Aviation gas turbines need a steady source of very hot, fast moving gas to spin the turbine and produce thrust. The combustor is the chamber where compressed air from the compressor meets sprayed fuel and burns in a controlled way. It must add a large amount of thermal energy while keeping the flow smooth enough for the turbine.
This makes the combustor one of the most important parts of a jet engine.
Inside an annular combustor, fuel nozzles spray a fine mist into swirling compressed air so the mixture can ignite and burn efficiently. Flame holders and recirculating flow keep part of the hot gas moving back toward the incoming mixture, which stabilizes the flame. Only some of the air burns directly with the fuel, while the rest cools the metal liner and dilutes the hot gases before they reach the turbine.
This careful air management allows combustion temperatures above the melting point of the liner material without destroying the engine.
Key Facts
- Combustor purpose: convert chemical energy in fuel into thermal energy of hot gas.
- Energy release rate can be estimated by P = mass flow rate of fuel x heating value.
- Air to fuel ratio by mass is AFR = mass of air / mass of fuel.
- Jet engines often run with overall AFR much higher than the stoichiometric value to control turbine inlet temperature.
- Combustion efficiency = actual heat released / maximum possible heat release.
- Film cooling uses a thin layer of cooler air along the liner to reduce heat transfer to the metal.
Vocabulary
- Combustor
- The engine section where compressed air and fuel mix, burn, and produce hot gas for the turbine.
- Annular combustor
- A ring shaped combustion chamber that surrounds the engine shaft and provides a continuous flame zone.
- Fuel nozzle
- A device that sprays fuel into tiny droplets so it can mix and burn quickly with air.
- Flame stabilization
- The process of keeping a flame anchored in fast moving air by using swirl, recirculation, or flame holders.
- Liner
- The thin metal wall inside the combustor that shapes the flame zone and is protected by cooling air.
Common Mistakes to Avoid
- Thinking all compressor air burns with the fuel. In most combustors, only part of the air enters the primary flame zone while the rest is used for cooling and dilution.
- Ignoring droplet size in fuel spray. Large droplets evaporate slowly, which can cause poor mixing, incomplete combustion, and hot spots.
- Assuming the flame simply travels downstream with the air. A stable combustor uses swirl and recirculation to keep burning gases near the incoming fuel and air.
- Forgetting liner cooling when comparing flame temperature to metal limits. Gas temperatures can exceed metal melting temperatures because film cooling and dilution air protect the liner.
Practice Questions
- 1 A combustor burns 0.80 kg/s of fuel with a heating value of 43,000,000 J/kg. Estimate the thermal power released if combustion is complete.
- 2 An engine sends 48 kg/s of air and 1.2 kg/s of fuel into the combustor. Calculate the overall air to fuel ratio by mass.
- 3 Explain why an aircraft combustor uses excess air and liner cooling instead of simply making the combustor walls from a very high melting point metal.