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- Title: FirsrLawSolns
- Type: Notes
- School: Michigan State University
- Course: ME 201
- Term: Spring
201 ME Thermodynamics Solutions First Law Practice Problems 1. Consider a balloon that has been blown up inside a building and has been allowed to come to equilibrium with the inside temperature of 25 C and inside pressure of 100 kPa. The diameter of the balloon is measured and found to be 0.13 m. The balloon is then taken outside and allowed to come to equilibrium with the outside temperature of -5 C and outside pressure of 100 kPa. Determine the boundary work, heat transfer, and final balloon diameter. Solution: System Type: Closed System Substance Type: Ideal gas Process: Isobaric Initial State: Fixed Final State: Fixed Q = UNKNOWN Wsh = 0 Wbnd = UNKNOWN Conservation of Mass: m1 = m2 1st Law: m(u 2 - u1 ) = Q - Wbnd Boundary Work: P(v 2 - v1 ) State 2 State 1 T1 = 25 C = 298 K T2 = -5 C = 268 K P1 = 100 kPa P2 = 100 kPa D1 = 0.13 m D2 = 0.1255 m -3 3 V1 = 1.150 x 10 m V2 = 1.034 x 10-3 m3 m1 = 1.345 x 10-3 kg m2 = 1.345 x 10-3 kg u1 = 209.06 kJ/kg u2 = 191.17 kJ/kg 1 = 1.6783 kJ/(kg K) 2 = 1.588 kJ/(kg K) v1 = 0.855 m3/kg v2 = 0.769 m3/kg Italicized values are from ideal gas relations. Bold values are calculated. Approach: We begin by evaluating the properties at state 1 and state 2 by using the air tables and the ideal gas law. We can then use our boundary work equation to calculate the boundary work. Finally, we use the conservation of energy to determine the heat transfer. 1 ME 201 Thermodynamics We start by determining our total volume at state 1. The volume of a sphere is given by 4 V1 = R 3 = 1.150 x 10 -3 m 3 3 Next we determine our mass. Using the ideal gas law we have P1V1 (100)(1.150 x 10 -3 ) m1 = = = 1.345 x 10 -3 kg RT1 (0.287)(298) V1 (1.150 x 10 -3 ) = = 0.855 m 3 /kg v1 = -3 m1 (1.345 x 10 ) We can go to the air tables and find u1 = 209.06 kJ/kg 1 = 1.6783 kJ/(kgK) u2 = 191.17 kJ/kg 2 = 1.5888 kJ/(kgK) The specific volume at state 2 must be given by the ideal gas law, so that RT2 (0.287)(268) = = 0.769 m 3 /kg v2 = P2 (100) The total final volume is then V2 = m2v2 = (1.345 x 10-3)(0.769) = 1.034 x 10-3 m3 The final diameter is 3V D 2 = 2 2 = 0.1255 m 4 The boundary work is calculated Wbnd = P(V2 - V1 ) = (100)(1.034 x 10 -3 - 1.150 x 10 -3 ) = - 0.0116 kJ and the heat transfer is Q = m(u 2 - u1 ) + Wbnd = (1.345 x 10 -3 )(191.17 - 209.06) + (-0.0116) 1/3 = - 0.0357 kJ 2 ME 201 Thermodynamics 2. Air at 1800 K and 800 kPa enters an ideal turbine at 2.3 kg/s. The power output required of this turbine is 700 kW. Determine the exhaust temperature and pressure. Solution: System Type: Control Volume System Substance Type: Ideal gas Process: Isentropic Initial State: Fixed Final State: Unknown Q=0 Wsh = 700 kW & & Conservation of Mass: m 2 = m1 = 2.3 kg/s & & 1st Law: m(h 2 - h1 ) = - Wsh State 1 State 2 T1 = 1800 K T2 = 1552 K P1 = 800 kPa P2 = 424 kPa h1 = 2003.3 kJ/kg h2 = 1698.95 kJ/kg 1 = 3.6684 kJ/(kg K) 2 = 3.4864 kJ/(kg K) Italicized values are from ideal gas relations. Bold values are calculated. Approach: We begin by evaluating the properties at state 1 by using the air tables and the ideal gas law. We can then use the conservation of energy to determine the enthalpy at state 2. The air tables will give T2. Finally, the isentropic relation will give the exit pressure At state 1 we can go to the air tables and find h1 = 2003.3 kJ/kg 1 = 3.6684 kJ/(kgK) From the first law we solve for h2 & W 700 h 2 = h1 - sh = 2003.3 = 1698.95 kJ/kg & m 2.3 The air tables can then give T2 = 1552 K 2 = 3.4864 kJ/(kgK) The isentropic relation is P s = 0 = 2 - 1 - R ln 2 P 1 Solving for P2 gives - 3.4864 - 3.6684 P2 = P1 exp 2 1 = (800)exp = 424 kPa 0.287 R 3 ME 201 Thermodynamics 3. Refrigerant-134a as saturated liquid at 32 C enters a valve and exits at 0.16 MPa. What is the fluid phase at the exit? Solution System Type: Control Volume System Substance Type: Compressible Process: Isenthalpic Initial State: Fixed Final State: Unknown Q=0 Wsh = 0 1st Law: h2 - h1 = 0 State 2 State 1 T1 = 32 C T2 = -15.62 C P1 = 0.81528 MPa P2 = 0.16 MPa h1 = 94.39 kJ/kg h2 = 94.39 kJ/kg Phase: sat.liq. Phase: 2 phase, x2 = 0.31 Italicized values are fromR-134a tables. Bold values are calculated. Approach: We begin by evaluating the properties at state 1 by using the refrigerant tables. We can then use the conservation of energy to determine the enthalpy at state 2. The refrigerant tables will give the fluid phase. At state 1 we can go to the refrigerant tables and find h1 = 94.39 kJ/kg P1 = 0.81528 MPa From the first law we solve for h2 h2 = h1 = 94.39 kJ/kg At 0.16 MPa, we find from the tables hf = 29.78 kJ/kg hg = 237.97 kJ/kg Since h2 is between these two values we have a two phase mixture at -15.62 C and with quality h -h 94.39 - 29.78 x2 = 2 f = = 0.31 h fg 208.18 4 ME 201 Thermodynamics 4. A tank contains Refrigerant-134a as saturated vapor at 100 kPa. There is heat transfer to the tank of 66 kJ/kg. Determine the final temperature and pressure. Solution: System Type: Closed System Substance Type: Compressible Process: Isotropic Initial State: Fixed Final State: Unknown q = 66 kJ/kg Wsh = 0 Wbnd = 0 Conservation of Mass: m1 = m2 1st Law: u 2 - u1 = q State 2 State 1 T1 = -26.43 C T2 = 60 C P1 = 100 kPa P2 = 140 kPa u1 = 212.18 kJ/kg u2 = 278.18 kJ/kg 3 v1 = 0.1917 m /kg v2 = 0.1917 m3/kg phase: sat.vap. phase: sup.vap. Italicized values are from tables. Bold values are calculated. Approach: We begin by evaluating the properties at state 1 by using the refrigerant tables. We can then use isotropic process to determine the specific volume at state 2. Next, we use the conservation of energy to determine the final internal energy. Finally, we go to the refrigerant tables and find the T and P. We can go to the refrigerant tables and find u1 = 212.18 kJ/kg T1 = -26.43 C v1 = 0.1917 m3/kg Since our process is isotropic, we have v2 = v1 = 0.1917 m3/kg The final internal energy is given by the 1st law or u2 = q + u1 = 66 + 212.18 = 278.18 kJ/kg So now we must go to the refrigerant tables and find the T and P that will correspond to these values of u and v. Since we had saturated vapor and we added heat, we will assume that we have gone to superheated vapor. Then scanning the tables we find that at 0.14 MPa and 60 C, we have u = 278.74 kJ/kg v = 0.1902 m3/kg which is close enough. 5 ME 201 Thermodynamics 5. Consider a 300 gallon hot water heater which is to provide water a 180 F and 20 psia. If a typical shower consumes 3 gallons/minute , last 15 minutes, and requires that the hot water should stay above 160 F, determine the heat transfer rate required. Water is supplied at 63 F and 22 psia and 2 gallons/minute. Solution: System Type: Transient System Substance Type: Incompressible Process: Isotropic Initial State: Fixed Final State: Fixed Inlet State: Fixed Exit State: Fixed Q = UNKNOWN Wsh = 0 Wbnd = 0 m - m1 & & Conserv. of Mass: 2 = m in - m out t m u - m1u1 & & & 1st Law: 2 2 = m in h in - m out h out + Q t State 1 State In State Out State 2 T1 = 180 F Tin = 63 F Tout = 170 F T2 = 160 F P1 = 20 psia Pin = 22 psia Pout = 20 psia P2 =20 psia u1 = 147.97 Btu/lbm hin = 31.43 Btu/lbm hout =138.01 Btu/lbm u2 = 127.94 Btu/lbm v1 = 0.01651 ft3/lbm vin = 0.01606 ft3/lbm vout = 0.01645 ft3/lbm v2 = 0.01640 ft3/lbm m1 = 51.08 lbm min = 0.0055 lbm/s mout = 0.0083 lbm/s m2 = 48.56 lbm Note that we have used the average temperature at state 1 and 2 to fix our outlet temperature Italicized values are from steam tables. Bold values are calculated. Approach: We begin by evaluating the properties at all states by using the steam tables. We can then use the conservation of mass to determine the mass at state 2. Finally the 1st law is used to determine the heat transfer rate. Even though we will treat water as an incompressible substance in this case, we can use the steam tables to determine the specific volume and internal energy since for an incompressible substance they only depend on temperature, we can take the values for saturated liquid at the given temperature. For the enthalpy, we can use the enthalpy of saturated liquid and then bring it up to the appropriate pressure or hincompressible sub. = hf + vf(P-Psat) 6 ME 201 Thermodynamics Our values are then entered on the table. To obtain the masses and mass flows we convert gallons to ft3 and divide by the specific volume. These values are entered on the table. The final state mass is given by conservation of mass or & & m 2 = t (m in - m out ) + m1 = (15)(60)(0.055 - 0.0083) + 51.08 = 48.56 lb m Then the heat transfer rate becomes & m u - m1u1 - m h + m h & in in & out out Q= 2 2 t (48.56)(127.94) - (51.08)(147.97) = (15)(60) - (0.0055)(31.43) + (0.0083)(138.01) = - 0.522 Btu/s = - 1881 Btu/hr 6. A piston cylinder system contains gas at 2300 K, 2500 and kPa, 0.03 liters. The gas then undergoes a polytropic expansion with a polytropic exponent of 1.15 to 0.3 liters. Compare the work performed in kJ for air as the gas versus hydrogen as the gas. Solution: We start this problem by working in air and then in hydrogen System Type: Closed System Substance Type:: Ideal gas Process: Polytropic with n=1.15 Initial State: Fixed Final State: UNKNOWN Q = UNKNOWN (but not asked for) Wsh = 0 Wbnd = UNKNOWN Conservation of Mass: m1 = m2 1st Law: m(u 2 - u1 ) = Q - Wbnd P2 v 2 - P1v1 (1 - n) State 1 State 2 T1 = 2300 K T2 = NA P1 = 2500 kPa P2 = 162.8 kPa -5 3 V1 = 3 x 10 m V2 = 3 x 10-4 m3 Italicized values are from ideal gas relations. Bold values are calculated. Boundary Work: Wbnd = Approach: At state 2 we can determine the pressure from the polytropic relation. We can then use our boundary work equation to calculate the boundary work 7 ME 201 Thermodynamics From the polytropic relationship we have n P2 V2 = P1V1n or solving for P2 1.15 V 0.03 1 P2 = P1 = (2300) = 162.8 kPa V 0.3 2 The boundary work is then given by (162.8)(3 x 10 -4 ) - (2300)(3 x 10 -5 ) = 0.134 kJ Wbnd = (1 - 1.15) Since the calculation above never used the fact that air was our ideal gas, we will obtain the same boundary work for hydrogen as the gas. n 7. In an open feedwater, subcooled liquid water is heated to saturated liquid by mixing directly with steam. In a given situation water at 10 MPa and 200 C enters an open feedwater heater at 12 kg/s. Steam at 10 MPa and 350 C is available to be added to this water to produce saturated liquid at 10 MPa. How much steam in kg/s must be added? Solution: System Type: Control Volume System Substance Type: Water (compressible substance) Process: Isobaric Inlet State: Fixed Exit State: Fixed Q=0 Wsh = 0 & & & Conserv. of Mass: m in -1 + m in -2 - m out = 0 & & & 1st Law: m in -1h in -1 + m in -2 h in -2 - m out h out = 0 State In-1 State In-2 State Out Tin-1 = 200 C Tin-2 = 350 C Tout = 311 C Pin-1 = 10 MPa Pin-2 = 10 MPa Pout = 10 MPa hin-1 = 856 kJ/kg hin-2 = 2923.4 kJ/kg hout =1407.56 kJ/kg & & & m in -1 = 12 kg/s m in -2 = 4.366 kg/s m out = 16.366 kg/s phase: sub.liq. phase: sub.liq. phase: sat.liq. Italicized values are from steam tables. Bold values are calculated. Approach: We begin by evaluating the properties at all states by using the steam tables. We can then use the conservation of mass to algebraically solve for the exit mass flow rate. Substituting this into the 1st law steam flow rate can be determined. 8 ME 201 Thermodynamics The enthalpies are read from the steam tables and entered into our table above. Solving for mout from conservation of mass & & & m out = m in -1 + m in -2 Substituting into the energy equation & & & & m in -1h in -1 + m in -2 h in -2 - (m in -1 + m in -2 )h out = 0 Solving for min-2 & m (h - h ) 12(856 - 1407.56) & m in -2 = in -1 in -1 out = = 4.366 kg h out - h in - 2 1407.56 - 2923.4 8. Most of the time during the winter Dr. Somerton turns down the thermostat to 50 F when he leaves in the morning. When he is in the house he likes to have the temperature at 68 F. The house may be considered to be composed of air, occupying a volume of 10,000 ft3, and structural material (mostly wood) of 11,000 lbm. Determine the total heat transfer required to bring the house up to 68 F. What fraction of this total goes to heating up the air and what fraction goes to heating up the structural material? Solution System Type: Closed System Substance Type: Ideal gas (air) and Incompressible (wood) Process: Isobaric Initial State: Fixed Final State: Fixed Q = UNKNOWN Wsh = 0 Wbnd = 0 Conservation of Mass: m1 = m2 1st Law: U2 - U1 = Q State 2 State 1 T1 = 50 F T2 = 68 F 3 V1,a = 10,000 ft V2,a = 10,000 ft3 m1,a = 779 lbm m2,a = 779 lbm m1,w = 11,000 lbm m2,w = 11,000 lbm Italicized values are from ideal gas relations. Bold values are calculated. Approach: We begin by determining the mass of air in the house. Then we can use the 1st law to calculate the heat transfer required. We determine our mass of air assuming a pressure of 14.7 psia. Using the ideal gas law we have PV (14.7)(10,000) m1,a = 1 1,a = = 779 lb m RT1 (10.73 / 29)(510) 9 ME 201 Thermodynamics We can calculate the heat transfer from the energy equation Q = U 2 - U1 = m a c v,a (T2 - T1 ) + m w c P, w (T2 - T1 ) = (779)(0.171)(68 - 50) + (11,000)(0.42)(68 - 50) = 85,558 Btu of which 97% goes into the structural material and 3% into the air. 9. Oil enters an ideal pump at 100 F and 12 psia and leaves at 17 psia. The oil flow rate is 0.04 lbm/s. The pump inlet has a diameter of 5 inches and the pump outlet has a diameter of 2 inches. What pumping power is required? Solution: System Type: Control Volume System Substance Type: Incompressible Process: Isentropic, can't neglect KE Initial State: Fixed Final State: Unknown Q=0 Wsh = UNKNOWN & & Conservation of Mass: m 2 = m1 = 0.04 lb m /s r2 r2 & h 2 + v 2 - h1 - v1 = - Wsh & 1st Law: m 2 2 State 1 State 2 T1 = 100 F T2 = 100 F P1 = 12 psia P2 = 17 psia r r v1 = 0.005 ft/s v 2 = 0.032 ft/s Bold values are calculated. Approach: We begin by using our process description to fix state 2. Then from the diameter and mass flow information the velocities are obtained. Finally, the 1st law is used to calculate the power. Since the process is isentropic and we have an incompressible substance, we write T s 2 - s1 = c P,avg ln 2 T 1 which yields T2 = T1 =100 F The velocities can be calculated from one of our continuity relations & & r m m v= = Ac D 2 4 10 ME 201 Thermodynamics Using a density for oil of 57 lbm/ft3, we find r v1 = 0.005 ft/s r v 2 = 0.032 ft/s The enthalpy change for an incompressible substance is given by h 2 - h1 = c P,avg (T2 - T1 ) + v avg (P2 - P1 ) Then the shaft work is calculated (using the density rather than the specific volume) r2 r2 & sh = - m P2 - P1 + v 2 - v1 & W 2 17 - 12 1 Btu 3 57 5.404 psia ft = - (0.04) (3600 s/hr) 2 2 (0.032) - (0.005) 1 Btu / lb m + 2 25.037 ft 2 / s 2 = - 2.338 Btu/hr 10.The exhaust process for an internal combustion engine may be modeled as transient system undergoing an isobaric process with boundary work. Just before the exhaust valve opens the cylinder of 0.5 liters contains air at 200 kPa and 500 K. At the end of exhaust the volume is 0.0833 liters. Assume that the process is adiabatic. Determine the final temperature and mass and the boundary work. Solution: System Type: Transient System Substance Type: Ideal gas Process: Isobaric Initial State: Fixed Outlet State: UNKNOWN Final State: UNKNOWN Q=0 Wsh = 0 Wbnd = P(V2-V1) Conserv. of Mass: m2-m1 = - mout 1st Law: m2u2-m1u1 = - mouthout - Wbnd State 1 State Out State 2 T1 = 500 K Tout = T2 = P1 = 200 kPa Pout = 200 kPa P2 =200 kPa -4 3 V1 = 5 x 10 m Vout = NA V2 =8.33 x 10-5 m3 Bold values are calculated. Approach: We begin by calculating the boundary work. We then use the ideal gas law and constant specific heat to determine the enthalpy and internal energies. 11 ME 201 Thermodynamics The boundary work is then Wbnd = P(V2-V1) = (200)[8.33 x 10-5 - 5 x 10-4] = -0.0833 kJ Now we want to solve for the final temperature, but we note that in the first law four different things depend on T2: u2, m2, hout, and mout. We can eliminate mout with the help of our conservation of mass so that m2u2-m1u1 = - (m1-m2)hout - Wbnd Since the exhaust air temperature is changing during the process, we will use our linear average approach to determine hout, or T + T2 h out = h air at 1 2 The mass at the final state will be given by PV m2 = 2 2 RT2 So substituting PV RT2 T + T2 m1u1 - m1 - 2 2 h air at 1 u2 = - Wbnd 2 RT2 P2 V2 When we consider that the air tables provide the relationships among u and T and h and T, we see that we do have a well posed problem, but we can't do the algebra, so we will have to solve the problem by iteration. Our process will be Guess T2 Evaluate hair and u2 from the air tables Calculate u2 from the above equation Compare the u2 at the guessed value of T2 to the calculated value of u2 Re-guess T2 and repeat the process until the difference between the values of u2 at the guessed T2 and the value of the calculated u2 is negligible I set this up in an Excel spreadsheet and the results are shown below T2,guess 500 525 550 600 650 625 u2,guess 147.31 165.99 184.82 222.95 261.71 242.25 Tout 500 512.5 525 550 575 562.5 hout 205.28 218.19 231.14 257.14 283.3 270.2 m2 0.000117 0.000111 0.000106 9.71E-05 8.96E-05 9.32E-05 u2 u2,guess-u2,calc 572.3208 -425.0108163 522.2173 -356.2273109 464.1406 -279.3206002 324.0476 -101.097563 151.6535 110.0564586 241.9062 0.343817527 So it would appear that our final temperature is about 625 K. 12
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