Brayton cycle (or Joule Cycle) is a thermodynamic cycle upon which a Gas turbine works. Gas turbines are used to generate power at many places. Brayton cycle is named after George Brayton, an American engineer who developed it.
Below are P-V and T-S Diagrams of the Brayton (or Joule) Cycle.
Brayton Cycle is comprised of four processes
Process 1-2
It is isentropic compression process. Here a little rise in the temperature of gas occurs due to compression. Since it is a compression process, volume of the gas decreases.
Process 2-3
It is isobaric heat addition process. A little increase in volume happens due to heat addition. Since it is a heat addition process, temperature of the gas increases.
Process 3-4
It is isentropic expansion process. Here a little dip in temperature occurs due to expansion. Since it is an expansion process, volume of the gas increases.
Process 4-1
It is isobaric heat rejection process. A little decrease in volume happens due to heat rejection. Since it is a heat rejection process, temperature of the gas decreases.
Below is the table which shows heat and work interactions of the gas turbine, along with the change in the internal energy.
| Process | Change in Internal Energy | Work Interaction | Heat Interaction |
| Process 1-2 | CV(T2-T1) | (h2-h1) | 0 |
| Process 2-3 | CV(T3-T2) | 0 | (h3-h2) |
| Process 3-4 | CV(T4-T3) | (h3-h4) | 0 |
| Process 4-1 | CV(T1-T4) | 0 | (h4-h1) |
Note: Negative value of heat interaction indicates heat rejected by the system and positive value of heat interaction indicates heat added to the system. Positive value of work interaction indicates work done by the system and negative value of work interaction indicates work done on the system.
Efficiency of Brayton Cycle
Efficiency of the Brayton Cycle is the ratio of work output to the heat input.
Work output = [(h3-h2) – (h4-h1)]
Heat Input = (h3-h2)
Efficiency = Work Output/Heat Input
After putting values of heat input and work output in the above formula, we get
η = 1 – [(h4-h1)/(h3-h2)]
also
η = 1 – [(T4-T1)/(T3-T2)]
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