forgive my lack of caps and poper quoting …second post
ok AS in fire technology here let see if i can clear up a few misconceptions.
KCB615 you are on the right track. The big thing that everyone i think is missing except for joe cool is that only a gas will actually burn. You kill a fire by cooling the fuel to the point that its vapor pressure is exceeded by atmospheric pressure. Gasoline will not ignite if it is so cold that no evaporation is taking place. Once ignition is achieved then the heat of the fire will warm the fuel source, increasing the vapor pressure and further accellerating combustion. Substances like kerosene have very low vapor pressure and only burn well when they have alot of surface area (saturating a wick or in a mist like in jet engines) You can literally put out matches in a bucket of kerosene…just dont try this at home please.
The main reason why a smurf-sized campfire wouldn’t last very long is the surface area/volume ratio of the fuel.
Contrary to popular belief, in a fire, solid objects are not directly reacting with oxygen. Instead, the fuel heats up and vaporizes or decomposes into organic gases. These gases are what actually react with oxygen. Again, WOOD does NOT react with oxygen! Heating wood causes it to decompose into various gases which DO react with oxygen.
So, you have now a smurf-sized fire made out of small sticks. This fire might be, for example, 1/10 the height of a human-sized campfire. The volume of fuel will be 1/1000 of a human-sized campfire, but the surface area of the fuel will be 1/100 of the human-sized campfire.
Assuming that this minifire is kept at the same temperature as the big fire, it will release flammable gases at 1/100 of the rate of the large fire, because of the difference in surface area. However, the fuel is only 1/1000 of the full-sized fire’s fuel, so it will all be converted to flammable gas in 1/10 the time of the large fire.
So if the big one lasts a couple hours, the 1/10 scale will last about 12 minutes. And so on…
Damn they have a BS in fire science??
<sigh> bow humbly
[sarcasm] guess i need to call one of those “instant diploma” sites so I can have one too [/sarcasm]
Sure do. University of New Haven’s where I got mine. They also have a master’s program, but I didn’t want my masters in fire science (FPE gets you a bit more money). From what I’ve heard (but not verified), University of Cincinatti has a Doctorate of fire science program. God help the poor soul who’s a “fire doctor.”
I think some of you may be confusing fire, burning, and oxidation. Most CHEM101 books have an entire chapter devoted to oxidation reactions. Entire volumes have been written on the subject. And don’t get me started on organic chemistry and oxidation!
As stated, oxygen doesn’t ‘burn’. But it does oxidize to O3, ozone. Likewise, iron and steel do not burn but oxidize, as any owner of an MG or Triumph can tell you. Things that normally smolder in ‘low heat’ [coal, rags, some plastics, that gal from “Traffic” with the great bazoomies] burst into flame in oxygen-enriched amospheres.
Cooling liquid fuels will impede their ability to ignite to flame because of vapor pressure considerations [see earlier posts]. Your car is hard to start in winter because of too lean a fuel-air mixture [not enough fuel, too much air] caused by the increased dnsity of cold air. That’s why there is a ‘choke’, to lessen the amount of air fed into the combustion chamber [I’m simplifying-- you ever take apart a carb? Wow!]. When the engine warms, the choke is released because there is a huge heat blanket warming the air in the engine. In colder climes [Siberia, Point Barrow], fuels gel and cannot be pumped to the engine at all.
Ultra-cold ‘burning’ is possible, unless the idiots at Fox are correct and the Moon landing was a hoax; see, the rockets in space use fire to fly [simplifying again].
On the miniaturization front; it would make it easier for all concerned if time feels different at smaller sizes. Then a 20-second match burn would feel like an hour to a teeny-tiny life form. They would live their lives out in a week and not need more than a box of kitchen matches. A bag of charcoal could last a civilization long enough to develop writing, music, art, and George Bush [pick either one of them].
None of this explains why my bong goes out when it gets back to me, though. Now where’s the damned lighter…
I have an experiment related to the tablespoon of fire dipped in cold water post. (Forgive me if I am behind here, I just joined today) This cannot be done in your kitchen, but can be done in your back yard.
Start a small campfire. Let it burn until you have a nice even fire going. Take three paper cups. Dixie cups will work, but the larger kind that yo udrink out of will work better. These are the kind made out of waxed paper, sold in every grocery store. The first cup is dry. It is your control. The second is wet, inside and out. The third cup is 1/2 to 2/3 full of water. Set these three cups in the fire, on the coals and watch. The first cup will ignite and burn quickly. The second will sizzle briefly, then ignite and burn. The third will burn down to the water level and stay intact allowing you to actually boil the water in the paper cup. Fuel isn’t a factor, the wax and paper are readily available to the fire. Oxegen is also readily available on the outside of the cup. The water, even boiling, is below the temperature neccessary for the waxed paper to burn so it stays intact. If you are very good with tongs, you can pull the cup out of the fire and it will be intact below the water line.
In the case of very small fires, the main problem is that the flame front becomes erratic to the point where a simple non-controlled flame can’t sustain itself, even under ideal conditions. That is, one molecule burning expands, pushing all the surrounding non-combusted molecules apart, inhibiting further combustion. To continue, you have to use a flame holder to introduce turbulence that will concentrate the combustion. If you present a small tube and expel flammable non-oxygenated gas out of it, ignite it, and then decrease the fuel flow, the gas starts out as a regular-shaped flame, decreases to a ball, and then before it blinks out, the ball breaks into a ring-shaped flame that burns around itself, concentric with the tube’s axis/end. The sizes of all this depend on the size of the tube and amount of fuel. The ring-flame is very small and very unstable, invisible to the eye and doesn’t occur under atmospheric conditions. It doesn’t occur at all in any conditions using larger tubes, due to its instability. A demonstration of this instability is the way that you can turn a gas stove burner down until it’s almost off, and the flame wil race around the burner in circles. - MC