The Brayton Cycle is the Theoretical Cycle for the Gas Turbine.
Actually, there are two types of Brayton Cycles. They are Open Brayton Cycle and the Closed Brayton Cycle.
- The Open Cycle is the representation for the Internal Combustion Engine, where the working fluid will not be reused and it will be exhausted to the atmosphere. This principle is used in the jet propulsion. This open Gas turbine cycle is the Ideal for the Brayton cycle.
- Whereas the closed cycle represents the Gas Turbine where the working fluid will be reused. This principle is used in the Electric power generations.
Check the P-V diagram and the T-S Diagram for the both the Ideal Brayton Cycle and the Diesel Cycle.
From the above graphs, we can observe that the Isentropic expansion of the Diesel Cycle is further extended in the Brayton Cycle to increase the efficiency.
Lets see how the all the four process will work in the open Gas Turbine.
1 → 2
This 1 → 2 is the Isentropic Compression process. In this
2 → 3
1 → 2 is the constant Pressure Heat addition. In this process, the Fuel and the higher pressure air from the compressor are sent to a combustion chamber, where fuel is burned at constant pressure. The resulting high-temperature gases are sent to a turbine.
3 → 4
3 → 4 is the Isentropic Expansion process. The high-temperature gases expand to the ambient pressure in the turbine and produce power. During this expansion process will be continued till the combustion gasses expands till its lowest pressure to get the maximum efficiency.
4 → 1
4 → 1 is the Constant pressure heat rejection processes. In this
How the efficiency will be Increase by extending the Expansion stroke for the lowest pressure?
During the Expansion process and the exhaust process 3 → 4 & 4 → 1 respectively, the indicated power generated supposed to be completely utilised, but it is not happening in the Diesel cycle. So If we extend the expansion stroke till the combustion particle to expand till its lowest pressure then we can utilise the entire indicated power generated in the cycle to increase the efficiency of the cycle.
Efficiency of the Ideal Brayton Cycle
Thermodynamically, efficiency of the Diesel Cycle is given by
Work done by the system = Heat supplied (QS)- Heat rejected (QR)
W = QS – QR
Note: r is the compression Ratio where as the rp is the pressure ratio
Considering the 3 → 4 process
Similarly from the 1 → 2 process
By substituting the T1 ,T4 in the above Efficiency formula
From the relation between the Compression ratio and the pressure ratio.
From the above equation the efficiency equation can be written as
From the above equation, we can observe that the efficiency of the Brayton cycle
In the Brayton
We have discussed how the Brayton Cycle is extended to get the maximum efficiency from the engine and also we have derived the efficiency equation for the Brayton Cycle. Please let us know your thoughts in the comment section below.