Answer to Question #210821 in Mechanical Engineering for Mohamed sadique

Question

Air at 1 bar and 27°C is heated in a non flow system at constant pressure to 177°C. Heat is supplied from a constant temperature reservoir at 577 °C. The atmospheric temperature is 20 °C. Calculate the available energy.

Answer 1

Heat received by the air is given by $Q = m_ac_v (T_2′ – T_1′)$

Say Q=600, $\implies 600 = 1× 0.81 (T_2′ – 400)$

$T_ 2′ =\frac{ 600}{ 3 × 0.81} + 400 = 646.9 K \space say \space 647 K$

Available energy with the source $(1200 – 290) × \frac{ 600}{ 1200} = 455 kJ$

Change in entropy of the air $= m_ac_v log _e \frac{T_2}{ T_1} = 3 × 0.81 × log_e \frac{647}{400} = 1.168 kJ/K$

Unavailability of the air $= 290 × 1.168 = 338.72 kJ$

Available energy with the air $= 600 – 338.72 = 261.28 kJ$