Engineering Board Exam Reviewer · Thermodynamics · hard
One kilogram of air is compressed polytropically (n = 1.3) from 300 K to 400 K. What is the heat transfer? (cv = 0.7186 kJ/kg· K, k = 1.4)
- 23.95 kJ (heat added)
- 71.86 kJ (heat added)
- 0 kJ
- -23.95 kJ (heat rejected)
Answer
D. -23.95 kJ (heat rejected)
Solution
Polytropic specific heat cn = cv(n - k)/(n - 1) = 0.7186(-0.1)/0.3 = -0.2395 kJ/kg· K. Q = m cn T = -0.2395(100) = -23.95 kJ, i.e., heat is rejected.
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