What causes change in air temperature?

Calculating entropy change for air flow given pressure and temperature at both points?

• There is air flow through an insulated duct. At point X, the pressure/temperature is 150kPa/187 degrees celcius. At another point Y, the pressure/temperature is 130kPa/160 degrees ...show more

• Answer:

  We would appear here to have air expanding in an adiabatic process according (in the usual symbolic) to PV^γ = K (a constant). For air, γ = 1.4 Otherwise we take the behaviour of air to be that of an ideal gas. There is no energy loss across the system boundary (insulated) Consider 1m³ of air at X. At Y this will have a volume of 1×(150/130).(433/460) = 1.086 m³ Work done by this quantity of gas = W = 150000×[1 – (1.086)^(-0.4)]/0.4 = 12,173. J. Based on the density of air at 100 kPa and 0˚C the mass of the 1m³ at X = 1.269×1.5×273/460 = 1.13 kg. So the specific energy loss = 12173/1.13 =10776 J Entropy change = 10776/27 = 399 J/K Flow is from X to Y (higher to lower pressure). If this (or indeed any other answer) helps you, kindly take a moment to award Best Answer. We answer to try to assist you and this award is a gratifying indication that we met with some success.