Compressor stalls on jets: why flame out the back?

Entertaining training video here from Boeing, instructing pilots on how to deal with engine compressor stalls. They can be pretty violent events, and apparently pilots have been known to do the wrong thing in response, e.g. aborting takeoff, or unnecessarily shutting down an engine in mid-flight (sometimes the wrong one).

Once you understand that the blades of the compressor section are airfoils functioning pretty much like little wings to stuff air into the engine, then you can grasp that, just as a wing stall means the wing is no longer shoving air downward very well, a compressor stall means the engine’s compressor section is no longer stuffing air into the engine, and air that’s already been stuffed into the engine reverses direction and comes back out through the front. Unburnt mixture from the combustor section heads that way too, making for an exciting sound and light show for all on board (fast forward the above video to 3:40 and imagine yourself as a window-seat passenger).

Clear enough.

The puzzling thing to me is this: why does a compressor stall also result in a blast of flame out of the back of the engine?

Air flow hasn’t reversed. I don’t think airflow has completely stopped either. The flame out the back would be from the bypass air that flows around the turbine.

Wiki mentions two types of stalls. Only one involves the air flow reversing. It’s possible that video doesn’t involve that type of stall/surge. Even if it does, my guess would be that the engine started back up and ‘swallowed’ the flame out right back into the engine. Also, at speed, the flames would have to push pretty hard for a sustained amount of time to get out of the way of the engine.

:confused: During normal operation, there is no flammable material in front of the engine. If flames suddenly appear in front of the engine during a stall, then they must have come from inside the engine, ergo flow must be from the combustor section forward through the compressor section. This is the very definition of flow reversal.

See video at 1:02 and 1:17. The flames are clearly coming only from the central jet nozzle, not the bypass air.

With the loss of compression, presumably the velocity of the exhaust stream is reduced. Given that flames aren’t blown out the back during normal operation, how can they be blown out the back when the exhaust stream velocity is lower?

Sorry if this sounds combative; that’s not my intent. There are conflicts between the claims presented so far and my understanding/assumptions of what’s happening in the engine; something is wrong here, and it may well be me. :smiley:

When the compressor stalls, it stops supplying air into the combustion chamber, but the fuel keeps flowing. You then have a very fuel-rich mixture, which may actually stop burning due to lack of oxygen. And once the mixture is pushed out of the engine, it auto-ignites.

True, but the wing analogy is misleading. Airfoils mostly don’t “shove air downward” although flaps do; airfoils provide lift by virtue of lower air pressure over the top of the wing than under it due to the higher airflow speed over top of the airfoil shape.

It’s the definition of flow reversal in the front half of the engine, forward of the combustion chamber. Expanding hot gases still flow backward in the rear half of the engine. You can actually see this illustrated in the early part of the video.

Sure they do. The upward force on a wing is exactly counterbalanced by the acceleration of air downwards.

How that acceleration happens is by a variety of (partially overlapping) mechanisms, but regardless, airfoils absolutely “shove air downwards” as their primary purpose in life.

I’ve seen massive internet argument about whether the pressure differential on the wing’s upper/lower surfaces causes the air to move downward, or whether the downward deflection of air causes the pressure differential to happen, but the question of which is cause and which is effect is irrelevant; suffice it to say that both phenomena are present, and they are inextricably linked. Wings absolutely do make air go down, which is why the plane doesn’t. Search YouTube for videos of helicopters hovering just above the ground, and take note of the massive downwash of air coming from the rotor (which is an airfoil). Planes cause downwash too, but you generally don’t notice it because it’s spread out over the path of travel.

Take a look at this Blue Angels sneak pass; he pulls up HARD as he passes the camera’s location, and the downwash scatters beach umbrellas.

Unless you are in a plane and following another plane too closely–in which case the downwash manifests itself as wake turbulence, and can get you in quite a lot of trouble.

Yes, you are both correct. I was (mistakenly) trying to draw a distinction between lift generated by the Bernoulli pressure differential over an airfoil and the lift generated by angle of attack of the underside of an airfoil or a flat surface. But fundamentally lift must obey Newton’s third law so you have to get the downflow of air one way or another. IOW, never mind. I was wrong. So wrong. sniff

But I stand by the comment about the bidirectional flow in a stalled jet engine. :slight_smile:

Unless you are a glider pilot, in which case you have plenty of practice with this. Gliders on tow routinely follow the tow plane closely, and pilots get plenty of practice, both in how to avoid the downwash from the tow plane, and dealing with it if you happen to get into it.

The simple answer is compressor stalls are not as simple as 100% of the airflow reverses out the front of the engine. Some does. Some doesn’t.

Fuel is being injected just as always. Whenever the chaotic flow gets diluted enough to burn it’s ignited by the rest of fire that’s always present inside the engine. So flammable fuel air mixture is coming out both ends of the engine & igniting.

Sometimes the problem is real obvious in the cockpit. Particularly for tail-mounted engines it may not be very obvious at all. A bit of odd noise and vibration and jumpy gauges. One the pilots clue onto the gauges the rest of the remedy is pretty obvious.

During a stall, the combustor is temporarily starved of air, making the mixture temporarily fuel-rich. The unburned fuel catches fire as soon as it gets more oxygen, from the atmosphere surrounding the exhaust, so you get a flame there. The flame is visible at first because it’s still somewhat oxygen-starved at first.

Yes, air does flow forward as well, from the high-pressure region in the combustor to the low-pressure region in front, once the compressor stops pushing air aft. The forward rush of air brings flame from the combustor with it, making a flame out the front, too. The heat can cause some serious internal damage to the compressor, too.

Does this mean the flame in the combustor section winks out completely during a stall? If so, does this mean the igniters are running all the time to ensure that the fire relights right away after blowing out?

The flame gets pretty ragged looking, and it can go out if the stall lasts long enough. The igniters are normally only used in starting and in icing conditions, but will come back on in the restart process too.