[Stoves] Thickness of flame front

[AJH]

On Wed, 16 Jan 2008 00:00:23 +0200, Crispin Pemberton-Pigott wrote:

>Where the gases are first produced in a piece of wood, there is no flame.

I'll bite on that, it will depend on how the wood is being ignited but
in general if you heat a piece of wood up the surface must dry, then
pyrolyse then ignite.

>The gases emerge driven by heat from outside.  When it comes to the carbon
>starting to 'burn' there is a lot of radiant heat involved.

Yes because char is a good black body. In fact I see a super black
material has been constructed of carbon nanotubes which are black
holes in themselves. Point this material at the sun and it should get
very hot.
>
>Heat does not have to conduct 'backwards' into the fuel.

Yes it does because the centre has to be raised to ignition
temperature. It's how this happens that's interesting because there is
a difference between a dry log and a wet one, the dry one tends to
retain its shape and turn to char and the wet one shrinks as the outer
layers burn off.


>  I would not be
>surprised at all if half of it was radiated. 

The char burning at the surface will radiate a lot of heat but the
offgas will be carrying some away also, as I said, strike a match and
try and burn the char out, the charred stick is thin and has a high
surface to volume ratio, it radiates heat so quickly that the char
falls below its ignition temperature.
AJH