Last Name: ___________________ First Name: Middle Initial:

ME 200 – Thermodynamics I, Spring 2015, Exam 3, 8 p.m. to 9 p.m. on April 14, 2015

CIRCLE YOUR LECTURE BELOW: 7:30 a.m. 10:30 a.m. 12:30 p.m. 2:30 p.m. 4:30 p.m. Joglekar Chen Chen Kittel Naik

INSTRUCTIONS 1. This is a closed book examination. You are provided with an equation sheet and all the property

tables needed. 2. Do not hesitate to ask the instructor if you do not understand a problem statement. 3. Start each problem on the same page as the problem statement. Write on only one side of the

page. Materials on the back side of the page will not be graded. There are blank pages following problems 2 and 3 for your work. If you need extra pages, ask the instructor.

4. Put only one problem on a page. Another problem on the same page will not be graded. 5. Label your system and list relevant assumptions for problems 2 and 3. 6. If you give multiple solutions, you will receive only a partial credit although one of the solutions

might be correct. Delete the solution you do not want graded. 7. For your own benefit, please write clearly and legibly. Maximum credit for each problem is

indicated below. 8. After you have completed the exam, at your seat put your papers in order. This may mean

that you have to remove the staple and re-staple. Do not turn in loose pages. 9. Once time is called you must stop working on the exam. Points will be subtracted if you

continue working on the exam.

Problem Possible Score

1 30

2 35

3 35

Total 100

1

Last Name: ___________________ First Name: Middle Initial: Problem 1: (30/100 points) Each part of this problem is worth 6 points. There is no partial credit and your answer must be placed in the box. The standard problem solving procedure is not required for Problem 1. For parts (1), (2) and (3) of Problem 1, refer to the following T-s diagram of a Carnot refrigeration cycle.

(1) What is the value of QH (heat rejected to the high temperature reservoir), in J? (2) Consider Process 1 to 2. If the working fluid is an ideal gas, how should the pressure vary?

(a) Pressure will increase (b) Pressure will remain constant (c) Pressure will decrease (d) Not enough information is given

(3) What are some potential sources of irreversibility that would lead to entropy production if we tried to build the refrigeration cycle in real life?

(a) Heat transfer through a finite temperature difference (b) Throttling process (c) Friction between piston and cylinder (d) All of the above

(4) What assumption(s) is(are) required for the following equation to be valid: (p2/p1) = (pr2/pr1)? (a) Ideal gas (b) Constant specific heat (c) Isentropic (d) Open system

(5) What assumption(s) is(are) required for the following equation to be valid? (a) Internally reversible (b) Adiabatic (c) Closed system (d) None of the above

J

(K)

2 2

1 1+ ∆ = ∫ ∫

du PdvsT T

2

Last Name: ___________________ First Name: Middle Initial: Problem 2 (35/100 points) Given: A piston-cylinder assembly contains 2.0 lbm of saturated liquid and vapor water mixture

at 660ºR and quality of 0.2 (State 1). It undergoes an isothermal expansion until it becomes saturated vapor (State 2) while receiving heat from a hot reservoir. Additional information is provided in the table below.

State v (ft3/lbm) u (Btu/lbm) s (Btu/lbm·oR)

1 349.272 0.59044

2

Find:

(a) Complete the table. (b) Evaluate the work done by the system, in Btu. (c) Determine the heat transfer into the system, in Btu. (d) Calculate the reservoir temperature if the entropy produced is 0.1365 Btu/ºR, in oR. (e) Draw the process on T-s diagram and label states 1 and 2.

Place your final answers in the boxes below: System sketch: (Label the system under your consideration, such as control mass/volume)

Assumptions: (e)

Q

Hot Reservoir TH = ? ºR

1 2

T1 = T2 = 660ºR

T

s

(b) Btu (c) Btu (d) ºR

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Last Name: ___________________ First Name: Middle Initial: Problem 2 (continued) Basic equations: Solution: (b)

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Last Name: ___________________ First Name: Middle Initial: Problem 2 (continued)

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Last Name: ___________________ First Name: Middle Initial: Problem 3 (35/100 points) Given: Air flowing steadily at the rate of 5 kg/s enters a well-insulated compressor at 1 bar and

300 K (State 1). Air is compressed to a pressure of 6.07 bar (State 2). The compressor requires power input of 1168.95 kW. The specific heat is not constant. Do not interpolate; use the closest table value.

Molecular weight of air = 28.97 kg/kmol Universal gas constant = 8.314 kJ/kmol·K

Find: (a) Calculate temperature of air at the exit of the compressor, in K. (b) Find the rate of entropy generation for the compressor, in kW/K. (c) Determine the isentropic efficiency of the compressor, in %. (d) Show the isentropic and actual process for air on T-s diagram. Label states and constant

pressure lines. Place your final answers in the boxes below: System sketch: (Label the system under your consideration, such as control mass/volume)

Assumptions:

(d)

Compressor

P1 = 1 barT1 = 300 K

compressorW

P2 = 6.07 bar

(b) kW/K (c) % (a) K

s

T

6

Last Name: ___________________ First Name: Middle Initial: Problem 3 (continued) Basic equations: Solution: (a)

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Last Name: ___________________ First Name: Middle Initial: Problem 3 (continued)

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