What is the Brayton cycle?

Short Answer:

The Brayton cycle is the thermodynamic cycle on which a gas turbine engine operates. It describes the process of converting heat energy into mechanical work through the compression of air, combustion of fuel at constant pressure, and expansion of hot gases in a turbine.

In simple words, the Brayton cycle explains how gas turbines produce power. It consists of four main processes — isentropic compression, constant pressure heat addition, isentropic expansion, and constant pressure heat rejection. This cycle is widely used in gas turbine power plants, jet engines, and combined-cycle power systems.

Detailed Explanation :

Brayton Cycle

The Brayton cycle is a thermodynamic cycle that represents the working principle of a gas turbine engine. It is also known as the Joule cycle. In this cycle, air is used as the working fluid, and the energy is added by burning fuel at constant pressure. The Brayton cycle explains how fuel energy is converted into useful mechanical energy in gas turbines, jet engines, and other power-producing systems.

This cycle consists of four main processes — two adiabatic (isentropic) and two constant pressure processes. The Brayton cycle can be used in two forms:

  1. Open Cycle: The working fluid (air) enters the compressor, passes through the turbine once, and is then discharged to the atmosphere.
  2. Closed Cycle: The same working fluid circulates continuously within the system, and heat is added and rejected externally.

The open cycle is mostly used in aircraft engines and gas turbine power plants, while the closed cycle is used in stationary power plants and nuclear gas-cooled reactors.

Processes of Brayton Cycle

The ideal Brayton cycle consists of four thermodynamic processes, as shown below:

  1. Isentropic Compression (Process 1–2)
  • In this process, air from the atmosphere enters the compressor, where it is compressed adiabatically (without heat exchange).
  • As a result, both the pressure and temperature of the air increase significantly.
  • This high-pressure air is then sent to the combustion chamber.
  • Work is required to drive the compressor, and this energy is supplied by the turbine shaft.

Mathematical Relation:

Where,
= temperatures before and after compression,
= pressures before and after compression,
= ratio of specific heats (Cₚ/Cᵥ).

  1. Constant Pressure Heat Addition (Process 2–3)
  • The compressed air enters the combustion chamber, where fuel is injected and burned at constant pressure.
  • The chemical energy of the fuel is converted into thermal energy, increasing the air’s temperature drastically.
  • The pressure remains almost constant during this process, but the volume and temperature rise sharply.
  • The high-temperature and high-pressure gases produced are then directed toward the turbine.

Heat Added per kg of Air:

  1. Isentropic Expansion (Process 3–4)
  • The high-pressure gases now enter the turbine, where they expand adiabatically.
  • During expansion, the gases perform work on the turbine blades, causing the shaft to rotate.
  • The expansion process reduces both pressure and temperature of the gases.
  • The turbine produces mechanical power, part of which drives the compressor, and the rest is used for external load such as generating electricity or propelling an aircraft.

Mathematical Relation:

Work Done by Turbine:

  1. Constant Pressure Heat Rejection (Process 4–1)
  • After expansion, the exhaust gases leave the turbine and reject heat to the surroundings at constant pressure.
  • In an open cycle, the gases are discharged into the atmosphere.
  • In a closed cycle, the working fluid releases heat in a heat exchanger and returns to its original state before entering the compressor again.

Heat Rejected per kg of Air:

Representation of Brayton Cycle

  1. On Pressure-Volume (P–V) Diagram:
    • The area enclosed by the cycle shows the net work output.
    • The compression and expansion processes are curved (isentropic), while heat addition and rejection are straight horizontal lines (constant pressure).
  2. On Temperature-Entropy (T–S) Diagram:
    • The isentropic processes are vertical lines (no change in entropy).
    • The constant pressure processes are sloped lines showing heat transfer.
    • The enclosed area represents the net work done by the cycle.

Work and Efficiency of Brayton Cycle

Net Work Output:

Thermal Efficiency (η):
The thermal efficiency of an ideal Brayton cycle depends on the pressure ratio (r_p) of the compressor.

Where,
= compressor pressure ratio.

From this equation, it is clear that the efficiency increases with an increase in pressure ratio. However, practical limitations exist due to material strength and temperature limits.

Factors Affecting Brayton Cycle Performance

  1. Pressure Ratio:
    Increasing pressure ratio improves efficiency up to an optimum limit. Beyond that, compressor work increases too much.
  2. Turbine Inlet Temperature:
    Higher inlet temperature increases turbine output and overall efficiency.
  3. Regeneration:
    Using a regenerator (heat exchanger) to recover exhaust heat and preheat air from the compressor enhances efficiency.
  4. Intercooling and Reheating:
    Intercooling reduces compression work, and reheating increases turbine work, improving net output.
  5. Fuel Type and Quality:
    Clean fuels such as natural gas improve combustion efficiency and reduce maintenance.

Applications of Brayton Cycle

  1. Gas Turbine Power Plants:
    Used for electricity generation where high efficiency and fast operation are required.
  2. Aircraft Jet Engines:
    Jet propulsion systems are based on the open Brayton cycle.
  3. Marine Propulsion:
    Used in ships for direct mechanical drive due to compact size and high power output.
  4. Combined Cycle Plants:
    Exhaust gases from the Brayton cycle are used to generate steam for a Rankine cycle, increasing total plant efficiency.
  5. Industrial Applications:
    Used in mechanical drives for compressors and pumps in oil and gas industries.

Advantages of Brayton Cycle

  • Continuous and smooth power generation.
  • High power-to-weight ratio.
  • Suitable for both stationary and mobile applications.
  • Quick start-up and operation.
  • Simple construction with fewer moving parts.

Limitations of Brayton Cycle

  • Lower efficiency at part loads.
  • Requires high-quality materials to withstand high temperatures.
  • Compressor consumes a large portion of generated power.
  • Efficiency highly depends on pressure ratio and temperature limits.
Conclusion

In conclusion, the Brayton cycle is the fundamental thermodynamic cycle used in gas turbine engines for converting heat energy into mechanical power. It consists of four main processes — isentropic compression, constant pressure heat addition, isentropic expansion, and constant pressure heat rejection. The cycle’s efficiency depends on pressure ratio and turbine inlet temperature. Due to its high power output, fast response, and reliability, the Brayton cycle is widely applied in gas turbine power plants, aircraft engines, and combined-cycle systems.