Hi again, just wondering about this:

"Air is compressed in a reversible polytrophic process (pv^n=constant) from an initial state of 15 bar and

100 ° to a final pressure of 50 bar. The polytropic exponent is n=1.3. Calculate the change in

entropy per mass of air."

Would we use perfect gas relationships - specific - pv=RT at state 1. Looking at entropy in tables though, they show a change in entropy. Not sure about this one.

Hi,

dS can be directly calculated from equation,

ds = Cp*ln(T2/T1) - R*ln(P2/P1)Cp = 1.005 kJ/kg.K

R = 0.287 kJ/kg.K

T2 can be calculated using the relation

T2/T1 = (P2/P1)^((n-1)/n)

I got,

T2= 492.46 K and dS = -0.0663 kJ/kgK

No problem. Please do let your friends also know about Thermodynamics Forum. :)