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duke
Topic Author
Posts: 1177
Joined: Tue Sep 21, 1999 9:52 pm

Afterburners

Wed Jan 09, 2002 8:58 am

I don't understand how an afterburner can efficiently work if the gas it re-heats
is not compressed.

Also, do any modern jet fighters use "augmented turbofans"? Is that more
efficient than an afterburner?
 
Staffan
Posts: 3879
Joined: Tue Mar 18, 2008 9:21 am

RE: Afterburners

Wed Jan 09, 2002 10:14 am

The gas it heats is indeed compressed, it's the jet blast that is heated up more by spraying fuel into it.


 
Dash 80
Posts: 294
Joined: Wed Nov 07, 2001 4:26 am

RE: Afterburners

Wed Jan 09, 2002 1:44 pm

Its like adding a second explosion after the one in the burner cans. Extra bang for the buck.
...where the rubber hits the runway...
 
aeroguy
Posts: 66
Joined: Wed Aug 15, 2001 1:33 am

RE: Afterburners

Tue Jan 15, 2002 2:09 am

That's correct, the air in an afterburner or augmentor has been compressed already by the compressor. Consider what happens to the air pressure after the compressor: There is a small amount of pressure loss through the combustor. Then the air is expanded through the turbine, (obviously) to a pressure that is lower than the compressor exit pressure but higher than the exhaust nozzle exit pressure. So basically the afterburner or augmentor burns compressed air, but at a lower pressure than the combustor does.
 
aeroguy
Posts: 66
Joined: Wed Aug 15, 2001 1:33 am

RE: Afterburners

Tue Jan 15, 2002 2:16 am

Oh and to answer the second part, yes modern fighters use augmented turbofans. From a component standpoint, I don't know if augmentors are more efficient than afterburners. I guess augmentors run cooler thanks to the injection of bypass air, but I don't know what that does to efficiency. Either way, overall a turbofan is more efficient than a turbojet.
 
lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

RE: Afterburners

Fri Jan 18, 2002 8:40 am

Either way, overall a turbofan is more efficient than a turbojet.

If this is the case, then tell me why did NASA choose a turbojet w/afterburner rated at 80,000lbs for their HSCT and didn't make the noise requirements? Why didn't they choose a turbofan, it would have been quieter and certainly less fuel is used through the sequence.

Also, do you guys happen to know the by-pass-ratio of augumented turbofans on fighters or bombers?
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
aeroguy
Posts: 66
Joined: Wed Aug 15, 2001 1:33 am

RE: Afterburners

Fri Jan 18, 2002 1:52 pm

The important distinction here is what speed are we talking about. The HSCT choice of a turbojet had to do with speed. Turbofans are generally more efficient than turbojets at subsonic speeds. Turbojets are more efficient at supersonic speeds.

Most fighters and bombers spend most of their time in subsonic cruise, so the primary powerplant consideration is efficient subsonic cruise. Low bypass ratio turbofans are chosen because they sit in the grey area between high bypass turbofans and turbojets, and sort of share the best of both worlds. Low bypass turbofans have the efficiency advantage of a bypass engine while still retaining the supersonic capability of a turbojet. On the other hand, for the HSCT, the primary powerplant consideration was efficient supersonic cruise, so by optimizing for that condition led to choosing a turbojet. Not meeting noise requirements is a consequence of optimizing for supersonic cruise. I suppose they could have made noise more of a priority and chosen a turbofan, which would have also offered a fuel burn advantage in the subsonic portions of the flight. But then the fuel burn in supersonic cruise would have been higher than what a turbojet would offer. Someone must have ran the numbers and realized the cheaper option in the end was to optimize for cruise.

I believe the GE4 afterburning turbojet was chosen for the old Boeing 2707 SST. It was chosen over an augmented turbofan offered by Pratt if I'm not mistaken.


Here's some bypass ratios of some popular low bypass turbofans:

PW F100 (F-15, F-16): 0.4
GE F101 (B-1B): 1.91
GE F110 (F-16): 0.8
GE F404 (F-18): 0.34
PW TF33 (B-52H): 1.55

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