Question

A cylinder/piston contains 100 L of air at 110 kPa, 25°C. The air is compressed in a reversible polytropic process to a final state of 800 kPa, 200°C. Assume the heat transfer is with the ambient at 25°C and determine the polytropic exponent n and the final volume of the air. Find the work done by the air, the heat transfer and the total entropy generation for the process. 

Answer 1

C.V. Air of constant mass m2 = m1 = m out to ambient.  

Thus the unknown is the exponent n.

m = P₁V₁ /(RT₁) = 110 × 0.1/(0.287 × 298.15) = 0.1286 kg 

The relation from the process and ideal gas is

 From the entropy equation 

₁S_2,gen = m(s₂ - s₁) - ₁Q₂/T₀ 

 = 0.1286 × (-0.106) + (5.144/298.15) = 0.00362 kJ/K