If an ideal Brayton cycle is to use air as the working fluid. Consider that the

Question

If an ideal Brayton cycle is to use air as the working fluid. Consider that the specific heats are constant. Air enters the compressor at 0.2 MPa and 350 K and is discharged to 1.0 MPa. Gas entering the turbine has been heated to 1000 k. a. What is the temperature at each point in the cycle? b. What is the thermal efficiency for this Brayton cycle?

Explanation / Answer

The thermal efficiency of the ideal Brayton cycle is given by the ratio of the absolute temperatures before and after compression [1] ? = 1 - (T1/T2) In the ideal cycle the compression o reversible and adiabatic, i.e. isentropic. For an ideal gas undergoing an isentropic process [2]: T·V^(? -1) = constant => (T1/T2) = (V2/V1)^(? -1) ? is the heat capacity ratio. For a diatomic ideal gas it is [3]: ? = 7/5 I f the volume is halved during compression, i.e 1 - (V2/V1)=1/2 the thermal efficiency of the cycle is: ? = 1 - (V2/V1)^(? -1) = 1 - (1/2)^(2/5) = 0.242 = 24.2%

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