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Name: Rahul B.
Status: other
Grade: other
Country: India
Date: Fall 2012



Question:
Why does the flame of a candle have particular shape, like an irregular elliptical?


Replies:
The shape of the candle flame is determined by the shape of the heat of the flame. The surrounding air/flame is cooler than the center of the candle flame. Since the center of the flame is hotter than the outer edges, this part of the flame is less dense and so rises at a faster rate than outer parts.

You raise an interesting question. What is the shape of the envelope, assuming there is no turbulence? I am sure that has been determined, but I do not have the work at my “finger tips”. A good observation on your part!!

Vince Calder


You are seeing the shape that results from hot, expanding gases moving away from a fuel source. They move away from the fuel source as they expand, and since the gases are hot, they tend to rise. That explains the "elliptical" you refer to... the irregular part is due to turbulence causes by the moving air currents (and perhaps also caused by irregularities in the fuel source/candle).

Hope this helps, Burr Zimmerman


I oversimplified the factors determining the shape of a flame. A candle is one extreme. The fuel is fed into the combustion region by diffusion of the “hot” wax. And the shape of the flame envelope is largely determined by the diffusion of paraffin fragments into the combustion region. The temperature profile is determined “color” of the flame envelope. But that is not the only combustion configuration. At the other extreme is a flame produced by an oxygen / acetylene “torch”. Here the color of the flame is blue, not the yellow of a candle flame, and the flow rate of fuel and oxygen is “forced”. It is no longer a diffusion of the candle wax. The dynamics of such a flame are very different than the diffusion of a candle.

Vince Calder


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