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jetmech
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Joined: Wed Mar 29, 2006 2:14 am

Jet Engine Heat Usage

Tue Jul 20, 2010 7:34 am

G'day Techies,

I found an interesting diagram about the P&W F135 engine, which powers the F-35 aircraft. I find it interesting that the waste heat is supposedly the same wether the lift fan is operating or not. I thought the amount of waste heat may be less with the lift fan operating. What do you think?

http://upload.wikimedia.org/wikipedia/commons/6/68/JetEngineGraph-LiftFan.PNG

http://en.wikipedia.org/wiki/Pratt_&_Whitney_F135

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Jet Engine Heat Usage

Tue Jul 20, 2010 10:03 am

Quoting jetmech (Thread starter):
I thought the amount of waste heat may be less with the lift fan operating. What do you think?

If the fan is running, it's extracting power from the turbines, which means less energy entering the nozzle, but it also means a bigger pressure drop in the turbines so the nozzle can do less work. At least as far as PW concerns, the energy extracted to the lift fan is exactly balanced by the energy not extracted by the nozzle, so the amount of waste heat holds constant. That makes some sense in my head, although I doubt it works out exactly that well in real life.

Tom.
 
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Faro
Posts: 1505
Joined: Sun Aug 12, 2007 1:08 am

RE: Jet Engine Heat Usage

Tue Jul 20, 2010 1:36 pm

Quoting tdscanuck (Reply 1):
If the fan is running

Is the F135 a pure turbojet or am I seeing things? I can't seem to find a fan stage ahead of the compressors...

Faro
The chalice not my son
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: Jet Engine Heat Usage

Wed Jul 21, 2010 12:17 am

Quoting faro (Reply 2):
Is the F135 a pure turbojet or am I seeing things? I can't seem to find a fan stage ahead of the compressors...

That looks like a fairly generic jet engine diagram. There are good cutaways on P&W's site:

http://www.pw.utc.com/StaticFiles/Pr.../Images/f135-ctol-cutaway-high.jpg

http://www.pw.utc.com/StaticFiles/Pr...Images/f135-stovl-cutaway-high.jpg

You can clearly see the bypass area (small though it may be, at about .57).
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Jet Engine Heat Usage

Thu Jul 22, 2010 1:37 am

Quoting faro (Reply 2):
Is the F135 a pure turbojet or am I seeing things? I can't seem to find a fan stage ahead of the compressors...

I meant the lift fan...the shaft going out the front in the OP's image feeds a RR lift fan ahead of the engine.

Tom.
 
jetlife2
Posts: 184
Joined: Sun Jul 09, 2006 11:32 am

RE: Jet Engine Heat Usage

Thu Jul 22, 2010 1:07 pm

Horribly irrelevant diagram on wikipedia. Single shaft turbojet used to illustrate a turbofan - lift fan combination! LOL. Someone with the time and energy should replace it. JetMech you are just the man for the job.
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