Short Answer:
The processes in Brayton cycle are four main thermodynamic stages that describe how a gas turbine converts heat energy into mechanical work. These processes are isentropic compression, constant pressure heat addition, isentropic expansion, and constant pressure heat rejection.
In simple words, air is compressed in a compressor, heated at constant pressure in the combustion chamber, expanded in a turbine to produce power, and finally cooled or released to the atmosphere. Together, these four processes form a continuous cycle used in gas turbines and jet engines for power generation and propulsion.
Detailed Explanation :
Processes in Brayton Cycle
The Brayton cycle, also called the Joule cycle, is a thermodynamic cycle that explains how a gas turbine engine produces mechanical or electrical power from fuel energy. It is based on the continuous flow of air and fuel through four main processes — compression, heat addition, expansion, and heat rejection.
The working fluid used in the Brayton cycle is usually air, which is compressed, heated, expanded, and then exhausted. The cycle can be open (air is discharged into the atmosphere) or closed (the same air circulates continuously).
In the ideal Brayton cycle, the processes are assumed to be frictionless and adiabatic for compression and expansion, while heat is added and rejected at constant pressure. These assumptions simplify the analysis and represent the ideal working of a gas turbine plant.
The Four Processes in Brayton Cycle
The four main processes that occur in the Brayton cycle are described below:
- Isentropic Compression (Process 1–2)
- This process takes place in the compressor.
- Air from the atmosphere enters the compressor at low pressure and temperature.
- The air is compressed adiabatically (without heat transfer), which increases its pressure and temperature.
- The work required to drive the compressor is supplied by the turbine output.
- The compression process is reversible and isentropic, meaning entropy remains constant.
Effect on Air:
- Pressure increases from to
- Temperature increases from to
- Volume decreases from to
Mathematical Relation:
Where = ratio of specific heats (Cₚ/Cᵥ).
This process prepares the air for combustion by compressing it to a high pressure.
- Constant Pressure Heat Addition (Process 2–3)
- This process takes place in the combustion chamber or heat exchanger.
- The compressed air enters the chamber, and fuel (such as natural gas or kerosene) is injected and burned at constant pressure.
- The heat produced from combustion is absorbed by the air, increasing its temperature and internal energy.
- The pressure remains constant because the combustion process is designed to allow expansion of gases.
Effect on Air:
- Temperature increases from to
- Volume increases from to
- Pressure remains constant at
Heat Added per unit mass of Air:
The air after this process becomes a high-temperature, high-energy gas ready to expand in the turbine.
- Isentropic Expansion (Process 3–4)
- This process occurs in the turbine section of the gas turbine plant.
- The high-pressure and high-temperature gases from the combustion chamber expand adiabatically through the turbine blades.
- As the gases expand, they do work on the turbine rotor, converting heat energy into mechanical energy.
- The expansion is isentropic, meaning it is reversible and adiabatic (no heat loss).
- The gas temperature and pressure both decrease during this process.
Effect on Gas:
- Pressure decreases from to
- Temperature decreases from to
- Volume increases from to
Mathematical Relation:
Work Done by the Turbine:
Part of this turbine work drives the compressor, and the rest is used for external load or power generation.
- Constant Pressure Heat Rejection (Process 4–1)
- This is the final process of the cycle.
- After expansion, the exhaust gases leave the turbine and release their remaining heat at constant pressure.
- In an open Brayton cycle, the gases are discharged directly to the atmosphere.
- In a closed Brayton cycle, the gases transfer heat to a cooling medium (like water or air) in a heat exchanger before returning to the compressor.
Effect on Gas:
- Temperature decreases from to
- Volume decreases from to
- Pressure remains constant at
Heat Rejected per unit mass of Air:
This process completes the cycle by restoring the working fluid to its original condition.
Summary of Brayton Cycle Processes
| Process | Type of Process | Component | Nature of Process | Main Effect |
| 1–2 | Isentropic Compression | Compressor | Adiabatic | Pressure and temperature increase |
| 2–3 | Constant Pressure Heat Addition | Combustion Chamber | Heat added | Temperature increases |
| 3–4 | Isentropic Expansion | Turbine | Adiabatic | Work produced, pressure drops |
| 4–1 | Constant Pressure Heat Rejection | Exhaust/Heat Exchanger | Heat rejected | Temperature decreases |
(Note: The explanation is in text form, not a table.)
Work and Efficiency
- Net Work Output:
- Thermal Efficiency:
The efficiency of the Brayton cycle depends on the pressure ratio (r_p):
Increasing the pressure ratio and turbine inlet temperature improves efficiency.
Energy Flow in the Brayton Cycle
- Compressor: Mechanical work input to compress air.
- Combustion Chamber: Chemical energy of fuel converted into heat energy.
- Turbine: Expansion converts heat energy into mechanical power.
- Exhaust: Heat released back to the atmosphere or recovery system.
This continuous energy conversion makes the Brayton cycle suitable for gas turbines and jet propulsion.
Applications of Brayton Cycle
- Gas turbine power plants for electricity generation.
- Jet engines and aero engines for aircraft propulsion.
- Marine propulsion systems in ships and submarines.
- Combined cycle power plants to increase overall efficiency.
- Industrial drives for compressors and pumps.
Conclusion
In conclusion, the processes in Brayton cycle include isentropic compression, constant pressure heat addition, isentropic expansion, and constant pressure heat rejection. These four processes together describe how air and fuel energy are converted into useful mechanical work in gas turbine systems. The cycle operates continuously and efficiently, making it ideal for aircraft engines, power generation, and industrial applications. Improvements such as regeneration, intercooling, and reheating can further increase its efficiency and performance in modern gas turbines.