PURPOSE AND REQUIREMENTS
The function of the combustion system is to produce a hot stream of gas for the turbines and for the jet nozzle.
It produces this stream of hot gas by the continuous combustion of a fuel-air mixture.
Combustion is a very difficult process on aircraft engines because of many opposing factors and requirements.
For safe and efficient operation of the engine the combustion chamber must fulfill the following requirements:
1. The ignition of the air-fuel mixture in flight, on the ground and in all operating conditions of the engine must be safe in the combustion system configuration.
2. The combustion must also be complete. This means, that no un-burnt fuel should leave the combustion chamber.
3. Other combustion chamber requirements are to give equal temperature distribution at the combustion chamber outlet. This is important for the first turbine stage.
4. Combustion should happen with a minimum of pressure loss in the combustion chamber to increase engine efficiency.
5. The combustion chamber must be as small and light as possible to save weight and it must have a dependable cooling system for all materials which get in contact with the hot gas flow.
6. Finally, it should have a high operating lifetime because the replacement of these engine components is very time consuming.
COMBUSTION CHAMBER COMPONENTS
Several types of combustion chambers are used on jet engines.
However, some basic parts are similar in all types.
Each combustion chamber has two main components:
1. The combustion chamber casing
2. The flame tube.
Note that the casing can be split into an inner and an outer casing.
The combustion chamber casing is the outer shield of the combustion section. It takes the air pressure loads and protects the internal and external engine parts from the hot combustion gases.
The housing also gives support to the flame tube and other combustion chamber components like fuel nozzles and igniter plugs.
The flame tube controls and guides the flame.
The air leaving the rear of the high-pressure compressor is traveling at too high a velocity for combustion.
The diffuser section after the compressor reduces air velocity to around 500 ft/ sec as it enters the combustion section. This is far too high a velocity.
As the air enters the diverging combustion section it splits, 18% entering the snout of the flame tube as primary combustion air and the rest as secondary and tertiary air passing at approximately 200 ft/ sec into the annular space between the flame tube and the air casing.
The combustion chamber initially diffuses the 18% primary airflow down to around 80 ft/ sec and then further reduces it’s velocity in the primary zone of the flame tube to close to the flame speed of the kerosene fuel which at the normal 15:1 air fuel ratio can be as low as 2ft/ sec.
This is done by use of a perforated flare and swirl vanes in the primary bum zone.
The swirl vanes create a toroidal vortex inside the flame tube that has a very low pressure core that induces the spinning air and the air entering through the flare to reverse flow back into the low pressure zone.
This effectively anchors the flame while a conical pattern, atomized fuel spray from the spray nozzle intersects the recirculating airflow.
The air turbulence in the primary zone breaks down the fuel droplets exposing more of their surface area to the heat.
This quickly vaporizes the fuel and brings it to ignition temperature.
The turbulence and the vaporization of the fuel cause a slight pressure loss in the chamber of around 8%. Once ignited, combustion is self-sufficient.
Adjacent to the burning zone is the secondary air holes. About 20% of the air passing outside of the tube passes through these holes into the prim zone.
The secondary air is attracted in by the very low-pressure vortex core and injects deep into the zone to provide air to complete the combustion of the fuel.
COMBUSTION SYSTEM CHEMICAL REACTION
Combustion is actually a chemical reaction where the oxygen in the air chemically combines at high temperature with the hydrogen and the carbon in the hydrocarbon fuel.
This ideally results in the formation of carbon dioxide (CO2) and water (H2O).
It is important, therefore, that the air fuel ratio and the time allowed for combustion are correct.
Insufficient oxygen or time results in incomplete combustion. This gives high levels of carbon monoxide (CO) and carbon (C) or soot
Injecting secondary air into the primary zone fully oxidizes the fuel. The time is also critical.
Too short a burn time and the fuel cannot fully oxidise.
Too long a burn time under high combustion chamber pressure and temperature allows the nitrogen in the air to oxidise into
nitrogen dioxide (NO2) which is the infamous NOX emission limited by the Environmental Protection Agency.
The critical factors are time, pressure and temperature.
The air used in combustion is primary and secondary air and that combustion should be carried out fully in the primary zone of the combustion chamber at a fuel: air ratio of 15:1 by weight.
Aviation fuel in liquid form has a high ignition temperature of well over 1 000°C.
The temperature in the flame region is 2000°C so this is not a problem. However, this is far too hot for the materials in the turbine section.
A further 20% of the air from the tertiary airflow enters the chamber through the large dilution air holes to cool the gas stream down to around 900°C in the dilution zone before it enters the high-pressure turbine nozzle guide vanes of the turbine section.
The problem is, if combustion is not completed in the primary zone, the un-burnt hydrocarbons passing into the dilution zone can be cooled below their ignition temperature and will end up as soot.
Dilution air does, however, provide a back-up to fully complete the conversion of CO to CO2 and HO to H2O if this has not already been achieved.
So far we have only accounted for 58% of the airflow. 18% primary air, 20% secondary air and 20% of tertiary air used for dilution.
Most of the remaining 42% of the tertiary air is used as cooling air for the flame tube before joining the main gas stream.