Combustion Section
Chamber types, primary/secondary air, flame stabilisation and dilution cooling.
Combustion section, summary notes
- 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.
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.
Combustion concept map
Combustion Section
Combustion section quiz
Combustion Section, quiz
1. In a gas turbine combustion chamber, the primary air is used to:
Mix with the fuel and support combustionCool the turbine discsDrive the starter2. Dilution air in a combustion chamber:
Cools the gases to a temperature the turbine can acceptIncreases the flame temperatureProvides the whole burning air3. In the combustion chamber shown, the air entering through the holes marked C towards the downstream end is:
Dilution air, cooling the gas before the turbinePrimary air for combustionCooling air for the compressor4. Combustion in a gas turbine occurs at approximately:
Constant pressureConstant volumeFalling pressure5. Roughly what proportion of the airflow actually takes part in combustion?
About one quarter (primary air)All of itAbout 90%6. The flame is held in place by:
A recirculation zone from the swirlerThe fuel pumpThe turbine blades7. The modern standard combustor type is the:
Annular chamberMultiple can chamberOpen flame tube8. Secondary/dilution air is used to:
Cool the gases to a turbine-safe temperatureIncrease the flame temperatureLubricate the liner