[Erasmus00]

I've been thinking about this for a few minutes, and an answer isn't coming to me. Why does fire have a well-defined outline? Any ideas?
-Will

[Tormod]

The first question that popped into my mind when I read your question was, "does it really"?

[Erasmus00]

Originally Posted by Tormod

The first question that popped into my mind when I read your question was, "does it really"?

well, I've been lighting matches and staring at them for awhile, and its not a gradual fall off, but a fairly sharp outline. It flickers a bit, but its sharp.
-Will

[infamous]

Originally Posted by Tormod

The first question that popped into my mind when I read your question was, "does it really"?

If you abserve the shape of the flame given off by a candle, one could reckon a rather distinct shape. This is however only one example, if one chooses to find a recognizable shape for say, a house fire, this becomes less defined. My own opinion is that; the shape is determined by the area of super heated gas which has risen to the point of combustion. Outside this boundry, one may still find combustable gases but tempertures fall below the required flash point of said gas.

[infamous]

Also, maybe as the heat of the fire rises, a vortex is created, kind of like a minature tornado which gives the appearance of a defined surface. Similiar to how we can recognize the shape of the tornado.

[Erasmus00]

Originally Posted by infamous

If you abserve the shape of the flame given off by a candle, one could reckon a rather distinct shape. This is however only one example, if one chooses to find a recognizable shape for say, a house fire, this becomes less defined. My own opinion is that; the shape is determined by the area of super heated gas which has risen to the point of combustion. Outside this boundry, one may still find combustable gases but tempertures fall below the required flash point of said gas.

A quick google image search for house fire reveals that even in the case of house fire, you have a well defined region. It would seem to me that the temperature would drop off gradually away from the center of the reaction. This gradual temperature drop off, I would think, would result in a gradual radiation drop off.
-Will

[C1ay]

The first thing that came to my mind when I saw this was that someone was yelling 'Fire' in a crowded forum

[CraigD]

Originally Posted by Erasmus00

Why does fire have a well-defined outline?

The glowing region of a flame does appear well-defined, particularly a small flame, such as that of a small brazing torch or a butane lighter adjusted to its lowest working setting.

Flame is plasma, atoms where the electrons have become disconnected from their nuclei. The glow is photons emitted by electrons as they become reconnected to nuclei in the plasma. Because electrons in plasma readily lose energy due to interaction with surrounding air (this is, of course, the source of the energy for convective heating), the plasma containing region is confined to an elongated spheroid region close to the combustion.

In a near vacuum, plasma loses far less energy to surrounding molecules, diffusing further and into more irregular shapes. Large fires involve stronger convective air currents, whipping the plasma envelopes into wilder shapes – in a large fire, such as a bonfire, small envelopes of plasma can be observed detaching from the main envelope and existing independently for very brief durations.

[Erasmus00]

Originally Posted by CraigD

Flame is plasma

Ahh, there is my error. I had been thinking of the glow of the fire as thermal radiation.
-Will

[Bo]

a flame has a well defined shape in space, for the same reason for example a water droplet as a well defined shape: the flame is kept together by the intermolocular forces the shape of the flame is the shap that minimalises the intermolocular potential.

I'm not completely sure if a flame realy should be seen as a plasma. An ionized cloud seems more likely to me.

Bo