Question

In an experimaental set-up, air flows between two stations $P$ and $Q$ adiabatically. The direction of flow depdns on the pressure and temperature conditions maintained at $P$ and $Q$. The conditions at $P$ are $150kPa$ and $35K$. The temperature at station $Q$ is $300K$. The following are the properties and relations pertaining to air:
Specific heat at constant pressure,
$c_p=1.005kG/kgK;$
Specific heat at constnat volume,
$c_v=0.718kJ/kgK;$
Characteristic gas constnat,
$R=0.287kJ/kgK$
Enthalpy, $h=c_{p}T.$
Internal energy, $u=c_{v}T.$

If the pressure at station $Q$ is $50kPa,$ the change in entropy $(s_Q-s_P)$ in $kJ/kgK$ is

• A. $0$
• B. $0.355$
• C. $-0.155$
• D. $0.160$

Answer 1

The correct option is D $0.160$

If $P_Q=50kPa$
Chage in entropy,
$S_Q-S_P=c_{P}In\frac{T_Q}{T_P}-RIn\frac{p_Q}{p_P}$
$=1.005In\frac{300}{350}-0.287In\frac{50}{150}$
$=1.005(-0.1541)-0.287(-1.098)$
$=-0.1548+0.3151=0.160kJ/kgK$