Brayton Cycle - Models

Models

A Brayton-type engine consists of three components:

1. a gas 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 heated (by compression), 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:

• isentropic process - ambient air is drawn into the compressor, where it is pressurized.
• 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.
• 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.
• isobaric process - heat rejection (in the atmosphere).

Actual Brayton cycle:

• adiabatic process - compression.
• isobaric process - heat addition.
• adiabatic process - expansion.
• isobaric process - heat rejection.

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

The efficiency of the ideal Brayton cycle is, where is the heat capacity ratio. 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.