captjetblast
Topic Author
Posts: 288
Joined: Thu Aug 30, 2001 5:59 am

### Why So Much Air Entering The Turbines?

Why is it that turbines must pressurize lots of air before it enters the combustion chamber?
Is it necessary in order to burn lots of fuel every second?

Thanks

Staffan
Posts: 3879
Joined: Tue Mar 18, 2008 9:21 am

### RE: Why So Much Air Entering The Turbines?

To get more power out of the engine, it's the same with a car engine equipped with a turbo.

Staffan

NKP S2
Posts: 1665
Joined: Mon Dec 20, 1999 3:16 am

### RE: Why So Much Air Entering The Turbines?

It's the compressor that pressurizes the air, not the turbine. Since a jet engine is of the continuous Brayton cycle, it must be be burning an air/fuel mixture cntinuously to be self-sustaining. The amount of thrust in a big fan engine is dependent on how much heat energy/velocity of combustion can be extracted by the turbine. More air + more fuel = more thrust. More fuel WITHOUT more air = Lots of smoke and potentially melted turbine blades.

Staffan
Posts: 3879
Joined: Tue Mar 18, 2008 9:21 am

### RE: Why So Much Air Entering The Turbines?

Sorry, didn't notice the mistake with the turbine vs the compressor. NKP_S2 is right though!

Staffan

Twotterwrench
Posts: 1087
Joined: Tue Jan 16, 2001 5:57 pm

### RE: Why So Much Air Entering The Turbines?

It is a little more complicated than that. The air is compressed for several reasons.

1- Provide a sufficient volume of air for the amount of power you want to extract.

2- Compression heats the air to a calculated value that is most efficient for combustion.

3- Only about 30% of the compressed volume is actually used for combustion. The rest is used for:
-Shaping the flame cone in the combustion chamber directing it away from combustion chamber surfaces.
-Cooling of turbine vanes and guide vanes
-Some of it can be used to power airframe systems such as pressurization, surface de-ice and/or anti-ice, etc.
-A small amount of it is redirected to the fuel control to give a speed/pressure reference for fuel scheduling.

These are just general explanations and numbers, as every engine does it in a different way. Hope that helps

prebennorholm
Posts: 6592
Joined: Tue Mar 21, 2000 6:25 am

### RE: Why So Much Air Entering The Turbines?

Twotterwrench gave great info. Just adding to that:

1. Fuel is your only energy source.
2. Burning one pound of fuel requires the oxygen from 15 pounds of air.
3. Since only about 30% of the air is used for combustion, then 50 pounds of air has to pass.
4. Until now we have only talked about the engine core. On a modern turbofan engine typically 6 times more air passes the fan only.
5. So one pound of fuel typically "moves" 350 pounds of air. That's a lot of air!!!
6. A mid sized tubofan engine burns several pounds of fuel per second at take-off thrust at sea level.

Moving air is the whole name of the game.

Accellerating a 100 tons heavy airliner from zero to 500 mph means for instance that 100 tons of air has to be accellerated backwards at a speed of 500 mph. This is just a hypotetical example which doesn't account for drag. Lifting the same airliner from the runway to 30,000 feet requires much more.

Now you may think that it is important to accellerate the air to as great speed as possible. That's not the case. It would be terribly noisy. Therefore it is better to accellerate more air to a lower speed.

The very largest engines may "treat" as much as roughly 100,000 ft3 air per second. Quite amazing, even if only some 4 or 5% of that is used for combustion. At lower power settings the percentage of "burned air" is lower.

Regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs

Staffan
Posts: 3879
Joined: Tue Mar 18, 2008 9:21 am

### RE: Why So Much Air Entering The Turbines?

"Now you may think that it is important to accellerate the air to as great speed as possible. That's not the case. It would be terribly noisy. Therefore it is better to accellerate more air to a lower speed."

If I'm not wrong, that's why jet-fighters are so noisy?

Regards,

Staffan

lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

### RE: Why So Much Air Entering The Turbines?

During a severe conflict, the last thing anyone will complain about is the noise military aircraft make; it is not so much that the engines are loud, it is that they suffer less restrictions. This fact is why no one here in San Diego wants our international airport to be moved to MCAS Miramar, because they think it's going to be louder that a bunch of F/A-18's.
The meaning of life is curiosity; we were put on this planet to explore opportunities.

delta-flyer
Posts: 2631
Joined: Mon Jul 30, 2001 9:47 am

### RE: Why So Much Air Entering The Turbines?

Another reason military aircraft seem loud is when they use "afterburner" or, as our brothers across the pond call it, "reheat".

This is accomplished by spraying fuel vapor into the exhaust duct leaving the engine to obtain additional thrust.

Cheers,
Pete
"In God we trust, everyone else bring data"

EssentialPowr
Posts: 1646
Joined: Wed Sep 06, 2000 10:30 pm

### RE: Why So Much Air Entering The Turbines?

No one answered the question: "Why are jet fighters so noisy?"

Here's why.

For any jet, the exit velocity must be at least as fast as the velocity of the a/c (in reality, much greater to account for losses and "slippage".) That makes them loud; very high exhaust velocities in order to be capable of near sonic to, in most cases, supersonic velocities.

Even an a/c flying at .80 mach must have a jet exit velocity > than .80, of course combined with sufficient mass flow in lbs per sec. A sea level, std day, supersonic jet the diameter of pencil beam won't propel a 777 to .80...

Cheers-

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