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It's purpose is contain the burning mixture of air and fuel to allow maximum heat release at a almost constant pressure so that the turbine receives a uniformly expanded, heated and accelerated stream of gas.
The temperature is limited by the materials from which the Nozzle Guide Vanes and Turbine are manufactured.
We need to slow down the speed of the airflow before it enters the combustion chamber to prevent the flame being extinguished immediately. This is done by increasing the pressure and decreasing the velocity of the air by guiding it through the 'snout' before being divided to go through the flare and swirl vanes in the chamber.
Primary Air
- 20% of the flow coming into the chamber
- Is the air which is mixed with fuel
- Passed through the flare and swirl vanes to reduce the velocity
Secondary Air
- 20% of the flow coming into the chamber, it's the air not being picked up by the snout and goes into the space between flame tube and casing
- Some is allowed into the flame tube by secondary air holes and reacts with the primary air to form a vortex, a region of low velocity airflow which stabilises the flame and prevents it being dragged down the flame tube too far
Tertiary Air
- Remaining 60% of the airflow
- Used to cool and dilute the gases before they are allowed to go into the turbine assembly
Components
Most turbine engines only have 2 igniters. The flame is passed between the different combustion chambers by the inter-connector. After light up, the pressure in the chamber increases and the pressure difference between the chambers is what drives the burning gases through the inter-connector (here they ignite the mixture).
The sealing ring allows the chamber to expand (but it's unable to expand in direction of compressor)
A drain tube is provided at the bottom of the chamber to remove all unburned fuel that is left.
Different arrangement methods
- Multiple combustion chamber system
Multiple combustion chambers are fitted around the engine.
- Tubo-annular combustion chamber system
Does not have a individual air casing for each flame tube. A number of them are fitted into a common air casing which provides a more compact unit.
- Annular combustion chamber system
Only has one flame tube which is contained by an inner and outer air casing. It has no separate combustion chambers, no inter-connectors and is therefore lighter in design.
Combustion Stability
During normal running operations, combustion is self-supporting meaning that the ignition system can be switched off as soon as the engine has attained self sustaining speed. Some conditions however require ignition (flame out, water intake, weak mixture, ...).
Combustion stability means smooth burning coupled with the ability to remain alight over a large range of air/fuel ratios and air mass flows.
Relighting
Ability to relight will vary according to height and forward speed of the aircraft.
The airflow through the engine will cause it to rotate (windmilling), so the compressor will supply sufficient air. All the relight switch has to do is add HP fuel and operate the ignition system.
Fuel spray nozzles
They need to vaporise the fuel so that it is completely burned, giving the high combustion efficiency that the combustion chamber has.
- The airspray system
The high velocity air from the compressor is used to break up the fuel flow.
It needs relatively low pressures and can therefore use a light pump.
- The duplex system
At low pressures, a pressurising valve closes off the main fuel feed to the nozzles, the only supply is now coming from the primary fuel line.
This primary fuel line feeds the primary orifice (a much smaller hole capable of providing a fine spray at lower pressures).
At higher pressures (engine accelerates), pressurising valve opens to allow fuel to feed the main orifice.
- The vaporising tube system
The fuel is sprayed from the feed tubes into vaporising tubes which are located inside the flame tube. Fuel is turned through 180° and as the tubes are heated by combustion, it vaporises before passing into the flame tube.
With an engine start, there is no combustion yet so no heat for the tubes. We therefore pre-heat the fuel by either a coil around the tube or by compression in the fuel pump.
The temperature is limited by the materials from which the Nozzle Guide Vanes and Turbine are manufactured.
We need to slow down the speed of the airflow before it enters the combustion chamber to prevent the flame being extinguished immediately. This is done by increasing the pressure and decreasing the velocity of the air by guiding it through the 'snout' before being divided to go through the flare and swirl vanes in the chamber.
Primary Air
- 20% of the flow coming into the chamber
- Is the air which is mixed with fuel
- Passed through the flare and swirl vanes to reduce the velocity
Secondary Air
- 20% of the flow coming into the chamber, it's the air not being picked up by the snout and goes into the space between flame tube and casing
- Some is allowed into the flame tube by secondary air holes and reacts with the primary air to form a vortex, a region of low velocity airflow which stabilises the flame and prevents it being dragged down the flame tube too far
Tertiary Air
- Remaining 60% of the airflow
- Used to cool and dilute the gases before they are allowed to go into the turbine assembly
Components
Most turbine engines only have 2 igniters. The flame is passed between the different combustion chambers by the inter-connector. After light up, the pressure in the chamber increases and the pressure difference between the chambers is what drives the burning gases through the inter-connector (here they ignite the mixture).
The sealing ring allows the chamber to expand (but it's unable to expand in direction of compressor)
A drain tube is provided at the bottom of the chamber to remove all unburned fuel that is left.
Different arrangement methods
- Multiple combustion chamber system
Multiple combustion chambers are fitted around the engine.
- Tubo-annular combustion chamber system
Does not have a individual air casing for each flame tube. A number of them are fitted into a common air casing which provides a more compact unit.
- Annular combustion chamber system
Only has one flame tube which is contained by an inner and outer air casing. It has no separate combustion chambers, no inter-connectors and is therefore lighter in design.
Combustion Stability
During normal running operations, combustion is self-supporting meaning that the ignition system can be switched off as soon as the engine has attained self sustaining speed. Some conditions however require ignition (flame out, water intake, weak mixture, ...).
Combustion stability means smooth burning coupled with the ability to remain alight over a large range of air/fuel ratios and air mass flows.
Relighting
Ability to relight will vary according to height and forward speed of the aircraft.
The airflow through the engine will cause it to rotate (windmilling), so the compressor will supply sufficient air. All the relight switch has to do is add HP fuel and operate the ignition system.
Fuel spray nozzles
They need to vaporise the fuel so that it is completely burned, giving the high combustion efficiency that the combustion chamber has.
- The airspray system
The high velocity air from the compressor is used to break up the fuel flow.
It needs relatively low pressures and can therefore use a light pump.
- The duplex system
At low pressures, a pressurising valve closes off the main fuel feed to the nozzles, the only supply is now coming from the primary fuel line.
This primary fuel line feeds the primary orifice (a much smaller hole capable of providing a fine spray at lower pressures).
At higher pressures (engine accelerates), pressurising valve opens to allow fuel to feed the main orifice.
- The vaporising tube system
The fuel is sprayed from the feed tubes into vaporising tubes which are located inside the flame tube. Fuel is turned through 180° and as the tubes are heated by combustion, it vaporises before passing into the flame tube.
With an engine start, there is no combustion yet so no heat for the tubes. We therefore pre-heat the fuel by either a coil around the tube or by compression in the fuel pump.
