Question

Air is compressed in the compressor of a turbojet engine. Air enters the compressor at 270 K and 58 kPa and exits the compressor at 465 K and 350 kPa. Take the average specific heat at constant pressure (cp,avg) of air as 1.015 kJ/ kg.K and Rair = 0.287 kJ/ kg.K. Find the mass specific entropy change associated with the compression process

Answer 1

Recall the Entropy Balance (Second Law of Thermodynamics) equations for the system below

And for ideal gas, we know that

Change in Entropy of the air kJ/Kg.K, ΔS = S₁ - S ₂ = Cp Ln (T₂/T₁) - R Ln(P₂/P₁)

where R is gas constant =0.287kJ/kg.K and given

Cp=1.015 kJ/ kg.K

T₂ = 465k T₁=270k, P1=58kPa, P2=350kPa

substituting these values into the eqn above, we have

S = (1.015x 0.544) - (0.287x1.798)

S = 0.0363kJ/Kg.K

Hence, the mass specific entropy change associated with the compression process = 0.0363kJ/Kg.K