Combustion Chambers

22.1 Purpose and Airflow

• The combustion chamber receives high-pressure air from the compressor, mixes it with fuel,
• and burns the mixture to add a massive amount of heat energy to the air, causing it to expand
• and accelerate rearwards towards the turbine.
•  Airflow Distribution: Only about 20% of the air entering the combustion chamber is
• actually used for combustion. The rest is used for cooling and control.
• Primary Air (20%): Enters the front of the flame tube and is mixed with fuel for burning.
• Swirl vanes create a vortex to stabilize the flame.
• Secondary Air (20%): Enters through holes in the flame tube to form a region of low-
• velocity, recirculating air that anchors the flame and ensures complete combustion.
• Tertiary Air (60%): The largest portion of the air flows between the outer casing and the
• flame tube, providing a cooling film of air for the metal walls. It is then gradually mixed with
• the hot gases to cool them down from a peak of ~2000°C to a safe temperature for the turbine
• (1000-1500°C).

22.2 Types of Combustion Chambers

•  Multiple Can: A system of individual flame tubes, each in its own air casing, arranged
• around the engine. Flame propagates between the cans via interconnectors.
•  Tubo-Annular (Can-Annular): A hybrid system with multiple individual flame tubes
• arranged within a single, common air casing.
•  Annular: A single, continuous ring-shaped flame tube contained within a common air
• casing. This is the most modern and efficient design, offering the best combustion efficiency,
• temperature distribution, and a shorter engine length.

22.3 Combustion Stability and Relight

•  Combustion Stability: The ability of the flame to remain alight over a wide range of engine
• perating conditions. If the air/fuel ratio becomes too lean (e.g., during a rapid throttle
• reduction) or too rich, a flameout can occur.
•  Relight Envelope: An engine can only be relit in flight within a specific range of altitudes
• and airspeeds, known as the relight envelope. Outside this envelope, the airflow may be too
• fast or the air density too low to support ignition.