Joule or Brayton Cycle

The ideal cycle for the simple gas turbine is the Joule or Brayton cycle which is represented by the cycle 1234 in the p-v and T-S diagram (Figure 4.3). The cycle comprises of the following process.

1-2 is the isentropic compression occuring in the compressor, 2-3 is the constant pressure heat addition in the combustion chamber, 3-4 is the isentropic expansion in the turbine releasing power output, 4-1 is the rejection of heat at constant pressure - which closes the cycle. Strictly speaking, the process 4-1 does not occur within the plant. The gases at the exit of the turbine are lost into the atmosphere; therefore it is an open cycle.

	C- Compressor B- Burner or Combustion Chamber T- Turbine L- Load

In a steady flow isentropic process,

Thus, the

Compressor work per kg of air

Turbine work per kg of air

Heat supplied per kg of air

The cycle efficiency is,

or,

Making use of the isentropic relation , we have,

Where, r is pressure ratio. The cycle efficiency is then given by,

Thus, the efficiency of a simple gas turbine depends only on the pressure ratio and the nature of the gas.
Figure 4.4 shows the relation between η and r when the working fluid is air (γ =1.4), or a monoatomic gas such as argon( γ =1.66).

The specific work output w, upon which the size of plant for a given power depends, is found to be a function not only of pressure ratio but also of maximum cycle temperature T₃.

Thus, the specific work output is,