8 A wellinsulated pistoncylinder device contains 10 L of air

8. A well-insulated piston–cylinder device contains 10 L of air at 100 kPa and 35 °C. An 18 W electric heater is used to heat the air for 5 minutes. Assuming the surroundings are at 20 °C and 100 kPa, determine (a) the exergy destroyed (kJ) during this process if the pressure of air is maintained constant (5 points). (b) Draw the T-s diagram of this process (3 points).

Solution

Here, power = 18 watt and time = 5 minute

So, heat, Q = 18*5*60 = 5.4kJ, m = 10L = 10kg

given, T_h= 35⁰C, T₀= 20⁰C, V = 10L

therefore, available energy(W_max) = Q(1-T₀/T_h) = 5.4(1-20/35) = 2.314

Now exergy destroyed (X_destroyed) = T₀(S_gen) = mT₀(s₂-s₁)

from steam table s2 and s1 can be calculated and final result can be obtained.