Brayton cycle

INTRODUCTION:
The Brayton cycle is a thermodynamic cycle that describes the workings of a constant pressure heat engine. Gas turbine engines and airbreathing jet engines use the Brayton Cycle. Although the Brayton cycle is usually run as an open system (and indeed mustbe run as such if internal combustion is used), it is conventionally assumed for the purposes of thermodynamic analysis that the exhaust gases are reused in the intake, enabling analysis as a closed system.

More:

A Brayton-type engine consists of three components:

1. a compressor
2. a mixing chamber
3. an expander

In the original 19th-century Brayton engine, ambient air is drawn into a piston compressor, where it is compressed; ideally an isentropic process. The compressed air then runs through a mixing chamber where fuel is added, an isobaric process. The pressurized air and fuel mixture is then ignited in an expansion cylinder and energy is released, causing the heated air and combustion products to expand through a piston/cylinder; another ideally isentropic process. Some of the work extracted by the piston/cylinder is used to drive the compressor through a crankshaft arrangement.

The term Brayton cycle has more recently been given to the gas turbine engine. This also has three components:

1. a gas compressor
2. a burner (or combustion chamber)
3. an expansion turbine

Ideal Brayton cycle:

1. isentropic process - ambient air is drawn into the compressor, where it is pressurized.
2. isobaric process - the compressed air then runs through a combustion chamber, where fuel is burned, heating that air—a constant-pressure process, since the chamber is open to flow in and out.
3. isentropic process - the heated, pressurized air then gives up its energy, expanding through a turbine (or series of turbines). Some of the work extracted by the turbine is used to drive the compressor.
4. isobaric process - heat rejection (in the atmosphere).

Actual Brayton cycle:

1. adiabatic process - compression.
2. isobaric process - heat addition.
3. adiabatic process - expansion.
4. isobaric process - heat rejection.

Idealized Brayton cycle

Since neither the compression nor the expansion can be truly isentropic, losses through the compressor and the expander represent sources of inescapable workinginefficiencies. In general, increasing the compression ratio is the most direct way to increase the overall power output of a Brayton system.^[8]

The efficiency of the ideal Brayton cycle is , where  is the heat capacity ratio.^[9] Figure 1 indicates how the cycle efficiency changes with an increase in pressure ratio. Figure 2 indicates how the specific power output changes with an increase in the gas turbine inlet temperature for two different pressure ratio values.

Figure 1: Brayton cycle efficiency

Figure 2: Brayton cycle specific power output

The highest temperature in the cycle occurs at the end of the combustion process, and it is limited by the maximum temperature that the turbine blades can withstand. This also limits the pressure ratios that can be used in the cycle. For a fixed turbine inlet temperature, the net work output per cycle increases with the pressure ratio (thus the thermal efficiency) and the net work output. With less work output per cycle, a larger mass flow rate (thus a larger system) is needed to maintain the same power output, which may not be economical. In most common designs, the pressure ratio of a gas turbine ranges from about 11 to 16.^[10]