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It's possible to blow out a candle flame so how come once it's blown out, if the wick is still smouldering, blowing on it just makes it glow brighter instead of stopping the smouldering reaction.

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According to the Wikipedia article Smouldering, smouldering is a combustion reaction that occurs only at the surface unlike flaming combustion. I won't explain why a smouldering reaction can't be blown out in general because the answer would be too be too complicated. For that reason, I will assume a certain combustion reaction is smouldering and explain why it can't be blown out under that assumption. Suppose you have a diamond sphere whose entire surface is smouldering by reacting with air. According to https://chemistry.stackexchange.com/questions/67228/can-diamond-undergo-a-self-sustaining-combustion-reaction-all-the-way-to-carbon/67236#67236, diamond can undergo a self sustaining combustion reaction all the way to carbon dioxide. Unfortunately, that answer never said whether all the steps of the reaction occur fast enough for the reaction to be perceived as smouldering.

Suppose that reaction is a smouldering reaction, then the air right at the surface of the diomond has nitrogen gas and carbon dioxide gas in it but no oxygen gas as a result of the reaction. The further from the surface, the higher the concentration of oxygen gas in the air. It's only diffusion of the air that enables more oxygen to get to the surface to react with the diamond. There are 2 kinds of diffusion of heat, conductive diffusion and radiative diffusion. When ever the size of the sphere is within a certain range and the wind speed is more than 1 m/s and less than half the speed of sound in air, the Reynold number is large and the radiative diffusion of heat can be neglected and the normal turbulent properties of the airflow occur.

Under these conditions, the equilibrium temperature of the diamond sphere from smoldering is independent of its size and the wind speed. If it's a bit hotter, air will suck heat out faster than heat gets produced by smoldering. If it's a bit cooler, smoldering will produce heat faster than the air sucks heat out. At that temperature, the rate the air sucks heat out of the diamond sphere varies as the wind speed to the power of $\frac{1}{2}$ and as the size of the sphere to the power of $\frac{3}{2}$. At that temperature, the rate that the smouldering reaction produces heat also varies as the wind speed to the power of $\frac{1}{2}$ and as the size of the sphere to the power of $\frac{3}{2}$.

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