Minimum size flame

[Externet]

Hi.

I think learning long ago, that a flame cannot be smaller than a certain limit. How is the theory -or fact-?

 

Perhaps that is the principle of operation of flame arresters, which are a bunch of small holes.

If an explosion happens at the end of a hose carrying -say gasoline- vapors, would the flame propagate inside the hose towards the source of vapors and blowup everything; or if the hose is small enough it won't happen ?

 

Is a burning explosive wavefront also prevented to propagate trough a flame arrester, of that is only for flames ? -if they are not the same-

 

What prevents the flame of a bunsen burner to ignite the vapors in the hose all the way back to its butane tank and... kaboom ? Yes, no oxygen in there.... What if a mixture of combustible vapors and air are flowing in the hose ?

Miguel

[CaptainPanic]

I'll just answer the 1st question... I think you have already understood the rest (your guess is right what happens if there is a combustible mixture in the hose, and there's a fire a the end: Kaboom). I know little of flame arresters actually.

 

A flame or fire needs the usual 3 things:

 

1. Fuel

2. Oxygen

3. Heat

 

Points 1 and 2, fuel and oxygen, are molecules. There is no size limitation there. But the heat becomes a problem when the flame is too small. Very very small flames just cool down too fast.

 

Why?

 

I will simplify things a little. I neglect a lot of things I learned about heat transfer here, and my old teachers would definitely frown upon this... but I think it is simplification that can make this problem look easy:

 

Cooling down happens at the surface of the flame, where the flame meets the outside (cold) air. Let's say that the energy lost to the outside air is proportional to the outside surface of the flame.

The heating up of the flame happens inside the flame. Let's say that the energy produced is proportional to the volume of the flame.

 

Area is a 2nd order function of length (or radius). Area is measured in square meters (m2).

Volume is a 3rd order function of length (or radius). volume is measured in cubic meters (m3).

 

So, when a radius becomes smaller, the volume is decreasing faster than the surface. The volume decreases be the 3rd order while the outside surface decreases by a second order function.

 

This simply means that the energy produced becomes less faster. The heat loss to the outside air drops too, but not so fast. At some point, the heat loss becomes so much larger (relatively to the production of heat)... the flame goes out.

 

You should realize that air is of course oxygen, and therefore the flame and the air mix quite a bit... and this means that the theory of heat transfer from the flame to the air is a little more tricky than I just described. But I think it still holds that heat transfer is a surface phenomenon. Heat production is a volume phenomenon.

[hermanntrude]

in other words, smaller flames have a higher surface area to volume ratio, as do all small things compared to big things.

 

Seems like a sensible assumption to me. while it's true that the flame and the air mix somewhat, they still have to do it through the "surface" of the flame.

[chemkid]

If you were to provide oxygen to a fuel source and warm said oxygen you may be able to sustain a reaction at very small size. It would be interesting making a campfire under a microscope using threads of cotton! fuel will be a major problem. Fuel most be provided very fast as very small things burn very qucikly.

 

Chemkid

[Mr Skeptic]

You also need to maintain a proper fuel-air mixture. If you try to make your flame to small, likely lots of the fuel will diffuse to quickly.