HW30.SOLNS

# HW30.SOLNS - HW#30 Solutions 7-79C The entropy of a gas can...

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HW#30 Solutions 7-79C The entropy of a gas can change during an isothermal process since entropy of an ideal gas depends on the pressure as well as the temperature. 7-85E The final temperature of air when it is expanded isentropically is to be determined. Assumptions Air is an ideal gas with constant specific heats. Properties The specific heat ratio of air at an anticipated average temperature of 300 ° F is k = 1.394 (Table A-2Eb). Analysis From the isentropic relation of an ideal gas under constant specific heat assumption, R 609 = + = = - 394 . 1 / 394 . 0 / ) 1 ( 1 2 1 2 psia 100 psia 20 R) 460 (500 k k P P T T Discussion The average air temperature is (960+609)/2=785 R=325 ° F, which is sufficiently close to the assumed average temperature of 300 ° F. 7-87 The final temperatures of neon and air when they are expanded isentropically are to be compared. Assumptions Neon and air are ideal gases with constant specific heats.

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