Basic Considerations

A flame in a hurricane

Air leaves the last stage of a compressor fast. A hydrocarbon flame, meanwhile, eats its way into fresh fuel–air mixture slowly: it is a wave of chemistry, and it moves relative to the gas it sits in. Before reading on, make a guess.

So here is the puzzle in its rawest form. Spray fuel into that airstream and light it: which way does the flame go? Try it.

A few things should jump out.

In a bare duct, a flame can only stand still when the flow speed exactly equals the flame speed. That is a knife-edge: slightly faster and the flame is washed downstream, slightly slower and it creeps upstream toward the fuel supply (flashback). At compressor-exit velocities the flow wins by a factor of hundreds, and even turbulence, which wrinkles the flame and speeds it up several-fold, doesn't come close to closing the gap.

Press Watch the idea for a guided tour, or take the controls yourself; the challenges under each simulation tick themselves off when you achieve them.

Add a flame holder and everything changes. Behind the blunt body the flow separates and curls back on itself, trapping a pocket of hot, burned gas. That pocket is a permanent pilot light: it touches the fresh mixture flowing past and re-ignites it continuously. The flame no longer has to outrun the flow. It only has to spread sideways from a fixed anchor, leaning back at an angle $\sin\alpha = \cflame{S_T}/\cair{U}$, flame speed over air speed.

In a real combustor that region is created not by a V-gutter but by a swirler and its central recirculation zoneCentral recirculation zoneA bubble of reversed flow on the combustor axis, created by vortex breakdown, that returns hot products to ignite fresh mixture., which spins the air hard enough that the flow on the axis turns around. And even the best recirculation zone has a limit: push the air too fast and it blows off too. That is why, before anything else, the air must be slowed down. Hold on to both of those facts; they become Lessons 3 and 4.

Seven requirements that fight each other

Holding the flame is necessary, but nowhere near sufficient. Lefebvre lists what a combustor must deliver, simultaneously, for tens of thousands of hours:

1. High combustion efficiency. Every bit of fuel should burn: >99.9% at take-off, and still high at idle.
2. Low pressure loss. Typically only 3–6% of the inlet total pressure can be spent.
3. Reliable ignition and altitude relight. On a cold ground start, and after a flame-out at 35,000 ft.
4. Wide stability limits. Lit from idle to take-off, through rain, hail and rapid throttle changes.
5. Uniform exit temperature. A hot streak 200 K above the mean can destroy a turbine vane.
6. Low emissions. NOₓ, CO, unburned hydrocarbons and smoke, all regulated, and tighter every certification cycle.
7. Durability. A liner sitting in 2300 K gas must survive thousands of flight cycles.

The trouble is that the physical levers that improve one of these almost always hurt another. Hotter primary zones are more stable and more efficient, and they make more NOₓ and cook the liner. More liner pressure drop mixes better, and costs engine efficiency. More cooling air saves the wall and starves the flame and the dilution jets. Try to have it all:

You will not reach the dashed ring. No one does. Each preset is a different engine's answer to the same impossible question, and much of this course is about understanding the physics behind each trade so you can reason about them instead of memorising them.

Three ways to wrap a flame around a shaft

A gas turbine is a ring of machinery around a shaft, so the combustor must be a ring too. Engineers have packaged it three ways.

• Tubular (can) combustors: 6–16 separate cylindrical cans, each in its own casing. You can develop one can on a small rig, but the result is long, heavy and lossy.
• Can-annular: individual flame tubes inside one shared annular casing. It keeps single-can testing and gains a stiffer structure; still common in heavy-duty industrial machines.
• Annular: a single continuous liner with a ring of fuel injectors. Shortest, lightest, lowest pressure loss, most uniform exit, and the standard for virtually every modern aero engine. The price is a full-annulus test rig and a large, thin shell that is hard to keep from distorting.

There is one more wonderful, practical difference: how the flame gets around the engine at start-up. Only one or two igniters light a small fraction of the burners, and the rest must catch from their neighbours.

Inside the liner: four zones, one budget of air

Whatever the outer packaging, look inside any combustor and you find the same internal logic. Follow the air:

The diffuser slows the compressor air. The primary zone anchors the flame in its swirl-driven recirculation, burning near stoichiometric. The intermediate zone adds air to finish burning CO, hydrogen and soot. The dilution zone injects large jets to bring the gas down to what the turbine can survive, and to smooth out its temperature profile. Wrapped around all of it, a film of cooling air protects the liner wall.

Next, we zoom into the primary zone and ask what a flame actually is: why temperature is the master variable, why a few hundred kelvin can change reaction rates by orders of magnitude, and where the bell-shaped curve that governs this whole course comes from.