lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

Improving Fuel Burn With Spray Nozzle Direction?

Sun Aug 27, 2006 7:14 am

When fans and compressor fansets spin around, so does the air that goes through them thoughout the enigne; so why do injectors spray fuel straight backwards? I realize the compressor outlet guide vanes and turbine inlet guide vanes help with flow direction, I'm talking about the spray head itself, what direction is it oriented along the axis of the engine axial? Looking at a picture of an annular combustion liner (below), I don't see any indication that while the flow goes around, that the injectors spray anywhere but along the chamber length.

Or is the difference negligible?



Side question: does engine RPM vary the amount of airflow spin?

[Edited 2006-08-27 00:16:26]
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
kaddyuk
Posts: 3697
Joined: Thu Nov 08, 2001 1:04 am

RE: Improving Fuel Burn With Spray Nozzle Direction?

Sun Aug 27, 2006 9:35 am

The nozzles have to jettison fuel rearwards in a straight line to avoid the flame of the combustion chamber touching the side walls.

Air needs to be swirled around the flames and combuster to reduce their temperature to make sure they don't melt. When they're out of alignment, expect to see extensive damage on the combuster chamber wall... The flame will burn through anything...
Whoever said "laughter is the best medicine" never had Gonorrhea
 
lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

RE: Improving Fuel Burn With Spray Nozzle Direction?

Sun Aug 27, 2006 12:09 pm

I wasn't talking about varying the spray either towards the axile or the outer lining, I meant around the chamber itself, in the direction of circulation and still straight back with respect to an engine cross section.
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
Tristarsteve
Posts: 3364
Joined: Tue Nov 22, 2005 11:04 pm

RE: Improving Fuel Burn With Spray Nozzle Direction?

Sun Aug 27, 2006 5:53 pm

The airflow through the engine is basically in a straight line. Although the blades rotate the air goes straight through. In a combustor the idea is that the flame must not touch the sidewall. If it does it will burn straight through it and damage the engine. All the holes you see on the sides introduce air to keep the flame off the walls until it hits the NGVs at a temp lower enough for them to survive. The flame will melt the combustor lining.
 
N231YE
Posts: 2620
Joined: Tue Jul 25, 2006 9:24 am

RE: Improving Fuel Burn With Spray Nozzle Direction?

Mon Aug 28, 2006 12:35 am

The airflow, whether originating from a centrifugal or axial flow compressor, does have a spin to it as it enters a diffuser. But the air is straitened and slowed inside the diffuser; the side effect of this is that the pressure is raised prior to entering the combustion stage. Just like humans + high altitude (lower pressure means lower oxygen content)=less performance, the same occurs with engines. It is because of the diffuser that air enters the combustion chamber strait, without any "swirl" to it.

Besides the pressure, the diffuser is needed to slow the airflow down in order to "keep the flames attached" to the nozzles. Imagine a candle, with you blowing at it. If you blow too much, the flame is lifted off the wick, and immediately goes out since the flame is no longer attached to the wick; its "nozzle" if you will (the candle flame burns the wax via capillary action up the wick).

So to answer your question, the nozzles are mounted strait back in relation to the airflow, as the airflow has been straitened by the diffuser.

Hope this helps
N231YE

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