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High Bypass Turbofan  
User currently offlineModesto2 From United States of America, joined Jul 2000, 2819 posts, RR: 5
Posted (10 years 6 months 3 weeks 3 days 2 hours ago) and read 6848 times:

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.

6 replies: All unread, jump to last
 
User currently offlineJustplanesmart From United States of America, joined Mar 2001, 724 posts, RR: 2
Reply 1, posted (10 years 6 months 3 weeks 3 days 1 hour ago) and read 6802 times:

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

http://travel.howstuffworks.com/turbine.htm



"So many planes; so little time..."
User currently offlineDl757md From United States of America, joined May 2004, 1562 posts, RR: 17
Reply 2, posted (10 years 6 months 3 weeks 3 days 1 hour ago) and read 6802 times:

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!
User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 25
Reply 3, posted (10 years 6 months 3 weeks 2 days 23 hours ago) and read 6730 times:

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


User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2648 posts, RR: 17
Reply 4, posted (10 years 6 months 3 weeks 1 day 13 hours ago) and read 6457 times:
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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?

User currently offlineAtlamt From United States of America, joined Sep 2001, 241 posts, RR: 3
Reply 5, posted (10 years 6 months 3 weeks 1 day 12 hours ago) and read 6444 times:

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


Fwd to MCO and Placard
User currently offlineDl757md From United States of America, joined May 2004, 1562 posts, RR: 17
Reply 6, posted (10 years 6 months 3 weeks 1 day 12 hours ago) and read 6442 times:

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.



757 Most beautiful airliner in the sky!
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