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cargil48
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Jet engine combustion chambers

Thu Sep 19, 2019 10:46 am

Gentlemen,

I'm going through some reports about the evolution of the jet engine. Maybe we find here people who know more in-depth how the so-called "hot section" works to give an answer to my questions.

1. Why are there two types of disposing the injector nozzles inside the combustion chambers, one "annular type" and an "can-annular type"? I know the difference of both, just asking why there are two slightly different injection systems. Has one of them any advantage (or disadvantage) over the other?

2. Since in all jet engines from the 50s on there is a (fresh air) bypass, why isn't there (as far as I know) a ring of air inlets just before the HP turbine to let part of the cold air in to lower the TIT and the harmful emissions by mixing fresh air with the exaust gases? Wouldn't that increase also the life span of the HP turbine and of the entire "hot section", thus improving the TBO of the engines?

3. Is it true that there are some kind of high power jet engines with a double system of annular injector rings? One main (bigger radius) annular ring and concentrical with it another one with some more injection nozzles to increase power when needed? When starting on the oxygen rich altitude of the airport and with MTOW (and maybe a somewhat short rwy) all injector nozzles are used, when the cruising altitude is reached, with less oxygen, the EICAS automatically shuts off the inner circle of injectors. I think I've read something like this somewhere.

Would be interesting to discuss these matters, I think. Thanks in advance.
 
WIederling
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Re: Jet engine combustion chambers

Thu Sep 19, 2019 4:00 pm

cargil48 wrote:
2. Since in all jet engines from the 50s on there is a (fresh air) bypass, why isn't there (as far as I know) a ring of air inlets just before the HP turbine to let part of the cold air in to lower the TIT and the harmful emissions by mixing fresh air with the exaust gases? Wouldn't that increase also the life span of the HP turbine and of the entire "hot section", thus improving the TBO of the engines?


Potentially available Carnot efficiency is given by
nCarnot = 1 - ( Tcold / Thot )

The Temp effective at the turbine inlet is Thot. Not what happens in the "can".

(excessive) cooling after the burner can but before the turbine inlet would cost fuel. Quite a bit actually.
Murphy is an optimist
 
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cargil48
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Re: Jet engine combustion chambers

Thu Sep 19, 2019 5:48 pm

Does that mean that if one cools the temperature of the expanded hot air inside the burning chamber before it reaches the HP turbine (to make it spin) one looses energy, thus wasting burnt fuel? Is that what you mean? (Interesting stuff, btw...)

PS: I'm not an engineer.
 
LH707330
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Re: Jet engine combustion chambers

Thu Sep 19, 2019 9:15 pm

Yes, that's exactly it. As it stands, most (if not all) modern designs do use some bleed air from the compressor for cooling, adding it in to create a cooler sheath around the turbine blades to prevent them from melting. One of the current limiters on efficiency is the temperature tolerance of today's unobtanium, which is why the engine OEMs are working on CMCs to raise that temperature to get a higher delta-T and hence efficiency.
 
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Starlionblue
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Re: Jet engine combustion chambers

Fri Sep 20, 2019 12:48 am

As I understand it, can annular combustion chambers are a thing of the past, and have long since been superseded by "pure" annular chambers. They were an intermediate stage of development between cans and annular chambers..

cargil48 wrote:

...

3. Is it true that there are some kind of high power jet engines with a double system of annular injector rings? One main (bigger radius) annular ring and concentrical with it another one with some more injection nozzles to increase power when needed? When starting on the oxygen rich altitude of the airport and with MTOW (and maybe a somewhat short rwy) all injector nozzles are used, when the cruising altitude is reached, with less oxygen, the EICAS automatically shuts off the inner circle of injectors.


Nitpick: EICAS doesn't control the engine. FADEC does.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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jetmech
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Re: Jet engine combustion chambers

Fri Sep 20, 2019 5:29 am

cargil48 wrote:
Does that mean that if one cools the temperature of the expanded hot air inside the burning chamber before it reaches the HP turbine (to make it spin) one looses energy, thus wasting burnt fuel? Is that what you mean? (Interesting stuff, btw...)
PS: I'm not an engineer.

That's exactly it. Approximately 30% of the air entering the core engine is "lost" to a combination of cooling (blades, discs, shafts etc) and leakage (inter-stage, bearing and balance chambers etc.). As you state, burning precious fuel to heat the air-stream and then immediately adding a significant amount of cooler fluid results in a marked reduction in thermal efficiency.

cargil48 wrote:
Is it true that there are some kind of high power jet engines with a double system of annular injector rings? One main (bigger radius) annular ring and concentrical with it another one with some more injection nozzles to increase power when needed? When starting on the oxygen rich altitude of the airport and with MTOW (and maybe a somewhat short rwy) all injector nozzles are used, when the cruising altitude is reached, with less oxygen, the EICAS automatically shuts off the inner circle of injectors. I think I've read something like this somewhere.

GE have been using their TAPS (Twin Annular Premixed Swirl) combustion chamber on the CFM56 and GE90 and will have it on the GE9X.

https://www.faa.gov/about/office_org/he ... Report.pdf

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
WIederling
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Re: Jet engine combustion chambers

Fri Sep 20, 2019 6:31 am

cargil48 wrote:
Does that mean that if one cools the temperature of the expanded hot air inside the burning chamber before it reaches the HP turbine (to make it spin) one looses energy, thus wasting burnt fuel? Is that what you mean? (Interesting stuff, btw...)
.


you don't lose energy. ( that is impossible. :-)
you just don't "extract work" and instead dissipate it.

see: https://en.wikipedia.org/wiki/Entropy#Carnot_cycle

not really intuitive, I know.
Murphy is an optimist
 
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cargil48
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Re: Jet engine combustion chambers

Fri Sep 20, 2019 1:04 pm

Starlionblue wrote:
Nitpick: EICAS doesn't control the engine. FADEC does.


No "nitpick" at all, my friend! Pure correction of a stupid confusion on my part!... In my age (71y) one should avoid writing about challenging stuff if one is tired... :)
 
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cargil48
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Re: Jet engine combustion chambers

Fri Sep 20, 2019 1:10 pm

Gentlemen, my sincerest thanks for your detailed explanations about this matter!

(Offtopic @ Wiederling: Man, where on earth did you get that nickname from??? Even "Regenwurm" would be less "agressive" for us to read... :D :D :D
 
WIederling
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Re: Jet engine combustion chambers

Fri Sep 20, 2019 4:44 pm

cargil48 wrote:
Gentlemen, my sincerest thanks for your detailed explanations about this matter!

(Offtopic @ Wiederling: Man, where on earth did you get that nickname from??? Even "Regenwurm" would be less "agressive" for us to read... :D :D :D


hehe!

peruse the Duden on meaning of "wider" and "wieder" :-)
Murphy is an optimist
 
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lightsaber
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Re: Jet engine combustion chambers

Tue Sep 24, 2019 4:30 pm

As someone with a master's degree in combustion I will try to answer:
1. Annular combustors are modern combustors, as they have less surface area to flow than multiple cans, they save fuel (via less cooling needed).

As noted, they are a thing of the past. They have fewer accoustic modes and we're thus quicker to develop. Now that we know how to develop annular combustors (e.g., fuel injector coupling, rotational accoustic modes and how to dampen. Each can is tough to lite. Not so with annular. Annular is lighter and cheaper too.

2. As already noted, Carnot cycle efficiency is driven by having the turbine powered by the hottest gas possible. Cooling air must be under greater pressure than flow path air, so only the highest pressure air can cool the high turbine. In fact, the centrifugal effect is used to pump the air to a higher pressure to help cool the 1st stage turbine.

3. Double annular injectors have problems clogging. Yes, they have been done when mixing was marginal. Pratt applied for a Patent off technology I started for that, but smaller fuel injectors have easier to clog passages, more surface area (heat is more of a problem) per flow area. Minimizing the number of fuel injectors is more reliable. More injectors is lower pollution emissions. Pick you poison.

Lightsaber
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