The main function of a jet engine combustion chamber is to supply a steady stream of hot gas which is able to release its energy to the turbine and nozzle section of the machine. After an increase in pressure through the compressor section, heat is added to the airflow by the burning of the flammable gaseous mixture of vaporized fuel and highly-compressed air. The combustion process is confined to the cramped volume of the combustion chamber and has to be carried out at a minimal loss of pressure.
When discharged from the compressor, the air mass flow enters the combustion chamber at a velocity of around 150 m/s (490 ft/s), which is far too high to sustain a flame for combustion. Therefore, what is required in the first place is a slowing down of the airflow. This is achieved in the forward section of the combustion chamber that is formed as a diffuser. It means the flow passage cross-section increases in the downstream direction. As a result, there is a decrease of airflow velocity, but there is also a further increase in pressure at the same time.
The second essential function of the combustion chamber is to supply the right fuel/air mixture. The mass ratio of the two components that react in the combustion process varies with the operating conditions of the aircraft and may range between ratios of 1:45 to 1:130. However, the fuel/air ratio for efficient combustion is around 1:15, which means that only a fraction of the incoming air is required for the combustion process. Apportioning the air for combustion is accomplished through a short air duct (snout), which is fitted with a number of drag-producing swirl vanes at its exit to reduce flow velocity.
Combustion efficiency
Usually, the injected fuel does not burn completely. It means it produces less heat than would be possible theoretically. The reason is that proportional distribution of the exact amount of air necessary for complete combustion is very hard to accomplish, especially with respect to the wide range of aircraft operating conditions. The degree of actual fuel usage is characterized by a combustion efficiency factor giving the amount of heat released by combustion in relation to the heat theoretically available in the fuel. Modern combustion chambers efficiency ranges between 90 and 98%.
Below, diagram of flow in a jet engine combustion chamber.
