Thermodynamics - College of Engineering - Wayne State University

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27 Οκτ 2013 (πριν από 3 χρόνια και 10 μήνες)

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Wayne State University

College of Engineering

Mechanical Engineering Department

Ph. D. Preliminary Qualifying Examination

Tuesday, January 26, 2010

Thermodynamics

You are required to solve four out of five problems. Clearly indicate which four problems
you are
selecting. Show all work on the exam sheets provided and write your student number on each sheet. Do
not write your name on any sheet.


Student PID Number…………………………..




QUESTION 1


A gas turbine for an automobile is designed with a regenerator

(
heat exchanger)
. Air enters th
e
compressor

(point 1)

at 100 kPa, and 25
o
C

. The compressor

(and turbine)

pressure
ratio is 10
; th
e
maximum cycle temperature is 9
00
o
C . The cold air
stream

(
point
3

in fig.
)

leaves the regenerator 10
o
C
cooler than the hot a
ir stream

(
point
5
)

at the inlet of regenerator. Assuming b
oth compressor and turbine
to have

isentropic

efficiency of 90%
, determine the rate of heat addition and rejection for this cycle,

overall thermal efficiency,
when it produces 200 kW

power
.


Wa
yne State University

College of Engineering

Mechanical Engineering Department

Ph. D. Preliminary Qualifying Examination

Tuesday, January 26, 2010

Thermodynamics

You are required to solve four out of five problems. Clearly indicate which four problems yo
u are
selecting. Show all work on the exam sheets provided and write your student number on each sheet. Do
not write your name on any sheet.


Student PID Number…………………………..



Question 2
.


A piston cylinder device with a set of stops initially contains 0.
20 Kg steam at 3.5 MPa and

300
o
C steam. Heat is lost to the surroundings

at 298K

and piston moves down hitting a set of
stops at which point the cylinder co
ntains steam with a quality of 1
0%. Heat t
ransfer continues
till the final
temperature is 200C.


De
termine (a) final pressure and quality
,

if 2 phase (b) Heat transfer for the proc
ess (c) Entropy
change for the
process (system +surroundings)









Wayne State University

College of Engineering

Mechanical Engineering Department

Ph. D. Preliminary Qua
lifying Examination

Tuesday, January 26, 2010

Thermodynamics

You are required to solve four out of five problems. Clearly indicate which four problems you are
selecting. Show all work on the exam sheets provided and write your student number on each shee
t. Do
not write your name on any sheet.


Student PID Number…………………………..



Question 3
.


A 6
-
m x 6
-
m x 9
-
m room is to be heated by electric resistance heater placed in a short duct in the
room. Initially room

air temperature

is 10
o
C and local pressure is 10
0 kP
a. The room is losing
heat to the outside at a rate of 240 kJ/min. A 200
-
W fan circulates the air
steadily through the
duct. The duct can be assumed to be adiabatic and there is no air leaking in or out of the room. If
it takes 25 minutes for the room
air to reach an average temperature of 25
o
C. Determine (a) the
power rating of electric heater (b) the temperature rise
that
air experiences each time it passes
through heater.





Wayne State University

College of Engineering

Mechanical Engineering Depa
rtment

Ph. D. Preliminary Qualifying Examination

Tuesday, January 26, 2010

Thermodynamics

You are required to solve four out of five problems. Clearly indicate which four problems you are
selecting. Show all work on the exam sheets provided and write yo
ur student number on each sheet. Do
not write your name on any sheet.


Student PID Number…………………………..



Question 4
.


An insulated vertical piston
-
cylinder device initially contains 0.2 m
3
of air at 200 kPa and
25
o
C. At this stage, a linear spring touches

the piston but exerts no force on it. The
cylinder is connected by a valve to a line that supplies air at 800 kPa and 25
o
C. The valve is
opened and air from high pr
essure line is allowed to enter the cylinder. The valve is closed,
when pressure in the cy
linder reaches 600 kPa. If the enclosed vo
lume inside the cylinder
triples

during this process, determine (a) mass of air that entered the cylinder (b) the final
temperature of air inside the cylinder (c) entropy change for the system and surroundings


You

may assume specific heat of air to be constant during this process.




Wayne State University

College of Engineering

Mechanical Engineering Department

Ph. D. Preliminary Qualifying Examination

Tuesday, January 26, 2010

Thermodynamics

You are required t
o solve four out of five problems. Clearly indicate which four problems you are
selecting. Show all work on the exam sheets provided and write your student number on each sheet. Do
not write your name on any sheet.


Student PID Number…………………………..



Ques
tion 5:


A balloon initially contains 75

m
3

helium (MW=4) gas at
100 kPa and 20
o
C. The balloon is
connected by a valve to the reservoir t
hat supplies helium 200 kPa, 25
o
C. Now the valve is
opened until the pressure in the balloon reaches 150 kPa, at which

point valve is closed. The
material of balloon is such that its volume increases linearly with pressure. If no heat transfer
takes place during this process, determine (a) final temperature
in the balloon
(b) change of
entropy within balloon.