lehpron
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Basic Combustion Question

Mon Jul 26, 2004 4:06 am

At least I think it should be really easy. All this time I've assumed that because it doesn't happen in reality, there is no problem, but:

ASSUMPTION: The combustion pressures are greater than intake pressures [after ignition].

QUESTION: If the above is correct, why doesn't the combustion go back up the intake and the outlet at the same time?


I would figure with engine systems that have a physical block (compressor turbine or a intake value in 4-cycle piston system) will prevent flow going out the front.

What about a ramjet? That is actually what I am getting at. What prevents forward outflow? The incoming dynamic pressure is less than combustion pressure such that a net force results over the outlet area, correct?
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Klaus
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Lehpron

Mon Jul 26, 2004 4:44 am

Lehpron : What about a ramjet? That is actually what I am getting at. What prevents forward outflow? The incoming dynamic pressure is less than combustion pressure such that a net force results over the outlet area, correct?

No, I don´t think so. The total pressure (static+dynamic) at the combustion chamber inlet is greater than the pressure exerted by the combustion process (at least partially due to inertia at high speeds). As far as I understand, that´s the whole deal with (SC)RamJets: At lower speeds you need a turbocompressor to prevent a stall; Only at very high speeds the ram air compression alone is sufficient to do that.
 
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Starlionblue
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RE: Basic Combustion Question

Mon Jul 26, 2004 5:17 am

As I understand my physics, the intake pressure HAS to be greater than the combusion area pressure, or thrust would not be generated.

However, energy has to be added somewhere, and combustion does this, in both turbofans and ramjets.

I'm sure the more technical members can explain this better.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Klaus
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RE: Basic Combustion Question

Mon Jul 26, 2004 5:22 am

In a SCRamJet, the combustion process is supersonic anyway; I don´t think there would be even a theoretical chance of a stall any more as long as the flow speed is kept that fast.
 
MITaero
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RE: Basic Combustion Question

Mon Jul 26, 2004 8:24 am

Most combustion processes in engines can be assumed to take place at constant pressure. See "brayton cycle": http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/ThermLaw2/HeatEngines.html
 
Klaus
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MITaero

Mon Jul 26, 2004 10:38 am

MITaero: Most combustion processes in engines can be assumed to take place at constant pressure.

In the sense of "continuous" at constant load? Sure. The point here were the different (even if individually constant) pressures at different places within the engine.

And - correct me if I´m wrong - one of the reasons for (turbine- or ram-effect-driven) compressors in a jet engine was to keep the combustion exhaust from pushing forward through the compressor instead of aft through the nozzle or turbines, right?
 
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Starlionblue
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RE: Basic Combustion Question

Mon Jul 26, 2004 11:06 am

Right. There's no thrust produced if pressure isn't lower at the ass end.
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"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Goldenshield
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RE: Basic Combustion Question

Mon Jul 26, 2004 3:38 pm

If I remember correctly, the pressure will be the same, but since the air that leaves the combustion chambers is of a higher temperature, it will need much more area to remain at the same pressure. This is why jet engines suck in less air than they put out, as the air that is taken in, is the same mass of air as exhuasted, but becuase it is heated, it expands in order to maintain it's original pressure, thus exiting at a higher speed.
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MITaero
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RE: Basic Combustion Question

Mon Jul 26, 2004 4:47 pm

>The point here were the different (even if individually constant) pressures at different places within the engine.

The combustor inlet and exit are two different places in the engine. But I understand what you're talking about.

Flows can still move from low to high pressure. This is called an adverse pressure gradient; it happens all the time. You guys are right to be concerned, as separation is much more likely in the compressor due to a higher pressure at the back end of the compressor. However, because the incoming flow has enough momentum to overcome the adverse pressure gradient, the flow doesn't come close to being reversed.

Starlionblue, there is always thrust produced if the momentum of the exiting air is greater than the momentum of the air coming into the engine. There are nozzles that expand the exhaust to atmospheric pressure (called "ideally expanded" nozzles), and thrust is definitely produced.

Goldenshield, there's no way for an engine to suck in less air than it puts out; this would violate conservation of mass. The inlet and exit mass flow rates are the same (ignoring the small fuel mass flow rate). The increment in momentum of the exiting air causes the thrust.

[Edited 2004-07-26 10:07:48]
 
SlamClick
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RE: Basic Combustion Question

Tue Jul 27, 2004 2:27 am

Physics aside (for the benefit of those of us not well schooled in physics) don't forget the role of velocity in this. All jet engines have a velocity to the gas stream and it would take a lot of pressure to back up against this.

There was a variant of the ramjet, called a pulse jet. It used shutters at the intake which were momentarily closed by the combustion-induced overpressures. It would open, admit some air for the next combustion cycle, and close while that was taking place. Produced a characteristic pulse sound.

Extra points for anyone knowing where it was used.
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MITaero
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RE: Basic Combustion Question

Tue Jul 27, 2004 2:43 am

>Physics aside (for the benefit of those of us not well schooled in physics) don't forget the role of velocity in this. All jet engines have a velocity to the gas stream and it would take a lot of pressure to back up against this.

That's right (that's what I meant by momentum, which is just the product of velocity and mass).

>Extra points for anyone knowing where it was used.

I saw an episode of JAG where they used it over Korea on a secret mission, and for some reason they let the (lawyer) main character guy fly it. Hilarious.
 
Vorticity
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RE: Basic Combustion Question

Tue Jul 27, 2004 2:46 am

The germans used the pulse jet in their V-1 buzz bombs that they used to attack England. They had a very characteristic sound because of the pulse jet.

Anyway, compressors play a big role in all of this, compressing the air that goes into the combustion chamber. Ramjets and Scramjets attempt to compress the air without using compressor turbines. Lots of solutions for flight.
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Goldenshield
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RE: Basic Combustion Question

Tue Jul 27, 2004 4:29 am

If only I could edit what I wrote. > Sad

It should read, "This is why [it seems] that jet engines suck in more air than they put out."
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MITaero
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RE: Basic Combustion Question

Tue Jul 27, 2004 5:17 am

Heh, it's ok, I knew what you meant, just didn't want other people to get confused.

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