BMCT 2123 THERMODYNAMICS ITUTORIAL 06

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FACULTY OF MECHANICAL ENGINEERINGBMCT 2123 THERMODYNAMICS I

TUTORIAL 06

1. The entropy of a hot baked potato decreases as it cools. Is this a violation of the increaseof entropy principle? Explain.

2. Is it possible to create entropy? Is it possible to destroy it?

3. Is it possible for the entropy change of a closed system to be zero during an irreversibleprocess? Explain.

4. Air is compressed by a 12 kW compressor from P1 to P2. The air temperature ismaintained constant at 25oC during this process as a result of heat transfer to thesurrounding medium at 10oC. Determine the rate of entropy change of the air. State theassumption made in solving this problem.

5. During the isothermal heat addition process of a Carnot cycle, 900 kJ of heat is added tothe working fluid from a source at 400oC. Determine:

a) The entropy change of the working fluidb) The entropy change of the sourcec) The total entropy change for the process.

6. Refrigerant-134a enters the coils of the evaporator of a refrigeration system as a saturatedliquid-vapor mixture at a pressure of 200 kPa. The refrigerant absorbs 120 kJ of heat fromthe cooled space, which is maintained at -5oC, and leaves as saturated vapor at the samepressure. Determine:

a) The entropy change of the refrigerantb) The entropy change of the cooled spacec) The total entropy change for this process.

7. The radiator of a steam heating system has a volume of 20 L and is filled with superheatedwater vapor at 200 kPa and 200oC. At this moment both the inlet and the exit valves tothe radiator are closed. After a while the temperature of the steam drops to 80oC as aresult of heat transfer to the room air. Determine the entropy change of the steam duringthis process in kJ/K.

UNIVERSITI TEKNIKAL MALAYSIA MELAKAKARUNG BERKUNCI 1200

AYER KEROH75450 MELAKA

BMCT 2123 THERMODYNAMICS ITUTORIAL 06

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8. A 0.5 m3 rigid tank contains refrigerant-134a initially at 200 kPa and 25 percent quality.Heat is transferred now to the refrigerant from a source at 35oC until the pressure rises to400 kPa. Determine:

a) The entropy change of the refrigerantb) The entropy change of the heat sourcec) The total entropy change for this process.

9. An insulated piston-cylinder device contains 5 L of saturated liquid water at a constantpressure of 150 kPa. An electric resistance heater inside the cylinder is now turned on,and 2200 kJ of energy is transferred to the steam. Determine the entropy change of thewater during this process in kJ/K.

10. Air is compressed in a piston-cylinder device from 90 kPa and 20oC to 400 kPa in areversible isothermal process. Determine:

a) The entropy change of airb) The work done.

11. Air is compressed in a piston-cylinder device from 100 kPa and 17oC to 800 kPa in areversible, adiabatic process. Determine the final temperature and the work done duringthis process, assuming:

a) Constant specific heatsb) Variable specific heats for air.

12. A quantity of gas has an initial pressure, volume and temperature of 140 kPa, 0.14 m3, and25oC respectively. It is compressed to a pressure of 1.4 kPa according to the lawPV1.25 = constant, determine the change of entropy.