Academy · Jet Engines · Lesson 4/10
The combustor
A fire that never goes out, in a wind stronger than any storm. ~5 min
Between compressor and turbine sits a ring of chambers doing something that shouldn't be possible: keeping a flame permanently alight inside a hurricane. The air arrives at hundreds of km/h — try lighting a match in that — yet the combustor holds a continuous fire for ten-hour flights, relights it on demand at altitude, and hands the turbine gas at exactly the temperature the metal can survive. This is the room where the energy enters.
Sheltering the flame
The trick is to build the fire a cave. Inside the outer casing hangs a perforated flame tube (liner), and only a fraction of the compressor's air — the primary air, ~20% — is admitted through swirl vanes at its head. The swirler spins this air into a doughnut-shaped vortex that recirculates backwards, creating a sheltered pocket where the local flow speed is near zero: the flame anchors there, continuously reignited by its own recirculating hot gas. Fuel sprays into this pocket as a fine mist and burns at a ferocious ~2,000 °C — the flame never touches the walls it lives inside.
The other 80%: armour and dilution
The rest of the compressor's air never meets the flame directly. Part of it pours through the liner's countless small holes to form a cooling film hugging the walls — an air blanket between fire and metal. The remainder floods in further downstream as dilution air, mixing with the 2,000 °C combustion products and taming them to the ~1,400–1,600 °C the turbine's first row can (barely) endure, stirred carefully so no scorching hot streak sneaks through to burn a blade. The combustor, in short, burns at a temperature its own downstream neighbour cannot survive — then launders the evidence.
Steady, relightable, clean-ish
Two igniters — glorified spark plugs — fire only at start-up and for relights; in flight the flame sustains itself. If it ever does blow out (violent weather, ice ingestion), the relight procedure exploits the windmilling engine's own airflow, and combustors are certified to relight at altitude. Meanwhile the same little room carries modern aviation's emissions burden: staged lean-burn injection, better mixing, and sustainable fuels are all combustor stories. Not bad for the only part of a jet engine with no moving parts at all.