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

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