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Aircraft Glossary: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Jet Engine Combustion Chamber

Although practical model jet engines have been around since the early 1980's it has only been in the past few years that home construction has become viable. Careful design has made simple home construction possible using basic tools and a small centre lathe.


Model engines are small and the time available to heat up the air passing through the combustion chamber is very short. Successful combustion has taken a lot of development effort especially for liquid fuels to be used safely and efficiently.

In a gas turbine engine, the combustor or combustion chamber is fed high pressure air from the compressor. The combustor then mixes this air with fuel amd ignites the mixture. The hot combustion mix of burnt fuel and air then passes from the combustor through the nozzle guide vanes to the turbine.

The combustion chamber must contain and maintain stable combustion despite very high air flow rates. To do so combustors are carefully designed to first mix and ignite the air and fuel, and then mix in more air to complete the combustion process.

Types of combustion chamber:

  • Can - self-contained cylindrical combustion chambers, each with it′s own fuel injector, liner, interconnectors, casing and each with it′s own air source.
  • Cannular - like the can type combustor, can annular combustors have discrete combustion zones contained in separate liners with their own fuel injectors. Unlike the can combustor, all the combustion zones share a common air casing.
  • Annular - this does away with the separate combustion zones and simply has a continuous liner and casing in a ring (an annulus).

Early full size gas turbine engines used a single chamber known as a can type combustor.

The main components of all combustion chambers are: a diffuser, a casing, a liner, a fuel injector and a cooling arrangement.

Diffuser

The air entering the combustion chamber will have a high velocity. It is important to diffuse this air or it will be difficult to maintain the flame.

There are two main types of diffuser:

  1. Aerodynamic diffuser - the flow is slowed down gradually and with minimal losses.
  2. Dump diffuser - the air is slowed down quickly and with greater losses, but is also smaller. The compact nature means it is mainly used in aircraft jet engines.

Casing and Liner

After the diffuser, the airflow is split up by the liner. One part goes through the region between the liner and the casing - this is called the annulus. The other part enters the mixing chamber, where fuel is injected.

The air flow is split to achieve the correct air-fule ratio and to reduce the flow velocity. The part of the flow that will be ignited has to be slowed down even further.

Fuel Injector

It is important to ensure the fuel is vaporized before it enters the flame zone so that it combusts correctly.

To get the fuel to vaporise it is first atomised, the fuel should be atomized. To accomplish this, an atomizer is used. To atomize fuel, it has to be given a high relative velocity, with respect to the airflow:

  • Pressure-assist atomizers give the fuel a high velocity.
  • Air blast atomizers inject slow-moving fuel into a high-velocity air stream.

Cooling Arrangement

The liner is exposed to high temperatures during the combustion process and as this is continuous it needs to be cooled. The most obvious way is to use the airflow around the liner to do this and to bleed air through into the combustion chamber.

Examples

This shows an early gas turbine design, this one made by John Wright.

Over the years the combustion chamber designs have matured and some examples are shown below.


Schreckling FD-3 64

KJ66

See also: Engine, Jet Engines.

Aircraft Glossary: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z