Air enters a nozzle steadily at 280 kPa and 77°C with a velocity of 50 m/s and exits at 85 kPa and 320 m/s. The heat losses from the nozzle to the surroundings medium at 20°C are estimated to be 3.2 kJ/kg. Determine (a) the exit temperature and (b) the entropy generation for this process. Assume that the air has variable specific heats.

Respuesta :

The input values are the following

[tex]\left.T_{1}=350 K\right\ then[/tex]

[tex]h_{1}=350.49 \frac{k j}{k g}, s_{1}=1.85708 \frac{K j}{K g \cdot K}[/tex]

By using the energy equilibrium

[tex]\dot{E}_{i n}-\dot{E}_{o u t}=\Delta \dot{E}_{s y s t e m}=0[/tex] , [tex]\dot{E}_{i n}=\dot{E}_{o u t}[/tex]

we have[tex]T_{2}=297.2 K[/tex]

eq (1)  [tex]\dot{m}\left(h_{1}+\frac{V_{1}^{2}}{2}\right)=\dot{m}\left(h_{2}+\frac{V_{2}^{2}}{2}\right)+\dot{Q}_{o u t}[/tex] ∴

[tex]0=q+h_{2}-h_{1}+\frac{V_{2}^{2}-V_{1}^{2}}{2}[/tex]

Now, for specific energy h2:

[tex]h_{2}=h_{1}-q_{o u t}-\frac{V_{2}^{2}-V_{1}^{2}}{2}[/tex]

By replacing the eq (1) we have

[tex]h_{2}=350.49 \frac{k j}{k g}-3.2-\frac{\left(320 \frac{m}{s}\right)^{2}-\left(50 \frac{m}{m}\right)^{2}}{2}\left(\frac{1 \frac{k j}{k g}}{1000 \frac{m^{2}}{s^{2}}}\right)[/tex]

[tex]h_{2}=297.34 \frac{k j}{k g}[/tex]

By using a standard ideal-gas properties of air we have

[tex]T_{2}=297.2 K[/tex]