 5th 28 new 17 new 27 51 53 40 37 44 42 new 38 39 20 23 mod 43 24 45 new new 49 mod 55 36 new 58 60 new 59 61 New 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 5th new new new new new 67 mod new 68 mod 62 72 mod 63 new new 79 new 64 new 65 new new new 69 new new 74 76 new 66 new 46 New 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 5th new 84 77 30 54 82 new 89 87 new 90 new 86 new new new 22 29 57 35 31 32 48 56 18 new 83 new 85
Sonntag, Borgnakke and van Wylen
The english unit problem set corresponds to the 5th edition as New 139 140 141 142 143 144 145 146 147 148 149 150 5th new new new new new new new 102 103 104 mod 105 mod 104 mod New 151 152 153 154 155 156 157 158 159 160 161 162 5th 107 108 106 new 112 115 111 110 109 113 114 118 New 163 164 165 166 167 168 169 170 171 172 173 174 5th 124 119 new 120 new 122 121 new 125 130 129 123 New 175 176 177 178 179 180 181 182 5th 127 new 131 132 135 new 136 134
Sonntag, Borgnakke and van Wylen
Concept-Study Guide Problems
5.1 What is 1 cal in SI units and what is the name given to 1 N-m?
Look in the conversion factor table A.1 under energy: 1 cal (Int.) = 4.1868 J = 4.1868 Nm = 4.1868 kg m2/s2 This was historically defined as the heat transfer needed to bring 1 g of liquid water from 14.5oC to 15.5oC, notice the value of the heat capacity of water in Table A.4 1 N-m = 1 J or Force times displacement = energy = Joule
5.2 In a complete cycle what is the net change in energy and in volume? For a complete cycle the substance has no change in energy and therefore no storage, so the net change in energy is zero. For a complete cycle the substance returns to its beginning state, so it has no change in specific volume and therefore no change in total volume.
5.3 Why do we write E or E2 E1 whereas we write 1Q2 and 1W2? E or E2 E1 is the change from state 1 to state 2 and depends only on states 1 and 2 not upon the process between 1 and 2. 1Q2 and 1W2 are amounts of energy transferred during the process between 1 and 2 and depend on the process path.
Sonntag, Borgnakke and van Wylen
5.4 When you wind a spring up in a toy or stretch a rubber band what happens in terms of work, energy and heat transfer? Later when they are released, what happens then? In both processes work is put into the device and the energy is stored as potential energy. If the spring or rubber is inelastic some of the work input goes into internal energy (it becomes warmer) and not its potential energy and being warmer than the ambient air it cools slowly to ambient temperature. When the spring or rubber band is released the potential energy is transferred back into work given to the system connected to the end of the spring or rubber band. If nothing is connected the energy goes into kinetic energy and the motion is then dampened as the energy is transformed into internal energy.
5.5 Explain in words what happens with the energy terms for the stone in Example 5.2. What would happen if it were a bouncing ball falling to a hard surface? In the beginning all the energy is potential energy associated with the gravitational force. As the stone falls the potential energy is turned into kinetic energy and in the impact the kinetic energy is turned into internal energy of the stone and the water. Finally the higher temperature of the stone and water causes a heat transfer to the ambient until ambient temperature is reached. With a hard ball instead of the stone the impact would be close to elastic transforming the kinetic energy into potential energy (the material acts as a spring) that is then turned into kinetic energy again as the ball bounces back up. Then the ball rises up transforming the kinetic energy into potential energy (mgZ) until zero velocity is reached and it starts to fall down again. The collision with the floor is not perfectly elastic so the ball does not rise exactly up to the original height loosing a little energy into internal energy (higher temperature due to internal friction) with every bounce and finally the motion will die out. All the energy eventually is lost by heat transfer to the ambient or sits in lasting deformation (internal energy) of the substance.
Sonntag, Borgnakke and van Wylen
5.6 Make a list of at least 5 systems that store energy, explaining which form of energy. A spring that is compressed. Potential energy (1/2)kx2 A battery that is charged. Electrical potential energy. V Amp h A raised mass (could be water pumped up higher) Potential energy mgH A cylinder with compressed air. Potential (internal) energy like a spring. A tank with hot water. Internal energy mu A fly-wheel. Kinetic energy (rotation) (1/2)I2 A mass in motion. Kinetic energy (1/2)mV2
5.7 A 1200 kg car is accelerated from 30 to 50 km/h in 5 s. How much work is that? If you continue from 50 to 70 km/h in 5 s is that the same? The work input is the increase in kinetic energy. E2 E1 = (1/2)m[V2 - V1] = 1W2 km2 = 0.5 1200 kg [502 302] h 1000 m2 = 600 [ 2500 900 ] kg 3600 s = 74 074 J = 74.1 kJ The second set of conditions does not become the same 2 2 1000 m2 E2 E1 = (1/2)m[V2 - V1] = 600 [ 702 502 ] kg 3600 s = 111 kJ
2 2
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5.8 A crane use 2 kW to raise a 100 kg box 20 m. How much time does it take?
. L Power = W = FV = mgV = mg t mgL 100 kg 9.807 m/s2 20 m = 9.81 s t= . = 2000 W W
5.9 Saturated water vapor has a maximum for u and h at around 235oC. Is it similar for other substances? Look at the various substances listed in appendix B. Everyone has a maximum u and h somewhere along the saturated vapor line at different T for each substance. This means the constant u and h curves are different from the constant T curves and some of them cross over the saturated vapor line twice, see sketch below.
P C.P. T u=C T v C.P. P=C u=C v
Constant h lines are similar to the constant u line shown.
Notice the constant u(h) line becomes parallel to the constant T lines in the superheated vapor region for low P where it is an ideal gas. In the T-v diagram the constant u (h) line becomes horizontal.
Sonntag, Borgnakke and van Wylen
5.10 A pot of water is boiling on a stove supplying 325 W to the water. What is the rate of mass (kg/s) vaporizing assuming a constant pressure process? To answer this we must assume all the power goes into the water and that the process takes place at atmospheric pressure 101 kPa, so T = 100oC. Energy equation dQ = dE + dW = dU + PdV = dH = hfg dm
dQ dm = hfg dt dt . 325 W dm Q = h = 2257 kJ/kg = 0.144 g/s dt fg The volume rate of increase is dV dm 3 dt = dt vfg = 0.144 g/s 1.67185 m /kg = 0.24 10-3 m3/s = 0.24 L/s
5.11 A constant mass goes through a process where 100 W of heat transfer comes in and 100 W of work leaves. Does the mass change state? Yes it does. As work leaves a control mass its volume must go up, v increases As heat transfer comes in at a rate equal to the work out means u is constant if there are no changes in kinetic or potential energy.
Sonntag, Borgnakke and van Wylen
5.12 I have 2 kg of liquid water at 20oC, 100 kPa. I now add 20 kJ of energy at a constant pressure. How hot does it get if it is heated? How fast does it move if it is pushed by a constant horizontal force? How high does it go if it is raised straight up? a) Heat at 100 kPa. Energy equation: E2 E1 = 1Q2 1W2 = 1Q2 P(V2 V1) = H2 H1= m(h2 h1) h2 = h1 + 1Q2/m = 83.94 + 20/2 = 94.04 kJ/kg Back interpolate in Table B.1.1: T2 = 22.5oC (We could also have used T = 1Q2/mC = 20 / (2*4.18) = 2.4oC) b) Push at constant P. It gains kinetic energy. 0.5 m V2 = 1W2 V2 = 2 1W2/m = 2 20 1000 J/2 kg = 141.4 m/s c) Raised in gravitational field m g Z2 = 1W2 Z2 = 1W2/m g = 20 000 J = 1019 m 2 kg 9.807 m/s2
2
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5.13 Water is heated from 100 kPa, 20oC to 1000 kPa, 200oC. In one case pressure is raised at T = C, then T is raised at P = C. In a second case the opposite order is done. Does that make a difference for 1Q2 and 1W2? Yes it does. Both 1Q2 and 1W2 are process dependent. We can illustrate the work term in a P-v diagram. P Cr.P. S 1000 100 L a 1 20 200 2 V T
P
T
C.P.
1553 kPa 1000
a 1000 100 1
2
20 C
200 C
200 20 a 1
2
b v
100
b
180 C
v
In one case the process proceeds from 1 to state "a" along constant T then from "a" to state 2 along constant P. The other case proceeds from 1 to state "b" along constant P and then from "b" to state 2 along constant T.
Sonntag, Borgnakke and van Wylen
5.14 Two kg water at 120oC with a quality of 25% has its temperature raised 20oC in a constant volume process. What are the new quality and specific internal energy? Solution: State 1 from Table B.1.1 at 120oC v = vf + x vfg = 0.001060 + 0.25 0.8908 = 0.22376 m3/kg State 2 has same v at 140oC also from Table B.1.1 v - vf 0.22376 - 0.00108 x= v = = 0.4385 0.50777 fg u = uf + x ufg = 588.72 + 0.4385 1961.3 = 1448.8 kJ/kg
P C.P.
140 C 120 C
T
C.P.
361.3 198.5
T v
140 120 v
Sonntag, Borgnakke and van Wylen
5.15 Two kg water at 200 kPa with a quality of 25% has its temperature raised 20oC in a constant pressure process. What is the change in enthalpy? Solution: State 1 from Table B.1.2 at 200 kPa h = hf + x hfg = 504.68 + 0.25 2201.96 = 1055.2 kJ/kg State 2 has same P from Table B.1.2 at 200 kPa T = T + 20 = 120.23 + 20 = 140.23oC
2 sat
so state 2 is superheated vapor (x = undefined) from Table B.1.3 20 h2 = 2706.63 + (2768.8 2706.63)150 - 120.23 = 2748.4 kJ/kg h2 h1 = 2748.4 1055.2 = 1693.2 kJ/kg
P C.P.
140 C
T
C.P. 200 kPa
200
120.2 C
T v
140 120
v
5.16 You heat a gas 10 K at P = C. Which one in table A.5 requires most energy? Why? A constant pressure process in a control mass gives (recall Eq.5.29) 1q2 = u2 - u1 + 1w2 = h2 - h1 Cp T The one with the highest specific heat is hydrogen, H2. The hydrogen has the smallest mass but the same kinetic energy per mol as other molecules and thus the most energy per unit mass is needed to increase the temperature.
Sonntag, Borgnakke and van Wylen
5.17 Air is heated from 300 to 350 K at V = C. Find 1q2? What if from 1300 to 1350 K? Process: V = C Energy Eq.:
1W2 = 1q2 = u2 - u1
u2 - u1 = 1q2 0
Read the u-values from Table A.7.1 a) 1q2 = u2 - u1 = 250.32 214.36 = 36.0 kJ/kg b) 1q2 = u2 - u1 = 1067.94 1022.75 = 45.2 kJ/kg case a) Cv 36/50 = 0.72 kJ/kg K , see A.5 case b) Cv 45.2/50 = 0.904 kJ/kg K (25 % higher)
5.18 A mass of 3 kg nitrogen gas at 2000 K, V = C, cools with 500 W. What is dT/dt? Process: V=C
1W2= 0
. dE dU dU dT . = dt = m dt = mCv dt = Q W = Q = -500 W dt du u u2100 - u1900 1819.08 - 1621.66 Cv 2000 = dT = = = = 0.987 kJ/kg K 200 T 2100-1900 . dT Q -500 W K = mC = = -0.17 dt s v 3 0.987 kJ/K Remark: Specific heat from Table A.5 has Cv 300 = 0.745 kJ/kg K which is nearly 25% lower and thus would over-estimate the rate with 25%.
Sonntag, Borgnakke and van Wylen
5.19 A drag force on a car, with frontal area A = 2 m2, driving at 80 km/h in air at 20oC is Fd = 0.225 A airV2. How much power is needed and what is the traction force? . W = FV km 1000 V = 80 h = 80 3600 ms-1 = 22.22 ms-1 P 101 AIR = RT = = 1.20 kg/m3 0.287 293 Fd = 0.225 AV2 = 0.225 2 1.2 22.222 = 266.61 N . W = FV = 266.61 N 22.22 m/s = 5924 W = 5.92 kW
Sonntag, Borgnakke and van Wylen
Kinetic and Potential Energy
5.20 A hydraulic hoist raises a 1750 kg car 1.8 m in an auto repair shop. The hydraulic pump has a constant pressure of 800 kPa on its piston. What is the increase in potential energy of the car and how much volume should the pump displace to deliver that amount of work? Solution: C.V. Car. No change in kinetic or internal energy of the car, neglect hoist mass. E2 E1 = PE2 - PE1 = mg (Z2 Z1) = 1750 9.80665 1.8 = 30 891 J The increase in potential energy is work into car from pump at constant P. W = E2 E1 = P dV = P V V =
E2 E1 30891 = 800 1000 = 0.0386 m3 P
Sonntag, Borgnakke and van Wylen
5.21 A piston motion moves a 25 kg hammerhead vertically down 1 m from rest to a velocity of 50 m/s in a stamping machine. What is the change in total energy of the hammerhead? Solution: C.V. Hammerhead The hammerhead does not change internal energy (i.e. same P, T), but it does have a change in kinetic and potential energy. E2 E1 = m(u2 u1) + m[(1/2)V2 2 0] + mg (h2 - 0) = 0 + 25 (1/2) 502 + 25 9.80665 (-1) = 31250 245.17 = 31005 J = 31 kJ
Sonntag, Borgnakke and van Wylen
5.22 Airplane takeoff from an aircraft carrier is assisted by a steam driven piston/cylinder device with an average pressure of 1250 kPa. A 17500 kg airplane should be accelerated from zero to a speed of 30 m/s with 30% of the energy coming from the steam piston. Find the needed piston displacement volume. Solution: C.V. Airplane. No change in internal or potential energy; only kinetic energy is changed. E2 E1 = m (1/2) (V2 - 0) = 17500 (1/2) 302 = 7875 000 J = 7875 kJ The work supplied by the piston is 30% of the energy increase.
2
W = P dV = Pavg V = 0.30 (E2 E1) = 0.30 7875 = 2362.5 kJ W 2362.5 V = P = 1250 = 1.89 m3 avg
Sonntag, Borgnakke and van Wylen
5.23 Solve Problem 5.22, but assume the steam pressure in the cylinder starts at 1000 kPa, dropping linearly with volume to reach 100 kPa at the end of the process. Solution: C.V. Airplane.
P
E2 E1 = m (1/2) (V22 - 0) = 3500 (1/2) 302 = 1575000 J = 1575 kJ W = 0.25(E2 E1) = 0.25 1575 = 393.75 kJ W = P dV = (1/2)(Pbeg + Pend) V W 2362.5 V = P = 1/2(1000 + 100) = 4.29 m3 avg
1000
1 2 V
100
W
Sonntag, Borgnakke and van Wylen
5.24 A 1200 kg car accelerates from zero to 100 km/h over a distance of 400 m. The road at the end of the 400 m is at 10 m higher elevation. What is the total increase in the car kinetic and potential energy? Solution:
KE = m (V2 - V1) V2 = 100 km/h = = 27.78 m/s 100 1000 m/s 3600
2
2
KE = 1200 kg (27.782 02) (m/s)2 = 463 037 J = 463 kJ PE = mg(Z2 Z1) = 1200 kg 9.807 m/s2 ( 10 - 0 ) m = 117684 J = 117.7 kJ
Sonntag, Borgnakke and van Wylen
5.25 A 25 kg piston is above a gas in a long vertical cylinder. Now the piston is released from rest and accelerates up in the cylinder reaching the end 5 m higher at a velocity of 25 m/s. The gas pressure drops during the process so the average is 600 kPa with an outside atmosphere at 100 kPa. Neglect the change in gas kinetic and potential energy, and find the needed change in the gas volume. Solution: C.V. Piston (E2 E1)PIST. = m(u2 u1) + m[(1/2)V2 2 0] + mg (h2 0) = 0 + 25 (1/2) 252 + 25 9.80665 5 = 7812.5 + 1225.8 = 9038.3 J = 9.038 kJ Energy equation for the piston is: E2 E1 = Wgas - Watm = Pavg Vgas Po Vgas (remark Vatm = Vgas so the two work terms are of opposite sign) Vgas = 9.038/(600 100) = 0.018 m3
V Po g
P
H
Pavg
1 2 V
Sonntag, Borgnakke and van Wylen
5.26 The rolling resistance of a car depends on its weight as: F = 0.006 mg. How far will a car of 1200 kg roll if the gear is put in neutral when it drives at 90 km/h on a level road without air resistance? Solution: The car decreases its kinetic energy to zero due to the force (constant) acting over the distance. m (1/2V2 1/2V1) = -1W2 = - F dx = -FL V2 = 0,
2 2 2
km 90 1000 V1 = 90 h = 3600 ms-1 = 25 ms-1
-1/2 mV1 = -FL = - 0.006 mgL 0.5 V1 0.5252 m2/s2 = 5311 m L = 0.0006g = 0.0069.807 m/s2 Remark: Over 5 km! The air resistance is much higher than the rolling resistance so this is not a realistic number by itself.
2
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5.27 A mass of 5 kg is tied to an elastic cord, 5 m long, and dropped from a tall bridge. Assume the cord, once straight, acts as a spring with k = 100 N/m. Find the velocity of the mass when the cord is straight (5 m down). At what level does the mass come to rest after bouncing up and down? Solution: Let us assume we can neglect the cord mass and motion. 1: V1 = 0, 3: V3 = 0, 1 2: Z1= 0 2 : V2, Z2= -5m
Z3= -L , Fup = mg = ks L 2 2 mV1 + mg Z1 = V2 + mgZ2
Divide by mass and left hand side is zero so V2 + g Z2 = 0 V2 = (-2g Z2)1/2 = ( -2 9.807 (-5)) 1/2 = 9.9 m/s State 3: m is at rest so Fup = Fdown
2
ks L = mg mg 5 9.807 kg ms-2 L = k = 100 = 0.49 m s Nm-1 L = Lo + L = 5 + 0.49 = 5.49 m So: Z2 = -L = - 5.49 m
BRIDGE m V
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Properties (u, h) from General Tables
5.28 Find the missing properties. a. H2O T = 250C, v = 0.02 m3/kg b. c. d. N2 H2O R-134a Solution: T = 120 K, P = 0.8 MPa T = -2C, P = 100 kPa P = 200 kPa, v = 0.12 m3/kg vf < v < vg P=? u=? x=? h=? u=? v=? u=? T=? P = Psat = 3973 kPa
a) Table B.1.1 at 250C:
x = (v - vf)/ vfg = (0.02 0.001251)/0.04887 = 0.38365 u = uf + x ufg = 1080.37 + 0.38365 1522.0 = 1664.28 kJ/kg b) Table B.6.1 Table B.6.2: P is lower than Psat so it is super heated vapor and we find the state in Table B.6.2 h = 114.02 kJ/kg
=> x = undefined
c) Table B.1.1 : T < Ttriple point => B.1.5: P > Psat so compressed solid u ui = -337.62 kJ/kg v vi = 1.0910-3 m3/kg
approximate compressed solid with saturated solid properties at same T. d) Table B.5.1 v > vg superheated vapor => Table B.5.2.
T ~ 32.5C = 30 + (40 30) (0.12 0.11889)/(0.12335 - 0.11889) u = 403.1 + (411.04 403.1) 0.24888 = 405.07 kJ/kg
P L S T c v a P C.P. b d a c T v c a T C.P. b P=C d
C.P. V d b
v
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5.29 Find the missing properties of T, P, v, u, h and x if applicable and plot the location of the three states as points in the T-v and the P-v diagrams a. Water at 5000 kPa, u = 800 kJ/kg b. Water at 5000 kPa, v = 0.06 m3/kg c. R-134a at 35oC, v = 0.01 m3/kg Solution: a) Look in Table B.1.2 at 5000 kPa u < uf = 1147.78 Table B.1.4: => compressed liquid between 180 oC and 200 oC
800 - 759.62 T = 180 + (200 - 180) 848.08 - 759.62 = 180 + 20*0.4567 = 189.1 C v = 0.001124 + 0.4567 (0.001153 - 0.001124) = 0.001137 b) Look in Table B.1.2 at 5000 kPa v > vg = 0.03944 => superheated vapor Table B.1.3: between 400 oC and 450 oC. T = 400 + 50*(0.06 - 0.05781)/(0.0633 - 0.05781) = 400 + 50*0.3989 = 419.95 oC h = 3195.64 + 0.3989 *(3316.15 - 3195.64) = 3243.71 c) B.5.1: v f < v < vg => 2-phase, P = Psat = 887.6 kPa,
x = (v - vf ) / vfg = (0.01 - 0.000857)/0.02224 = 0.4111 u = uf + x ufg = 248.34 + 0.4111*148.68 = 309.46 kJ/kg
P C.P. T C.P. P = const. a T c v b a c v b
States shown are placed relative to the two-phase region, not to each other.
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5.30 Find the missing properties and give the phase of the ammonia, NH3. a. T = 65oC, P = 600 kPa b. T = 20oC, P = 100 kPa c. T = 50oC, v = 0.1185 m3/kg Solution: a) Table B.2.1 P < Psat u=? v=? u=? v=? x=? u=? P=? x=?
=> superheated vapor Table B.2.2:
v = 0.5 0.25981 + 0.5 0.26888 = 0.2645 m3/kg u = 0.5 1425.7 + 0.5 1444.3 = 1435 kJ/kg b) Table B.2.1: P < Psat => x = undefined, superheated vapor, from B.2.2: v = 1.4153 m3/kg ; u = 1374.5 kJ/kg
c) Sup. vap. ( v > vg) Table B.2.2. P = 1200 kPa, x = undefined u = 1383 kJ/kg
P C.P. T c C.P. c T b v
States shown are placed relative to the two-phase region, not to each other.
1200 kPa 600 kPa a b v
a
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5.31 Find the phase and missing properties of P, T, v, u, and x. a. Water at 5000 kPa, u = 1000 kJ/kg (Table B.1 reference) b. R-134a at 20oC, u = 300 kJ/kg c. Nitrogen at 250 K, 200 kPa Show also the three states as labeled dots in a T-v diagram with correct position relative to the two-phase region. Solution: a) Compressed liquid: B.1.4 interpolate between 220oC and 240oC. T = 233.3oC, v = 0.001213 m3/kg, x = undefined b) Table B.5.1: u < ug => two-phase liquid and vapor x = (u - uf)/ufg = (300 - 227.03)/162.16 = 0.449988 = 0.45 v = 0.000817 + 0.45*0.03524 = 0.01667 m3/kg c) Table B.6.1: T > Tsat (200 kPa) so superheated vapor in Table B.6.2 x = undefined v = 0.5(0.35546 + 0.38535) = 0.3704 m3/kg, u = 0.5(177.23 + 192.14) = 184.7 kJ/kg
P C.P. T a b c T v b C.P. P = const. c
States shown are placed relative to the two-phase region, not to each other.
a
v
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5.32 Find the missing properties and give the phase of the substance a. b. c. d. e. H2O H2O N2 NH3 N2 T = 120C, v = 0.5 m3/kg T = 100C, P = 10 MPa T = 200 K, P = 200 kPa T = 100C, v = 0.1 m3/kg T = 100 K, x = 0.75 u=? P=? x=? u=? x=? v=? v=? u=? P=? x=? v=? u=?
Solution: a) Table B.1.1: vf < v < vg => L+V mixture, P = 198.5 kPa, x = (0.5 - 0.00106)/0.8908 = 0.56, u = 503.48 + 0.56 2025.76 = 1637.9 kJ/kg b) Table B.1.4: compressed liquid, v = 0.001039 m3/kg, u = 416.1 kJ/kg c) Table B.6.2: 200 K, 200 kPa u = 147.37 kJ/kg
v = 0.29551 m3/kg ; d) Table B.2.1: v > vg
=> superheated vapor, x = undefined
0.1 - 0.10539 B.2.2: P = 1600 + 400 0.08248-0.10539 = 1694 kPa e) Table B.6.1: 100 K, x = 0.75
v = 0.001452 + 0.75 0.02975 = 0.023765 m3/kg u = -74.33 + 0.75 137.5 = 28.8 kJ/kg
P C.P. T c a e v d T b C.P. a e v c >
States shown are placed relative to the two-phase region, not to each other.
b
P = const. d
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5.33 Find the missing properties among (T, P, v, u, h and x if applicable) and give the phase of the substance and indicate the states relative to the two-phase region in both a T-v and a P-v diagram. a. R-12 P = 500 kPa, h = 230 kJ/kg b. c. Solution: a) Table B.3.2: h > hg = > superheated vapor, look in section 500 kPa and interpolate T = 68.06C, v = 0.04387 m3/kg, u = 208.07 kJ/kg R-22 R-134a T = 10oC, u = 200 kJ/kg T = 40oC, h = 400 kJ/kg
b) Table B.4.1: u < ug => L+V mixture, P = 680.7 kPa u - uf 200 - 55.92 x = u = 173.87 = 0.8287, fg v = 0.0008 + 0.8287 0.03391 = 0.0289 m3/kg, h = 56.46 + 0.8287 196.96 = 219.7 kJ/kg c) Table B.5.1: h < hg => two-phase L + V, look in B.5.1 at 40C: h - hf 400 - 256.5 x = h = 163.3 = 0.87875 fg P = Psat = 1017 kPa, v = 0.000 873 + 0.87875 0.01915 = 0.0177 m3/kg u = 255.7 + 0.87875 143.8 = 382.1 kJ/kg
P C.P. T C.P. P=C a b, c T v b, c a
States shown are placed relative to the two-phase region, not to each other.
v
Sonntag, Borgnakke and van Wylen
5.34 Saturated liquid water at 20oC is compressed to a higher pressure with constant temperature. Find the changes in u and h from the initial state when the final pressure is a) 500 kPa, b) 2000 kPa, c) 20 000 kPa Solution: State 1 is located in Table B.1.1 and the states a-c are from Table B.1.4 State 1 a b c u [kJ/kg] 83.94 83.91 83.82 82.75 h [kJ/kg] 83.94 84.41 85.82 102.61 u = u - u1 -0.03 -0.12 -1.19 h = h - h1 0.47 1.88 18.67 (Pv) 0.5 2 20
For these states u stays nearly constant, dropping slightly as P goes up. h varies with Pv changes. P c b a 1 T = 20 C v v
o
T
c,b,a,1
P L T
cb
C.P. V
c b a S
1
v
Sonntag, Borgnakke and van Wylen
Energy Equation: Simple Process
5.35 A 100-L rigid tank contains nitrogen (N2) at 900 K, 3 MPa. The tank is now cooled to 100 K. What are the work and heat transfer for this process? Solution: C.V.: Nitrogen in tank. Energy Eq.5.11: m2 = m1 ; => / 1W2 = 0
m(u2 - u1) = 1Q2 - 1W2
Process: V = constant, v2 = v1 = V/m
Table B.6.2: State 1: v1 = 0.0900 m3/kg => m = V/v1 = 1.111 kg u1 = 691.7 kJ/kg State 2: 100 K, v2 = v1 = V/m, look in Table B.6.2 at 100 K 200 kPa: v = 0.1425 m3/kg; u = 71.7 kJ/kg 400 kPa: v = 0.0681 m3/kg; u = 69.3 kJ/kg so a linear interpolation gives: P2 = 200 + 200 (0.09 0.1425)/(0.0681 0.1425) = 341 kPa 0.09 0.1425 u2 = 71.7 + (69.3 71.7) 0.0681 0.1425 = 70.0 kJ/kg,
1Q2 = m(u2 - u1) = 1.111 (70.0 691.7) = -690.7 kJ
Sonntag, Borgnakke and van Wylen
5.36 A rigid container has 0.75 kg water at 300oC, 1200 kPa. The water is now cooled to a final pressure of 300 kPa. Find the final temperature, the work and the heat transfer in the process. Solution: C.V. Water. Constant mass so this is a control mass Energy Eq.: Process eq.: =>
o
U2 - U1 = 1Q2 - 1W2 V = constant. (rigid)
1W2 = P dV = 0
P 1200 1
State 1: 300 C, 1200 kPa => superheated vapor Table B.1.3 v = 0.21382 m3/kg, u = 2789.22 kJ/kg
300
2 v
State 2: 300 kPa and v2 = v1 T2 = Tsat = 133.55oC
from Table B.1.2
v2 < vg
two-phase
v2 - vf 0.21382 - 0.001073 = = 0.35179 x2 = v 0.60475 fg u2 = uf + x2 ufg = 561.13 + x2 1982.43 = 1258.5 kJ/kg
1Q2 = m(u2 - u1) + 1W2 = m(u2 - u1)
= 0.75 (1258.5 - 2789.22) = -1148 kJ
Sonntag, Borgnakke and van Wylen
5.37 A cylinder fitted with a frictionless piston contains 2 kg of superheated refrigerant R134a vapor at 350 kPa, 100oC. The cylinder is now cooled so the R-134a remains at constant pressure until it reaches a quality of 75%. Calculate the heat transfer in the process. Solution: C.V.: R-134a Energy Eq.5.11
m2 = m1 = m; m(u2 - u1) = 1Q2 - 1W2
Process: P = const. 1W2 = PdV = PV = P(V2 - V1) = Pm(v2 - v1) P T 1 2 V State 1: Table B.5.2 State 2: Table B.5.1 V
2
1
h1 = (490.48 + 489.52)/2 = 490 kJ/kg h2 = 206.75 + 0.75 194.57 = 352.7 kJ/kg (350.9 kPa)
1Q2 = m(u2 - u1) + 1W2 = m(u2 - u1) + Pm(v2 - v1) = m(h2 - h1) 1Q2 = 2 (352.7 - 490) = -274.6 kJ
Sonntag, Borgnakke and van Wylen
5.38 Ammonia at 0C, quality 60% is contained in a rigid 200-L tank. The tank and ammonia is now heated to a final pressure of 1 MPa. Determine the heat transfer for the process. Solution: C.V.: NH3 P 2
1 V Continuity Eq.: Energy Eq.5.11: m2 = m1 = m ; m(u2 - u1) = 1Q2 - 1W2 v2 = v1 &
1W2 = 0
Process: Constant volume State 1: Table B.2.1 two-phase state.
v1 = 0.001566 + x1 0.28783 = 0.17426 m3/kg u1 = 179.69 + 0.6 1138.3 = 862.67 kJ/kg m = V/v1 = 0.2/0.17426 = 1.148 kg State 2: P2 , v2 = v1 superheated vapor Table B.2.2 T2 100C, u2 1490.5 kJ/kg So solve for heat transfer in the energy equation
1Q2 = m(u2 - u1) = 1.148(1490.5 - 862.67) = 720.75 kJ
Sonntag, Borgnakke and van Wylen
5.39 Water in a 150-L closed, rigid tank is at 100C, 90% quality. The tank is then cooled to -10C. Calculate the heat transfer during the process. Solution: C.V.: Water in tank. Energy Eq.5.11:
m2 = m1 ; m(u2 - u1) = 1Q2 - 1W2
Process: V = constant, v2 = v1, 1W2 = 0 State 1: Two-phase L + V look in Table B.1.1 v1 = 0.001044 + 0.9 1.6719 = 1.5057 m3/kg u1 = 418.94 + 0.9 2087.6 = 2297.8 kJ/kg State 2: T2, v2 = v1 mix of saturated solid + vapor Table B.1.5 => x2 = 0.003224 v2 = 1.5057 = 0.0010891 + x2 466.7 m = V/v1 = 0.15/1.5057 = 0.09962 kg
1Q2 = m(u2 - u1) = 0.09962(-345.34 - 2297.8) = -263.3 kJ
u2 = -354.09 + 0.003224 2715.5 = -345.34 kJ/kg
P C.P.
T
C.P. P = const. 1
1 T 2 v
2
v
P L T S 2 1
C.P. V L+V S+V v
Sonntag, Borgnakke and van Wylen
5.40 A piston/cylinder contains 1 kg water at 20oC with volume 0.1 m3. By mistake someone locks the piston preventing it from moving while we heat the water to saturated vapor. Find the final temperature and the amount of heat transfer in the process. Solution: C.V. Water. This is a control mass Energy Eq.: m (u2 - u1 ) = 1Q2 - 1W2 Process : State 1:
1W2 = 0 T, v1 = V1/m = 0.1 m3/kg > vf so two-phase
V = constant
v1 - vf 0.1-0.001002 x1 = v = 57.7887 = 0.0017131 fg u1 = uf + x1 ufg = 83.94 + x1 2318.98 = 87.913 kJ/kg State 2: v2 = v1 = 0.1 & x2 =1 found in Table B.1.1 between 210C and 215 C 0.1-0.10441 T2 = 210 + 5 0.09479-0.10441 = 210 + 5 0.4584 = 212.3C u2 = 2599.44 + 0.4584 (2601.06 2599.44) = 2600.2 kJ/kg From the energy equation 1Q2 = m(u2 - u1) = 1( 2600.2 87.913) = 2512.3 kJ
P 2 1 V
T 2 1 V
Sonntag, Borgnakke and van Wylen
5.41 A test cylinder with constant volume of 0.1 L contains water at the critical point. It now cools down to room temperature of 20C. Calculate the heat transfer from the water. Solution: C.V.: Water P m2 = m1 = m ; 1 Energy Eq.5.11: m(u2 - u1) = 1Q2 - 1W2
2 v
Process: Constant volume v2 = v1 Properties from Table B.1.1 State 1: v1 = vc = 0.003155 m3/kg, u1 = 2029.6 kJ/kg m = V/v1 = 0.0317 kg State 2: T2, v2 = v1 = 0.001002 + x2 57.79
x2 = 3.710-5, u2 = 83.95 + x2 2319 = 84.04 kJ/kg Constant volume => / 1W2 = 0
1Q2 = m(u2 - u1) = 0.0317(84.04 - 2029.6) = -61.7 kJ
Sonntag, Borgnakke and van Wylen
5.42 A 10-L rigid tank contains R-22 at -10C, 80% quality. A 10-A electric current (from a 6-V battery) is passed through a resistor inside the tank for 10 min, after which the R-22 temperature is 40C. What was the heat transfer to or from the tank during this process? Solution: C.V. R-22 in tank. Control mass at constant V. Continuity Eq.: m2 = m1 = m ; Energy Eq.: Process: m(u2 - u1) = 1Q2 - 1W2 1 V Constant V v2 = v1 => no boundary work, but electrical work State 1 from table B.4.1 v1 = 0.000759 + 0.8 0.06458 = 0.05242 m3/kg u1 = 32.74 + 0.8 190.25 = 184.9 kJ/kg m = V/v = 0.010/0.05242 = 0.1908 kg State 2: Table B.4.2 at 40C and v2 = v1 = 0.05242 m3/kg => sup.vapor, so use linear interpolation to get P2 = 500 + 100 (0.05242 0.05636)/(0.04628 0.05636) = 535 kPa, u2 = 250.51 + 0.35 (249.48 250.51) = 250.2 kJ/kg
1W2 elec = power t = Amp volts t =
P 2
10 6 10 60 = 36 kJ 1000
1Q2 = m(u2 u1) + 1W2 = 0.1908 ( 250.2 184.9) 36 = 23.5 kJ
Sonntag, Borgnakke and van Wylen
5.43 A piston/cylinder contains 50 kg of water at 200 kPa with a volume of 0.1 m3. Stops in the cylinder are placed to restrict the enclosed volume to a maximum of 0.5 m3. The water is now heated until the piston reaches the stops. Find the necessary heat transfer. Solution: C.V. H2O m = constant Energy Eq.5.11: m(e2 e1) = m(u2 u1) = 1Q2 - 1W2 Process : P = constant (forces on piston constant) 1W2 = P dV = P1 (V2 V1) P
1 0.1 Properties from Table B.1.1
2 0.5
V
State 1 : v1 = 0.1/50 = 0.002 m3/kg => 2-phase as v1 < vg v1 vf 0.002 0.001061 x= = 0.001061 0.88467 vfg = h = 504.68 + 0.001061 2201.96 = 507.02 kJ/kg State 2 : v2= 0.5/50 = 0.01 m3/kg also 2-phase same P v2 vf 0.01 0.001061 = = 0.01010 x2 = v 0.88467 fg h2 = 504.68 + 0.01010 2201.96 = 526.92 kJ/kg Find the heat transfer from the energy equation as 1Q2 = m(u2 u1) + 1W2 = m(h2 h1)
1Q2 = 50 kg (526.92 507.02) kJ/kg = 995 kJ
[ Notice that
1W2 = P1 (V2 V1) = 200 (0.5 0.1) = 80 kJ ]
Sonntag, Borgnakke and van Wylen
5.44 A constant pressure piston/cylinder assembly contains 0.2 kg water as saturated vapor at 400 kPa. It is now cooled so the water occupies half the original volume. Find the heat transfer in the process. Solution: C.V. Water. This is a control mass. Energy Eq.5.11: m(u2 u1) = 1Q2 1W2 Process: P = constant =>
1W2 = Pm(v2 v1)
So solve for the heat transfer: 1Q2 = m(u2 - u1) + 1W2 = m(u2 - u1) + Pm(v2 - v1) = m(h2 - h1) State 1: Table B.1.2 v1 = 0.46246 m3/kg; h1 = 2738.53 kJ/kg State 2: v2 = v1 / 2 = 0.23123 = vf + x vfg from Table B.1.2 x2 = (v2 vf) / vfg = (0.23123 0.001084) / 0.46138 = 0.4988 h2 = hf + x2 hfg = 604.73 + 0.4988 2133.81 = 1669.07 kJ/kg
1Q2 = 0.2 (1669.07 2738.53) = 213.9 KJ
Sonntag, Borgnakke and van Wylen
5.45 Two kg water at 120oC with a quality of 25% has its temperature raised 20oC in a constant volume process as in Fig. P5.45. What are the heat transfer and work in the process? Solution: C.V. Water. This is a control mass Energy Eq.: m (u2 - u1 ) = 1Q2 - 1W2 Process : V = constant
1W2 =
P dV = 0
State 1:
T, x1 from Table B.1.1 v1 = vf + x1 vfg = 0.00106 + 0.25 0.8908 = 0.22376 m3/kg u1 = uf + x1 ufg = 503.48 + 0.25 2025.76 = 1009.92 kJ/kg
State 2:
T2, v2 = v1< vg2 = 0.50885 m3/kg
so two-phase
v2 - vf2 0.22376 - 0.00108 = = 0.43855 x2 = v 0.50777 fg2 u2 = uf2 + x2 ufg2 = 588.72 + x2 1961.3 = 1448.84 kJ/kg From the energy equation 1Q2 = m(u2 - u1) = 2 ( 1448.84 1009.92 ) = 877.8 kJ
P C.P.
140 C 120 C
T
C.P.
361.3 198.5
T v
140 120 v
Sonntag, Borgnakke and van Wylen
5.46 A 25 kg mass moves with 25 m/s. Now a brake system brings the mass to a complete stop with a constant deceleration over a period of 5 seconds. The brake energy is absorbed by 0.5 kg water initially at 20oC, 100 kPa. Assume the mass is at constant P and T. Find the energy the brake removes from the mass and the temperature increase of the water, assuming P = C. Solution: C.V. The mass in motion. E2 - E1= E = 0.5 mV = 0.5 25 25 /1000 = 7.8125 kJ C.V. The mass of water. m(u2 - u1) H2O = E = 7.8125 kJ Assume u2 = uf => u2 - u1 = 7.8125 / 0.5 = 15.63 T2 23.7oC, T = 3.7oC u2 = u1 + 15.63 = 83.94 + 15.63 = 99.565 kJ/kg then from Table B.1.1:
2 2
We could have used u2 - u1 = CT with C from Table A.4: C = 4.18 kJ/kg K giving T = 15.63/4.18 = 3.7oC.
Sonntag, Borgnakke and van Wylen
5.47 An insulated cylinder fitted with a piston contains R-12 at 25C with a quality of 90% and a volume of 45 L. The piston is allowed to move, and the R-12 expands until it exists as saturated vapor. During this process the R-12 does 7.0 kJ of work against the piston. Determine the final temperature, assuming the process is adiabatic. Solution: Take CV as the R-12. Continuity Eq.: m2 = m1 = m ; Energy Eq.5.11: State 1: (T, x) m(u2 - u1) = 1Q2 - 1W2 Tabel B.3.1 =>
v1 = 0.000763 + 0.9 0.02609 = 0.024244 m3/kg m = V1/v1 = 0.045/0.024244 = 1.856 kg u1 = 59.21 + 0.9 121.03 = 168.137 kJ/kg State 2: (x = 1, ?) We need one property information. Apply now the energy equation with known work and adiabatic so / 1Q2 = 0 = m(u2 - u1) + 1W2 = 1.856 (u2 - 168.137) + 7.0 => u2 = 164.365 kJ/kg = ug at T2 Table B.3.1 gives ug at different temperatures: T2 -15C P T
1
1 2 v
2 v
Sonntag, Borgnakke and van Wylen
5.48 A water-filled reactor with volume of 1 m3 is at 20 MPa, 360C and placed inside a containment room as shown in Fig. P5.48. The room is well insulated and initially evacuated. Due to a failure, the reactor ruptures and the water fills the containment room. Find the minimum room volume so the final pressure does not exceed 200 kPa. Solution: Solution: C.V.: Containment room and reactor. Mass: m2 = m1 = Vreactor/v1 = 1/0.001823 = 548.5 kg Energy: m(u2 - u1) = 1Q2 - 1W2 = 0 - 0 = 0 v1 = 0.001823 m3/kg, u1 = 1702.8 kJ/kg Energy equation then gives u2 = u1 = 1702.8 kJ/kg State 1: Table B.1.4 State 2: P2 = 200 kPa, u2 < ug => Two-phase Table B.1.2 x2 = (u2 - uf)/ ufg = (1702.8 504.47)/2025.02 = 0.59176 v2 = 0.001061 + 0.59176 0.88467 = 0.52457 m3/kg V2 = m2 v2 = 548.5 0.52457 = 287.7 m3 P 1 200 2 v T 1 2 200 kPa u = const v
P L T
1
C.P. 200 kPa 2 v
Sonntag, Borgnakke and van Wylen
5.49 A piston/cylinder arrangement contains water of quality x = 0.7 in the initial volume of 0.1 m3, where the piston applies a constant pressure of 200 kPa. The system is now heated to a final temperature of 200C. Determine the work and the heat transfer in the process. Take CV as the water. Continuity Eq.: Energy Eq.5.11: Process: P = constant
m2 = m1 = m ; m(u2 - u1) = 1Q2 - 1W2 1W2 = PdV = Pm(v2 - v1)
State 1: Table B.1.2 T1 = Tsat at 200 kPa = 120.23C v1 = vf + xvfg = 0.001061 + 0.7 0.88467 = 0.62033 m3 h1 = hf + xhfg = 504.68 + 0.7 2201.96 = 2046.05 kJ/kg Total mass can be determined from the initial condition, m = V1/v1 = 0.1/0.62033 = 0.1612 kg T2 = 200C, P2 = 200 kPa (Table B.1.3) gives v2 = 1.08034 m3/kg h2 = 2870.46 kJ/kg (Table B.1.3) V2 = mv2 = 0.1612 kg 1.08034 m3/kg = 0.174 m3 Substitute the work into the energy equation 1Q2 = U2 - U1 + 1W2 = m ( u2 u1 + Pv2 Pv1) = m(h2 - h1)
1Q2= 0.1612 kg (2870.46-2046.05) kJ/kg = 132.9 kJ (heat added to system).
P 1 2
T 2 1 V
V
Sonntag, Borgnakke and van Wylen
5.50 A piston/cylinder arrangement has the piston loaded with outside atmospheric pressure and the piston mass to a pressure of 150 kPa, shown in Fig. P5.50. It contains water at -2C, which is then heated until the water becomes saturated vapor. Find the final temperature and specific work and heat transfer for the process. Solution: C.V. Water in the piston cylinder. Continuity: m2 = m1, Energy Eq. per unit mass: Process: P = constant = P1, u2 - u1 = 1q2 - 1w2
2
=>
1w2 = P dv = P1(v2 - v1)
1
State 1: T1 , P1 => Table B.1.5 compressed solid, take as saturated solid. v1 = 1.0910-3 m3/kg, v2 = vg(P2) = 1.1593 m3/kg, From the process equation -3 1w2 = P1(v2 -v1) = 150(1.1593 -1.0910 ) = 173.7 kJ/kg From the energy equation
1q2 = u2 - u1 + 1w2 = 2519.7 - (-337.62) + 173.7 = 3031 kJ/kg
u1 = -337.62 kJ/kg
State 2: x = 1, P2 = P1 = 150 kPa due to process => Table B.1.2 T2 = 111.4C ; u2 = 2519.7 kJ/kg
P L S T 1 L+V S+V v 2 C.P. V
P C.P. 1
T
C.P. P=C
2 v
2 v
1
Sonntag, Borgnakke and van Wylen
5.51 A piston/cylinder assembly contains 1 kg of liquid water at 20oC and 300 kPa. There is a linear spring mounted on the piston such that when the water is heated the pressure reaches 1 MPa with a volume of 0.1 m3. Find the final temperature and the heat transfer in the process. Solution: Take CV as the water. m2 = m1 = m ; m(u2 - u1) = 1Q2 - 1W2 u1 = uf = 83.94 kJ/kg so T2 = Tsat = 179.9C State 1: Compressed liquid, take saturated liquid at same temperature. v1 = vf(20) = 0.001002 m3/kg, => Two phase as v2 < vg State 2: v2 = V2/m = 0.1/1 = 0.1 m3/kg and P = 1000 kPa x2 = (v2 - vf) /vfg = (0.1 - 0.001127)/0.19332 = 0.51145 u2 = uf + x2 ufg = 780.08 + 0.51147 1806.32 = 1703.96 kJ/kg Work is done while piston moves at linearly varying pressure, so we get
1W2 = P dV = area = Pavg (V2 - V1)
= 0.5 (300 + 1000)(0.1 - 0.001) = 64.35 kJ Heat transfer is found from the energy equation
1Q2 = m(u2 - u1) + 1W2 = 1 (1703.96 - 83.94) + 64.35 = 1684 kJ
P P2 P 1 1
cb
2
v
Sonntag, Borgnakke and van Wylen
5.52 A closed steel bottle contains ammonia at -20C, x = 20% and the volume is 0.05 m3. It has a safety valve that opens at a pressure of 1.4 MPa. By accident, the bottle is heated until the safety valve opens. Find the temperature and heat transfer when the valve first opens. Solution: C.V.: NH3 : m2 = m1 = m ; Energy Eq.5.11: m(u2 - u1) = 1Q2 - 1W2 P 2 Process: constant volume process 1W2 = 0 State 1: (T, x) Table B.2.1 v1 = 0.001504 + 0.2 0.62184 = 0.1259 m3/kg => m = V/v1 = 0.05/0.1259 = 0.397 kg u1 = 88.76 + 0.2 1210.7 = 330.9 kJ/kg State 2: P2 , v2 = v1 => superheated vapor, interpolate in Table B.2.2: 1 V
T 110C = 100 + 20(0.1259 0.12172)/(0.12986 0.12172), u2 = 1481 + (1520.7 1481) 0.51 = 1501.25 kJ/kg
1Q2 = m(u2 - u1) = 0.397(1501.25 330.9) = 464.6 kJ
Sonntag, Borgnakke and van Wylen
5.53 Two kg water at 200 kPa with a quality of 25% has its temperature raised 20oC in a constant pressure process. What are the heat transfer and work in the process?
C.V. Water. This is a control mass Energy Eq.: m (u2 - u1 ) = 1Q2 - 1W2 Process : P = constant
1W2 =
P dV = mP (v2 - v1)
State 1: Two-phase given P,x so use Table B.1.2 v1 = 0.001061 + 0.25 0.88467 = 0.22223 m3/kg u1 = 504047 + 0.25 2025.02 = 1010.725 kJ/kg T = T + 20 = 120.23 + 20 = 140.23 State 2 is superheated vapor 20 v2 = 0.88573 + 150-120.23 (0.95964 0.88573 ) = 0.9354 m3/kg 20 u2 = 2529.49 + 150-120.23 (2576.87- 2529.49) = 2561.32 kJ/kg From the process equation we get
1W2 = mP (v2 - v1) = 2 200 ( 0.9354 - 0.22223) = 285.3 kJ
From the energy equation
1Q2 = m (u2 - u1) + 1W2
= 2 ( 2561.32 1010.725 ) + 285.3 = 3101.2 + 285.27 = 3386.5 kJ
P 1 2
T 2 1 V
V
Sonntag, Borgnakke and van Wylen
5.54 Two kilograms of nitrogen at 100 K, x = 0.5 is heated in a constant pressure process to 300 K in a piston/cylinder arrangement. Find the initial and final volumes and the total heat transfer required. Solution: Take CV as the nitrogen. Continuity Eq.: m2 = m1 = m ; Energy Eq.5.11: Process: P = constant State 1: Table B.6.1 v1 = 0.001452 + 0.5 0.02975 = 0.01633 m3/kg, h1 = -73.20 + 0.5 160.68 = 7.14 kJ/kg State 2: (P = 779.2 kPa , 300 K) => sup. vapor interpolate in Table B.6.2 v2 = 0.14824 + (0.11115-0.14824) 179.2/200 = 0.115 m3/kg, V2 = 0.23 m3 h2 = 310.06 + (309.62-310.06) 179.2/200 = 309.66 kJ/kg Now solve for the heat transfer from the energy equation
1Q2 = m(u2 - u1) + 1W2 = m(h2 - h1) = 2 (309.66 - 7.14) = 605 kJ
m(u2 - u1) = 1Q2 - 1W2 1W2 = PdV = Pm(v2 - v1) V1 = 0.0327 m3
P 1 2
T 2 1 V
V
Sonntag, Borgnakke and van Wylen
5.55 A 1-L capsule of water at 700 kPa, 150C is placed in a larger insulated and otherwise evacuated vessel. The capsule breaks and its contents fill the entire volume. If the final pressure should not exceed 125 kPa, what should the vessel volume be? Solution: C.V. Larger vessel. Continuity: m2 = m1 = m = V/v1 = 0.916 kg Process: expansion with 1Q2 = 0 , 1W2 = 0 / / Energy: m(u2 - u1) = 1Q2 - 1W2 = 0 u2 = u1 / u1 uf = 631.66 kJ/kg 631.66 444.16 = 0.09061 2069.3 State 1: v1 vf = 0.001091 m3/kg; State 2: P2 , u2 x2 =
v2 = 0.001048 + 0.09061 1.37385 = 0.1255 m3/kg V2 = mv2 = 0.916 0.1255 = 0.115 m3 = 115 L
P 1 200 2 v
T 1 2 200 kPa u = const v
P L T
1
C.P. 200 kPa 2 v
Sonntag, Borgnakke and van Wylen
5.56 Superheated refrigerant R-134a at 20C, 0.5 MPa is cooled in a piston/cylinder arrangement at constant temperature to a final two-phase state with quality of 50%. The refrigerant mass is 5 kg, and during this process 500 kJ of heat is removed. Find the initial and final volumes and the necessary work. Solution: C.V. R-134a, this is a control mass. Continuity: m2 = m1 = m ; Energy Eq.5.11: m(u2 -u1) = 1Q2 - 1W2 = -500 - 1W2 v1 = 0.04226 m3/kg ; u1 = 390.52 kJ/kg State 1: T1 , P1 Table B.5.2,
=> V1 = mv1 = 0.211 m3 State 2: T2 , x2 Table B.5.1 u2 = 227.03 + 0.5 162.16 = 308.11 kJ/kg, v2 = 0.000817 + 0.5 0.03524 = 0.018437 m3/kg => V2 = mv2 = 0.0922 m3
1W2 = -500 - m(u2 - u1) = -500 - 5 (308.11 - 390.52) = -87.9 kJ
P 2 1
T 2 1
v
v
Sonntag, Borgnakke and van Wylen
5.57 A cylinder having a piston restrained by a linear spring (of spring constant 15 kN/m) contains 0.5 kg of saturated vapor water at 120C, as shown in Fig. P5.57. Heat is transferred to the water, causing the piston to rise. If the piston cross-sectional area is 0.05 m2, and the pressure varies linearly with volume until a final pressure of 500 kPa is reached. Find the final temperature in the cylinder and the heat transfer for the process. Solution: C.V. Water in cylinder. Continuity: m2 = m1 = m ; Energy Eq.5.11: m(u2 - u1) = 1Q2 - 1W2 v1 = 0.89186 m3/kg, 15 0.5 u1 = 2529.2 kJ/kg ksm State 1: (T, x) Table B.1.1 => Process: State 2: => P2 = P1 +
(v - v ) = 198.5 + (v - 0.89186) Ap2 2 1 (0.05)2 2
P2 = 500 kPa and on the process curve (see above equation). v2 = 0.89186 + (500 - 198.5) (0.052/7.5) = 0.9924 m3/kg => T2 = 803C; u2 = 3668 kJ/kg
(P, v) Table B.1.3
P1 + P2 W12 = PdV = 2 m(v2 - v1) 198.5 + 500 = 0.5 (0.9924 - 0.89186) = 17.56 kJ 2
1Q2 = m(u2 - u1) + 1W2 = 0.5 (3668 - 2529.2) + 17.56 = 587 kJ
P 2 1 ksm A2 p v
T 2 1 v
Sonntag, Borgnakke and van Wylen
5.58 A rigid tank is divided into two rooms by a membrane, both containing water, shown in Fig. P5.58. Room A is at 200 kPa, v = 0.5 m3/kg, VA = 1 m3, and room B contains 3.5 kg at 0.5 MPa, 400C. The membrane now ruptures and heat transfer takes place so the water comes to a uniform state at 100C. Find the heat transfer during the process. Solution: C.V.: Both rooms A and B in tank. A B
Continuity Eq.: Energy Eq.:
m2 = mA1 + mB1 ; m2u2 - mA1uA1 - mB1uB1 = 1Q2 - 1W2 mA1 = VA/vA1 = 1/0.5 = 2 kg
State 1A: (P, v) Table B.1.2,
v vf 0.5 - 0.001061 = = 0.564 xA1 = v 0.88467 fg uA1 = uf + x ufg = 504.47 + 0.564 2025.02 = 1646.6 kJ/kg State 1B: Table B.1.3, vB1 = 0.6173, uB1 = 2963.2, VB = mB1vB1 = 2.16 m3 Process constant total volume: m2 = mA1 + mB1 = 5.5 kg State 2: T2 , v2 Table B.1.1 x2 = Vtot = VA + VB = 3.16 m3 and 1W2 = 0 / => v2 = Vtot/m2 = 0.5746 m3/kg
two-phase as v2 < vg
v2 vf 0.5746 0.001044 = = 0.343 , 1.67185 vfg
u2 = uf + x ufg = 418.91 + 0.343 2087.58= 1134.95 kJ/kg Heat transfer is from the energy equation
1Q2 = m2u2 - mA1uA1 - mB1uB1 = -7421 kJ
Sonntag, Borgnakke and van Wylen
5.59 A 10-m high open cylinder, Acyl = 0.1 m2, contains 20C water above and 2 kg of 20C water below a 198.5-kg thin insulated floating piston, shown in Fig. P5.59. Assume standard g, Po. Now heat is added to the water below the piston so that it expands, pushing the piston up, causing the water on top to spill over the edge. This process continues until the piston reaches the top of the cylinder. Find the final state of the water below the piston (T, P, v) and the heat added during the process. Solution: C.V. Water below the piston. Piston force balance at initial state: F = F = PAA = mpg + mBg + P0A State 1A,B: Comp. Liq. v vf = 0.001002 m3/kg; VA1 = mAvA1 = 0.002 m3; mass above the piston u1A = 83.95 kJ/kg
mtot = Vtot/v = 1/0.001002 = 998 kg (198.5+996)*9.807 = 218.5 kPa 0.1*1000
mB1 = mtot - mA = 996 kg
PA1 = P0 + (mp + mB)g/A = 101.325 + State 2A:
mpg PA2 = P0 + A = 120.8 kPa ; vA2 = Vtot/ mA= 0.5 m3/kg
xA2 = (0.5 - 0.001047)/1.4183 = 0.352 ; T2 = 105C uA2 = 440.0 + 0.352 2072.34 = 1169.5 kJ/kg Continuity eq. in A: Process: mA2 = mA1
P
Energy: mA(u2 - u1) = 1Q2 - 1W2 P linear in V as mB is linear with V 1 1W2 = PdV = 2(218.5 + 120.82)(1 - 0.002) = 169.32 kJ
1Q2 = mA(u2 - u1) + 1W2 = 2170.1 + 169.3 = 2340.4 kJ
1 2 V
W
cb
Sonntag, Borgnakke and van Wylen
5.60 Assume the same setup as in Problem 5.48, but the room has a volume of 100 m3. Show that the final state is two-phase and find the final pressure by trial and error. C.V.: Containment room and reactor. Mass: m2 = m1 = Vreactor/v1 = 1/0.001823 = 548.5 kg Energy: m(u2 - u1) = 1Q2 - 1W2 = 0 - 0 = 0 u2 = u1 = 1702.8 kJ/kg v2 = Vroom/m2 = 0.1823 m3/kg Total volume and mass =>
State 2: u2 , v2 Table B.1.1 see Figure. Note that in the vicinity of v = 0.1823 m3/kg crossing the saturated vapor line the internal energy is about 2585 kJ/kg. However, at the actual state 2, u = 1702.8 kJ/kg. Therefore state 2 must be in the two-phase region. T Trial & error v = vf + xvfg ; u = uf + xufg
1060 kPa 1060 kPa u=2585
v2 - vf u2 = 1702.8 = uf + v ufg fg Compute RHS for a guessed pressure P2:
sat vap 0.184
v
P2 = 600 kPa: RHS = 669.88 + P2 = 550 kPa: RHS = 655.30 +
0.1823-0.001101 1897.52 = 1762.9 0.31457 0.1823-0.001097 1909.17 = 1668.1 0.34159 P2 568.5 kPa
too large too small
Linear interpolation to match u = 1702.8 gives
Sonntag, Borgnakke and van Wylen
Energy Equation: Multistep Solution
5.61 10 kg of water in a piston cylinder arrangement exists as saturated liquid/vapor at 100 kPa, with a quality of 50%. It is now heated so the volume triples. The mass of the piston is such that a cylinder pressure of 200 kPa will float it, as in Fig. 4.68. Find the final temperature and the heat transfer in the process. Solution: Take CV as the water. m2 = m1 = m ; m(u2 - u1) = 1Q2 - 1W2 Process: v = constant until P = Plift , then P is constant. State 1: Two-phase so look in Table B.1.2 at 100 kPa u1 = 417.33 + 0.5 2088.72 = 1461.7 kJ/kg, v1 = 0.001043 + 0.5 1.69296 = 0.8475 m3/kg State 2: v2, P2 Plift => v2 = 3 0.8475 = 2.5425 m3/kg ; Interpolate: T2 = 829C, u2 = 3718.76 kJ/kg => V2 = mv2 = 25.425 m3
1W2 = Plift(V2 -V1) = 200 10 (2.5425 - 0.8475) = 3390 kJ 1Q2 = m(u2 - u1) + 1W2 = 10(3718.76 - 1461.7) + 3390 = 25 961 kJ
Po
cb
P 2 P2
H2O
P1
1
cb
V
Sonntag, Borgnakke and van Wylen
5.62 Two tanks are connected by a valve and line as shown in Fig. P5.62. The volumes are both 1 m3 with R-134a at 20C, quality 15% in A and tank B is evacuated. The valve is opened and saturated vapor flows from A into B until the pressures become equal. The process occurs slowly enough that all temperatures stay at 20C during the process. Find the total heat transfer to the R-134a during the process. Solution: C.V.: A + B State 1A: vA1 = 0.000817 + 0.15 0.03524 = 0.006103 m3/kg uA1 = 227.03 + 0.15 162.16 = 251.35 kJ/kg mA1 = VA/vA1 = 163.854 kg Process: Constant temperature and constant total volume. m2 = mA1 ; V2 = VA + VB = 2 m3 ; v2 = V2/m2 = 0.012206 m3/kg
1W2 =
P dV = 0
State 2: T2 , v2 x2 = (0.012206 0.000817)/0.03524 = 0.3232 u2 = 227.03 + 0.3232 162.16 = 279.44 kJ/kg
1Q2 = m2u2 - mA1uA1 - mB1uB1 + 1W2 = m2(u2 - uA1)
= 163.854 (279.44 - 251.35) = 4603 kJ
A
B
Sonntag, Borgnakke and van Wylen
5.63 Consider the same system as in the previous problem. Let the valve be opened and transfer enough heat to both tanks so all the liquid disappears. Find the necessary heat transfer. Solution: C.V. A + B, so this is a control mass. State 1A: vA1 = 0.000817 + 0.15 0.03524 = 0.006 103 m3/kg uA1 = 227.03 + 0.15 162.16 = 251.35 kJ/kg mA1 = VA/vA1 = 163.854 kg Process: Constant temperature and total volume. m2 = mA1 ; V2 = VA + VB = 2 m3 ; v2 = V2/m2 = 0.012 206 m3/kg State 2: x2 = 100%, v2 = 0.012206 T2 = 55 + 5 (0.012206 0.01316)/(0.01146 0.01316) = 57.8C u2 = 406.01 + 0.56 (407.85 406.01) = 407.04 kJ/kg
1Q2 = m2(u2 - uA1) = 163.854 (407.04 - 251.35) = 25 510 kJ
A
B
Sonntag, Borgnakke and van Wylen
5.64 A vertical cylinder fitted with a piston contains 5 kg of R-22 at 10C, shown in Fig. P5.64. Heat is transferred to the system, causing the piston to rise until it reaches a set of stops at which point the volume has doubled. Additional heat is transferred until the temperature inside reaches 50C, at which point the pressure inside the cylinder is 1.3 MPa. a. What is the quality at the initial state? b. Calculate the heat transfer for the overall process. Solution: C.V. R-22. Control mass goes through process: 1 -> 2 -> 3 As piston floats pressure is constant (1 -> 2) and the volume is constant for the second part (2 -> 3). So we have: v3 = v2 = 2 v1 State 3: Table B.4.2 (P,T) v3 = 0.02015 m3/kg, u3 = 248.4 kJ/kg P P o
cb
3
R-22
1
2 V
So we can then determine state 1 and 2 Table B.4.1: v1 = 0.010075 = 0.0008 + x1 0.03391 => b) u1 = 55.92 + 0.2735 173.87 = 103.5 kJ/kg State 2: v2 = 0.02015 m3/kg, P2 = P1 = 681 kPa
2
x1 = 0.2735 this is still 2-phase.
1W3 = 1W2 = PdV = P1(V2 - V1) = 681 5 (0.02 - 0.01) = 34.1 kJ 1
1Q3 = m(u3-u1) + 1W3 = 5(248.4 - 103.5) + 34.1 = 758.6 kJ
Sonntag, Borgnakke and van Wylen
5.65 Find the heat transfer in Problem 4.67. A piston/cylinder contains 1 kg of liquid water at 20C and 300 kPa. Initially the piston floats, similar to the setup in Problem 4.64, with a maximum enclosed volume of 0.002 m3 if the piston touches the stops. Now heat is added so a final pressure of 600 kPa is reached. Find the final volume and the work in the process. Solution: Take CV as the water. Properties from table B.1 m2 = m1 = m ; m(u2 - u1) = 1Q2 - 1W2 State 1: Compressed liq. v = vf (20) = 0.001002 m3/kg, u = uf = 83.94 kJ/kg so 2-phase T = Tsat = 158.85 C State 2: Since P > Plift then v = vstop = 0.002 and P = 600 kPa For the given P : vf < v < vg v = 0.002 = 0.001101 + x (0.3157-0.001101) => x = 0.002858 u = 669.88 + 0.002858 1897.5 = 675.3 kJ/kg Work is done while piston moves at Plift= constant = 300 kPa so we get
1W2 = P dV = m Plift (v2 -v1) = 1300(0.002 - 0.001002) = 0.299 kJ
Heat transfer is found from energy equation
1Q2 = m(u2 - u1) + 1W2 = 1(675.3 - 83.94) + 0.299 = 591.66 kJ
P P o
cb
H2O
1
2 V
Sonntag, Borgnakke and van Wylen
5.66 Refrigerant-12 is contained in a piston/cylinder arrangement at 2 MPa, 150C with a massless piston against the stops, at which point V = 0.5 m3. The side above the piston is connected by an open valve to an air line at 10C, 450 kPa, shown in Fig. P5.66. The whole setup now cools to the surrounding temperature of 10C. Find the heat transfer and show the process in a Pv diagram. C.V.: R-12. Control mass. Continuity: m = constant, Energy Eq.5.11: m(u2 - u1) = 1Q2 - 1W2 Process: F = F = P A = PairA + Fstop R-22 if V < Vstop Fstop = 0 / This is illustrated in the P-v diagram shown below. State 1: v1 = 0.01265 m3/kg, u1 = 252.1 kJ/kg m = V/v = 39.523 kg State 2: T2 and on line compressed liquid, see figure below. v2 vf = 0.000733 m3/kg V2 = 0.02897 m3; u2 = uf = 45.06 kJ/kg
Air line
1W2 = PdV = Plift(V2 - V1) = 450 (0.02897 - 0.5) = -212.0 kJ ;
Energy eq.
1Q2 = 39.526 (45.06 - 252.1) - 212 = -8395 kJ
P 2 MPa T = 10 450 kPa 2 v 11.96 10 1 150 ~73
T
P = 2 MPa 1 P = 450 kPa
2
v
Sonntag, Borgnakke and van Wylen
5.67 Find the heat transfer in Problem 4.114. A piston/cylinder (Fig. P4.114) contains 1 kg of water at 20C with a volume of 0.1 m3. Initially the piston rests on some stops with the top surface open to the atmosphere, Po and a mass so a water pressure of 400 kPa will lift it. To what temperature should the water be heated to lift the piston? If it is heated to saturated vapor find the final temperature, volume and the work, 1W2. Solution: C.V. Water. This is a control mass. m2 = m1 = m ; m(u2 - u1) = 1Q2 - 1W2 P P o H2O
1a 1
2
V
State 1: 20 C, v1 = V/m = 0.1/1 = 0.1 m3/kg x = (0.1 - 0.001002)/57.789 = 0.001713 u1 = 83.94 + 0.001713 2318.98 = 87.92 kJ/kg To find state 2 check on state 1a: P = 400 kPa, Table B.1.2: v = v1 = 0.1 m3/kg vf < v < vg = 0.4625 m3/kg
State 2 is saturated vapor at 400 kPa since state 1a is two-phase. v2 = vg = 0.4625 m3/kg , V2 = m v2 = 0.4625 m3, u2 = ug= 2553.6 kJ/kg Pressure is constant as volume increase beyond initial volume.
1W2 =
P dV = P (V2 - V1) = Plift (V2 V1) = 400 (0.4625 0.1) = 145 kJ
1Q2 = m(u2 - u1) + 1W2 = 1 (2553.6 87.92) + 145 = 2610.7 kJ
Sonntag, Borgnakke and van Wylen
5.68 A rigid container has two rooms filled with water, each 1 m3 separated by a wall. Room A has P = 200 kPa with a quality x = 0.80. Room B has P = 2 MPa and T = 400C. The partition wall is removed and the water comes to a uniform state, which after a while due to heat transfer has a temperature of 200C. Find the final pressure and the heat transfer in the process. Solution: C.V. A + B. Constant total mass and constant total volume. Continuity: m2 mA1 mB1= 0 ; V2= VA+ VB= 2 m3 =>
1W2 = 0
Energy Eq.5.11: U2 U1 = m2u2 mA1uA1 mA1uA1 = 1Q2 1W2 = 1Q2 Process: V = VA + VB = constant State 1A: Table B.1.2 uA1= 504.47 + 0.8 2025.02 = 2124.47 kJ/kg, vA1= 0.001061 + 0.8 0.88467 = 0.70877 m3/kg State 1B: Table B.1.3 u B1= 2945.2, vB1= 0.1512 mB1= 1/vB1= 6.614 kg mA1= 1/vA1= 1.411 kg
State 2: T2, v2 = V2/m 2= 2/(1.411 + 6.614) = 0.24924 m3/kg Table B.1.3 superheated vapor. 800 kPa < P2 < 1 MPa Interpolate to get the proper v2 0.24924-0.2608 P2 800 + 0.20596-0.2608 200 = 842 kPa From the energy equation
1Q2 = 8.025 2628.8 1.411 2124.47 6.614 2945.2 = - 1381 kJ
u2 2628.8 kJ/kg
P PB1
B1
A Q
B
PA1
2
A1 v
Sonntag, Borgnakke and van Wylen
5.69 The cylinder volume below the constant loaded piston has two compartments A and B filled with water. A has 0.5 kg at 200 kPa, 150oC and B has 400 kPa with a quality of 50% and a volume of 0.1 m3. The valve is opened and heat is transferred so the water comes to a uniform state with a total volume of 1.006 m3. a) Find the total mass of water and the total initial volume. b) Find the work in the process c) Find the process heat transfer. Solution: Take the water in A and B as CV. Continuity: m2 - m1A - m1B = 0 Energy: Process: m2u2 - m1Au1A - m1Bu1B = 1Q2 - 1W2 P = constant = P1A if piston floats (VA positive) i.e. if V2 > VB = 0.1 m3
State A1: Sup. vap. Table B.1.3 v = 0.95964 m3/kg, u = 2576.9 kJ/kg => V = mv = 0.5 0.95964 = 0.47982 State B1: Table B.1.2 v = (1-x) 0.001084 + x 0.4625 = 0.2318 m3/kg => m = V/v = 0.4314 kg u = 604.29 + 0.5 1949.3 = 1578.9 kJ/kg State 2: 200 kPa, v2 = V2/m = 1.006/0.9314 = 1.0801 m3/kg Table B.1.3 => close to T2 = 200oC and u2 = 2654.4 kJ/kg So now V1 = 0.47982 + 0.1 = 0.5798 m3, m1 = 0.5 + 0.4314 = 0.9314 kg Since volume at state 2 is larger than initial volume piston goes up and the pressure then is constant (200 kPa which floats piston).
1W2 = P dV = Plift (V2 - V1) = 200 (1.006 - 0.57982) = 85.24 kJ 1Q2 = m2u2 - m1Au1A - m1Bu1B + 1W2
= 0.9314 2654.4 - 0.5 2576.9 - 0.4314 1578.9 + 85.24 = 588 kJ
Sonntag, Borgnakke and van Wylen
5.70 A rigid tank A of volume 0.6 m3 contains 3 kg water at 120oC and the rigid tank B is 0.4 m3 with water at 600 kPa, 200oC. They are connected to a piston cylinder initially empty with closed valves. The pressure in the cylinder should be 800 kPa to float the piston. Now the valves are slowly opened and heat is transferred so the water reaches a uniform state at 250oC with the valves open. Find the final volume and pressure and the work and heat transfer in the process. C.V.: A + B + C. Only work in C, total mass constant. m2 - m1 = 0
1W2 =
C
=>
m2 = mA1 + mB1
U2 - U1 = 1Q2 - 1W2 ;
A
B
PdV = Plift (V2 - V1)
1A: v = 0.6/3 = 0.2 m3/kg => xA1 = (0.2 - 0.00106)/0.8908 = 0.223327 u = 503.48 + 0.223327 2025.76 = 955.89 kJ/kg 3/kg => m = 0.4/0.35202 = 1.1363 kg ; u = 2638.91 kJ/kg 1B: v = 0.35202 m B1 m2 = 3 + 1.1363 = 4.1363 kg V2 = VA+ VB + VC = 1 + VC Locate state 2: Must be on P-V lines shown State 1a: 800 kPa, V +V v1a = Am B = 0.24176 m3/kg 800 kPa, v1a => T = 173C => too low. v = 0.29314 m3/kg > v1a OK
P2 V
and
P 1a 2
Assume 800 kPa: 250C
Final state is : 800 kPa; 250C => u2 = 2715.46 kJ/kg W = 800(0.29314 - 0.24176) 4.1363 = 800 (1.2125 - 1) = 170 kJ Q = m2u2 - m1u1 + 1W2 = m2u2 - mA1uA1 - mB1uB1 + 1W2 = 4.1363 2715.46 - 3 955.89 - 1.1363 2638.91 + 170 = 11 232 - 2867.7 - 2998.6 + 170 = 5536 kJ
Sonntag, Borgnakke and van Wylen
5.71 Calculate the heat transfer for the process described in Problem 4.60. A cylinder containing 1 kg of ammonia has an externally loaded piston. Initially the ammonia is at 2 MPa, 180C and is now cooled to saturated vapor at 40C, and then further cooled to 20C, at which point the quality is 50%. Find the total work for the process, assuming a piecewise linear variation of P versus V. Solution: C.V. Ammonia going through process 1 - 2 - 3. Control mass. Continuity: m = constant, Energy Eq.5.11: m(u3 - u1) = 1Q3 - 1W3 Process: P is piecewise linear in V State 1: (T, P) Table B.2.2: v1 = 0.10571 m3/kg, u1 = 1630.7 kJ/kg State 2: (T, x) Table B.2.1 sat. vap. P2 = 1555 kPa, v2 = 0.08313 m3/kg
P 2000 1555 857 3 1 2
o
180 C o 40 C 20 C
v
o
State 3: (T, x)
P3 = 857 kPa, u3 = (272.89 + 1332.2)/2 = 802.7 kJ/kg
v3 = (0.001638+0.14922)/2 = 0.07543
3 1
Process: piecewise linear P versus V, see diagram. Work is area as: W13 = PdV ( = P2 + P3 + P1 P2 ) m(v2 - v1) + ( 2 ) m(v3 - v2) 2
2000 + 1555 1555 + 857 1(0.08313 - 0.10571) + 1(0.07543 - 0.08313) 2 2
= -49.4 kJ From the energy equation, we get the heat transfer as:
1Q3 = m(u3 - u1) + 1W3 = 1 (802.7 - 1630.7) - 49.4 = -877.4 kJ
Sonntag, Borgnakke and van Wylen
5.72 Calculate the heat transfer for the process described in Problem 4.70. A piston cylinder setup similar to Problem 4.24 contains 0.1 kg saturated liquid and vapor water at 100 kPa with quality 25%. The mass of the piston is such that a pressure of 500 kPa will float it. The water is heated to 300C. Find the final pressure, volume and the work, 1W2. Solution: Take CV as the water: m2 = m1 = m Energy Eq.5.11: Process: m(u2 - u1) = 1Q2 - 1W2 P = Plift v = constant until
Plift P1
P 1a 1
cb
2
To locate state 1: Table B.1.2 v1 = 0.001043 + 0.251.69296 = 0.42428 m3/kg u1 = 417.33 + 0.252088.7 = 939.5 kJ/kg
V
State 1a: 500 kPa, v1a = v1 = 0.42428 > vg at 500 kPa, so state 1a is superheated vapor Table B.1.3 T1a = 200C State 2 is 300C so heating continues after state 1a to 2 at constant P = 500 kPa. 2: T2, P2 = Plift => Tbl B.1.3 v2 =0.52256 m3/kg; u2 = 2802.9 kJ/kg From the process, see also area in P-V diagram
1W2 = Plift m(v2 - v1) = 500 0.1 (0.5226 - 0.4243) = 4.91 kJ
From the energy equation
1Q2 = m(u2 - u1) + 1W2 = 0.1(2802.9 - 939.5) + 4.91 = 191.25 kJ
Sonntag, Borgnakke and van Wylen
5.73 A cylinder/piston arrangement contains 5 kg of water at 100C with x = 20% and the piston, mP = 75 kg, resting on some stops, similar to Fig. P5.73. The outside pressure is 100 kPa, and the cylinder area is A = 24.5 cm2. Heat is now added until the water reaches a saturated vapor state. Find the initial volume, final pressure, work, and heat transfer terms and show the Pv diagram. Solution: C.V. The 5 kg water. Continuty: m2 = m1 = m ; Energy: m(u2 - u1) = 1Q2 - 1W2 Process: V = constant if P < Plift otherwise P = Plift see P-v diagram. 75 9.807 P3 = P2 = Plift = P0 + mp g / Ap = 100 + 0.00245 1000 = 400 kPa
P 2 3 143 C 100 C v
o o
P o
cb
H2O
cb
1
State 1: (T,x) Table B.1.1 v1 = 0.001044 + 0.2 1.6719, V1 = mv1 = 5 0.3354 = 1.677 m3 u1 = 418.91 + 0.2 2087.58 = 836.4 kJ/kg State 3: (P, x = 1) Table B.1.2 => v3 = 0.4625 > v1, u3 = 2553.6 kJ/kg Work is seen in the P-V diagram (if volume changes then P = Plift)
1W3 = 2W3 = Pextm(v3 - v2) = 400 5(0.46246 - 0.3354) = 254.1 kJ
Heat transfer is from the energy equation
1Q3 = 5 (2553.6 - 836.4) + 254.1 = 8840 kJ
Sonntag, Borgnakke and van Wylen
Energy Equation: Solids and Liquids
5.74 Because a hot water supply must also heat some pipe mass as it is turned on so it does not come out hot right away. Assume 80oC liquid water at 100 kPa is cooled to 45oC as it heats 15 kg of copper pipe from 20 to 45oC. How much mass (kg) of water is needed? Solution: C.V. Water and copper pipe. No external heat transfer, no work. Energy Eq.5.11: U2 U1 = Ucu + UH2O = 0 0 From Eq.5.18 and Table A.3: kJ Ucu = mC = 15 kg 0.42 kg K (45 20) K = 157.5 kJ From the energy equation mH2O = - Ucu / uH2O mH2O = Ucu / CH2O(- H2O) = or using Table B.1.1 for water 157.5 mH2O = Ucu / ( u1- u2) = 334.84 188.41 = 1.076 kg Cu pipe Water The real problem involves a flow and is not analyzed by this simple process. 157.5 = 1.076 kg 4.18 35
Sonntag, Borgnakke and van Wylen
5.75 A house is being designed to use a thick concrete floor mass as thermal storage material for solar energy heating. The concrete is 30 cm thick and the area exposed to the sun during the daytime is 4 m 6 m. It is expected that this mass will undergo an average temperature rise of about 3C during the day. How much energy will be available for heating during the nighttime hours? Solution: C.V. The mass of concrete. Concrete is a solid with some properties listed in Table A.3 V = 4 6 0.3 = 7.2 m3 ; m = V = 2200 kg/m3 7.2 m3 = 15 840 kg From Eq.5.18 and C from table A.3 kJ U = m C T = 15840 kg 0.88 kg K 3 K = 41818 kJ = 41.82 MJ
Sonntag, Borgnakke and van Wylen
5.76 A copper block of volume 1 L is heat treated at 500C and now cooled in a 200-L oil bath initially at 20C, shown in Fig. P5.76. Assuming no heat transfer with the surroundings, what is the final temperature? Solution: C.V. Copper block and the oil bath. Also assume no change in volume so the work will be zero. Energy Eq.: U2 - U1 = mmet(u2 - u1)met + moil(u2 - u1)oil = 1Q2 - 1W2 = 0 Properties from Table A.3 and A.4 mmet = V = 0.001 m3 8300 kg/m3 = 8.3 kg, moil = V = 0.2 m3 910 kg/m3 = 182 kg Solid and liquid Eq.5.17: u Cv T, Table A.3 and A.4: kJ kJ Cv met = 0.42 kg K, Cv oil = 1.8 kg K
The energy equation for the C.V. becomes mmetCv met(T2 - T1,met) + moilCv oil(T2 - T1,oil) = 0 8.3 0.42(T2 - 500) + 182 1.8 (T2 - 20) = 0 331.09 T2 1743 6552 = 0 T2 = 25 C
Sonntag, Borgnakke and van Wylen
5.77 A 1 kg steel pot contains 1 kg liquid water both at 15oC. It is now put on the stove where it is heated to the boiling point of the water. Neglect any air being heated and find the total amount of energy needed. Solution: Energy Eq.: U2 - U1= 1Q2 - 1W2
The steel does not change volume and the change for the liquid is minimal, so 1W2 0.
State 2:
T2 = Tsat (1atm) = 100oC u2 = 418.91 kJ/kg
Tbl B.1.1 : u1 = 62.98 kJ/kg, Tbl A.3 : Cst = 0.46 kJ/kg K
Solve for the heat transfer from the energy equation 1Q2 = U2 - U1 = mst (u2 - u1)st + mH2O (u2 - u1)H2O = mstCst (T2 T1) + mH2O (u2 - u1)H2O
1Q2 = 1 kg 0.46 kg K (100 15) K + 1 kg (418.91 62.98) kJ/kg
kJ
= 39.1 + 355.93 = 395 kJ
Sonntag, Borgnakke and van Wylen
5.78 A car with mass 1275 kg drives at 60 km/h when the brakes are applied quickly to decrease its speed to 20 km/h. Assume the brake pads are 0.5 kg mass with heat capacity of 1.1 kJ/kg K and the brake discs/drums are 4.0 kg steel. Further assume both masses are heated uniformly. Find the temperature increase in the brake assembly. Solution: C.V. Car. Car loses kinetic energy and brake system gains internal u. No heat transfer (short time) and no work term. m = constant; Energy Eq.5.11: E2 - E1 = 0 - 0 = mcar 2(V2 - V1) + mbrake(u2 - u1)
1 2 2
The brake system mass is two different kinds so split it, also use Cv from Table A.3 since we do not have a u table for steel or brake pad material.
2 1000 msteel Cv T + mpad Cv T = mcar 0.5 (602 - 202) 3600 m2/s2 kJ (4 0.46 + 0.5 1.1) K T = 1275 kg 0.5 (3200 0.077 16) m2/s2
= 157 406 J = 157.4 kJ => T = 65.9 C
Sonntag, Borgnakke and van Wylen
5.79 Saturated, x = 1%, water at 25C is contained in a hollow spherical aluminum vessel with inside diameter of 0.5 m and a 1-cm thick wall. The vessel is heated until the water inside is saturated vapor. Considering the vessel and water together as a control mass, calculate the heat transfer for the process. Solution: C.V. Vessel and water. This is a control mass of constant volume. Continuity Eq.: m2 = m1 Energy Eq.5.11: Process: U2 - U1 = 1Q2 - 1W2 = 1Q2 used above V = constant => 1W2 = 0
State 1: v1 = 0.001003 + 0.01 43.359 = 0.4346 m3/kg u1 = 104.88 + 0.01 2304.9 = 127.9 kJ/kg State 2: x2 = 1 and constant volume so v2 = v1 = V/m vg T2 = v1 = 0.4346 => T2 = 146.1C; u2 = uG2 = 2555.9 0.06545 VINSIDE = 6 (0.5)3 = 0.06545 m3 ; mH2O = 0.4346 = 0.1506 kg Valu = 6((0.52)3 - (0.5)3) = 0.00817 m3 malu = aluValu = 2700 0.00817 = 22.065 kg From the energy equation 1Q2 = U2 - U1 = mH2O(u2 - u1)H2O + maluCv alu(T2 - T1) = 0.1506(2555.9 - 127.9) + 22.065 0.9(146.1 - 25) = 2770.6 kJ
Sonntag, Borgnakke and van Wylen
5.80 A 25 kg steel tank initially at 10oC is filled up with 100 kg of milk (assume properties as water) at 30oC. The milk and the steel come to a uniform temperature of +5 oC in a storage room. How much heat transfer is needed for this process? Solution: C.V. Steel + Milk. This is a control mass. Energy Eq.5.11: U2 - U1 = 1Q2 - 1W2 = 1Q2 Process: V = constant, so there is no work
1W2 = 0.
Use Eq.5.18 and values from A.3 and A.4 to evaluate changes in u
1Q2 = msteel (u2 - u1)steel + mmilk(u2 - u1)milk
kJ kJ = 25 kg 0.466 kg K [5 - (-10)] + 100 kg 4.18 kg K (5 - 30) = 172.5 - 10450 = -10277 kJ
Sonntag, Borgnakke and van Wylen
5.81 An engine consists of a 100 kg cast iron block with a 20 kg aluminum head, 20 kg steel parts, 5 kg engine oil and 6 kg glycerine (antifreeze). Everything begins at 5oC and as the engine starts we want to know how hot it becomes if it absorbs a net of 7000 kJ before it reaches a steady uniform temperature. Energy Eq.: U2 - U1= 1Q2 - 1W2
Process: The steel does not change volume and the change for the liquid is minimal, so 1W2 0. So sum over the various parts of the left hand side in the energy equation mFe (u2 - u1) + mAl (u2 - u1)Al + mst (u - u1)st + moil (u2 - u1)oil + mgly (u2 - u1)gly = 1Q2 Tbl A.3 : CFe = 0.42 , CAl = 0.9, Cst = 0.46 all units of kJ/kg K Tbl A.4 : Coil = 1.9 , Cgly = 2.42 all units of kJ/kg K So now we factor out T2 T1 as u2 - u1 = C(T2 T1) for each term [ mFeCFe + mAlCAl + mstCst+ moilCoil + mglyCgly ] (T2 T1) = 1Q2 T2 T1 = 1Q2 / mi Ci 7000 100 0.42 + 20 0.9 + 20 0.46 + 5 1.9 + 6 2.42 7000 = 93.22 = 75 K = T2 = T1 + 75 = 5 + 75 = 80oC
Air intake filter Shaft power Fan Radiator Atm. air
Exhaust flow
Coolant flow
Sonntag, Borgnakke and van Wylen
Properties (u, h, Cv and Cp), Ideal Gas
5.82 Use the ideal gas air table A.7 to evaluate the heat capacity Cp at 300 K as a slope of the curve h(T) by h/T. How much larger is it at 1000 K and 1500 K. Solution : From Eq.5.24: dh h h320 - h290 = = 1.005 kJ/kg K Cp = dT = T 320 - 290 1000K Cp = 1500K Cp = h h1050 - h950 1103.48 - 989.44 = = = 1.140 kJ/kg K 100 T 1050 - 950 h h1550 - h1450 1696.45 - 1575.4 = = = 1.21 kJ/kg K 100 T 1550 - 1450
Notice an increase of 14%, 21% respectively. h C p 1500
Cp 300 300 1000 1500
T
Sonntag, Borgnakke and van Wylen
5.83 We want to find the change in u for carbon dioxide between 600 K and 1200 K. a) Find it from a constant Cvo from table A.5 b) Find it from a Cvo evaluated from equation in A.6 at the average T. c) Find it from the values of u listed in table A.8 Solution : a) b) u Cvo T = 0.653 (1200 600) = 391.8 kJ/kg 1 Tavg = 2 (1200 + 600) = 900, T 900 = 1000 = 1000 = 0.9
Cpo = 0.45 + 1.67 0.9 - 1.27 0.92 + 0.39 0.93 = 1.2086 kJ/kg K Cvo = Cpo R = 1.2086 0.1889 = 1.0197 kJ/kg K u = 1.0197 (1200 600) = 611.8 kJ/kg c) u = 996.64 392.72 = 603.92 kJ/kg u u1200
u600 300 600 1200
T
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5.84 We want to find the change in u for oxygen gas between 600 K and 1200 K. a) Find it from a constant Cvo from table A.5 b) Find it from a Cvo evaluated from equation in A.6 at the average T. c) Find it from the values of u listed in table A.8 Solution: a) b) u Cvo T = 0.662 (1200 - 600) = 397.2 kJ/kg 1 Tavg = 2 (1200 + 600) = 900 K, T 900 = 1000 = 1000 = 0.9
Cpo = 0.88 - 0.0001 0.9 + 0.54 0.92 - 0.33 0.93 = 1.0767 Cvo = Cpo - R = 1.0767 - 0.2598 = 0.8169 kJ/kg K u = 0.8169 (1200 - 600)= 490.1 kJ/kg c) u = 889.72 - 404.46 = 485.3 kJ/kg
u u1200
u600 300 600 1200
T
Sonntag, Borgnakke and van Wylen
5.85 Water at 20C, 100 kPa, is brought to 200 kPa, 1500C. Find the change in the specific internal energy, using the water table and the ideal gas water table in combination. Solution: State 1: Table B.1.1 u1 uf = 83.95 kJ/kg State 2: Highest T in Table B.1.3 is 1300C Using a u from the ideal gas tables, A.8, we get u1500 = 3139 kJ/kg u1300 = 2690.72 kJ/kg u1500 - u1300 = 448.26 kJ/kg We now add the ideal gas change at low P to the steam tables, B.1.3, ux = 4683.23 kJ/kg as the reference. u2 - u1 = (u2 - ux)ID.G. + (ux - u1) = 448.28 + 4683.23 - 83.95 = 5048 kJ/kg
Sonntag, Borgnakke and van Wylen
5.86 We want to find the increase in temperature of nitrogen gas at 1200 K when the specific internal energy is increased with 40 kJ/kg. a) Find it from a constant Cvo from table A.5 b) Find it from a Cvo evaluated from equation in A.6 at 1200 K. c) Find it from the values of u listed in table A.8 Solution : u = uA.8 Cv avg T Cvo T a) b) 40 T = u / Cvo = 0.745 = 53.69C = 1200 / 1000 =1.2 Cpo = 1.11 0.48 1.2 + 0.96 1.22 0.42 1.2 3 = 1.1906 kJ/kg K Cvo = Cpo R = 1.1906 0.2968 = 0.8938 kJ/kg K T = u / Cvo = 40 / 0.8938 = 44.75C c) u = u1 + u = 957 + 40 = 997 kJ/kg less than 1300 K so linear interpolation. 1300 1200 T = 1048.46 957 40 = 43.73C Cvo
(1048.46 957) / 100 = 0.915 kJ/kg K
So the formula in A.6 is accurate within 2.3%.
Sonntag, Borgnakke and van Wylen
5.87 For an application the change in enthalpy of carbon dioxide from 30 to 1500C at 100 kPa is needed. Consider the following methods and indicate the most accurate one. a. Constant specific heat, value from Table A.5. b. Constant specific heat, value at average temperature from the equation in Table A.6. c. Variable specific heat, integrating the equation in Table A.6. d. Enthalpy from ideal gas tables in Table A.8. Solution: a) b) h = CpoT = 0.842 (1500 - 30) = 1237.7 kJ/kg Tave = 2 (30 + 1500) + 273.15 = 1038.15 K; = T/1000 = 1.0382 Table A.6 Cpo =1.2513 h = Cpo,ave T = 1.2513 1470 = 1839 kJ/kg c) For the entry to Table A.6: h = h2- h1 = Cpo dT = [0.45 (2 - 1) + 1.67 2 (22 - 12) 1 1 4 4 1.27 3 (23 - 13) + 0.39 4 (2 - 1 )] = 1762.76 kJ/kg d) h = 1981.35 217.12 = 1764.2 kJ/kg
1 1
2 = 1.77315 ; 1 = 0.30315
The result in d) is best, very similar to c). For large T or small T at high Tavg, a) is very poor.
Sonntag, Borgnakke and van Wylen
5.88 An ideal gas is heated from 500 to 1500 K. Find the change in enthalpy using constant specific heat from Table A.5 (room temperature value) and discuss the accuracy of the result if the gas is a. Argon b. Oxygen c. Carbon dioxide Solution: T1 = 500 K, T2 = 1500 K, h = CP0(T2-T1) a) Ar : h = 0.520(1500-500) = 520 kJ/kg Monatomic inert gas very good approximation. b) O2 : h = 0.922(1500-500) = 922 kJ/kg Diatomic gas approximation is OK with some error. c) CO2: h = 0.842(1500-500) = 842 kJ/kg Polyatomic gas heat capacity changes, see figure 5.11 See also appendix C for more explanation.
Sonntag, Borgnakke and van Wylen
Energy Equation: Ideal Gas
5.89 A 250 L rigid tank contains methane at 500 K, 1500 kPa. It is now cooled down to 300 K. Find the mass of methane and the heat transfer using a) ideal gas and b) the methane tables. Solution: a) Assume ideal gas, P2 = P1 (2 / 1) = 1500 300 / 500 = 900 kPa 1500 0.25 m = P1V/RT1 = 0.5183 500 = 1.447 kg Use specific heat from Table A.5 u2 - u1 = Cv (T2 T1) = 1.736 (300 500) = 347.2 kJ/kg
1Q2 = m(u2 - u1) = 1.447(-347.2) = 502.4 kJ
b) Using the methane Table B.7, v1 = 0.17273 m3/kg, u1 = 872.37 kJ/kg m = V/v1 = 0.25/0.17273 = 1.4473 kg State 2: v2 = v1 and 300 K is found between 800 and 1000 kPa 0.17273 0.19172 u2 = 467.36 + (465.91 467.36) 0.15285 0.19172 = 466.65 kJ/kg
1Q2 = 1.4473 (466.65 872.37) = 587.2 kJ
Sonntag, Borgnakke and van Wylen
5.90 A rigid insulated tank is separated into two rooms by a stiff plate. Room A of 0.5 m3 contains air at 250 kPa, 300 K and room B of 1 m3 has air at 150 kPa, 1000 K. The plate is removed and the air comes to a uniform state without any heat transfer. Find the final pressure and temperature. Solution: C.V. Total tank. Control mass of constant volume. Mass and volume: Energy Eq.: Process Eq.: Ideal gas at 1: Ideal gas at 2: m2 = mA + mB; V = VA + VB = 1.5 m3 Insulated Q = 0 U2 U1 = m2 u2 mAuA1 mBuB1 = Q W = 0 V = constant W = 0; mA = PA1VA/RTA1 = 250 0.5/(0.287 300) = 1.452 kg u A1= 214.364 kJ/kg from Table A.7 mB = PB1VB/RT B1= 150 1/(0.287 1000) = 0.523 kg u B1= 759.189 kJ/kg from Table A.7 m2 = mA + mB = 1.975 kg u2 = mAuA1 + mBuB1 1.452 214.364 + 0.523 759.189 = = 358.64 kJ/kg 1.975 m2 => Table A.7.1: T2 = 498.4 K
P2 = m2 RT2 /V = 1.975 0.287 498.4/1.5 = 188.3 kPa
A
B
cb
Sonntag, Borgnakke and van Wylen
5.91 A rigid container has 2 kg of carbon dioxide gas at 100 kPa, 1200 K that is heated to 1400 K. Solve for the heat transfer using a. the heat capacity from Table A.5 and b. properties from Table A.8 Solution: C.V. Carbon dioxide, which is a control mass. Energy Eq.5.11: Process: U2 U1 = m (u2- u1) = 1Q2 - 1W2 V = 0 1W2 = 0 a) For constant heat capacity we have: u2- u1 = Cvo (T2- T1) so
1Q2 mCvo (T2- T1) = 2 0.653 (1400 1200) = 261.2 kJ
b) Taking the u values from Table A.8 we get
1Q2 = m (u2- u1) = 2 (1218.38 996.64) = 443.5 kJ
Sonntag, Borgnakke and van Wylen
5.92 Do the previous problem for nitrogen, N2, gas. A rigid container has 2 kg of carbon dioxide gas at 100 kPa, 1200 K that is heated to 1400 K. Solve for the heat transfer using a. the heat capacity from Table A.5 and b. properties from Table A.8 Solution: C.V. Nitrogen gas, which is a control mass. Energy Eq.5.11: Process: U2 U1 = m (u2- u1) = 1Q2 - 1W2 V = 0 1W2 = 0
a) For constant heat capacity we have: u2- u1 = Cvo (T2 - T1) so
1Q2 mCvo (T2- T1) = 2 0.745 (1400 1200) = 298 kJ
b) Taking the u values from Table A.8, we get
1Q2 = m (u2- u1) = 2 (1141.35 957) = 368.7 kJ
Sonntag, Borgnakke and van Wylen
5.93 A 10-m high cylinder, cross-sectional area 0.1 m2, has a massless piston at the bottom with water at 20C on top of it, shown in Fig. P5.93. Air at 300 K, volume 0.3 m3, under the piston is heated so that the piston moves up, spilling the water out over the side. Find the total heat transfer to the air when all the water has been pushed out. Solution: Po P H2O P1 P0 air
V1 Vmax
1 2 V
cb
The water on top is compressed liquid and has volume and mass VH2O = Vtot - Vair = 10 0.1 - 0.3 = 0.7 m3 mH2O = VH2O/vf = 0.7 / 0.001002 = 698.6 kg The initial air pressure is then 698.6 9.807 P1 = P0 + mH2Og/A = 101.325 + 0.1 1000 = 169.84 kPa 169.84 0.3 and then mair = PV/RT = 0.287 300 = 0.592 kg State 2: No liquid water over the piston so P2 = P0 + 0 = 101.325 kPa, / State 2: P2, V2 V2 = 100.1 = 1 m3 T1P2V2 300101.3251 T2 = P V = 169.840.3 = 596.59 K 1 1
The process line shows the work as an area 1 1 PdV = 2 (P1 + P2)(V2 - V1) = 2 (169.84 + 101.325)(1 - 0.3) = 94.91 kJ 1W2 = The energy equation solved for the heat transfer becomes
1Q2 = m(u2 - u1) + 1W2 mCv(T2 - T1) + 1W2
= 0.592 0.717 (596.59 - 300) + 94.91 = 220.7 kJ Remark: we could have used u values from Table A.7: u2 - u1 = 432.5 - 214.36 = 218.14 kJ/kg versus 212.5 kJ/kg with Cv.
Sonntag, Borgnakke and van Wylen
5.94 Find the heat transfer in Problem 4.43. A piston cylinder contains 3 kg of air at 20oC and 300 kPa. It is now heated up in a constant pressure process to 600 K. Solution: Ideal gas PV = mRT State 1: T1, P1 State 2: T2, P2 = P1 P2V2 = mRT2 Process:
1
V2 = mR T2 / P2 = 30.287600 / 300 = 1.722 m3
P = constant, W2 = PdV = P (V2 - V1) = 300 (1.722 0.8413) = 264.2 kJ
Energy equation becomes U2 - U1 = 1Q2 - 1W2 = m(u2 - u1)
1 2
Q = U2 - U1 + 1W2 = 3(435.097 209.45) + 264.2 = 941 kJ
P 2 T1 T2 293 v 600 1 v T 2 300 kPa
300
1
Sonntag, Borgnakke and van Wylen
5.95 An insulated cylinder is divided into two parts of 1 m3 each by an initially locked piston, as shown in Fig. P5.95. Side A has air at 200 kPa, 300 K, and side B has air at 1.0 MPa, 1000 K. The piston is now unlocked so it is free to move, and it conducts heat so the air comes to a uniform temperature TA = TB. Find the mass in both A and B, and the final T and P. C.V. A + B Force balance on piston: PAA = PBA So the final state in A and B is the same. State 1A: Table A.7 uA1 = 214.364 kJ/kg, mA = PA1VA1/RTA1 = 200 1/(0.287 300) = 2.323 kg State 1B: Table A.7 uB1 = 759.189 kJ/kg, mB = PB1VB1/RTB1 = 1000 1/(0.287 1000) = 3.484 kg For chosen C.V. 1Q2 = 0 , 1W2 = 0 so the energy equation becomes mA(u2 - u1)A + mB(u2 - u1)B = 0 (mA + mB)u2 = mAuA1 + mBuB1 = 2.323 214.364 + 3.484 759.189 = 3143 kJ u2 = 3143/(3.484 + 2.323) = 541.24 kJ/kg From interpolation in Table A.7: T2 = 736 K kJ P = (mA + mB)RT2/Vtot = 5.807 kg 0.287 kg K 736 K/ 2 m3 = 613 kPa
A
B
Sonntag, Borgnakke and van Wylen
5.96 A piston cylinder contains air at 600 kPa, 290 K and a volume of 0.01 m3. A constant pressure process gives 54 kJ of work out. Find the final temperature of the air and the heat transfer input. Solution: C.V AIR control mass Continuity Eq.: m2 m1 = 0 Energy Eq.: Process: 1 : P1 , T1,V1 P=C m (u2 - u1) = 1Q2 - 1W2 so
1W2 = P dV = P(V2 V1)
2 : P1 = P2 , ? m1 = P1V1/RT1 = 600 0.01 / 0.287 290 = 0.0721 kg
1W2 = P(V2 V1) = 54 kJ
V2 V1 = 1W2 / P = 54 kJ / 600 kPa = 0.09 m3 V2 = V1 + 1W2 / P = 0.01 + 0.09 = 0.10 m3 Ideal gas law : P2V2 = mRT2 P2V2 0.10 T2 = P2V2 / mR = P V T1 = 0.01 290 = 2900 K 1 1 Energy equation with u's from table A.7.1 1Q2 = m (u2 - u1 ) + 1W2 = 0.0721 ( 2563.8 207.2 ) + 54 = 223.9 kJ
Sonntag, Borgnakke and van Wylen
5.97 A cylinder with a piston restrained by a linear spring contains 2 kg of carbon dioxide at 500 kPa, 400C. It is cooled to 40C, at which point the pressure is 300 kPa. Calculate the heat transfer for the process. Solution: C.V. The carbon dioxide, which is a control mass. Continuity Eq.: m2 m1 = 0 Energy Eq.: Process Eq.: m (u2 - u1) = 1Q2 - 1W2 P = A + BV (linear spring)
1 1W2 = PdV = 2(P1 + P2)(V2 - V1)
Equation of state: State 1: State 2:
PV = mRT (ideal gas)
V1 = mRT1/P1 = 2 0.18892 673.15 /500 = 0.5087 m3 V2 = mRT2/P2 = 2 0.18892 313.15 /300 = 0.3944 m3
1 1W2 = 2(500 + 300)(0.3944 - 0.5087) = -45.72 kJ
To evaluate u2 - u1 we will use the specific heat at the average temperature. From Figure 5.11: Cpo(Tavg) = 45/44 = 1.023 Cvo = 0.83 = Cpo - R For comparison the value from Table A.5 at 300 K is Cvo = 0.653 kJ/kg K
1Q2 = m(u2 - u1) + 1W2 = mCvo(T2 - T1) + 1W2
= 2 0.83(40 - 400) - 45.72 = -643.3 kJ
P 2 CO 2
1
v
Remark: We could also have used the ideal gas table in A.8 to get u2 - u1.
Sonntag, Borgnakke and van Wylen
5.98 Water at 100 kPa, 400 K is heated electrically adding 700 kJ/kg in a constant pressure process. Find the final temperature using a) The water tables B.1 b) The ideal gas tables A.8 c) Constant specific heat from A.5 Solution : Energy Eq.5.11: Process: u2 - u1 = 1q2 - 1w2 =>
1w2 = P ( v2 - v1 )
P = constant
1q2 = h2 - h1
Substitute this into the energy equation to get Table B.1: h1 2675.46 +
126.85 - 99.62 150 - 99.62 (2776.38 2675.46) = 2730.0 kJ/kg
h2 = h1 + 1q2 = 2730 + 700 = 3430 kJ/kg T2 = 400 + ( 500 400 ) 3488.09 - 3278.11 = 472.3C Table A.8: h2 = h1 + 1q2 = 742.4 + 700 = 1442.4 kJ/kg T2 = 700 + (750 700 ) 1443.43 - 1338.56 = 749.5 K = 476.3C Table A.5 h2 - h1 Cpo ( T2 - T1 ) T2 = T1 + 1q2 / Cpo = 400 + 700 / 1.872 = 773.9K = 500.8C
1442.4 - 1338.56 3430 - 3278.11
Sonntag, Borgnakke and van Wylen
5.99 A piston/cylinder has 0.5 kg air at 2000 kPa, 1000 K as shown. The cylinder has stops so Vmin = 0.03 m3. The air now cools to 400 K by heat transfer to the ambient. Find the final volume and pressure of the air (does it hit the stops?) and the work and heat transfer in the process. Solution: We recognize this is a possible two-step process, one of constant P and one of constant V. This behavior is dictated by the construction of the device. Continuity Eq.: m2 m1 = 0 Energy Eq.5.11: Process: m(u2 - u1) = 1Q2 - 1W2 P = constant = F/A = P1 if V > Vmin V = constant = V1a = Vmin State 1: (P, T) if P < P1 V1 = mRT1/P1 = 0.5 0.287 1000/2000 = 0.07175 m3
The only possible P-V combinations for this system is shown in the diagram so both state 1 and 2 must be on the two lines. For state 2 we need to know if it is on the horizontal P line segment or the vertical V segment. Let us check state 1a: State 1a: P1a = P1, V1a = Vmin V1a 0.03 Ideal gas so T1a = T1 V = 1000 0.07175 = 418 K 1 We see that T2 < T1a and state 2 must have V2 = V1a = Vmin = 0.03 m3. T2 V1 400 0.07175 P2 = P1 T V = 2000 1000 0.03 = 1913.3 kPa 1 2 The work is the area under the process curve in the P-V diagram
3 2 1W2 = 1 P dV = P1 (V1a V1) = 2000 kPa (0.03 0.07175) m = 83.5 kJ
Now the heat transfer is found from the energy equation, u's from Table A.7.1,
1Q2 = m(u2 - u1) + 1W2 = 0.5 (286.49 - 759.19) 83.5 = -319.85 kJ
P 1a P1 P2 2 1 T1 T1a V T2
T 1 1a 2
V
Sonntag, Borgnakke and van Wylen
5.100 A spring loaded piston/cylinder contains 1.5 kg of air at 27C and 160 kPa. It is now heated to 900 K in a process where the pressure is linear in volume to a final volume of twice the initial volume. Plot the process in a P-v diagram and find the work and heat transfer. Take CV as the air. m2 = m1 = m ;
m(u2 -u1) = 1Q2 - 1W2
Process: P = A + BV => 1W2 = P dV = area = 0.5(P1 + P2)(V2 -V1) State 1: Ideal gas. V1 = mRT1/P1 = 1.5 0.287 300/160 = 0.8072 m3 Table A.7 u1 = u(300) = 214.36 kJ/kg so ratio it to the initial state properties State 2: P2V2 = mRT2
P2V2 /P1V1 = P22 /P1 = mRT2 /mRT1 = T2 /T1 => P2 = P1 (T2 /T1 )(1/2) = 160 (900/300) (1/2) = 240 kPa Work is done while piston moves at linearly varying pressure, so we get
3 1W2 = 0.5(P1 + P2)(V2 -V1) = 0.5(160 + 240) kPa 0.8072 m = 161.4 kJ
Heat transfer is found from energy equation
1Q2 = m(u2 - u1) + 1W2 = 1.5(674.824 - 214.36) + 161.4 = 852.1 kJ
P 1 W
2
T 1 V
2
V
Sonntag, Borgnakke and van Wylen
5.101 Air in a piston/cylinder at 200 kPa, 600 K, is expanded in a constant-pressure process to twice the initial volume (state 2), shown in Fig. P5.101. The piston is then locked with a pin and heat is transferred to a final temperature of 600 K. Find P, T, and h for states 2 and 3, and find the work and heat transfer in both processes. Solution: C.V. Air. Control mass m2 = m3 = m1 Energy Eq.5.11: Process 1 to 2: u2 - u1 = 1q2 - 1w2 ; P = constant =>
1w2 =
P dv = P1(v2 -v1) = R(T2 -T1)
Ideal gas Pv = RT T2 = T1v2/v1 = 2T1 = 1200 K P2 = P1 = 200 kPa, Table A.7 Process 23:
1w2 = RT1 = 172.2 kJ/kg
h2 = 1277.8 kJ/kg, v3 = v2 = 2v1
h3 = h1 = 607.3 kJ/kg
1q2 = u2 - u1 + 1w2 = h2 - h1 = 1277.8 - 607.3 = 670.5 kJ/kg
2w3 = 0,
P3 = P2T3/T2 = P1T1/2T1 = P1/2 = 100 kPa 2q3 = u3 - u2 = 435.1 - 933.4 = -498.3 kJ/kg P 1 T 1200 3 v 600 1 3 v
P o
cb
200 100
2
2
Air
Sonntag, Borgnakke and van Wylen
5.102 A vertical piston/cylinder has a linear spring mounted as shown so at zero cylinder volume a balancing pressure inside is zero. The cylinder contains 0.25 kg air at 500 kPa, 27oC. Heat is now added so the volume doubles. a) Show the process path in a P-V diagram b) Find the final pressure and temperature. c) Find the work and heat transfer. Solution: Take CV around the air. This is a control mass. Continuity: m2 = m1 = m ; Energy Eq.5.11: m(u2 -u1) = 1Q2 - 1W2 Process: P linear in V so, P = A + BV, since V = 0 => P = 0 => A = 0 now: P = BV; B = P1/V1 State 1: P, T Ideal gas :
mRT 0.25 0.287 300 V= P = 500 b) = 0.04305 m3 State 2: V2 = 2 V1 ; ?
a)
P2 P1 0 0
P 2 1 V
must be on line in P-V diagram, this substitutes for the question mark only one state is on the line with that value of V2 P2 = BV2 = (P1/V1)V2 = 2P1 = 1000 kPa.
V1
2V1
PV 2P12V1 4P1V1 T2 = mR = mR = mR = 4 T1 = 1200 K c) The work is boundary work and thus seen as area in the P-V diagram:
1W2 = P dV = 0.5(P1 + P2 )( 2V1 - V1) = 0.5(500 + 1000) 0.04305 = 32.3 kJ 1Q2 = m(u2 - u1) + 1W2 = 0.25(933.4 - 214.4) + 32.3 = 212 kJ
Internal energy u was taken from air table A.7. If constant Cv were used then (u2 - u1) = 0.717 (1200 - 300) = 645.3 kJ/kg (versus 719 above)
Sonntag, Borgnakke and van Wylen
Energy Equation: Polytropic Process
5.103 A piston cylinder contains 0.1 kg air at 300 K and 100 kPa. The air is now slowly compressed in an isothermal (T = C) process to a final pressure of 250 kPa. Show the process in a P-V diagram and find both the work and heat transfer in the process. Solution : Process: T = C & ideal gas PV = mRT = constant
V2 P1 mRT W2 = PdV = V dV = mRT ln V = mRT ln P 1 1 2 = 0.1 0.287 300 ln (100 / 250 ) = -7.89 kJ since T1 = T2 u2 = u1 The energy equation thus becomes
1Q2 = m (u2 - u1 ) + 1W2 = 1W2 = -7.89 kJ
P 2
P = C v -1
T 2
T=C 1
1 v v
Sonntag, Borgnakke and van Wylen
5.104 Oxygen at 300 kPa, 100C is in a piston/cylinder arrangement with a volume of 0.1 m3. It is now compressed in a polytropic process with exponent, n = 1.2, to a final temperature of 200C. Calculate the heat transfer for the process. Solution: Continuty: m2 = m1 Energy Eq.5.11: m(u2 - u1) = 1Q2 - 1W2 State 1: T1 , P1 & ideal gas, small change in T, so use Table A.5 P1V1 300 0.1 m3 m = RT = 0.25983 373.15 = 0.309 kg 1 Process: PVn = constant
1W2 = 1-n (P2V2 - P1V1) = 1-n (T2 - T1) =
1
mR
0.309 0.25983 (200 - 100) 1 - 1.2
= -40.2 kJ
1Q2 = m(u2 - u1) + 1W2 mCv(T2 - T1) + 1W2
= 0.3094 0.662 (200 - 100) - 40.2 = -19.7 kJ P 2 P = C v -1.2 T2 1 T1 v T 2 1 v
T=Cv
-0.2
Sonntag, Borgnakke and van Wylen
5.105 A piston/cylinder contains 0.001 m3 air at 300 K, 150 kPa. The air is now compressed in a process in which P V1.25 = C to a final pressure of 600 kPa. Find the work performed by the air and the heat transfer. Solution: C.V. Air. This is a control mass, values from Table A.5 are used. Continuty: m2 = m1 Energy Eq.5.11: Process : State 2: m(u2 - u1) = 1Q2 - 1W2 PV1.25 = const. V2 = V1 ( P1/P2 )1.25= 0.00033 m3 600 0.00033 T2 = T1 P2V2/(P1V1) = 300 150 0.001 = 395.85 K
1W2 = n-1(P2 V2 P1V1) = n-1 (600 0.00033 150 0.001) = - 0.192 kJ
1
1
P1V1 1Q2 = m(u2 u1) + 1W2 = RT Cv (T2 T1) + 1W2 1 = 0.001742 0.717 95.85 0.192 = - 0.072 kJ
Sonntag, Borgnakke and van Wylen
5.106 Helium gas expands from 125 kPa, 350 K and 0.25 m3 to 100 kPa in a polytropic process with n = 1.667. How much heat transfer is involved? Solution: C.V. Helium gas, this is a control mass. Energy equation: m(u2 u1) = 1Q2 1W2 Process equation: Ideal gas (A.5): PV = constant = P1V1 = P2V2 m = PV/RT = 125 0.25 = 0.043 kg 2.0771 350
n n n
Solve for the volume at state 2 1250.6 = 0.25 100 = 0.2852 m3 100 0.2852 T2 = T1 P2V2/(P1V1) = 350 125 0.25 = 319.4 K Work from Eq.4.4 V2 = V1 (P1/P2)
1/n
1W2 =
P2V2- P1 V1 100 0.2852 - 125 0.25 = kPa m3 = 4.09 kJ 1-n 1 - 1.667
Use specific heat from Table A.5 to evaluate u2 u1, Cv = 3.116 kJ/kg K
1Q2 = m(u2 u1) + 1W2 = m Cv (T2 T1) + 1W2
= 0.043 3.116 (319.4 350) + 4.09 = -0.01 kJ
Sonntag, Borgnakke and van Wylen
5.107 A piston/cylinder in a car contains 0.2 L of air at 90 kPa, 20C, shown in Fig. P5.107. The air is compressed in a quasi-equilibrium polytropic process with polytropic exponent n = 1.25 to a final volume six times smaller. Determine the final pressure, temperature, and the heat transfer for the process. Solution: C.V. Air. This is a control mass going through a polytropic process. Continuty: m2 = m1 Energy Eq.5.11: m(u2 - u1) = 1Q2 - 1W2 Process: Pvn = const. 1.25 P1v1n = P2v2n P2 = P1(v1/v2)n = 90 6 = 845.15 kPa Substance ideal gas: Pv = RT T2 = T1(P2v2/P1v1) = 293.15(845.15/90 6) = 458.8 K P 2 P=Cv 1 v PV 90 0.210-3 m = RT = 0.287 293.15 = 2.1410-4 kg The work is integrated as in Eq.4.4 1 R Pdv = 1 - n (P2v2 - P1v1) = 1 - n (T2 - T1) 1w2 = 0.287 = 1 - 1.25(458.8 - 293.15) = -190.17 kJ/kg The energy equation with values of u from Table A.7 is 1q2 = u2 - u1 + 1w2 = 329.4 - 208.03 190.17 = -68.8 kJ/kg
1Q2 = m 1q2 = -0.0147 kJ (i.e a heat loss) -1.25
T 2 T=Cv 1 v
-0.25
Sonntag, Borgnakke and van Wylen
5.108 A piston/cylinder has nitrogen gas at 750 K and 1500 kPa. Now it is expanded in a polytropic process with n = 1.2 to P = 750 kPa. Find the final temperature, the specific work and specific heat transfer in the process. C.V. Nitrogen. This is a control mass going through a polytropic process. Continuty: m2 = m1 Energy Eq.5.11: Process: Substance ideal gas: m(u2 - u1) = 1Q2 - 1W2 Pvn = constant Pv = RT
0.2 n-1 n = 750 750 1.2 = 750 0.8909 = 668 K T2 = T1 (P2/P1) 1500
The work is integrated as in Eq.4.4 1 R 1w2 = Pdv = 1 - n (P2v2 - P1v1) = 1 - n (T2 - T1) 0.2968 = 1 - 1.2 (668 - 750) = 121.7 kJ/kg The energy equation with values of u from Table A.8 is
1q2 = u2 - u1 + 1w2 = 502.8 - 568.45 + 121.7 = 56.0 kJ/kg
If constant specific heat is used from Table A.5
1q2 = C(T2 - T1) + 1w2 = 0.745(668 750) + 121.7 = 60.6 kJ/kg
Sonntag, Borgnakke and van Wylen
5.109 A piston/cylinder arrangement of initial volume 0.025 m3 contains saturated water vapor at 180C. The steam now expands in a polytropic process with exponent n = 1 to a final pressure of 200 kPa, while it does work against the piston. Determine the heat transfer in this process. Solution: C.V. Water. This is a control mass. State 1: Table B.1.1 P = 1002.2 kPa, v1 = 0.19405 m3/kg, u1 = 2583.7 kJ/kg , m = V/v1 = 0.025/0.19405 = 0.129 kg Process: Pv = const. = P1v1 = P2v2 ; polytropic process n = 1. Table B.1.3 T2 155C , u2 = 2585 kJ/kg v2 = v1P1/P2 = 0.19405 1002.1/200 = 0.9723 m3/kg State 2: P2, v2 v2 0.9723 W2 = PdV = P1V1 ln v = 1002.2 0.025 ln 0.19405 = 40.37 kJ 1
1 1Q2 = m(u2 - u1) + 1W2 = 0.129(2585 - 2583.7) + 40.37 = 40.54 kJ
P
Sat vapor line 1 P = C v -1 T=C 2 v
T 1 2 v
Notice T drops, it is not an ideal gas.
Sonntag, Borgnakke and van Wylen
5.110 Air is expanded from 400 kPa, 600 K in a polytropic process to 150 kPa, 400 K in a piston cylinder arrangement. Find the polytropic exponent n and the work and heat transfer per kg air using constant heat capacity from A.5. Solution: Process: P V n = P V n
1 1 2 2
Ideal gas: PV = RT V = R/ P P1 ln P = ln (V2 / V1)n = n ln (V2 / V1) = n ln 2 P1 n = ln P / ln
2
2 [ P2 T1 ] 1
T
P
[ P1 T2 ] = ln 400 / ln [ 400 400 ] 150 600 150 2 1
P
T
= 1.7047
The work integral is from Eq.4.4 R 0.287 1W2 = PdV = 1 - n (T2 T1) = -0.7047 (400 600) = 81.45 kJ/kg Energy equation from Eq.5.11
1q2 = u2 - u1 + 1w2 = Cv(T2 - T1) + 1w2 = 0.717 (400-600) + 81.45
= -61.95 kJ/kg
Sonntag, Borgnakke and van Wylen
5.111 A piston/cylinder has 1 kg propane gas at 700 kPa, 40C. The piston cross-sectional area is 0.5 m2, and the total external force restraining the piston is directly proportional to the cylinder volume squared. Heat is transferred to the propane until its temperature reaches 700C. Determine the final pressure inside the cylinder, the work done by the propane, and the heat transfer during the process. Solution: C.V. The 1 kg of propane. Energy Eq.5.11: m(u2 - u1) = 1Q2 - 1W2 Process: P = Pext = CV2 PV-2 = constant, polytropic n = -2
Ideal gas: PV = mRT, and process yields n 700+273.152/3 n-1 = 700 P2 = P1(T2/T1) 40+273.15 = 1490.7 kPa The work is integrated as Eq.4.4 2 P2V2 - P1V1 mR(T2 - T1) W2 = PdV = = 1 1-n 1-n 1 = 1 0.18855 (700 40) = 41.48 kJ 1 (2)
The energy equation with specific heat from Table A.5 becomes 1Q2 = m(u2 - u1) + 1W2 = mCv(T2 - T1) + 1W2 = 1 1.490 (700 - 40) + 41.48 = 1024.9 kJ P P=CV
2
T T=CV 2
3
2 V 1 V
1
Sonntag, Borgnakke and van Wylen
5.112 An air pistol contains compressed air in a small cylinder, shown in Fig. P5.112. Assume that the volume is 1 cm3, pressure is 1 MPa, and the temperature is 27C when armed. A bullet, m = 15 g, acts as a piston initially held by a pin (trigger); when released, the air expands in an isothermal process (T = constant). If the air pressure is 0.1 MPa in the cylinder as the bullet leaves the gun, find a. The final volume and the mass of air. b. The work done by the air and work done on the atmosphere. c. The work to the bullet and the bullet exit velocity. Solution: C.V. Air. Air ideal gas: mair = P1V1/RT1 = 1000 10-6/(0.287 300) = 1.1710-5 kg Process: PV = const = P1V1 = P2V2 V2 = V1P1/P2 = 10 cm3 P1V1 1W2 = PdV = V dV = P1V1 ln (V2/V1) = 2.303 J -6 1W2,ATM = P0(V2 - V1) = 101 (10 - 1) 10 kJ = 0.909 J 1 Wbullet = 1W2 - 1W2,ATM = 1.394 J = 2 mbullet(Vexit)2 Vexit = (2Wbullet/mB)1/2 = (2 1.394/0.015)1/2 = 13.63 m/s
Sonntag, Borgnakke and van Wylen
5.113 A spherical balloon contains 2 kg of R-22 at 0C, 30% quality. This system is heated until the pressure in the balloon reaches 600 kPa. For this process, it can be assumed that the pressure in the balloon is directly proportional to the balloon diameter. How does pressure vary with volume and what is the heat transfer for the process? Solution: C.V. R-22 which is a control mass. m2 = m1 = m ; Energy Eq.5.11: m(u2 - u1) = 1Q2 - 1W2 State 1: 0C, x = 0.3. Table B.4.1 gives P1 = 497.6 kPa v1 = 0.000778 + 0.3 0.04636 = 0.014686 m3/kg u1 = 44.2 + 0.3 182.3 = 98.9 kJ/kg Process: P D, V D3 => PV -1/3 = constant, polytropic n = -1/3. => V2 = mv2 = V1 ( P2 /P1 )3 = mv1 ( P2 /P1 )3 v2 = v1 ( P2 /P1 )3 = 0.014686 (600 / 497.6)3 = 0.02575 m3/kg State 2: P2 = 600 kPa, process : v2 = 0.02575 Table B.4.1 x2 = 0.647, u2 = 165.8 kJ/kg
1W2 = P dV =
P2V2 - P1V1 600 0.05137 - 498 0.02937 = = 12.1 kJ 1 - (-1/3) 1-n
1Q2 = m(u2- u1) + 1W2 = 2(165.8 - 98.9) + 12.1 = 145.9 kJ
Sonntag, Borgnakke and van Wylen
5.114 Calculate the heat transfer for the process described in Problem 4.55. Consider a piston cylinder with 0.5 kg of R-134a as saturated vapor at -10C. It is now compressed to a pressure of 500 kPa in a polytropic process with n = 1.5. Find the final volume and temperature, and determine the work done during the process. Solution: Take CV as the R-134a which is a control mass Continuity: m2 = m1 = m ; Energy: m(u2 -u1) = 1Q2 - 1W2 Process: 1: (T, x) Pv = constant. Polytropic process with n = 1.5 P = Psat = 201.7 kPa from Table B.5.1 u1 = 372.27 kJ/kg
(1/1.5) 1.5
v1 = 0.09921 m3/kg,
2: (P, process) v2 = v1 (P1/P2)
= 0.09921 (201.7/500)
0.667
= 0.05416
=> Table B.5.2 superheated vapor, T2 = 79C, u2 = 440.9 kJ/kg Process gives P = C v
(-1.5)
, which is integrated for the work term, Eq.4.4
1W2 = P dV = m(P2v2 - P1v1)/(1-1.5)
= -20.5 (5000.05416 - 201.70.09921) = -7.07 kJ
1Q2 = m(u2 -u1) + 1W2 = 0.5(440.9 - 372.27) + (-7.07) = 27.25 kJ
Sonntag, Borgnakke and van Wylen
5.115 A piston/cylinder setup contains argon gas at 140 kPa, 10C, and the volume is 100 L. The gas is compressed in a polytropic process to 700 kPa, 280C. Calculate the heat transfer during the process. Solution: Find the final volume, then knowing P1, V1, P2, V2 the polytropic exponent can be determined. Argon is an ideal monatomic gas (Cv is constant). P1 T2 140 553.15 V2 = V1 P T = 0.1 700 283.15 = 0.0391 m3 2 1 P1V1n = P2V2n PdV = 1W2 = P2 V1 1.6094 n = ln (P ) / ln (V ) = 0.939 = 1.714
1 2
P2V2 -P1V1 7000.0391 - 1400.1 = = -18.73 kJ 1-n 1 - 1.714
m = P1V1/RT1 = 140 0.1/(0.20813 283.15) = 0.2376 kg
1Q2 = m(u2 - u1) + 1W2 = mCv(T2 - T1) + 1W2
= 0.2376 0.3122 (280 - 10) - 18.73 = 1.3 kJ
Sonntag, Borgnakke and van Wylen
Energy Equation in Rate Form
5.116 A crane lifts a load of 450 kg vertically up with a power input of 1 kW. How fast can the crane lift the load? Solution : Power is force times rate of displacement . W = FV = mgV . W 1000 W V = mg = 450 9.806 N = 0.227 m/s
Sonntag, Borgnakke and van Wylen
5.117 A computer in a closed room of volume 200 m3 dissipates energy at a rate of 10 kW. The room has 50 kg wood, 25 kg steel and air, with all material at 300 K, 100 kPa. Assuming all the mass heats up uniformly, how long will it take to increase the temperature 10C? Solution: C.V. Air, wood and steel. m2 = m1 ; no work . Energy Eq.5.11: U2 - U1 = 1Q2 = Qt The total volume is nearly all air, but we can find volume of the solids. Vwood = m/ = 50/510 = 0.098 m3 ; Vair = 200 - 0.098 - 0.003 = 199.899 m3 mair = PV/RT = 101.325 199.899/(0.287 300) = 235.25 kg We do not have a u table for steel or wood so use heat capacity from A.3. U = [mair Cv + mwood Cv + msteel Cv ]T = (235.25 0.717 + 50 1.38 + 25 0.46) 10 . = 1686.7 + 690 +115 = 2492 kJ = Q t = 10 kW t => t = 2492/10 = 249.2 sec = 4.2 minutes Vsteel = 25/7820 = 0.003 m3
Sonntag, Borgnakke and van Wylen
5.118 The rate of heat transfer to the surroundings from a person at rest is about 400 kJ/h. Suppose that the ventilation system fails in an auditorium containing 100 people. Assume the energy goes into the air of volume 1500 m3 initially at 300 K and 101 kPa. Find the rate (degrees per minute) of the air temperature change. Solution: . . Q = n q = 100 400 = 40000 kJ/h = 666.7 kJ/min dEair dTair . = Q = mairCv dt dt mair = PV/RT = 101 1500 / 0.287 300 = 1759.6 kg dTair . dt = Q /mCv = 666.7 / (1759.6 0.717) = 0.53C/min
Sonntag, Borgnakke and van Wylen
5.119 A piston/cylinder of cross sectional area 0.01 m2 maintains constant pressure. It contains 1 kg water with a quality of 5% at 150oC. If we heat so 1 g/s liquid turns into vapor what is the rate of heat transfer needed? Solution: Control volume the water. Continuity Eq.: mtot = constant = mvapor + mliq . . . on a rate form: mtot = 0 = mvapor + mliq Vvapor = mvapor vg , Vliq = mliq vf Vtot = Vvapor + Vliq . . . . . Vtot = Vvapor + Vliq = mvaporvg + mliqvf . . = mvapor (vg- vf ) = mvapor vfg . . . W = PV = P mvapor vfg = 475.9 0.001 0.39169 = 0.1864 kW = 186 W
. . mliq = -mvapor
Sonntag, Borgnakke and van Wylen
5.120 The heaters in a spacecraft suddenly fail. Heat is lost by radiation at the rate of 100 kJ/h, and the electric instruments generate 75 kJ/h. Initially, the air is at 100 kPa, 25C with a volume of 10 m3. How long will it take to reach an air temperature of -20C? Solution:
C.M. Air . Q el C.V. . Qrad
dM Continuity Eq: dt = 0 . dE . Energy Eq: dt = Qel - Qrad
. W=0 . KE = 0 . PE = 0
. . . . . . E = U = Qel - Qrad = Qnet U2 - U1 = m(u2 - u1) = Qnet(t2 - t1) P1V1 100 10 Ideal gas: m = RT = 0.287 298.15 = 11.688 kg 1 u2 - u1 = Cv0(T2 - T1) = 0.717 (-20 - 25) = -32.26 kJ/kg . t2 - t1 = mCv0(T2-T1)/Qnet = 11.688 (-32.26)/(-25) = 15.08 h
Sonntag, Borgnakke and van Wylen
5.121 A steam generating unit heats saturated liquid water at constant pressure of 200 kPa in a piston cylinder. If 1.5 kW of power is added by heat transfer find the rate (kg/s) of saturated vapor that is made. Solution: Energy equation on a rate form making saturated vapor from saturated liquid
. . . . . . . . . U = (mu) = mu = Q - W = Q - P V = Q - Pmv . . . . m(u + vP ) = Q = mh = mhfg . . m = Q/ hfg = 1500 / 2201.96 = 0.681 kg/s
Sonntag, Borgnakke and van Wylen
5.122 A small elevator is being designed for a construction site. It is expected to carry four 75-kg workers to the top of a 100-m tall building in less than 2 min. The elevator cage will have a counterweight to balance its mass. What is the smallest size (power) electric motor that can drive this unit? Solution: m = 4 75 = 300 kg ; Z = 100 m ; t = 2 minutes . . Z 300 9.807 100 -W = PE = mg = 1000 2 60 = 2.45 kW t
Sonntag, Borgnakke and van Wylen
5.123 As fresh poured concrete hardens, the chemical transformation releases energy at a rate of 2 W/kg. Assume the center of a poured layer does not have any heat loss and that it has an average heat capacity of 0.9 kJ/kg K. Find the temperature rise during 1 hour of the hardening (curing) process. Solution:
. . . . . U = (mu) = mCvT = Q = mq . . T = q/Cv = 210-3 / 0.9 = 2.222 10-3 C/sec . T = Tt = 2.222 10-3 3600 = 8 C
Sonntag, Borgnakke and van Wylen
5.124 A 100 Watt heater is used to melt 2 kg of solid ice at -10oC to liquid at +5oC at a constant pressure of 150 kPa. a) Find the change in the total volume of the water. b) Find the energy the heater must provide to the water. c) Find the time the process will take assuming uniform T in the water. Solution: Take CV as the 2 kg of water. Energy Eq.5.11
m2 = m1 = m ;
m(u2 - u1) = 1Q2 - 1W2
State 1: Compressed solid, take sat. solid at same temperature. v = vi(-10) = 0.0010891 m3/kg, h = hi = -354.09 kJ/kg State 2: Compressed liquid, take sat. liquid at same temperature v = vf = 0.001, h = hf = 20.98 kJ/kg Change in volume: V2 - V1 = m(v2 - v1) = 2(0.001 - 0.0010891) = 0.000178 m3 Work is done while piston moves at constant pressure, so we get
1W2 =
P dV = area = P(V2 - V1) = -150 0.000178 = -0.027 kJ = -27 J
Heat transfer is found from energy equation
1Q2 = m(u2 - u1) + 1W2 = m(h2 - h1) = 2 [20.98 - (-354.09)] = 750 kJ
The elapsed time is found from the heat transfer and the rate of heat transfer . t = 1Q2/Q = (750/100) 1000 = 7500 s = 125 min = 2 h 5 min
P L S T 1 L+V S+V v 2 C.P. V
P C.P. 1
T
C.P. P=C
2 v
2 v
1
Sonntag, Borgnakke and van Wylen
5.125 Water is in a piston cylinder maintaining constant P at 700 kPa, quality 90% with a volume of 0.1 m3. A heater is turned on heating the water with 2.5 kW. What is the rate of mass (kg/s) vaporizing? Solution: Control volume water. Continuity Eq.: mtot = constant = mvapor + mliq . . . . . on a rate form: mtot = 0 = mvapor + mliq mliq = -mvapor . . . . . . Energy equation: U = Q - W = mvapor ufg = Q - P mvapor vfg . Rearrange to solve for mvapor . . . mvapor (ufg + Pvfg) = mvapor hfg = Q . . 2.5 kW mvapor = Q/hfg = 2066.3 kJ/kg = 0.0012 kg/s
Sonntag, Borgnakke and van Wylen
Review Problems
5.126 Ten kilograms of water in a piston/cylinder setup with constant pressure is at 450C and a volume of 0.633 m3. It is now cooled to 20C. Show the Pv diagram and find the work and heat transfer for the process. Solution: C.V. The 10 kg water. Energy Eq.5.11: m(u2 - u1) = 1Q2 - 1W2 Process: P=C
1W2 = mP(v2 -v1)
State 1: (T, v1 = 0.633/10 = 0.0633 m3/kg) P1 = 5 MPa, h1 = 3316.2 kJ/kg State 2: (P = P = 5 MPa, 20C) v2 = 0.000 999 5 m3/kg ; P 2 1
Table B.1.3
Table B.1.4 h2 = 88.65 kJ/kg T
1 5 MPa
v
2
v
The work from the process equation is found as
1W2 = 10 5000 (0.0009995 - 0.0633) = -3115 kJ
The heat transfer from the energy equation is 1Q2 = m(u2 - u1) + 1W2 = m(h2 - h1)
1Q2 = 10 (88.65 - 3316.2) = -32276 kJ
Sonntag, Borgnakke and van Wylen
5.127 Consider the system shown in Fig. P5.127. Tank A has a volume of 100 L and contains saturated vapor R-134a at 30C. When the valve is cracked open, R-134a flows slowly into cylinder B. The piston mass requires a pressure of 200 kPa in cylinder B to raise the piston. The process ends when the pressure in tank A has fallen to 200 kPa. During this process heat is exchanged with the surroundings such that the R-134a always remains at 30C. Calculate the heat transfer for the process. Solution: C.V. The R-134a. This is a control mass. Continuity Eq.: m2 = m1 = m ; Energy Eq.5.11: Process in B: m(u2 - u1) = 1Q2 - 1W2 P = Pfloat (piston must move) If VB > 0 then
1W2 = Pfloat dV = Pfloatm(v2 - v1) Work done in B against constant external force (equilibrium P in cyl. B) State 1: 30C, x = 1. Table B.5.1: v1 = 0.02671 m3/kg, u1 = 394.48 kJ/kg m = V/v1 = 0.1 / 0.02671 = 3.744 kg State 2: 30C, 200 kPa superheated vapor Table B.5.2 v2 = 0.11889 m3/kg, u2 = 403.1 kJ/kg From the process equation 1W2 = Pfloatm(v2 - v1) = 2003.744(0.11889 - 0.02671) = 69.02 kJ From the energy equation
1Q2 = m(u2 - u1) + 1W2 = 3.744 (403.1 - 394.48) + 69.02 = 101.3 kJ
Sonntag, Borgnakke and van Wylen
5.128 Ammonia, NH3, is contained in a sealed rigid tank at 0C, x = 50% and is then heated to 100C. Find the final state P2, u2 and the specific work and heat transfer. Solution: Continuity Eq.: Energy Eq.5.11: m2 = m1 ; E2 - E1 = 1Q2 ; / (1W2 = 0)
Process: V2 = V1 v2 = v1 = 0.001566 + 0.5 0.28783 = 0.14538 m3/kg Table B.2.2: v2 & T2 between 1000 kPa and 1200 kPa 0.14538 0.17389 P2 = 1000 + 200 0.14347 0.17389 = 1187 kPa
P 2 u2 = 1490.5 + (1485.8 1490.5) 0.935 = 1485.83 kJ/kg u1 = 179.69 + 0.5 1138.3 = 748.84 kJ/kg V Process equation gives no displacement: 1w2 = 0 ; The energy equation then gives the heat transfer as 1q2 = u2 - u1 = 1485.83 748.84 = 737 kJ/kg
1
Sonntag, Borgnakke and van Wylen
5.129 A piston/cylinder contains 1 kg of ammonia at 20C with a volume of 0.1 m3, shown in Fig. P5.129. Initially the piston rests on some stops with the top surface open to the atmosphere, Po, so a pressure of 1400 kPa is required to lift it. To what temperature should the ammonia be heated to lift the piston? If it is heated to saturated vapor find the final temperature, volume, and the heat transfer. Solution: C.V. Ammonia which is a control mass. m2 = m1 = m ; m(u2 -u1) = 1Q2 - 1W2 State 1: 20C; v1 = 0.10 < vg x1 = (0.1 0.001638)/0.14758 = 0.6665 u1 = uf + x1 ufg = 272.89 + 0.6665 1059.3 = 978.9 kJ/kg Process: Piston starts to lift at state 1a (Plift, v1) State 1a: 1400 kPa, v1 Table B.2.2 (superheated vapor) 0.1 0.09942 Ta = 50 + (60 50) 0.10423 0.09942 = 51.2 C P 1400 1200 857 1a 2 1 v T 1a 2 1 v
State 2: x = 1.0, v2 = v1 => V2 = mv2 = 0.1 m3 T2 = 30 + (0.1 0.11049) 5/(0.09397 0.11049) = 33.2 C u2 = 1338.7 kJ/kg;
1W2 = 0; 1Q2 = m1q2 = m(u2 u1) = 1 (1338.7 978.9) = 359.8 kJ/kg
Sonntag, Borgnakke and van Wylen
5.130 A piston held by a pin in an insulated cylinder, shown in Fig. P5.130, contains 2 kg water at 100C, quality 98%. The piston has a mass of 102 kg, with cross-sectional area of 100 cm2, and the ambient pressure is 100 kPa. The pin is released, which allows the piston to move. Determine the final state of the water, assuming the process to be adiabatic. Solution: C.V. The water. This is a control mass. Continuity Eq.: m2 = m1 = m ; Energy Eq.5.11: Process in cylinder: m(u2 - u1) = 1Q2 - 1W2 P = Pfloat (if piston not supported by pin) 102 9.807 = 200 kPa 10010-4 103
P2 = Pfloat = P0 + mpg/A = 100 +
We thus need one more property for state 2 and we have one equation namely the energy equation. From the equilibrium pressure the work becomes
1W2 =
Pfloat dV = P2 m(v2 - v1)
With this work the energy equation gives per unit mass u2 - u1 = 1q2 - 1w2 = 0 - P2(v2 - v1) or with rearrangement to have the unknowns on the left hand side u2 + P2v2 = h2 = u1 + P2v1 h2 = u1 + P2v1 = 2464.8 + 200 1.6395 = 2792.7 kJ/kg State 2: (P2 , h2) Table B.1.3 => T2 161.75C
Sonntag, Borgnakke and van Wylen
5.131 A piston/cylinder arrangement has a linear spring and the outside atmosphere acting on the piston, shown in Fig. P5.131. It contains water at 3 MPa, 400C with the volume being 0.1 m3. If the piston is at the bottom, the spring exerts a force such that a pressure of 200 kPa inside is required to balance the forces. The system now cools until the pressure reaches 1 MPa. Find the heat transfer for the process. Solution: C.V. Water. Continuity Eq.: Energy Eq.5.11: P 3 MPa 1 MPa 200 kPa
0
m2 = m1 = m ; m(u2 - u1) = 1Q2 - 1W2 1 State 1: Table B.1.3 v1 = 0.09936 m3/kg, u1 = 2932.8 kJ/kg m = V/v1 = 0.1/0.09936 = 1.006 kg V, v Process: Linear spring so P linear in v. P = P0 + (P1 - P0)v/v1
2
v2
v1
v2 =
(P2 - P0)v1 (1000 - 200)0.09936 = 0.02839 m3/kg 3000 - 200 P1 - P0 = T2 = 179.91C, u2 = 761.62 + x2 1821.97 = 1018.58 kJ/kg
State 2: P2 , v2 x2 = (v2 - 0.001127)/0.19332 = 0.141,
1
Process => 1W2 = PdV = 2 m(P1 + P2)(v2 - v1) = 2 1.006 (3000 + 1000)(0.02839 -0.09936) = -142.79 kJ Heat transfer from the energy equation
1Q2 = m(u2 - u1) + 1W2 = 1.006(1018.58 - 2932.8) - 142.79 = -2068.5 kJ 1
Sonntag, Borgnakke and van Wylen
5.132 Consider the piston/cylinder arrangement shown in Fig. P5.132. A frictionless piston is free to move between two sets of stops. When the piston rests on the lower stops, the enclosed volume is 400 L. When the piston reaches the upper stops, the volume is 600 L. The cylinder initially contains water at 100 kPa, 20% quality. It is heated until the water eventually exists as saturated vapor. The mass of the piston requires 300 kPa pressure to move it against the outside ambient pressure. Determine the final pressure in the cylinder, the heat transfer and the work for the overall process. Solution: C.V. Water. Check to see if piston reaches upper stops. Energy Eq.5.11: Process: m(u4 - u1) = 1Q4 - 1W4 If P < 300 kPa then V = 400 L, line 2-1 and below If P > 300 kPa then V = 600 L, line 3-4 and above If P = 300 kPa then 400 L < V < 600 L line 2-3
These three lines are shown in the P-V diagram below and is dictated by the motion of the piston (force balance). 0.4 State 1: v1 = 0.001043 + 0.21.693 = 0.33964; m = V1/v1 = 0.33964 = 1.178 kg u1 = 417.36 + 0.2 2088.7 = 835.1 kJ/kg 0.6 State 3: v3 = 1.178 = 0.5095 < vG = 0.6058 at P3 = 300 kPa Piston does reach upper stops to reach sat. vapor. State 4: v4 = v3 = 0.5095 m3/kg = vG at P4 => P4 = 361 kPa, From Table B.1.2 u4 = 2550.0 kJ/kg
1W4 = 1W2 + 2W3 + 3W4 = 0 + 2W3 + 0 1W4 = P2(V3 - V2) = 300 (0.6 - 0.4) = 60 kJ 1Q4 = m(u4 - u1) + 1W4 = 1.178(2550.0 - 835.1) + 60 = 2080 kJ
T
4 2 3 1
P4 P2= P3 = 300 P1
v
cb
Water
Sonntag, Borgnakke and van Wylen
5.133 A piston/cylinder, shown in Fig. P5.133, contains R-12 at - 30C, x = 20%. The volume is 0.2 m3. It is known that Vstop = 0.4 m3, and if the piston sits at the bottom, the spring force balances the other loads on the piston. It is now heated up to 20C. Find the mass of the fluid and show the Pv diagram. Find the work and heat transfer. Solution: C.V. R-12, this is a control mass. Properties in Table B.3 Continuity Eq.: m2 = m1 Energy Eq.5.11: Process: E2 - E1 = m(u2 - u1) = 1Q2 - 1W2 V < 0.4 m3, A = 0 (at V = 0, P = 0) P = A + BV,
State 1: v1 = 0.000672 + 0.2 0.1587 = 0.0324 m3/kg u1 = 8.79 + 0.2 149.4 = 38.67 kJ/kg m = m1 = = V1/v1 = 6.17 kg
P 2
System: on line V Vstop; Pstop = 2P1 =200 kPa State stop: (P,v) Tstop -12C TWO-PHASE STATE
2P 1 P1 0 0 1
T -5C
T stop -12.5C
V
0.2
0.4
Since T2 > Tstop v2 = vstop = 0.0648 m3/kg 2: (T2 , v2) Table B.3.2: Interpolate between 200 and 400 kPa P2 = 292.3 kPa ; u2 = 181.9 kJ/kg From the process curve, see also area in P-V diagram, the work is
1 1 1W2 = PdV = 2 (P1 + Pstop)(Vstop - V1) = 2 (100 + 200)0.2 = 30 kJ
From the energy equation
1Q2 = m(u2 - u1) + 1W2 = 913.5 kJ
Sonntag, Borgnakke and van Wylen
5.134 A piston/cylinder arrangement B is connected to a 1-m3 tank A by a line and valve, shown in Fig. P5.134. Initially both contain water, with A at 100 kPa, saturated vapor and B at 400C, 300 kPa, 1 m3. The valve is now opened and, the water in both A and B comes to a uniform state. a. Find the initial mass in A and B. b. If the process results in T2 = 200C, find the heat transfer and work. Solution: C.V.: A + B. This is a control mass. Continuity equation: m2 - (mA1 + mB1) = 0 ; Energy: m2u2 - mA1uA1 - mB1uB1 = 1Q2 - 1W2 System: if VB 0 piston floats PB = PB1 = const. if VB = 0 then P2 < PB1 and v = VA/mtot see P-V diagram 1W2 = PBdVB = PB1(V2 - V1)B = PB1(V2 - V1)tot State A1: Table B.1.1, x = 1 vA1 = 1.694 m3/kg, uA1 = 2506.1 kJ/kg mA1 = VA/vA1 = 0.5903 kg State B1: Table B.1.2 sup. vapor vB1 = 1.0315 m3/kg, uB1 = 2965.5 kJ/kg mB1 = VB1/vB1 = 0.9695 kg m2 = mTOT = 1.56 kg * At (T2 , PB1) v2 = 0.7163 > va = VA/mtot = 0.641 so VB2 > 0 so now state 2: P2 = PB1 = 300 kPa, T2 = 200 C => u2 = 2650.7 kJ/kg and V2 = m2 v2 = 1.56 0.7163 = 1.117 m3 (we could also have checked Ta at: 300 kPa, 0.641 m3/kg => T = 155 C)
1W2 = PB1(V2 - V1) = -264.82 kJ 1Q2 = m2u2 - mA1uA1 - mB1uB1 + 1W2 = -484.7 kJ
P a PB1 V2 2
Sonntag, Borgnakke and van Wylen
5.135 A small flexible bag contains 0.1 kg ammonia at 10oC and 300 kPa. The bag material is such that the pressure inside varies linear with volume. The bag is left in the sun with with an incident radiation of 75 W, loosing energy with an average 25 W to the ambient ground and air. After a while the bag is heated to 30oC at which time the pressure is 1000 kPa. Find the work and heat transfer in the process and the elapsed time. Solution: Take CV as the Ammonia, constant mass. Continuity Eq.: m2 = m1 = m ; Energy Eq.5.11: m(u2 - u1) = 1Q2 1W2 Process: P = A + BV (linear in V) State 1: Compressed liquid P > Psat, take saturated liquid at same temperature. v1 = vf(20) = 0.001002 m3/kg, State 2: Table B.2.1 at 30oC : u1 = uf = 133.96 kJ/kg P < Psat so superheated vapor
v2 = 0.13206 m3/kg, u2 = 1347.1 kJ/kg, V2 = mv2 = 0.0132 m3 Work is done while piston moves at increacing pressure, so we get
1W2 = (300 + 1000)*0.1(0.13206 0.001534) = 8.484 kJ
Heat transfer is found from the energy equation 1Q2 = m(u2 u1) + 1W2 = 0.1 (1347.1 133.96) + 8.484 = 121.314 + 8.484 = 129.8 kJ
P C.P. T 2 30 -10 v 1 v C.P. 2
NH3
1000 300
1
. Qnet = 75 25 = 50 Watts . 129800 t = 1Q2 / Qnet = 50 = 2596 s = 43.3 min
Sonntag, Borgnakke and van Wylen
5.136 Water at 150C, quality 50% is contained in a cylinder/piston arrangement with initial volume 0.05 m3. The loading of the piston is such that the inside pressure is linear with the square root of volume as P = 100 + CV 0.5 kPa. Now heat is transferred to the cylinder to a final pressure of 600 kPa. Find the heat transfer in the process. Continuty: m2 = m1 Energy: m(u2 - u1) = 1Q2 - 1W2 m = V/v1 = 0.254 kg State 1: v1 = 0.1969, u1 = 1595.6 kJ/kg
Process equation P1 - 100 = CV11/2 so (V2/V1)1/2 = (P2 - 100)/(P1 - 100)
2 P2 - 1002 500 V2 = V1 P - 100 = 0.05 475.8 - 100 = 0.0885 1
1/2)dV = 100(V - V ) + C(V 1.5 - V 1.5) 1W2 = PdV = (100 + CV 2 1 2 1 3 = 100(V2 - V1)(1 - 2/3) + (2/3)(P2V2 - P1V1)
1W2 = 100 (0.0885-0.05)/3 + 2 (600 0.0885-475.8 0.05)/3 = 20.82 kJ
2
State 2: P2, v2 = V2/m = 0.3484 u2 = 2631.9 kJ/kg,
1Q2 = 0.254 (2631.9 - 1595.6) + 20.82 = 284 kJ
T2 196C
P 1 100
P = 100 + C V 2
1/2
V
Sonntag, Borgnakke and van Wylen
5.137 A 1 m3 tank containing air at 25oC and 500 kPa is connected through a valve to another tank containing 4 kg of air at 60oC and 200 kPa. Now the valve is opened and the entire system reaches thermal equilibrium with the surroundings at 20oC. Assume constant specific heat at 25oC and determine the final pressure and the heat transfer. Control volume all the air. Assume air is an ideal gas. Continuity Eq.: m2 mA1 mB1 = 0 Energy Eq.: Process Eq.: State 1: PA1VA1 (500 kPa)(1m3) mA1 = RT = (0.287 kJ/kgK)(298.2 K) = 5.84 kg A1 mB1RTB1 (4 kg)(0.287 kJ/kgK)(333.2 K) = 1.91 m3 PB1 = (200 kN/m2) U2 - U1 = m2u2 mA1uA1 mB1uB1 = 1Q2 - 1W2 V = constant
1W2 = 0
VB1 =
State 2: T2 = 20C, v2 = V2/m2 m2 = mA1 + mB1 = 4 + 5.84 = 9.84 kg V2 = VA1 + VB1 = 1 + 1.91 = 2.91 m3 P2 = m2RT2 (9.84 kg)(0.287 kJ/kgK)(293.2 K) = 284.5 kPa V2 = 2.91 m3
Energy Eq.5.5 or 5.11: 1Q2 = U2 - U1 = m2u2 mA1uA1 mB1uB1 = mA1(u2 uA1) + mB1(u2 uB1) = mA1Cv0(T2 TA1) + mB1Cv0(T2 TB1) = 5.84 0.717 (20 25) + 4 0.717 (20 60) = -135.6 kJ The air gave energy out.
A
B
Sonntag, Borgnakke and van Wylen
5.138 A closed cylinder is divided into two rooms by a frictionless piston held in place by a pin, as shown in Fig. P5.138. Room A has 10 L air at 100 kPa, 30C, and room B has 300 L saturated water vapor at 30C. The pin is pulled, releasing the piston, and both rooms come to equilibrium at 30C and as the water is compressed it becomes twophase. Considering a control mass of the air and water, determine the work done by the system and the heat transfer to the cylinder. Solution: C.V. A + B, control mass of constant total volume. Energy equation: mA(u2 u1)A + mB(uB2 uB1) = 1Q2 1W2 Process equation: V = C 1W2 = 0 T = C (u2 u1)A = 0 (ideal gas) The pressure on both sides of the piston must be the same at state 2. Since two-phase: P2 = Pg H2O at 30C = PA2 = PB2 = 4.246 kPa Air, I.G.: VA2 = PA1VA1 = mARAT = PA2VA2 = Pg H2O at 30C VA2 100 0.01 3 3 4.246 m = 0.2355 m
Now the water volume is the rest of the total volume VB2 = VA1 + VB1 - VA2 = 0.30 + 0.01 - 0.2355 = 0.0745 m3 VB1 0.3 mB = v = 32.89 = 9.12110-3 kg => B1 vB2 = 8.166 m3/kg
8.166 = 0.001004 + xB2 (32.89 - 0.001) xB2 = 0.2483 uB2 = 125.78 + 0.2483 2290.8 = 694.5 kJ/kg, uB1 = 2416.6 kJ/kg Q = m (u u ) = 9.12110-3(694.5 - 2416.6) = -15.7 kJ
B B2 B1
1 2
A
B

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Chapter 17 Temperature and the Kinetic Theory of GasesConceptual Problems*1 (a) False. If two objects are in thermal equilibrium with a third, then they are in thermal equilibrium with each other. (b) False. The Fahrenheit and Celsius temperature

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Chapter 24 Electrostatic Energy and CapacitanceConceptual Problems*1 Determine the Concept The capacitance of a parallel-plate capacitor is a function of the surface area of its plates, the separation of these plates, and the electrical properties

University of Texas - PHY - 303K/303L

Chapter 15 Wave MotionConceptual Problems*1 Determine the Concept The speed of a transverse wave on a rope is given by v = F where F is the tension in the rope and is its linear density. The waves on the rope move faster as they move up because

University of Texas - PHY - 303K/303L

Chapter 16 Superposition and Standing WavesConceptual Problems*1 Picture the Problem We can use the speeds of the pulses to determine their positions at the given times.2 Picture the Problem We can use the speeds of the pulses to determine thei

University of Texas - PHY - 303K/303L

Chapter 11 GravityConceptual Problems*1 (a) False. Kepler's law of equal areas is a consequence of the fact that the gravitational force acts along the line joining two bodies but is independent of the manner in which the force varies with distanc

University of Texas - PHY - 303K/303L

Chapter 9 RotationConceptual Problems*1 Determine the Concept Because r is greater for the point on the rim, it moves the greater distance. Both turn through the same angle. Because r is greater for the point on the rim, it has the greater speed.

University of Texas - PHY - 303K/303L

Chapter 31 Properties of LightConceptual Problems1 Determine the Concept The population inversion between the state E2,Ne and the state 1.96 eV below it (see Figure 31-9) is achieved by inelastic collisions between neon atoms and helium atoms exci

University of Texas - PHY - 303K/303L

Chapter 3 Motion in Two and Three DimensionsConceptual Problems*1 Determine the Concept The distance traveled along a path can be represented as a sequence of displacements.Suppose we take a trip along some path and consider the trip as a sequen

University of Texas - PHY - 303K/303L

Chapter 4 Newton's LawsConceptual Problems*1 Determine the Concept A reference frame in which the law of inertia holds is called an inertial reference frame. If an object with no net force acting on it is at rest or is moving with a constant speed

University of Texas - PHY - 303K/303L

Chapter 2 Motion in One DimensionConceptual Problems1 Determine the Concept The "average velocity" is being requested as opposed to "average speed". The average velocity is defined as the change in position or displacement divided by the change in

University of Texas - PHY - 303K/303L

Chapter 23 Electrical PotentialConceptual Problems*1 Determine the Concept A positive charge will move in whatever direction reduces its potential energy. The positive charge will reduce its potential energy if it moves toward a region of lower el

Florida Coastal School of Law - LAW - CivPro

Day 5 2-9 Summary Judgments; Pretrial Conferences I. Motion to Dismiss Rule 12(b)(6), (c) a. Rule 12(b)(6)complaint only (failure to state a claim) *court does not look at the evidence they just look at the complaint -court looks and asks if pl. prov

Cal Poly Pomona - ARO - 322

Cal Poly Pomona - ARO - 322

Cal Poly Pomona - ARO - 322

Cal Poly Pomona - ARO - 322

Cal Poly Pomona - ARO - 322

ARO322AerospaceFeedbackControlSystems Homework#4 Due10/26/20071.Forthefollowingsystem,findtheclosedlooptransferfunctionandresponseofthe closedloopsystemforaunitstepinput.Findthefinalvalueandverifytheresultusing MATLABorSimulink. R(s) + B(s) Soluti

USC - ASTR - 100

Solar flares explosive outbursts of twisted-tube energy through the photosphere during sunspot maxima Prominences eruptions of solar material from the side view (a hook or curve) Filaments what prominences are called from the frontal view Coronal

USC - ITAL - 120

Italian 120 Spring 2006Nome_ Data_ESAME FINALE REVIEW Completa con luso appropriato del VOCABOLARIO.A. PAOLO FA SHOPPING. Completa con il vocabolario dellabbigliamento1. __ _!2. _ _!3. _ _!4. _ _!5. _ _!6. __ _!17. _ _!2E. CH

USC - ITAL - 120

Italiano- 120 . C VOCABOLARIO. (12 punti)Nome_ Data_ .Part 1.`Formate una frase con le seguenti espressioni. La frase deve spiegare (must explain) il significato dellespressione. Sceglietene TRE. (9 punti) Fare freddo Fare due passi Avere sete A

USC - ITAL - 120

Italiano- 1 C VOCABOLARIO. (12 punti)Nome_Part 1.`Formate una frase con le seguenti espressioni. La frase deve spiegare (must explain) il significato dellespressione. Sceglietene TRE. (9 punti) Fare caldo Dare una festa Avere sonno Avere fretta

USC - ITAL - 120

Italiano 120 Giuseppina De MasiNome: _ Data: _QUIZ 5A. Una storia damore. Ieri sei stato al matrimonio del tuo amico Kevin e della tua amica Sarah a Los Angeles. Scrivi une-mail al tuo amico italiano Andrea che non potuto venire al matrimonio:

USC - ITAL - 120

USC - ITAL - 120

Italian 120Nome: _ Data: _QUIZ. 3 A. Le attivit di un tipico studente a USC. Descrivi la giornata di uno studente a USC. Cosa studia o non studia? Cosa fa e cosa non fa? Quando? Con chi? (min. 8 linee, 24 punti.)_ _ _ __ _ _ __ _ _ __ _B. Una

USC - ITAL - 120

USC - ITAL - 150

Nome_Italiano 150 Autunno 2006MIDTERM REVIEW A. Immagina le seguenti situazioni: 1. Una vacanza particolare. Scrivi una mail un amico/ unamica e parlagli/le dettagliatamente di una vacanza bellissima o disastrosa che hai fatto. (minimo 9 righe; 2

USC - ITAL - 150

Italiano 150 Fall 2006 USC Cristina VillaNome_ Data_QUIZ # 2 REVIEW (100 punti)A. Un appuntamento al buio. Racconta ad un tuo compagno di classe lincontro al buio tra due persone che non si conosco e usa il vocabolario con il corpo umano per de

USC - ITAL - 150

Review Midterm #1. A. La storia di Marco e Sandra. Completa con il tempo e la forma corretta del verbo (pay attention at and underline all the time key-words such as ieri, domani, prossimo, prima, fra, fa etc. before deciding which tense to use). 1.

ASU - POS - 305

Assignment for Lost HorizonRead: Wong, Preface, pp. 1-55, Class lecture and movie synopsis, and class handouts. Typewritten answers to the following questions about Lost Horizon are due by class time on Sep 9, 2008.1. How does Lost Horizon support