Liquid octane (C_3H_18) enters a steady-flow combustion chamber at 77 degree F a

Question

Liquid octane (C_3H_18) enters a steady-flow combustion chamber at 77 degree F and 1 atm at a rate of 0.3 lbm/min. It is burned completely with stoichiometric amount of air that enters at 140 degree F and 1 atm. If combustion products leave at 1100 degree F and 1 atm, determine: (a) the heat transfer rate from combustion chamber, (b) the entropy generation rate, (c) the exergy destruction rate. Assume T_0 = 77 degree F. Fill the cells of following tables which you need the data to solve the problem.

Explanation / Answer

2. (a) Flowrate of octane = 0.3 lb/min

Inlet temperature of octane = 77 oF

Outlet temperature of combustion products = 1100 oF

Specific heat capacity of octane = 2.064 kJ/kg.K = 0.027 kJ/lb.oF

Heat transfer rate, Q = mCpT

= 0.3 (lb/min) * 0.027 (kJ/lb.oF) * (1100 - 77) oF

= 8.5 kJ/min

(b) Entropy generation rate = Q/To

= (8.5 kJ/min) / (77 oF) (since To = 77 oF as given in question)

= 0.11 kJ/min.oF

(c) Exergy destruction rate = - Entropy generation rate

= - Q/T

= - 0.11 kJ/min.oF

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