1 ME EN 2300 Thermodynamics I Department of Mechanical Engineering University of Utah Spring 2014 Final Exam Name:_________________________________________ 04/30/2014 8 problems, 13 pages ID:___________________________________________ Total of 55 points 1. (5 points) True or False (Circle your answers in the box below) (a) True False (b) True False (c) True False (d) True False (e) True False (a) The entropy of an open system can decrease by heat transfer only. (b) Irreversibilities such as friction always lead to an increase in entropy. (c) An ideal heat engine has a maximum thermal efficiency of 100%. (d) An isothermal reversible process is also isentropic. (e) Both the enthalpy and the entropy of an ideal gas are only a function of the temperature.
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ME EN 2300 Thermodynamics I Department of Mechanical Engineering University of Utah Spring 2014 Final Exam Name:_________________________________________ 04/30/2014 8 problems, 13 pages ID:___________________________________________ Total of 55 points 1. (5 points) True or False (Circle your answers in the box below)
(a) The entropy of an open system can decrease by heat transfer only.
(b) Irreversibilities such as friction always lead to an increase in entropy.
(c) An ideal heat engine has a maximum thermal efficiency of 100%.
(d) An isothermal reversible process is also isentropic.
(e) Both the enthalpy and the entropy of an ideal gas are only a function of the temperature.
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2. (3 points) Consider a Carnot heat engine. It is claimed that the thermal efficiency of this heat engine can be doubled by doubling the temperature of the energy source. Is this claim valid? Justify your answer.
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3. (3 points) Air at 1500 kPa and 327ºC is expanded adiabatically in a closed system. The process is internally reversible and the final temperature of air is 27ºC. Assuming constant specific heats for air, determine the final pressure.
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4. (6 points) Determine the change in the specific entropy for the following cases:
5. (Total of 9 points) Consider a heat pump operating with R-134a as the working fluid. The heat pump is used to keep a room at 25ºC by absorbing 5 kW of heat from the outdoor air at 10ºC. R-134a enters the evaporator at -6ºC with a quality of 10 percent and leaves at the same pressure as saturated vapor. The evaporator can be modeled as a steady-flow device and the power input to the compressor is 1.5 kW.
(a) (4 points) Determine the mass flow rate of R-134a.
(b) (2 points) Determine the rate of heat supplied to the room Q•
H .
(c) (2 points) Determine the actual and maximum coefficients of performance of the heat pump.
(d) (1 point) Determine the minimum power input to the compressor for the same rate of heat supplied to the room.
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6. (Total of 10 points) A piston-cylinder device initially contains steam at 200 kPa, 200ºC and 0.4 m3. At this initial state, a linear spring is touching the piston but exerts no force on it. Heat is now transferred to the steam, thus causing the pressure and the volume to rise to 300 kPa and 0.6 m3, respectively.
(a) (2 points) Determine the mass of steam contained in the spring-loaded piston-cylinder device.
(b) (2 points) Determine the final temperature of the steam.
(c) (2 points) Determine the boundary work in kJ.
(d) (3 points) Determine the heat transferred to the steam in kJ.
(e) (1 point) Show the process on P −υ diagram with respect to saturation lines.
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7. (Total of 10 points) Air enters an adiabatic compressor at 100 kPa and 17ºC at a rate of 2.4 m3/s, and it exits at 257ºC. The compressor has an isentropic efficiency of 83.086%. In your calculations, assume variable specific heats for air.
(a) (1 point) Determine the mass flow rate of air.
Note: If you are unable to determine the mass flow rate in part (a), please assume a value of 3 kg/s.
(b) (3 points) Determine the temperature of air if the process was isentropic.
(c) (2 points) Determine the exit pressure of air.
(d) (3 points) Determine the actual power required to drive the compressor.
(e) (1 point) Name a potential factor causing the entropy of air to increase during the compression.
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8. (Total of 9 points) Water at 0ºC is frozen by evaporating saturated R-134a liquid at -16ºC. Heat is transferred from the water to the refrigerant at a rate of 230 kW. The refrigerant leaves the evaporator as a saturated vapor at -16ºC. The changes in kinetic and potential energies are negligible.
(a) (2 points) Determine the enthalpy and entropy of R-134a at the inlet and outlet of the evaporator.
(b) (2 points) Determine the mass flow rate of refrigerant.
(c) (4 points) Determine the total rate of entropy generation during this process (i.e., include both the evaporator and its surroundings).
(d) (1 point) In part (c), would it be possible to have a negative rate of entropy generation?
