Combustion Section and Fuels
Domain 4 - Combustion Section and Fuels
The exam tests combustor types, fuel scheduling, fuel properties and contamination effects.
Core rules
- Can, can-annular and annular combustors are used; annular is standard on modern turbofans because it is shorter, lighter and gives a more uniform outlet temperature. Reverse-flow designs suit compact layouts.
- Zones: primary, where the flame is stabilised by swirlers at near-stoichiometric mixture; intermediate or secondary; and dilution, which mixes hot gas down to a turbine-safe pattern factor.
- Fuel nozzles are simplex, duplex or airblast. Coking on the nozzle face produces a hot streak, distorted pattern factor and shortened turbine life. It shows as EGT spread and is confirmed by hot-section borescope.
- Jet A-1: flash point about 38 °C minimum, freeze point −47 °C maximum, and a narrow viscosity band for metering. Water, microbial growth and particulate cause filter blockage and pump wear. Fuel is also the main heat sink for oil and IDG cooling.
- Light-off needs the right fuel/air ratio, ignition energy and airflow. A rich mixture gives a hot start; a lean one gives a hung or no-light start.
Worked example
An engine shows a 60 °C EGT spread at takeoff with normal total fuel flow. One nozzle is found coked. The coked nozzle passes less fuel, so the local flame runs lean and cooler; the FADEC richens the whole combustor to hold thrust, raising mean EGT and stressing every blade. Cleaning or replacing the nozzle restores the pattern.
Traps
- Quoting flash point as if it were autoignition temperature.
- Treating a Jet A-1 leak as harmless because volatility is low; it still wets and burns.
- Forgetting that fuel is a heat sink when troubleshooting high oil temperature.