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Why So Much Air Entering The Turbines?  
User currently offlineCaptjetblast From Argentina, joined Aug 2001, 281 posts, RR: 0
Posted (12 years 10 months 1 day 6 hours ago) and read 1944 times:

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


9 replies: All unread, jump to last
 
User currently offlineStaffan From , joined Dec 1969, posts, RR:
Reply 1, posted (12 years 10 months 1 day 6 hours ago) and read 1914 times:

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

Staffan


User currently offlineNKP S2 From United States of America, joined Dec 1999, 1714 posts, RR: 5
Reply 2, posted (12 years 10 months 1 day 6 hours ago) and read 1900 times:

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.

User currently offlineStaffan From , joined Dec 1969, posts, RR:
Reply 3, posted (12 years 10 months 1 day 5 hours ago) and read 1883 times:

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

Staffan


User currently offlineTwotterwrench From , joined Dec 1969, posts, RR:
Reply 4, posted (12 years 10 months 1 day 3 hours ago) and read 1858 times:

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



User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6429 posts, RR: 54
Reply 5, posted (12 years 10 months 1 day ago) and read 1829 times:

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, Preben Norholm
User currently offlineStaffan From , joined Dec 1969, posts, RR:
Reply 6, posted (12 years 10 months 23 hours ago) and read 1815 times:

"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


User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 7, posted (12 years 10 months 22 hours ago) and read 1813 times:

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.
User currently offlineDelta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 8, posted (12 years 10 months 22 hours ago) and read 1807 times:

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


User currently offlineEssentialPowr From United States of America, joined Sep 2000, 1820 posts, RR: 2
Reply 9, posted (12 years 9 months 4 weeks 1 day 5 hours ago) and read 1770 times:

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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