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Last Name ___________________________ First Name ___________________________

ME 300 Engineering Thermodynamics Exam #2

Spring 2008 March 28, 2008 Form A

Note : (i) Closed book, closed notes; one 8.5 x 11 sheet allowed. (ii) 60 points total; 60 minutes; points are indicated in parentheses.

(iii) Select only one answer for multiple choice questions unless indicated otherwise; show work for other questions.

(iv) Certain thermodynamic property data tables are provided. Part I: All questions are 1 point each, except where indicated. 1. A system expands at the rate of 5 m3/s at a constant pressure of 500kPa. What is the instantaneous power produced? a. 2500 W b. 2500 kW c. 100 kW d. 100 W e. 25 kW 2. The desirable form of energy for heat pump is: (a) QH (b) QL (c) W (d) none of the above 3. A reversible adiabatic process fixed mass system is always: (a) Isochoric (b) Isentropic (c) Additive (d) Multiplicative 4. The relationship (equation) between energy received (QH), energy rejected (QL) and work produced (W) for an engine operating in cyclic manner is: (1)5. Which of the following equations is always valid for a fixed mass system undergoing an irreversible or reversible process:

A) w = ∫ Pdv B) Δ S = δ Q / T C) Q =ΔE+ W D) None of these

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(3 pts for next two problems; read the description and problems first)

A heat engine producing 500 kW of power receives heat from a high temperature source at 1300˚C

and rejects heat to a sink at 350˚C. (1 pt) 6. The ideal (Carnot) efficiency of an engine operating between the two temperature reservoirs

above is: (Calculate) (2 pts) 7. The maximum thermal efficiency this (real) engine can possibly have is: a) 45% b) 55% c) 100% d) 30% e) 70% The reason for the above answer is: 100 kJ of heat enters a closed system “A” at a temperature of 500 K containing 5 kg of gas while it undergoes an internally reversible isothermal process. The heat is transferred, without the help of any engine or heat pump or refrigerator, from the surrounding which is at a temperature of 400 K. The next three questions refer to this process.

8. The specific entropy change, ΔsA, of the system is (don’t forget the units): 9. The (total) entropy change of the surroundings, ΔSsurr, is (don’t forget the units): 10. This process is possible/impossible because_______________

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11. Units of specific enthalpy, s, are _______________. (2 pts) 12. Rank the following energy sources from the highest quality (rank 1) to the lowest quality (rank 5) if surrounding temperature is 25 C: Energy at 1000 C: Rank ________ Energy at 5000 C: Rank ________ Energy at 25 C: Rank ________ Energy at 20 C: Rank ________ Mechanical Energy: Rank _______ They are all equal (True, false) circle one. Following five questions refer to the figure below showing devices A, B, C, and D operating in cyclic manner:

High Temperature Reservoir

Low Temperature Reservoir

A B D

Q1

W

Q2

Q

W W

Q3

Q4

C

13. If magnitude of the heat energy Q1>Q2, device A is very likely to be (i) Refrigerator (ii) Heat Engine (iii) Heat Pump (iv) impossible 14. If magnitude of the heat energy Q2>Q1, device A is very likely to be (i) Refrigerator (ii) Heat Engine (iii) Heat Pump (iv) impossible 15. Device B is impossible/possible (select one). Why? (Explain very briefly) 16. Device C is impossible/possible (select one). Why? (Explain very briefly)

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17. Device D is very likely a (i) Refrigerator (ii) Heat Engine (iii) Heat Pump (iv) None of the above 18. The specific kinetic energy of 5 kg mass at speed = 20 m/s is equal to a) 100 m2/s2 b) 200 m2/s2 c) 400 m2/s2 d) 1000 kJ e) 1 kJ 19. The kinetic energy of 5 kg mass at speed = 20 m/s is equal to a) 100 m2/s2 b) 200 m2/s2 c) 400 m2/s2 d) 1000 kJ e) 1 kJ Following two questions refer to work done in a polytropic process, PVn = constant from state 1 to 2. 20. W = __________________________________ in an isochoric process. 21. W = __________________________________ in a process if n = 1. 22. In terms of wactual (actual work per mass, input or output) and wideal (ideal work per mass, input or output) the efficiency of a turbine is given by, η = __________________and efficiency of a compressor is given by η = __________________. 23. Your car is parked in the direct sunlight on a hot summer day and you observe that the temperature inside the car is considerably higher than the surrounding temperature. This observation: a) proves that heat can flow unassisted from a low temperature body to a high temperature body

for short periods of time. b) implies that the Second Law of Thermodynamics does not apply to automobiles. c) is totally incorrect. The temperature inside the car cannot be higher than the surrounding

temperature. d) is correct but has absolutely nothing to do with the Second Law of Thermodynamics.

e) is correct and totally consistent with the Second Law of Thermodynamics

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(12 pts) Problem 1. In a power plant, steam leaves the boiler and enters the turbine at 8 MPa, 1000K (state 3), at a speed of 5 m/s. The steam flows through the turbine operating in steady state at 1.5 kg/s and loses heat at 50 kJ/s. The steam leaves the turbine as saturated vapor at 12 kPa with a speed of 10 m/s (state 4). Neglect potential energy change. The component diagram with turbine is shown below. Properties of H2O are provided.

(2 pts) A. Write first law equation for the turbine in the general form, and show which terms are neglected in this problem. (2 pts) B. Calculate and/or look up enthalpies at states 3 and 4. (4pts) C. Calculate the net work output rate in kW. (2 pts) D. If the turbine were reversible and adiabatic with the same conditions at turbine entry as given above, what would be its exit entropy? Would the ideal work be greater than, equal to, or the same as the turbine above?

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(12pts) Problem 2: 5 kg of saturated liquid H2O is contained at P1 = 40 kPa (state 1) in a frictionless piston-cylinder device as shown. The mixture is heated until it becomes saturated vapor (state 2). Property table is provided. Do not interpolate; use the nearest value.

P

V

Q

Mixture

(4pts)(a) What are the initial and final total internal energy (U1 and U2) and initial and final total volume (V1 and V2) of the system? Note: m = 5 kg. (1 pt) (b) Sketch the process 1-2 in the P-V diagram above. (3pts) (c) Calculate the work done by system in kJ. (4pts) (e) Calculate the heat transfer in the process 1-2 in kJ.

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(12pts) Problem 3: A. (6 pts) Starting with the first law equation for first law for a closed (fixed mass) system, Q – W =

ΔE, show that the heat input Q1-2 in a constant pressure process is equal to difference in the enthalpy, H2 – H1 , in the absence of potential and kinetic energy.

B. (6 pts) A house has 200 W refrigerator, TV 200 W, and lights 400 W; all are turned on. An air-conditioner removes heat at a rate of 2000W. On a certain day, the house gains heat from exterior at the rate of 600 W. Calculate the rate of change of temperature of the house in degree C per minute. Given: heat capacity of house (dU/dT) is 20kJ/degC. Hint: start with rate equation of first law and neglect kinetic and potential energy.