trav110
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Question On Engine Fire?

Sun Apr 08, 2012 7:41 am

How are engine fires able to happen in passenger jets? Why doesn't the extremely fast air thats coursing through the engine just blow the fire out? |What's actually burning during an engine fire?
 
rwessel
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Question On Engine Fire?

Sun Apr 08, 2012 7:46 am

The air flowing through the combustors is moving far slower, more on the order of 60mph. It is considerable compressed, however. Even so, the fuel injection and flame holders are designed to produce a good "swirl" to allow the fuel/air mixture to burn completely in the correct part of the engine.
 
JETPILOT
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Question On Engine Fire?

Sun Apr 08, 2012 8:28 am

When engines light on fire it's in the nacelle. The continuous loop detectors that sense a build up of heat are located in the nacelle. The extinguishing agents are also fired into the nacelle and not actually into the engine. Usually it's the accessory gear case or a leaking fuel/oil line that catches fire.
 
rwessel
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Question On Engine Fire?

Sun Apr 08, 2012 8:50 am

Well, I certainly completely misinterpreted *that* question...
 
Mir
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Question On Engine Fire?

Sun Apr 08, 2012 1:38 pm

A professor of mine once asked "do you want a fire in an engine?" The answer: of course you do - that's how the engine works! It's a very good thing to have a fire in an engine, so long as it's inside the combustion chamber (where there is a certain amount of airflow). But if it should move outside the combustion chamber (where there probably isn't going to be as much airflow to help put it out), then you've got problems, and that's what the fire suppression systems are for.

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CosmicCruiser
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Question On Engine Fire?

Sun Apr 08, 2012 1:48 pm

A broken bleed air line will produce similar results too.
 
Mir
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Question On Engine Fire?

Sun Apr 08, 2012 2:02 pm

Quoting CosmicCruiser (Reply 5):
A broken bleed air line will produce similar results too.

   Every fire detection system I've seen is actually a heat detection system. So anything that gets things too hot in places where it shouldn't be hot will set off a fire warning.

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longhauler
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Question On Engine Fire?

Sun Apr 08, 2012 3:07 pm

Quoting trav110 (Thread starter):
What's actually burning during an engine fire?

99% of the time, in an engine fire, it is fuel that is burning. If not fuel, then residual engine oil or hydraulic fluid. That is why, when pulling an "engine fire warning switch" (it is called something different in each aircraft), among other things, fuel, hydraulic fluid, and bleed air are removed from the engine.

And ... 99% of the time, that extinguishes the fire.

The engine fire warning switch also arms the agent bottles to extinguish the fire if it continues to burn.

Quoting rwessel (Reply 3):
Well, I certainly completely misinterpreted *that* question...

Well, you answered one part of his question, in that a lot of the air moving through a turbine engine is not going as fast as some people think.
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113312
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Question On Engine Fire?

Sun Apr 08, 2012 4:16 pm

The air/gases going through the engine are far faster than 60 mph!!! The velocity, at cruise, is very close to the speed of the aircraft but will always be subsonic. For aircraft flying at the speed of sound, or greater, the airflow entering the engine must be slowed through the inlet to subsonic. This has the additional effect of increasing pressure and assists the compressor section.
 
rwessel
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Question On Engine Fire?

Sun Apr 08, 2012 5:36 pm

Quoting 113312 (Reply 8):
The air/gases going through the engine are far faster than 60 mph!!! The velocity, at cruise, is very close to the speed of the aircraft but will always be subsonic.

Not at the combustors. At entry and exit the flow velocities are far higher, but the velocity is (surprisingly) low through the combustors. This is the result of the compression of the air stream - the *mass* flow rate is consistent* throughout the engine, but in the middle, that mass is quite compressed, and takes up much less volume, hence the low velocity through the compressors.


*IOW, if 100kg/s is entering the front of the engine core (ignoring bypass, bleeds, cooling flows, and such), 100kg/s is flowing through each compressor stage, the combustors, each turbine stage, and leaving the engine. The velocities, OTOH, vary widely.
 
EMBQA
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Question On Engine Fire?

Sun Apr 08, 2012 5:39 pm

Because the air around the core is hardly moving at all. When a fuel or oil line start leaking or break the air flow just makes it worse by atomizing the fluid.
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akiss20
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Question On Engine Fire?

Sun Apr 08, 2012 7:59 pm

Quoting 113312 (Reply 8):
The air/gases going through the engine are far faster than 60 mph!!! The velocity, at cruise, is very close to the speed of the aircraft but will always be subsonic. For aircraft flying at the speed of sound, or greater, the airflow entering the engine must be slowed through the inlet to subsonic. This has the additional effect of increasing pressure and assists the compressor section.

Typically at the compressor face, the inlet has diffused the flow down to about Mach 0.5, by the time it gets to the combustors, it is typically around Mach 0.1 or so
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wingscrubber
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Question On Engine Fire?

Tue Apr 10, 2012 8:56 pm

Quoting longhauler (Reply 7):
That is why, when pulling an "engine fire warning switch" (it is called something different in each aircraft), among other things, fuel, hydraulic fluid, and bleed air are removed from the engine.

Correct, but I would not say that fuel etc is 'removed', but shut off. Pulling the fire handle actuates all of your fire shut-off valves or FSOVs, for fuel, hydraulic fluid and bleed air - by the time you do so, there's probably already a leak of some sort, closing the FSOVs just cuts off any additional fuel for the fire.

The knock-on effects are that the engine will be shut down, the hydraulic system affected by the pump you've just closed off will loose pressure, the cabin pressure might even decrease some, you also lose some electrical power when the generator on that engine stops turning. But hopefully you have at least one other spare engine...
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N243NW
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Question On Engine Fire?

Tue Apr 10, 2012 10:56 pm

Quoting rwessel (Reply 9):
the *mass* flow rate is consistent* throughout the engine
Quoting rwessel (Reply 9):
IOW, if 100kg/s is entering the front of the engine core (ignoring bypass, bleeds, cooling flows, and such), 100kg/s is flowing through each compressor stage, the combustors, each turbine stage, and leaving the engine.

I'm sure you've got the right idea, but it's probably worth mentioning that you're talking about the air only. Total mass flow is actually not constant - the mass flow leaving the engine is greater than the mass flow entering the engine, because the fuel starts out stationary relative to the engine and is injected, burned and accelerated out the turbine and nozzle.
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tdscanuck
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Question On Engine Fire?

Wed Apr 11, 2012 1:45 am

Quoting N243NW (Reply 13):
Total mass flow is actually not constant - the mass flow leaving the engine is greater than the mass flow entering the engine, because the fuel starts out stationary relative to the engine and is injected, burned and accelerated out the turbine and nozzle.

Although total mass flow does vary through the engine, the bleed taps are also a significant contributor.

Total mass flow entering the engine always equals total mass flow leaving the engine (taking all inputs and outputs into account) but mass out the back of the engine is may be lower than what went in if the bleed offtake is removing more mass flow than the fuel is adding.

Fuel/air ratio in modern jets is so low that it's usually ignored for first-order analysis.

Tom.
 
prebennorholm
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Question On Engine Fire?

Wed Apr 11, 2012 2:12 am

Quoting N243NW (Reply 13):
Total mass flow is actually not constant - the mass flow leaving the engine is greater than the mass flow entering the engine, because the fuel starts out stationary relative to the engine and is injected, burned and accelerated out the turbine and nozzle.

Correct! But the difference is so small that it is hardly worth mentioning.

Only a minor part of the air passing through the core is used for combustion. By far the most air is cooling air - cooling the combustion chamber and mixing with the combusted gasses to lower the temperature to what the turbine can handle.

For 100 lbs of that minor part of air, which is actually taking part in the combustion, only 7 lbs fuel is burned. So the mass flow at the core exhaust is something like 1-2% higher than the intake. The rest of the mass flow is some 500-1000 lbs of cooling air, depending on actual engine operation parameters.

If we look at the total mass flow in a turbofan engine (not just the core) the addition is a small fraction of one percent.

But of course, at the end of the day every single fuel molecule added to the fuel tanks comes out of the nozzle.
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N243NW
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Question On Engine Fire?

Wed Apr 11, 2012 6:16 am

Quoting prebennorholm (Reply 15):
But of course, at the end of the day every single fuel molecule added to the fuel tanks comes out of the nozzle.

Well, hopefully not every molecule - let's say every molecule minus legal IFR reserves  

Thanks for the discussion - some good points raised by both of you...
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