modesto2
Posts: 2669
Joined: Sun Jul 16, 2000 3:44 am

High Bypass Turbofan

Sun Jun 06, 2004 5:17 pm

How does the bypass region of a turbofan work? I understand that some of the air enters the engine core while much of it is routed around the core (bypass). How does this re-routed air produce so much thrust? Also, what's the definition of "high" bypass? I've heard a 4:1 bypass ratio.

What compressors/turbines are identified as N1, N2, N3? (N3 is only applicable to RR engines, correct?) Finally, are there any good websites that explain engine operation? Thanks.
 
justplanesmart
Posts: 675
Joined: Thu Mar 15, 2001 3:25 pm

RE: High Bypass Turbofan

Sun Jun 06, 2004 5:31 pm

Finally, are there any good websites that explain engine operation?

http://travel.howstuffworks.com/turbine.htm
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dl757md
Posts: 1482
Joined: Mon May 24, 2004 9:32 am

RE: High Bypass Turbofan

Sun Jun 06, 2004 5:41 pm

You are correct about the routing of air in a high bypass turbofan. The reason that the bypass air produces so much thrust in a high bypass turbofan is because of the greater volume of air that bypasses the core. In a 4:1 bypass turbofan you have 80% air bypassing and 20% entering the core. The core airflow is responsible for about 30% thrust while the bypass produces about 70%. The reason the core produce a higher proportion of thrust compared to its airflow is because of the heat energy and added massflow from combustion.

The identification of the N1, N2, and N3 sections or spools is determined from the front of the engine to the back ie. the fan is N1, the intermediate compressor section is N2, and the high pressure compressor section is considered N3, that is if it has N3. The turbines are reverse order since the sections or spools are on concentric shafts the turbines driving N3 are first, then N2, and then N3.
757 Most beautiful airliner in the sky!
 
QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

RE: High Bypass Turbofan

Sun Jun 06, 2004 8:03 pm

How does the bypass region of a turbofan work?

Well, there aren't really any moving parts or major subsystems involved with bypass flow, so basically it's just a flow moving outside of the engine core. Having said that, a turbofan's reverse thrust is almost always provided by bypass air (via cascade or petal reversers), and there are various other subsystems operated by bypass air.

How does this re-routed air produce so much thrust?

Simply by the amount of it, as Dl757md mentioned. Per unit volume of airflow, turbofan cores produce much more thrust. However, there's a lot more bypass flow than core flow, and thus the bypass produces so much thrust. The core flow is high-thrust but inefficient, and vice versa for the bypass (accelerating a small amount of air a lot produces a lot of thrust, but inefficiently). That's why turbofan engines exist...

Also, what's the definition of "high" bypass? I've heard a 4:1 bypass ratio.

4:1 and up is indeed the general dividing line...

Cheers,
QantasA332
 
sovietjet
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Joined: Sat Mar 15, 2003 12:32 am

RE: High Bypass Turbofan

Tue Jun 08, 2004 5:52 am

But if the air that is bypassing enters at a certain velocity how does it speed up to make thrust? And what is mass flow?
 
atlamt
Posts: 244
Joined: Thu Sep 13, 2001 12:15 am

RE: High Bypass Turbofan

Tue Jun 08, 2004 6:26 am

The air is accelerated because the bypass area is tapered and reduces in size at the rear. The reduction in size is a combination of the shape of the reverser/cowl and the engine core which gets larger at the turbine section. If you got to pratt's site they have a good animation. http://www.pratt-whitney.com/how.htm
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dl757md
Posts: 1482
Joined: Mon May 24, 2004 9:32 am

RE: High Bypass Turbofan

Tue Jun 08, 2004 6:46 am

Sovietjet

The bypass air is accelerated by the fan in a manner similar to how a propeller accelerates air. The thrust from the bypass air is the result of this acceleration.

Mass flow is a term that describes the amount and velocity of matter flowing through a conduit. It relates to turbofans in that the amount of matter that enters the bypass region is the same as the amount of matter that exits it. All of the thrust realized from the bypass air is produced soley from the acceleration of that air.
The core flow is a different story. The air flowing through the core has fuel added to it. This increases its mass. So along with accelerating the air you are increasing its mass and therefore increasing it's mass flow or thrust. Much in the same manner that ejecting the propellants out of a rocket produces thrust. I'm not sure how much thrust is realized from the acceleration of this additional mass but I don't think it's negligible.
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