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777 GE-90 "dome" Combustion

Mon Mar 22, 2004 1:42 pm

I was flipping through a book about the 777 and in the section where they discuss the engines, the GE90 is described in detail. The section in question is:

"...The GE90 also has a new type of combustor known as a dual-dome. This is arranged in a ring, or annulus, around the engine core and consists of of two major zones of combustion. When the engine is at low power, or idle thrust, only one 'dome' is lit. The use of one dome cuts down the amount of time any unburned fuel spends in the combustor and therefore reduces the emissions of carbon monoxide and hydrocarbons by up to 50%. At higher power levels, the second dome kicks in. The unit is designed to cut emissions of nitrogen oxides by 35% and smoke emissions by 50%."

I was wondering if anyone would be able to simplify this "dome" of combustion concept for me.

What does this "annulus" look like, and how does it kick in from one dome to the second dome?

Any help would be great!
My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.
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RE: 777 GE-90 "dome" Combustion

Mon Mar 22, 2004 5:31 pm

"Annulus" means toroid in latin. It's basically a ring shape.

You can think of the combustion chamber as a donut that sits around the engine shaft. Air comes in from the front of the donut. Fuel is sprayed in by nozzles set in a ring. Combustion takes place. Exhaust gases (at a higher temperature and pressure due to combustion) leave by the rear.

With only one "dome" lit, only part of the air is used for combustion. Fewer nozzles are used. My guess is that if, at low power, all the nozzles were used with low fuel throughput, this would hamper their efficiency. Using only the nozzles on one dome, but with normal throughput, you get the same thrust with less fuel and emissions.

Just an educated guess.

Here's a nice drawing of a turbine engine:

Here's a nice drawing of the combustion area:

These are not specific to aircraft engines, but give you an idea.

"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
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RE: 777 GE-90 "dome" Combustion

Tue Mar 23, 2004 7:25 am

CFM56 engines are also available with dual annular combustors.

Its the standard configuration of the CFM56-7, and an option on the CFM56-5B.


[Edited 2004-03-22 23:28:04]
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RE: 777 GE-90 "dome" Combustion

Tue Mar 23, 2004 6:29 pm

I can't believe no one has posted a more technical explanation yet...  Wow!

I for one would be interested.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
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RE: 777 GE-90 "dome" Combustion

Tue Mar 23, 2004 8:10 pm

"Its the standard configuration of the CFM56-7, and an option on the CFM56-5B"

That's not true. The standard configuration is always the single annular combustor for both engine families.

Now I try to give you a bit of a more technical explanation based on the CFM56 (the combustor is GE technology, and therefore I assume that the GE90 is following a similar design principle).

First of all, here's a link to an ilustration on the CFM website:

The upper half shows a crossection of a dual annular combustor engine, while the lower half is a single annular combustor. Basically the difference is, that a set of 20 fuel nozzles is arranged circumferencially for both versions, but the dual annular combustor fuel nozzle have two spray tips (so this gives an inner and outer ring of 20 spray tips each) versus one spray tip on the single annular combustor version. These two spray tips are controlled independently. These allows different spray patterns for the various phases of a flight.

For a typical flight profile (takeoff, climb, cruise, descent to a lower cruise level, decent, landing, reverse thrust, taxi) the following logic applies:
In takeoff and climb, the full burn 20/20 mode is active followed by the cruise I mode also 20/20, the idle decent 20/10, cruise II 20/20, idle decent 20/0, landing 20/10, reverse thrust 20/20 and finally taxi 20/0. This active management of burners in use reduces the overall temperature in the combustion chamber which results in lower emissions (especially NOx).

The problem is, that the complexity of the hardware increases the engine price and it is also assumed that the maintenance costs are higher for dual annular combustors.

I hope this was technically enough....  Big grin

Cheers, Thomas
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RE: 777 GE-90 "dome" Combustion

Thu Mar 25, 2004 1:28 pm

I think the way to look at the dual annular combustor might be to consider the conflicting design requirements of low NOx emissions and low CO & unburned HC (Hydrocarbon) emissions. For example, decreasing flame temperature improves NOx emissions but makes CO and HC worse. Decreased residence time reduces NOx emissions, but hurts CO and HC emissions. Also, leaner fuel/air ratios decrease NOx but increase CO and HC. You can design a combustor with good NOx emissions and poor CO/HC, and visa versa. The tricky part is designing one that has both low NOx and CO/HC. You can consider the GE90 combustor a combination of 2 combustors each designed to reduce a particular type of emission.

What you end up with is the dual annular arrangement. The outer annulus is called the pilot and the inner is called the main. The outer (pilot) is designed for minimum CO and HC emissions (at low power) and the inner (main) is designed for minimum NOx emissions (at high power). The outer (pilot) annulus has the better starting capability I believe. The pilot burner is always lit. So at takeoff, most of the fuel is burned in the inner (main) annulus for low NOx emissions at high power. When transitioning to cruise, the fuel to the inner annulus is reduced (I think maybe even shut off). This leaves the outer (pilot) annulus to offer its superior low power CO and HC emissions capability during cruise.

Also if you get a good look at the GE90 combustor you will see that the inner (main) annulus is slightly shorter. This has to do with my earlier comment about residence time. A shorter combustor means that the gases leave the combustor before much combination of Nitrogen of Oxygen can take place, i.e. reduced NOx emissions due to reduced residence time. The longer outer (pilot) chamber contributes to more complete combustion, thus reducing CO and HC emissions.

 Smokin cool

[Edited 2004-03-25 05:39:05]

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