The Gas Turbine Engine

Chapter 22: Combustion Chambers

Technical General for Aviators — Capt. Pankaj Pahil

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).

 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.

 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.