ME 322 – Mechanical Engineering Thermodynamics
Final Exam
Fall 2015
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THROUGH THE EXAM QUESTIONS UNTIL YOU
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Date
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Dr. Dan Dollars Collected
EXAM INSTRUCTIONS – PLEASE READ THIS CAREFULLY
You will have several days to complete this exam. You may use your notes, the online
course resources, your computer (EES, Google, etc.) and pretty much any non-human
resource you can find. You will also be allowed the use of “Dr. Dan Dollars” as outlined in
the guide posted on our course schedule.
Some problems will be open-ended calculations where you need to provide numerical
answers. For these problems, make sure to attach all of your calculations (or EES
code/output) attached (in order) to the back of the exam.
For multiple choice questions, circle the correct answer(s). Do provide details about
where your answer came from. If I can’t tell where you messed up, I can’t give you partial
credit for a problem you get wrong. :o(
There will be a maximum of 170 points on this exam (including extra points from “Dr.
Dan Dollars.”
Part 1: Engineering Calculations – each problem is worth 15 points
You are designing a HVAC system for your house that is in a warm, humid climate. Air entering the HVAC system
is at a dry bulb temperature of 95°F and 80% relative humidity. You would like the air circulating through the
building to be similar to typical ambient conditions in Moscow, Idaho during the early summer, which is about a dry
bulb temperature of 75°F and 40% relative humidity. You will use a cooling process (to remove moisture from the
incoming air), then a heating process (to get the air to the desired temperature and humidity ratio). The incoming air
flow rate to the HVAC system is 2000 cubic feet per minute. Answer the following questions:
1) Use the ASHRAE English Unit psychrometric chart (a copy can be printed from our BbLearn site, under
the “Homework Solutions” menu) to draw the process. Clearly label the:
a. Start point
b. End point
c. Path taken for these two processes.
2) Calculate the mass flow rate of water condensing out of the air stream (in lbm/hr)?
3) If the cooling unit has a COPR of 4.0 and the heating unit is electric (with efficiency of 100%), calculate the
following:
a. How much power will it take for the cooling process?
b. How much power will it take for the heating process?
c. If energy costs $0.10 per kW-hr, how much dollar/hr will it take to make this house feel like
typical ambient conditions in Moscow?
4) You are working with a methane-fired boiler that needs to produce 3 x 10 9 Btu/hr of heat power. Assuming
100% conversion efficiency (from chemical energy to boiler heat) if you were burning a stoichiometric
mixture of air and methane (CH4, which has a lower heating value of 21,400 Btu/lbm), what mass flow of
air (in lbm, at standard pressure and temperature) would be consumed in the combustion reaction?
Part 2: Multiple Choice – each problem worth 5 points
Some problems may have multiple answers
5) An innovative power cycle is being designed to operate between heat reservoirs at 150 C and
55 C. What is most nearly its maximum theoretical thermal efficiency?
A.
B.
C.
D.
E.
16%
22%
33%
48%
63%
6) At a given thermodynamic state for a pure fluid, the following properties are found in a table:
T = 300K, P = 200kPa, v = .5500m3/kg, h = 1620kJ/kg, and s = 8.3500kJ/kg-K.
The internal energy is most nearly,
A.
B.
C.
D.
E.
885 kJ/kg
1455 kJ/kg
1510 kJ/kg
1730 kJ/kg
2500 kJ/kg
7) Which of the following do not change during a cooling process without dehumidification?
A. Specific enthalpy
B. Specific volume
C. Wet bulb temperature
D. Relative humidity
E. Humidity ratio
8) Combustion gases enter a turbine at 1007 C and 1 MPa, ultimately expanding to atmospheric pressure.
What is the maximum specific work output? Consider variable heat capacities. (HINT: Use the air tables.)
A.
B.
C.
D.
E.
480 kJ/kg
500 kJ/kg
550 kJ/kg
610 kJ/kg
640 kJ/kg
9) Steam flows steadily through an adiabatic turbine. The steam enters the turbine at 900°F,
750 psia. The exhaust pressure of the turbine is 2 psia. A table of steam properties is shown below at
several different states. The isentropic efficiency of this turbine is most nearly,
A. 65%
B. 76%
C. 84%
D. 92%
E. 100%
10) A moisture sensitive part that has just been removed from a cold chamber and placed in an environment
that has a dry bulb temperature of 60 F and a web bulb temperature of 50 F. Most nearly, what is the
minimum temperature of the part to avoid condensation?
A.
B.
C.
D.
E.
32 F
40 F
45 F
50 F
60 F
11) In an isentropic process, P1 = 7.25 N/cm2, P2 = 10.73 N/cm2, and T1 = 82 C.
The ratio of specific heats is 1.4. What is most nearly the value of T 2?
A.
B.
C.
D.
E.
92 C
111 C
124 C
142 C
252 C
12) The volume occupied by 1 lbm of air at 14.7 psia and 240 F is most nearly,
A.
B.
C.
D.
E.
15.0 ft3
17.5 ft3
19.0 ft3
20.5 ft3
22.0 ft3
13) Nitrogen (N2) and oxygen (O2) are mixed on a 50:50 basis by volume.
What is the mass fraction of oxygen?
A.
B.
C.
D.
E.
.43
.47
.50
.53
.57
Questions 14 – 15 refer to ethanol combustion shown below. (HINT: Keep track of the oxygen in the fuel!)
C2 H5OH  4.95  O2  3.76N2   2CO2  3H2 O  1.95O2  18.61N2
14) For this reaction, the percent theoretical air is most nearly,
A.
B.
C.
D.
E.
100%
124%
141%
165%
195%
15) For this reaction at 1 atm, the dew point of water in the exhaust is most nearly,
A. 110 F
B. 120 F
C. 130 F
D. 140 F
E. 150 F
16) Which of the following quantities can be derived from a P-v diagram?
A.
B.
C.
D.
E.
Heat transfer for individual processes
Net cycle heat transfer
Work transfer for individual processes
Net cycle work transfer
Cycle Efficiency
17) An insulated, rigid container is divided into two compartments separated by a partition. One compartment
contains air at 15 C and .101 MPa, the other compartment contains air at 40 C and .202 MPa. When the
dividing partition is removed, the total internal energy of the system
A.
B.
C.
D.
E.
increases
decreases
does not change
is converted into entropy
is converted into heat
18) A vapor compression refrigeration cycle utilizes R-134a as the working fluid. The coefficient of
performance of the cycle is 3.0. Under these conditions, the compressor requires a power input of 2 hp.
The refrigeration capacity of the cycle is most nearly,
A.
B.
C.
D.
E.
0.42 tons
0.59 tons
0.77 tons
1.27 tons
1.70 tons
Questions 19 – 21 refer to the following problem statement and array table for an air-standard Diesel cycle.
P
T
3
2
3
4
2
4
1
1
v
s
19) The entropy production during the expansion stroke is most nearly,
A.
B.
C.
D.
E.
-.2864 Btu/lbm-F
0 Btu/lbm-F
.0286 Btu/lbm-F
.1635 Btu/lbm-F
.2864 Btu/lbm-F
20) The compression work per unit mass of working fluid in the cylinder is most nearly,
A.
B.
C.
D.
E.
-244 Btu/lbm
-176 Btu/lbm
0 Btu/lbm
176 Btu/lbm
244 Btu/lbm
21) The heat transfer into the cycle during the combustion process per unit mass of working fluid in the
cylinder is most nearly,
A.
B.
C.
D.
E.
176 Btu/lbm
244 Btu/lbm
571 Btu/lbm
749 Btu/lbm
837 Btu/lbm
22) Which of the following is not a thermodynamic property?
A.
B.
C.
D.
E.
Heat capacity
Specific volume
Internal energy
Specific work
Quality
23) The Mollier diagram (enthalpy vs entropy) below shows a turbine expansion process in a steam power
plant. Which distance represents the actual work delivered by the turbine per unit mass of steam flowing
through the turbine?
P1
D.
h
1
B.
A.
P2
C.
T1
T2
2
2s
E.
s
24) An ideal gas undergoes a polytropic process in a closed system such that Pv = constant.
This process is,
A.
B.
C.
D.
E.
Isothermal
Isentropic
Isochoric
Isobaric
Isenthalpic
25) A sling psychrometer is used to directly measure which two moist air parameters? (choose two)
A.
B.
C.
D.
E.
Dry bulb temperature and relative humidity
Relative humidity and humidity ratio
Dry bulb temperature and wet bulb temperature
Wet bulb temperature and dew point temperature
Dew point temperature and relative humidity
26) A rigid, insulated tank is connected to a high-pressure
pipe as shown. The tank is initially evacuated. The
valve is then opened and some of the fluid in the
pipeline is allowed to enter the tank. After a period of
time, the valve is closed. Which equation below is the
best representation of the First Law for the system
identified in the sketch? [In the equations below, i =
flow entering the system and e = flow leaving the
system]


A. Q  W  m i h i  he  0
B.
m i  hi  he   0
C.
Q  m i hi 
D. m i hi 
E.
dU sys
dt
dU sys
dt
m i hi  m e he 
dU sys
dt
Rigid, insulated tank