Question

A piston–cylinder device contains 1.2 kg of nitrogen gas at 120 kPa and 27°C. The gas is now compressed slowly in a polytropic process during which PV1.3 = constant. The process ends when the volume is reduced by one-half. Determine the entropy change of nitrogen during this process.

Answer 1

Solution;

From the tables;

$R=0.297kJ/kgK$

$C_v=0.743kJ/kgK$

From the polytropic relation;

$\frac{T_2}{T_1}=(\frac{v_1}{v_2})^{n-1}$

$T_2=300(2)^{0.3}=369.34K$

Change in entropy;

$\Delta s=m[C_vln(\frac{T_2}{T_1})+Rln(\frac{v_2}{v_1})]$

$\Delta s=1.2[0.743ln(\frac{369.34}{300})+0.297ln(0.5)]$

$\Delta s=-0.0616kJ/K$