Actual Brayton Cycle

The actual turbine cycle differs from the theoretical cycle in the following manner.

• Due to frictional losses in the compressor and turbine, entropy increases in compression and expansion process. In ideal case, the compressor efficiency (η_c) and turbine efficiency (η_T) are 100% but in practice they are less than 100%.
• A small pressure loss occurs in combustion chamber. This loss is so little that it can be neglected for the sake of simplicity.
• The specific heat of combustion gas is slightly higher than that of air. This increase is so little that the specific heat of combustion gas may be taken as that of air for simplicity wherever necessary.

A T–s diagram for an actual Brayton cycle is shown in Figure 5.21.

Figure 5.21 Actual Brayton Cycle

1 – 2 Isentropic compression

1 –2′ Actual compression

3 – 4 Isentropic expansion

3 – 4′ Actual expansion

The compressor efficiency,

where W_T = Work done by turbine

            W_C = Work done by compressor

            Q_A = Actual heat addition