Air standard Brayton cycle

Schematic representation of an air standard Brayton cycle

Brayton cycle on (a) P-v diagram (b) T-s diagram

Processes: -

1-2: isentropic compression

2-3: constant pressure energy addition

3-4: isentropic expansion

4-1: constant pressure energy rejection

Energy added, Q₁= mC_p (T₃-T₂)

Energy rejected, Q₂= mC_p (T₄-T₁)

Thermal efficiency,

The pressure ratio of the Brayton cycle, r_p is defined as,

Then

The processes 1-2 and 3-4 are isentropic. Hence,

We get,

or

Work delivered by the cycle is given by W=hQ₁

Increasing Q₁ can increase work done by the cycle

Since the Turbine blade material cannot withstand very high temperature, T₃ and hence Q₁ is limited

The optimum pressure ratio for fixed values of T₁ and T₃, for which work is maximum, is obtained by,

For optimum pressure ratio,

or

or

or