Subject Engineering Transport Phenomena


Air is compressed from 1 atm and 20°C to 32 psi to pump up a car tire. You are to consider the pressure in the tire to be infinitesimally less than 32 psi and the exit condition into the tire as stagnant (the last little bit of air to be added) so that you consider the maximum effort required and there is no variation of conditions with time.
a) What is the temperature of the air immediately after it enters the tire if the process is isentropic?
b) What is the temperature of the air immediately after it enters the tire if during the actual process the entropy of the air decreases by 0.07 kJ/kg.K?
c) What is the specific work required for the isentropic process?
d) If the actual process is polytropic, what is the specific work required? Hint: determine the polytropic coefficient using the answer from part (b).
e) What is the heat transfer for the isentropic process? What is the heat transfer for the actual process?
f) It seems unexpected that the entropy would decrease in the real process (we normally say that the entropy increases due to losses). Explain why the entropy cannot increase in this case. Include any calculations or figures that are necessary to aid your argument.
g) Explain why the results from (c) and (d) cannot be used to calculate the isentropic efficiency for this compressor.

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Air Compression

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