Section 15.5

Combustion Section

Chamber types, primary/secondary air, flame stabilisation and dilution cooling.

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Main ideas

The combustor burns fuel at essentially constant pressure to raise the gas temperature before the turbine. Only about a quarter of the air (primary air) takes part in combustion at roughly the correct fuel/air ratio; the rest (secondary/dilution air) cools the flame and the liner.
Chamber types: multiple (can), tubo-annular (cannular) and annular. The annular type is now standard — lightest, best flow, smallest, with the fewest ignition points needed.
A swirler and the flare create a low-velocity recirculation zone that anchors (stabilises) the flame, because the flame speed is far lower than the airflow speed — without it the flame would blow out.
Dilution air dilutes and cools the combustion gases to a temperature the turbine can survive, and shapes the temperature profile so no local hot streak burns a turbine blade.
Combustion problems include hot streaks, cracking and distortion of the liner, and carbon build-up; the section runs at the highest gas temperatures in the engine.
⚠ Exam trap: only ~25% of the air (primary) is burned — the majority is secondary/dilution and cooling air; the flame is held by a recirculation zone from the swirler because airflow velocity greatly exceeds flame propagation speed.

Solved examples

Why does a combustion chamber use a swirler at the fuel nozzle, and what would happen without it?
The swirler creates a recirculating, low-velocity region that continuously re-ignites incoming fuel/air and anchors the flame in place. The bulk airflow moves far faster than a flame can propagate, so without the stabilising recirculation zone the flame would simply be blown downstream and out — a flame-out.

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