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Chapter 1: Introduction



Problem 1.4:

Two kilograms of air at an initial temperature and pressure of 30oC and 100 kPa undergoes an isentropic process, the final temperature attained being 850oC. Find the final pressure, the initial and final densities, and the initial and final volumes.

Solution:

Given: \( m=2\,\text{kg} \), \( T_{1}=30^{\circ}\text{C}=303\,\text{K} \), \( p_{1}=100\,\text{kPa} \), isentropic process, \( T_{2}=850^{\circ}\text{C}=1123\,\text{K} \).

To calculate: \( p_{2}=? \), \( \rho_{1}=? \), \( \rho_{2}=? \), \( v_{1}=? \), \( v_{2}=? \).

Using isentropic relation, with \( \gamma=1.4 \) for air,

$$ \frac{p_{2}}{p_{1}}=\left(\frac{T_{2}}{T_{1}}\right)^{\gamma/\left(\gamma-1\right)} $$ $$ p_{2}=p_{1}\left(\frac{T_{2}}{T_{1}}\right)^{\gamma/\left(\gamma-1\right)} $$ $$ p_{2}=100\times10^{3}\times\left(\frac{1123}{303}\right)^{1.4/\left(1.4-1\right)} $$ $$ \boxed{p_{2}=9801.216\,\text{kPa}}\ . $$

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The density \( \rho_{1} \) can be calculated using the ideal gas equation as,

$$ \rho_{1}=\frac{p_{1}}{R\,T_{1}}=\frac{100\times10^{3}}{287\times303} $$ $$ \boxed{\rho_{1}=1.15\,\text{kg/m}^{3}}\ . $$

The density \( \rho_{2} \) can be calculated using the ideal gas equation as,

$$ \rho_{2}=\frac{p_{2}}{R\,T_{2}}=\frac{9801.216\times10^{3}}{287\times1123} $$ $$ \boxed{\rho_{2}=30.41\,\text{kg/m}^{3}}\ . $$

Since the mass is constant \( m=2\,\text{kg} \), the volumes can be calculated as,

$$ \boxed{v_{1}=\frac{m}{\rho_{1}}=\frac{2}{1.15}=1.73913\,\text{m}^{3}} $$ $$ \boxed{v_{2}=\frac{m}{\rho_{2}}=\frac{2}{30.41}=0.0657678\,\text{m}^{3}} $$



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Sourabh Bhat