Combustor

A ring of can type combustors circles the mid section of this gas turbine.

A combustor is a small component or area of a gas turbine, ramjet or pulsejet engine where combustion takes place. It is also known as a burner or flame can depending on the design. In a gas turbine engine, the main combustor or combustion chamber is located between the (HP) compressor and the (HP) turbine of the gas generator.

Combustor design requires that part of the airflow be almost stationary (Mach~0.05) where the fuel and air are mixed and ignited. This area of the combustor is sometimes called a flame holder and allows a stable flame front to be established and maintained. In the flame holder the fuel and the air are injected with a stoichiometric ratio. The fuel and air are intermixed by means of spraying the fuel so that it forms small droplets. Then the speed of the flow is reduced in a diffuser so that it is slower then the flame front speed in this air-droplet mixture. The exhaust is too hot for the guiding vanes and the turbines, so that they have to emit plenty of cooling air on their leading edges, which is then sometimes used in the afterburner to burn more fuel. Again in the afterburner the speed of the flow has to be very low. Often the flame-front area is enlarged by turbulence or folding.

In scramjet engines the combustor inlet flow is supersonic. Only a few practical scram-jets have flown and their details are mostly classified. However, hydrogen fuel is believed to be injected into a sheltered region, below a reverse-facing step. Unlike a conventional combustor, the Mach number of the flow decreases going through the combustor.

There are two categories of combustors, annular and can. Can combustors look like cans and are mounted around the engine. They can be easily removed for maintenance and provide convenient plumbing for fuel. Annular combustors are more compact and embedded deep within the engine's casing. Modern Jet engines usually have annular combustors. Double annular combustors are being introduced to reduce emissions. At low throttle settings, one of the two discrete volumes in the combustor is unfueled.

Small gas turbine engines often have a reverse flow combustor, which is a very compact design. The gas path, from high radius entry to low radius exit, is 'S' shaped.

Some, mainly military, turbojets and turbofans have an afterburner located in the tailpipe, to provide thrust augmentation during Take-off and Combat. On unmixed turbofans, 'afterburning' in the bypass stream is often called Plenum Chamber Burning.

Combustors in rocket engines operate outside of the main rocket chamber and will usually be handling pre-combustion of fuel and LOX so that the mixture burns completely in the rocket motor combustion chamber.