Brayton or Joule Cycle

Brayton cycle is an ideal air standard cycle for the closed cycle gas turbine unit. Both the heat supplied and rejected from the cycle occur at constant pressure, therefore this cycle is also known as constant pressure cycle. The cycle consists of four processes:

1 to 2: Isentropic compression
2 to 3: Isobaric heat supply
3 to 4: Isentropic expansion
4 to 1: Isobaric heat rejection

Work input to the cycle (compressor), W1 and work output of the cycle (gas turbine), W2 are:

W1=m (h2-h1)
W2=m (h3-h4)

wher m is the mass flow of the cycle. Heat supplied to the cycle (heater), Q1 and heat rejected from the cycle (cooler), Q2 are:

Q1=m (h3-h2)

Q2=m (h4-h1)

The thermal efficiency of a Brayton cycle with a perfect gas as working fluid is:

= 1-(T4-T1)/(T3-T2)

It can be shown that:

= 1 -r

where,
r = P2/P1 = pressure ratio
n = -1+1/= a constant depending on specific heat capacity