physics question regarding flammable liquids

SkinnyJoe said:
Does surface area in any way determine how fast a given volume of liquid burns up completely?

For instance, if I am burning 20 cc of a liquid in a dish 2 inches in diameter, and 20 cc of same liquid in a dish with a 1 inch diameter, will the 20 cc take the same amount of time to completely burn up, in both dishes?

Thanks.
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Surface area exposed to air absolutely affects the combustion rate. As the others have mentioned, more surface area exposed increases the overall rate of combustion of a pool fire flame.

Coincidentally, my research project for my MS in mechanical engineering at UC Berkeley (18 years ago!) involved measuring the soot volume fraction in various pool fire flames. The reason that was important is because the amount of soot in a flame determines its rate of radiation heat transfer.

Along the way, I had to burn a variety of fuels in circular pans that I designed with some dimensional analysis to get the scaling right. I don't recall the details (although I would bet the combustion rate scaled with the square of the radius, i.e. the area), but the pools definitely had increasingly higher rates of combustion as the diameter increased (as was expected).

A liquid fuel's combustion rate is controlled by the rate of pyrolysis, the rate at which the fuel is vaporized near the surface so it can react with the oxidizer. The more area exposed to the oxidizer (air), the faster the pyrolysis rate.

Re: alcohol flames ... they are only invisbile when you are talking about flames from wicks, such as alcohol lamps or even the clothing of a race car driver's pit crew. For pool fires (i.e. liquid pools of fuel), the flame is clearly visible. It contains a significantly higher soot volume fraction and radiates a lot more heat and visible light.

I'm not sure how the term "catalyst" came into this, as I am unfamiliar with its usage in this context. In any event, I gather the suggestion was to preheat the fuel by having a conductor immersed in it. That would in fact increase the pyrolyis rate and thus the rate of combustion, but I'm not sure by how much. The close the fuel is to its boiling point, the easier it will vaporize for sure, but close to the reaction front it's pretty hot anyway.

However, I do know that in gas turbine engines, there are some designs (maybe most?) where the "cold" liquid fuel is circulated through the turbine blades for two reasons. One is that it cools the turbine blades so they won't melt as easily, and the other is that it preheats the fuel before it is injected into the combustion chamber. It probably makes a bigger difference there, especially with a very high octane fuel like aviation gas than it would with a simple pool fire flame.
 
While everyone here has the topic pretty well covered, to Codger:
I think that cat food can stove is about the worst heating stove you can get. Go w/ a jetted stove that will pressurize itself, and the heat output will go up by a lot. These ones work like the cat food can stove until they heat up, which is the first 30-40 secs, then take off and work like a burner on a gas range. BIG difference. Unless you are using the fiberglass wicking, then I don't know for sure, as I have never tried it.
 
Thanks for all the good answers.

I am considering building a simple device for staying warm (alive) inside a broken down vehicle during a blizzard, in addition to blankets, chocolate, and handwarmers, while the help arrives. Has happened to me before, and it's a very unsettling feeling.

Alcohol, as we mentioned before does not produce CO, so that's definitely a plus. I am leaning toward a steel or aluminum tubing of some kind, filled with 91% rubbing alcohol, maybe 1 inch in diameter and about a foot long, with one end closed, and fixed into a wooden base, with the tube pointing straight up.

The other day I poured some alcohol into a camping style cup and set the vapors on fire. I could not put it out by blowing on it, and the flames were maybe 10 inches tall, and definitely visible. Perhaps diluting it with water would be more economical and useful as I would not want to set the car on fire. :D
 
Alcohol, as we mentioned before does not produce CO, so that's definitely a plus.
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But any flame will use up the oxygen so you'll die of asphyxiation anyway, I don't know if that is more pleasant than from CO.

TLM
 
just for clarity, i meant to answer only the second question in my first post.


Does surface area in any way determine how fast a given volume of liquid burns up completely?
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yes.


For instance, if I am burning 20 cc of a liquid in a dish 2 inches in diameter, and 20 cc of same liquid in a dish with a 1 inch diameter, will the 20 cc take the same amount of time to completely burn up, in both dishes?
Click to expand...

no.
 
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