[Stoves] Thickness of flame front

[AJH]

On Fri, 11 Jan 2008 12:25:21 -0800, frank wrote:

>If the combustion zone height is constant (say 4 cm)

We'd need to devise some tests but if we can ignore heat losses from
the sides of the tlud stove then the thickness of the pyrolysis front
is only dependant on the time it takes to pyrolyse a particle to the
hottest temperature, which is the temperature the offgas leaves at.


> the 2.5 cm wood 
>would be zero at the top of the zone and 2.5 at the bottom as the zone 
>moved downwards. A triangle.  That would mean half is already burned in 
>that area at one time (I think) and the rate it moves down depends on 
>the 1) oxygen it gets 

I wouldn't like to say what the shape of the pyrolised part of the
particle would be, just that it would be thicker at the hottest spots.
Yes I agree oxygen is the key because this determines how much heat is
released at the pyrolysis front.


>2) cooling  effect  from air moving the heat up 
>and away (conductivity)  based on narrowness of the void space and

Well this depends on turbulence and I don't think there is much
turbulence in the primary air because its flow is quite small, so
while it cools the particles below the front it immediately gets hot
and expands as the oxygen dissociates and reacts with char and carries
heat upwards and away from the reaction zone. The main means that the
temperature is moved down must be radiation and conduction. Both these
are determined by the temperature at the pyrolysis front and we know
it is slow if primary air is minimised. So I think this could be
independent of particle size but dependant on particle surface area.
Thus the effect of a large particle will be dependant on what happens
on its surface and the rate at which its interior is heated to
pyrolysis temperature during which it is a heat absorber. This surface
heat transfer to internal heat transfer is related to the Biot number
of the particle I think. 

Plainly water content is significant here and we know that the
downward movement of the pyrolysis front can be halted by wood over a
certain moisture content, this points to the fact that the heat
feedback downward is weak.


> 3)  
>how fast the surface can get up to temperature  with heat being drawn 
>into the particles (size of particle).  At least that is the way I am 
>looking at it today.

Yes as above.

AJH