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Unformatted text preview: MEEN 315—Spring 2010, Exam 2 Name SO I U¥l 0114 PP” Section Number Please put your name on w page. RELAX, this is only an exam. Read over the whole exam, then decide which problems to
work first. ‘ The exam is closed book and notes. Tables are provided. No collaboration with others!
Include a sketch, and clearly state assumptions and equations used on problems requiring
detailed analysis. If you get stuck on a problem, go to the next one. Problems must be worked in the unit system in which they are speciﬁed. Failure to do so
will result in a lower score. Assure that you have 12 different pages, including the formula sheet as the last page. AN AGGlE DOES NOT LIE, CHEAT, 0R STEAL [a
0R TOLERATE THOSE WHO DO MultipleChoice:
Problem 1 Problem 2 Problem 3 Problem 4 MEEN 315—Spring 2010, Exam 2 Name
Section Number Multiple choice problems worth 2 pts each. Circle the answer that is the most appropriate or
closest (numerically) to your answer. 1. The kinetic energy of a fluid increases as it flows through an adiabatic nozzle. Where does this
energy come from?
,‘rom the flow energy and internal energy
'. from the surroundings
C. from work done on the system
D. from internal energy generation 2. An example of an isenthalpic device is:
A. an adiabatic turbine
B. an isothermal heat exchanger
an adiabatic capillary tube
D. an adiabatic and reversible compressor 3. A heat pump consumes 2 kW of power and takes 4 kW of energy from a cold reservoir. The COP
for this heat pump is A. 0.5 COP: my” = (/1 = 3.0 B. 1.0
C. 2.0 .30 4. The quantities "p,” and ”v," in the ideal gas property tables for air are functions of
A. Pressure and specific volume
B. Pressure and temperature ' (9 Temperature
. D. Temperature and specific volume 5. The efficiency of a Carnot heat engine increases as
A. The high and low temperatures increase
The high temperature increases and the low temperature decreases
C. The high temperature decreases and the low temperature increases
D. The high and low temperatures decrease 6. A rigid tank contains air at a constant 400 K that is being stirred by a paddle wheel. The paddle
wheel does 200 k] of work on the air. The tank has heat transfer such that the process is
isothermal. What is the entropy change ofthe air? A. 0.0 kJ/K Q 250 1’: £3 90.5kJ/k 53:7:= (“,0 K ‘0‘: /I<
. 1.0kJ/K D. 2.0kJ/k MEEN 315—Spring 2010, Exam 2 Name Section Number
7. The following is one of the Carnot principles:
.The efﬁciency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs. B. The efficiency of an irreversible heat engine is always less than the efficiency of a
reversible one operating between any two reservoirs. C. The efficiency of an irreversible heat engine is always more than the efficiency of a
reversible one operating between any two reservoirs. D. The efficiency of an irreversible heat engine is always more than the efﬁciency of a
reversible one operating between the same two reservoirs. 8. When none of the extensive properties of a system vary with time,rthe system is said to be:
A. Uniform B. In equilibrium
Steady
D. All ofthe above 9. Although there are many valid statements of the second law of thermodynamics, select the best
such statement from the following: A. Energy is conserved
B. Entropy is conserved
© Energy has both quantity and quality
D. Entropy is zero for a pure crystalline substance at O K 10. An engineer wishes to tabulate ideal gas properties of oxygen. The properties to be included are
speciﬁc enthalpy (h), specific volume (v), and specific entropy (5). Which of these properties can
be tabulated using only temperature as the independent variable? h
B. v
C. s
D. v ands MEEN 315—Spring 2010, Exam 2 Name Section Number 1. Consider a heat engine and a heat pump. A heat engine receives 300 Btu/min of energy from a
source at 440°F, converts 160 Btu/min of it to power, and rejects the rest as waste heat to the
surroundings at 80°F. A. Using numbers, show that this heat engine is or is not possible. (6 points) B. Power of 160 Btu/min is used to operate a heat pump. The heat pump uses energy from the
surroundings at 80°F to heat a hot house at 110°F. Determine the maximum rate of energy
(Btu/min) that could be supplied to the hot house. (8 points) C. For this part, consider only the interaction between the surroundings and the hot house.
Determine the rate of change of entropy (Btu/min°R) for the hot house and for the surroundings. Note: for full credit, you need to indicate for each system whether the entropy
change is an increase or decrease. (6 points) MEEN 315—Spring 2010, Exam 2 Name Section Number 2. Air at 500 kPa and 400 K enters an adiabatic and reversible nozzle at a velocity of 30 m/s and
leaves at 300 kPa. The inlet area or the nozzle is 100 cm2. Use constant properties. The ratio of specific heats may be assumed to be 1.40.
A. Determine the exit temperature (K) of the air. (5 points)
B. Determine the speciﬁc volume (ms/kg) at the inlet and outlet of the nozzle. (5 points)
C. Determine the velocity (m/s) at the end of the nozzle. (5 points)
D. Determine the exit area (cmz). (,5 points) (a) From entropy conservation
(k—1)/k
5 z 5 z (3 / 5)‘°""”“‘ = 0.8642
T. I:
T2 = 400 x 0.8642 = 345.68K (b) From ideal gas law
v1 = R7} /P{ : 0.287 x 400/ 500 = 0.2296m3 /kg From isentropic process P W! 5 1/1.4
v2 = v1 —1 = 0.2296[—] ‘ = 0.33O7m3 /kg
P2 3
(c) From energy conservation a+mwz=t+nwz V2 = 1/2711 +V12 —2h2 = ,lchag —T2)+Vl" =‘lzx1099><(400—345.68)+302 = 334.517m / s
(d) For mass conservation
A2 =A1V1v2 =100x 30 x 03307 =12cm2
V2121 334.517 0.2296 MEEN 315—Spring 2010, Exam 2 Name Section Number 3. A stream of refrigerant 134a at 1.4 MPa and 12°C is mixed with another stream at 1.4 MPa and
60°C. The mass flow rate of the cold stream is twice that of the hot one.
A. Provide a sketch of the three streams and label the states. (2 points)
B. Determine the temperature (°C) of the exiting refrigerant. (8 points)
C. State whether the exit state is compressed liquid, saturated, or superheated. If the exiting
state is saturated, determine the quality. (10 points) A.) ° 12%: We KM“ bpca $ 1 = Z ,
0.) ‘ . _,,,,,22.,2,, . 22, C .) Completing a mass balance and assuming steady state, the mass balance gives, mc+mH—mc,,=0 :> mCH=mC+mH=2mH+mH =3n'1H. (1)
Now using the energy analysis the ﬁnal enthalpy can be determined,
Q—Wmchc +mHhH ~mCHhCH =m(Au—Ake—Ape). (2)
Assuming the system is stationary, at steady state, adiabatic and no work is done insert Eq.(1) in
to Eq.(2),
rhChC +57th —mCHhCH = 0 :> ZmHhC +ri1HhH —3n'1HhCH = O; (3)
Dividing Eq.(3) by rizH
211C +11, —3hCH = 0 :> 110., 2—2126th . (4)
The enthalpy for the cold stream is found by approximating the value with the saturated ﬂuid
value th (Table A1 1),
In]
ha ~h.(@TC)=68.18 (E) (5)
and for the hot stream (Table A—13),
hH = 285.47 (E). (6)
kg
Substitute Eqs.(5,6) into Eq.(4),
ha, 2 2(68.18)3+ 285.47 214061 (kg). 43,530,394 [l (7)
g
The corresponding quality is found at the operating pressure of 1.4 (MPa), ‘ T: 52  '1 DC?
h — h 140. 1— 7.
x = F” f = —61—223 = 0.08992. (8) h ,g 148.90 .__. MEEN BIS—Spring 2010, Exam 2 Name Section Number 4. Steam enters an adiabatic turbine at 5.0 MPa and 700°C at a rate of 5.0 kg/s, and exits at 200 kPa
and 250°C. Potential and kinetic energy changes can be neglected.
A. Determine the power output (MW) of the turbine. (5 points)
B. Determine the isentropic efﬁciency (%) of the turbine. (6 points)
C. Determine the rate of entropy generation (kW/K). (6 points)
D. Determine whether the expansion is internally reversible, irreversible or impossible. (You
need to supply a quantitative reason for your selection). (3 points) c) , <1) »/ /
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