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Thermodynamic Modeling of a Combined Cycle

A simple Brayton cycle with a net power output of 1 MW is combined with a regenerative Rankine cycle operating with steam as the working fluid. Air at 20 °C and 100 kPa enters the compressor where it is pressurized to 1.2 MPa. The isentropic efficiency of the compressor is 80%. The gas turbine operating with an isentropic efficiency of 90% receives hot air at 1000 °C. The exhaust stream of the gas turbine at 110 kPa is sent to a heat exchanger to produce 0.546 kg/s steam at 485 °C and 4.5 MPa. The exhaust gas at 100 kPa is finally discharged to the atmosphere. Steam is extracted from the turbine at 550 kPa. The preheated water leaving the open heater is saturated liquid. The water leaving the condenser is at 36 °C and 9.5 kPa. The isentropic efficiency of the steam turbine is 89%. Both pumps operate with an identical isentropic efficiency of 75%. Determine

(a) the mass flow rate of the air;

(b) the mass flow rate of the extracted steam;

(c) the temperature at state 5;

(d) the quality at the exhaust of the steam turbine;

(e) the power output of the Rankine cycle;

(f) the thermal efficiency of the combined cycle.


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