Flame Question

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Flame Question

Post by serverlan » Sat Apr 18, 2009 1:57 pm

Can someone explain the way flames sit at the end of tubes, even though the fuel and air are mixed long before in the the tube?

Some examples -

The bunsen burner. Air and gas are introduced at the bottom, but never burn until they reach free air at the top.

I have a gas needle-yet, which sucks the air in to mix with the fuel, but it doesn't burn until it escapes the tube.

While attempting to burn my beard off today, I observed the same behaviour in a simple 1" pipe.
I could get the gas to burn inside the pipe, but the flame would often jump to the far end of the pipe, and stay there.

This behaviour seems to be largely independent of gas flow rate.

I used to think I had an explanation for this tucked away in a distant corner of my mind, but went I went looking for it I couldn't find it, got lost, and had to come back out for dinner.

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Re: Flame Question

Post by PyroJoe » Mon Apr 20, 2009 6:34 pm

Have you tested alcohol yet, Also try the propane in a larger pipe say above 1.5" diameter.

My guess is the process of combustion increases the local gas volume and a slight bit of pressure is created that will tend to push outward of a confined space/pipe.

There is probably some mention of flame speed, I find alcohol has less problems burning inside smaller pipes between 1" to 1/2"diameter. Also some very tiny Pulse jet engines run on acetylene or hydrogen.

Convection can also play a role in where a flame will burn.


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Re: Flame Question

Post by serverlan » Tue Apr 21, 2009 5:36 am

Here is a conversation i'/ had with someone building forges and the like using LPG burners

Hi Tim,

I was searching on the internet to find an explanation of the following

"why does the flame tend to sit at the end of a burner, rather than moving
back up the tube (back-burning)"

I found your article at http://www.abymc.com/
and you mentioned that the flame stays at the point of pressure drop.
Thank you for that information.
Do you have any information as to why the flame would do that?
Well, the most precise answer would surely involve the Navier-Stokes
equation (making suitable adjustments for the fact that the gas starts out
fairly normal, then the temperature suddenly explodes under a certain
condition... bleh, and solving that equation as-is is already a chore!), but
I suppose that would go well over the heads of almost all metalcasters (not
having used that equation, that includes myself!).

Another possible view is that, at the step, a vortex (wake) is created,
which retains burning gasses, thus igniting the incoming gas as it passes.
"Pressure step" is probably a concise alternate description of this.
Incidentially, if you inspect the flame closely, you will see that the
inside (where the flame starts) also burns in a cone, so that the edge
ignites first (as it passes the flare), the flame spreading inward as it
travels, reaching the center (at which point, the entire column of gas is
burning) a few inches past the flare.

More evidence to the utility of a step is, if you construct a perfectly
smooth burner pipe (say, by expanding the end of a copper tube), the flame
is very unstable. So it is neither a smooth transition nor a smooth taper
(of any slope, including the oft-quoted 1/12 slope) which stabilizes the
flame. Turbulence seems to be the way to go.


Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

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