Does fire cause things to catch fire because it's an open flame or because it's hot?

[The_Flying_Dutchman]

@scudsucker

The really hot bit is at the bottom, often invisible due to the visible flames surrounding it. The flame (while still hot) is substantially cooler as it is actively losing energy in both heat and light forms.

I always thought the hottest part was the tip of the flame. Given that no one else has disputed the quoted claim, should I assume I was wrong ?

[SigMan]

The hottest part of the flame is the blue area. This is where the most oxygen is.

[Isilder]

@filmore

Certainly things will burn if they get hot even if there is no flame. But does a flame burn things easier than a hot object at that same temperature?

For example, a match will cause paper to have a flame almost immediately. But I can put a piece of paper on hot coals and the paper will smoulder and smoke but not necessarily burst into flame. Is it that the match is hotter than the coals? Or is it that the open flame of the match can cause the paper to catch fire easier than it can on hot coals?

The “flame” itself is just hot gas. The reason you see it is that the gas is hot enough to glow. Its the glowing (eg infrared) that hits the fuel, makes it hot, which then boils the fuel (wood, paper, breaks down into gas and Carbon deposit.
Look now at the coals ? How much flame do they emit ? The carbon can sit there glowing as a solid until O2 hits the hot carbon atom and it can then become CO2… the coals burn slowly… The requirement is that the burning keeps the coals hot enough that the C will react with O2… With one single flake of coals, the heat emitted by the CO2 is in all directions and only some of hits the same piece… So on its own, it doesn’t keep burning and cools down. Its the space in between the coals where the heat from the CO2 will most likely make the coals hot so that is where the burning occurs… from the inside out…

[pudytat72]

I have heard that if there is a fire approaching your house, to take the drapes and window coverings down. They get hot from the heat of the flames and then they start to burn and set the inside of the house on fire where it might not catch fire if there were no flamable items near the windows. So that theory indicates that the heat causes the other item to burn.

[Peter_Morris]

@filmore

If I had an open flame and piece of metal of the same temperature, would they catch things on fire just as easily?

Lets say you want to light a candle wick with a match, versus a piece of hot metal.

A flame transfers heat to the wick by convection. The piece of metal transfers heat by conduction. Does this make a difference, with convection being a more efficient form of heating?

A flame can surround the wick, heating it from all sides. The piece of metal only transfers heat at the point of contact. Does this make a difference?

[DSeid]

Contact with a piece of metal dramatically reduces the amount of oxygen available at that point of maximal heat: the area in contact has little oxygen and thus cannot rapidly oxidize; the area with more exposure to oxygen is the area next to the point of contact being heating to a lesser degree by conduction through the material.

Convection of hot gases of the same temperature is carrying a constant supply of oxygen with the hot gas creating a larger area of maximal temperature and a continued inflow of oxygen.

More surface area being maximally heated with significantly more continued influx of oxygen with the flame compared to contact with the metal of the same temperature … the flame wins.

[DSeid]

Suddenly rememberedthe Alan Alda flame explanation challenge! Explain fire to an eleven year old.

It really is pretty dang good.

[DSeid]

Just to recap as the video for 11 year olds makes clear.

Heat makes the fuel turn into gas and the gas to move around with lots of energy. That’s pyrolysis.

The atoms hitting each other hard causes blue light to be emitted. That’s chemiluminescence. I do believe that’s the hydrogen electron having moved up to shell = 2 back down to shell = 1 and emitting a photon as a result. The blue is chemiluminescence from the hydrogen.

The new heat inside the blue area is coming from the energy released as the hydrogen and oxygen and carbon rearrange to form CO2 and water. That’s oxidation. That release of more heat creates new pyrolysis. It’s why the area that is blue is the hottest part of the flame. Note of course that oxygen is not the only thing that can oxidize and carbon based fuel not the only thing that can reduce: water can oxidize certain metals for example, releasing huge amounts of heat, and magnesium can have a redox reaction with nitrogen.

Not enough oxygen results in incomplete oxidation of the fuel, leftover carbon that forms bits of soot, which is nevertheless heated and glows - yellow or orange or red usually. That’s incandescence.

Heat spreads by three means: conduction, radiation, and convection. A solid piece of metal will spread heat by conduction and to a small degree by radiation. A flame is spreading heat extremely effectively by convection and also by radiation of heat from those floating particles of soot plus conduction of those particles hitting the new fuel.

Thus again flame then has much more heat hitting more fuel in the presence of more oxygen triggering both more pyrolysis and oxidation than contact with the hot metal.