Jet Engine > Combustor

The combustor or combustion chamber is where fuel is added to the compressed air and ignited. When we think about the air entering the combustor from the compressor, it is is very dense and warm due to compression (approx. 450°C) and travelling at about 150m/s.  In the primary zone, fuel is added via a spray nozzle which creates a vortex mixing the two together making combustion more efficient. This zone also slows down the air as once ignited by an electric spark, the air-fuel mix needs to have a self-sustaining flame and the air coming from the compressor is fast enough to extinguish the flame.

In the combustor what we are looking to do is heat the air instantaneously and suddenly increase its volume, harnessing the Venturi Effect and the Ideal Gas Law (See Jet Engine).  Working in our favour is that air is mostly nitrogen and oxygen which when concentrated are very flammable.  In almost all jet engines fuel is added in the combustor to ignite the flammable dense air, raising the temperature of the air to over 2000°C and suddenly expanding it massively, creating the change in momentum that results in thrust.

 

The combustor is probably the most extreme environment in the jet engine.  Looking at the temperature change, we see air going from 450°C out of the compressor, to 2000°C once ignited.  This causes issues as most metals start losing integrity at 1300°C.  Using ceramics instead means we face issues with stress. Ceramics are strong but brittle and cannot handle changing stresses and temperature that are coupled together in this specific situation.  Facing this, the solution has been to cool the combustor components.  Looking at the fan section we see a lot of incoming cold air.  Not all of this air goes through the centre of the engine to be ignited, with some bypassing the centre of the engine. This bypass air travels around the core and provides more than 50% of the propulsive thrust as the fan acts as a propeller (this basically means our engine is 50% a propeller engine and 50% a jet engine!).  Once ignited the air coming out of the combustor is about 2000°C, so before it goes through the turbines we add some ‘cool’ air bled from the compressor.  Also, we use this air to cool the walls of the combustion chamber through a number of channels, noting that this air is called ‘cool’ but after going through the fan section and travelling along the outside of the compressor it can still be up to 700°C.

There are three common designs of combustion chambers.  The first is multiple chambers where air from the compressor is fed into small interconnected tubes running parallel to the shaft.  This type of combustor is no longer used in aerospace but sometimes used in industrial applications.  The second type of combustor is a turbo-annular which looks like the multiple chamber design but instead of all the air coming in from one end and out the other, the turbo-annular has air coming in directly from the compressor but also some of this air enters the chamber through holes in the side.  The third type of combustor has one tubular chamber like that shown in the model above.  This is the most common in modern jet engines as it has lots of advantages such as no problems in propagation of combustion from one chamber to another, smaller wall area meaning less cooling needed, increased efficiency, and, also it can be made smaller which means a shorter, stiffer, lighter engine.  Making this type of combustor also has challenges as it is structurally weaker, more complex to make, and it is harder to control the air coming out.

The metals usually considered for the combustor are nickel based alloys due to their high temperature capability.  Alloy C263 is a nickel superalloy often used as it has high strength, good corrosion resistance, with good formability and high temperature ductility in welded structures. Haynes 230 is also used in these extreme conditions.  Ceramics are then used as thermal barrier coatings (TBCs) rather than as structural components harnessing their corrosion and heat resistance capability.

 

Hot stuff…

The current temperature of the air entering the compressor in a modern jet engine is about 700°C before being ignited.  This is how hot the air in Frank Whittle’s landmark jet engine was after combustion.