Thermodynamics: An Engineering Approach 8th Edition

Published by McGraw-Hill Education
ISBN 10: 0-07339-817-9
ISBN 13: 978-0-07339-817-4

Chapter 9 - Gas Power Cycles - Problems - Page 541: 9-57

Answer

$\dot{W}_{\text {net,out }} =87.6\text{ kW}$

Work Step by Step

Process 1-2: isentropic compression.$$ T_2=T_1\left(\frac{\boldsymbol{V}_1}{\boldsymbol{V}_2}\right)^{k-1}=(343 \mathrm{~K})(22)^{0.4}=1181 \mathrm{~K} $$ Process 2-3: $P= $constant heat addition.$$ \frac{P_3 \boldsymbol{v}_3}{T_3}=\frac{P_2 \boldsymbol{v}_2}{T_2} \longrightarrow T_3=\frac{\boldsymbol{v}_3}{\boldsymbol{V}_2} T_2=1.8 T_2=(1.8)(1181 \mathrm{~K})=2126 \mathrm{~K} $$ Process 3-4: isentropic expansion.$$ T_4=T_3\left(\frac{\boldsymbol{V}_3}{\boldsymbol{V}_4}\right)^{k-1}=T_3\left(\frac{2.2 \boldsymbol{V}_2}{\boldsymbol{V}_4}\right)^{k-1}=T_3\left(\frac{2.2}{r}\right)^{k-1}=(2216 \mathrm{~K})\left(\frac{1.8}{22}\right)^{0.4}=781 \mathrm{~K} $$For the cycle:$$ \begin{aligned} m & =\frac{P_1 V_1}{R T_1}=\frac{(97 \mathrm{kPa})\left(0.0024 \mathrm{~m}^3\right)}{\left(0.287 \mathrm{kPa} \cdot \mathrm{m}^3 / \mathrm{kg} \cdot \mathrm{K}\right)(343 \mathrm{~K})}=0.002365 \mathrm{~kg} \\ Q_{\text {in }} & =m\left(h_3-h_2\right)=m c_p\left(T_3-T_2\right) \\ & =(0.002365 \mathrm{~kg})(1.005 \mathrm{~kJ} / \mathrm{kg} \cdot \mathrm{K})(2216-1181) \mathrm{K} \\ & =2.246 \mathrm{~kJ} \\ Q_{\text {out }} & =m\left(u_4-u_1\right)=m c_v\left(T_4-T_1\right) \\ & =(0.002365 \mathrm{~kg})(0.718 \mathrm{~kJ} / \mathrm{kg} \cdot \mathrm{K})(781-343) \mathrm{K} \\ & =0.7438 \mathrm{~kJ} \\ W_{\text {net,out }} & =Q_{\text {in }}-Q_{\text {out }}=2.246-0.7438=1.502 \mathrm{~kJ} / \mathrm{rev} \\ \dot{W}_{\text {net,out }} & =\dot{n} W_{\text {net,out }}=(3500 / 60 \mathrm{rev} / \mathrm{s})(1.502 \mathrm{~kJ} / \mathrm{rev})=87.6 \mathrm{~kW} \end{aligned} $$
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