How does wind put out a fire?
Well, a wind will fan a forest fire and make it burn even better. Wind tends to spread fires, not put them out.
If you’re thinking about blowing out a match or candle, the blown air probably lowers the temperature below the kindling point. But that only works with small fires; once the fire reaches a certain size, the wind increases the oxygen, thus giving the fire more fuel.
“What we have here is failure to communicate.” – Strother Martin, anticipating the Internet.
The wind can shift and send the flames back over where it has already burned. No fuel, no fire.
my impression on blowing out a candle was the CO2 in your exhalation deprived the flame of any oxygen to burn, putting it out. but that’s just a WAG.
-ellis
As it seems for all answers regarding fire, it depends.
For a decent size fire, moving air will increase the burning rate, ie, make it worse. Thats what spreads forest fires, and makes structure fires on a windy day a royal pain in the rear end.
For a small fire such as a candle, however, there is something different going on. Bear with me on this one.
A) Combustion (fire) is the combination of an oxidizer (normally oxygen, present in air) and a fuel (the stuff thats burning). Heat must be added to start the process. This makes the famous “fire triangle” we all learned about in 5th grade.
B) Solids and liquids don’t burn. Only gasses burn. When a solid or liquid is heated enough, it begins to pyrolyze. This process releases combustible gasses from the solid/liquid being heated. When these gasses are heated enough, they will burn. When the burning gasses heat the solid/liquid enough to produce more gasses, the fire will become self sustaining (doesn’t require an additional heat source to burn).
C) Another phenomenon is that of lower and upper flammable limits. If you have a gas, it has to mix with oxygen (ambient in the air) before it can burn. A certian amount of oxygen is needed for a certain volume of the gas to burn. This relationship between the amount of oxygen needed and the amount of fuel is called the flammable range. Lets take, for instance, Propane. If the air contains 2.1% to 9.5% propane, the propane will burn. If there is less than 2.1% propane, there is not enough propane to mix with the oxygen in the air (too lean to burn). More than 9.5% propane, there is too much propane to mix with the oxygen in the air (too rich to burn). This works with all gasses that can burn.
Now for what happens when you blow on a candle
When you blow across the candle wick, the moving air takes the gasses being pyrolized from the wick and wax, and dilutes them to below their lower flammable limit. This reduction in burning gasses (combined with a cooling effect on the molten wax) stops the candle flame.
Simple, eh?
Jeremy…
Self Declared SDMB Resident Fire Service Expert
“Fire and wind come from the sky, from the gods of the sky. But Crom is your god. Crom, and he lives in the earth. Once giants lived in the earth, Conan, and in the darkness of Chaos they fooled Crom and took from him the secret of steel. And Crom was angered. And the earth shook. And fire and wind struck down these giants and they cast their bodies into the waters. But in their rage, the gods forgot steel and left it on the battlefield. And we who found it are just men. Not gods, not giants, just men. Steel has always carried with it an enigma. You must learn its riddle, Conan, learn its discipline. For no one – no one in this world can you trust, not men, not women, not beasts. This, you can trust.” from Conan the Barbarian, 1982, Universal Pictures
–Da Cap’n
“Playin’ solitaire 'til dawn
With a deck of fifty-one.”
Just a thought, but i’ve sure seen magnesium burn. Is it really gaseous magnesium burning (Boiling Point ©: 1107)? Is it really impossible for an oxidative combustion process to occur at a solid surface? Maybe it’s just a question of the exact nature of matter at the point of combustion which i’m considering.
Simple Explanation: Fire needs three factors fuel, oxygen, temperature. The relationship is often depicted as a triangle. The proper balance will permit combustion. There’s a margin for “error” where a flame may burn lean or rich. Outside that margin, it’s light’s out!
I didn’t want to go into Class D (metal) fires regarding the OP, but hey, since you asked…
Metal fires (magnesium, aluminum, thorium, uranium, plutonium, and a few others that presently elude me) don’t really burn. Yes, there is an oxidation reaction; and yes it emits energy in the form of heat and light. However, its not really a fire in the traditional sense.
A “real” fire (as opposed to those wannabe fake metal fires) involves pyrolysis, the release of combustible gasses from the fuel. When the gasses burn, they start forming free radicals (compounds with a serious urge to do some reacting) left and right. These free radicals form other compounds, which form other compounds, etc, etc. The final result is the “normal” combustion byproducts: water, CO2, CO, H2S, Acrolein, HCN, etc. A chemist would be able to tell you a lot more about it than I can. These reactions, which take place just below where the flame starts (but above the fuel), and the products they toss off when they’re done, are what is traditionally called a fire.
Metal fires are an oxidation reaction taking place on the surface of the metal itself. There is no free radical formation, just a straight jump from Mg to MgO. That jump, though, tosses off a huge amount of energy (heat and light) based on the size of the “burning” material.
Also, the extinguishment of metal fires is a tough one. They don’t like to have water applied to them (the energy of the fire splits the H2 from the O and makes the fire larger), and dry chemical/Purple K just makes them angry. There are special powders to apply to them (such as Metl-X) to put them out that interfere with the oxidation reaction. Most fire departments just let the stuff burn itself out and keep the exposures cool.
So yes, you have seen Mg burn. But it wasn’t a real fire in the true sense of the term like wood or diesel fuel.
Jeremy…
Self Declared SDMB Resident Fire Service Expert
KCB615 said
I think chemists would seriously disagree with this sweeping statement. Reactions, even bimolecular reactions, at metal surfaces are very complicated and I don’t think radicals can be completely dismissed as playing a part.