Nitrogen at 100ºC and 600 kPa expands in such a way that it can be approximated by a polytropic process with n = 1.2. Calculate the work and the heat transfer if the final pressure is 100 kPa.
Text Solution
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Sol. The final temperature is found to be
T 2 = T 1 
= (373) 
= 276.7 K
The specific volumes are
v 1 = 
= 
= 0.1846 m 3 /kg
v 2 = 
= 
= 0.822 m 3 /kg
The work is then
w = 
= (600) (0.1846) 1.2 
(0.822 –0.2 –0.1846 –0.2 ) = 143 kJ/k
The first law provides us with the heat transfer:
q – w = Δ u = c v (T 2 – T 1 ) q – 143 = (0.745)(276.7 – 373)
∴ q = 71.3 kJ/kg
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