It should be appreciated that the materials used in the construction of the combustion and turbine sections have temperature limitations.
Taken to extremes, they will melt. There are other considerations, however. Metallic materials will suffer degradation in their mechanical properties if exposed to excessive heat.
Nickel based alloys can become brittle if overheated. This would lead to fatigue cracking.
Materials under stress at high temperatures can suffer plastic deformation through creep.
To avoid confusion do note that flame tubes are often described as combustion liners. You will encounter both descriptions.
The flame temperature in the primary combustion zone is 2,000°C, which is above the melting point of the nickel alloy liner.
If the liner is exposed to temperatures approaching this value it will suffer a level of degradation consistent with the temperature reached and the time of exposure.
This can range from instant burn through to discolouration or ‘blueing’.
It seems like a good idea to prevent the flame touching the combustion liner. The re-circulating primary air from the swirl vanes and the secondary air interact to create a centralised flame region within the toroidal vortex.
The air entering from the flare and the cone angle of the atomised spray from the fuel nozzle also helps to shape the flame.
The tertiary air provides a cooling flow to insulate the flame tube from the air casings.
Corrugations in the flame tube allow some of this air to pass into the flame tube along the tube inner wall surfaces to insulate them from the high temperatures.
The melting point of the nickel alloy material of the tube is around 1800°C so you can appreciate how important this insulating flow is. The tube is cooled externally by the tertiary airflow that also creates an insulating layer to protect the air casings.
Cooling air is also introduced into the liner through corrugations or holes.
This air forms a cooling boundary layer on the inner walls of the liner to insulate them from the flame.
Later combustion liners have air circulating through cavities within double walls to provide more efficient cooling.
These are transpiration and effusion cooling methods in combustion chamber.
This reduces the cooling air requirement by around 50%.
The gas temperature after combustion is then diluted down to a value acceptable to the high-pressure nozzle guide vanes in the turbine section.
This is achieved by introducing air through the large dilution holes at the rear section of the liner.
The remaining tertiary airflow will pass out of the air casing and be passed through the hollow high-pressure turbine nozzle guide vanes to act as cooling air for the vanes.
We still refer to this air used for cooling the HP nozzle guide vanes as HP compressor delivery air, which is what it is really.
Combustion liners are firmly attached at the front but are only located in a sliding joint at the rear.
This allows the combustion liner to expand and slide rearwards in its rear location support within the nozzle box without fear of buckling stresses being set up.
New combustion chamber designs incorporate a flame tube with double walled sections that allows the passage of cooling air between the walls.
Air enters the region between the walls through small effusion holes and the process is called transpiration cooling.
The combustion system raises the temperature of the air from the compressors by adding heat energy from the fuel.
The temperature of the air leaving the high pressure compressor can already be in the region of 500ºC.
The normal turbine entrance temperature is around 900ºC, rising briefly up to about 11 00°C on accelerations.
The amount of fuel consumed, therefore, is controlled to give a temperature rise in the range of 400 – 600°C.
It is the materials in the hot section that limit the possible temperature rise.
The fuel is initially lit by means of two electrical igniters during the engine start cycle and combustion then becomes self supporting.
There are many combustion chamber designs in use but the combustion process used in each is the same as that described above.