Blackbird
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Combustion Chamber Question

Sat Feb 10, 2007 5:08 pm

When was it first physically possible to design a combustor that would not require any film air or anything like that?

Because the GENx uses such an arrangement with it's TAPS combustor, and the HSCT engine design also was to use a filmless combustion chamber with a premixing antechamber (a chamber ahead of the combustor) to mix the prevaporized fuel and air mixture in a lean, dropless environment prior to ignition.

What would be necessary for something like that to work?

Andrea K
 
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lightsaber
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RE: Combustion Chamber Question

Sat Feb 10, 2007 6:10 pm

Blackbird,

Backside cooling has been around for a while. Its easier in a lean burn design (such as TAPS) due to the lower peak flame temperature in the dome area of the combustor.

What's necessary is good air impingement onto the backside of the combustor liner. It also helps to have finned channels to cool the hottest sections.

Eventually the cooling air must be dumped somewhere. Now, I haven't seen the GEnX cross section to that level of detail. (If you have a link please share), but its possible to bypass most fo the combustor before dumping the cooling air at either the Turbine inlet vane roots (to create a film at the ID and OD, post combustion, to protect the turbine) or even to have it go into the turbine inlet vane.

You only care to keep the film air away from the dome and first 2/3rds of the combustor to prevent the quenching of CO. Once enough residence time has been given to burn out the CO, its ok to quench the combustor gases as they have to finish burning sometime.  Wink

What's necessary?
1. Good backside cooling designs
2. Good Thermal barrier coatings
3. Good casting processes (to create the backside heat transfer enhancement)
4. I'm assuming its still a double wall combustor (with impingement jets). There has been a tremendous amount of work in this area.


What helps? Efficient compressors. The more efficient the compressor, the cooler the gas entering the combustor and thus the less cooling required.

Lightsaber
"They did not know it was impossible, so they did it!" - Mark Twain
 
Blackbird
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RE: Combustion Chamber Question

Sun Feb 11, 2007 5:42 am

I was actually talking about the NOx emission rate. Some of NASA's HSR program went into the development of a combustor design that allegedly used no film air at all. According to their figures, film air would increase NOx levels. So they wished to develop a combustion chamber that used NO film air at all.

Andrea K
 
N231YE
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RE: Combustion Chamber Question

Sun Feb 11, 2007 9:48 am

From what I understand, the elimination of film air (or dilution air) would make a highly efficient jet engine-one with unimaginable efficiency by today's standards.

The problem is, no material yet exists that can withstand the temperatures of combustion, at an affordable price.
 
DeltaGuy
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RE: Combustion Chamber Question

Sun Feb 11, 2007 11:16 am

Quoting N231YE (Reply 3):
The problem is, no material yet exists that can withstand the temperatures of combustion, at an affordable price.

Or the right weight....combustion chambers are pretty heavy as is, especially in older applications.

DeltaGuy
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RE: Combustion Chamber Question

Sun Feb 11, 2007 12:07 pm

Quoting Blackbird (Reply 2):
I was actually talking about the NOx emission rate. Some of NASA's HSR program went into the development of a combustor design that allegedly used no film air at all. According to their figures, film air would increase NOx levels. So they wished to develop a combustion chamber that used NO film air at all.

Aaghh...

Yes, eliminating the film cooling air does lower NOx emissions in both RQL and lean burn combustors. However, with TAPs, if they are going sans cooling film (I cannot verify, this is the first I've heard of it), it would be to lower CO emissions, not NOx. Why? Film cooling is more effective than pure backside cooling. Thus, a more efficient cooling stream frees up process air for minimizing NOx. However, film cooling in lean burn combustors (like TAPs) will quench CO. As GE engines tend to have a little more CO than the others and thus if they went film free it would be for CO reduction, not NOx reduction.

In an RQL (rich burn, quick mix, lean burn) film cooling in the rich section will add to NOx production. However... its not that bad. (I designed RQL's for years for low emission applications.)

I'm still curious if TAPs does have a film less cooling design.

The only one's I'm familiar with... I'm not supposed to know about.  Wink

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Neil
"They did not know it was impossible, so they did it!" - Mark Twain

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