[Stoves] Thickness of flame front

[AJH]

On Mon, 14 Jan 2008 10:49:45 -0800, frank wrote:

>I think glass is the way to go to 
>study pyrolysis with the lower temperatures if we can find the right 
>container.

Tom Reed talked about patenting a glass container with a molecular
thickness gold coating for his experiments.

>>
>Is the flow small when the air is squeezed between the particles? We are 
>talking meaningless relative values here but the speed past the surface 
>in the pyralysis zone must be much faster than the flow exiting. I 
>visulize a lot of turbalence as it rushes between the particles.

I simply don't know, the gasification crowd talk of superficial
velocity to describe the apparent rate of flow through a fuel bed but
as you realise this is only meaningful if you know the packed density
of fuel verses air spaces.
>
>
>>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. 
>>
>>  
>>
>I think it is more particle density than surface area but surface area 
>is a big one. (and one I would like to find a way to measure).

We probably disagree on that then, mind density will greatly affect
the Biot number, I would expect the more dense particle to carry more
heat away from the surface to the interior.


> Tom sent 
>me a wood gas stove to do some studies with and I found something I 
>think interesting.  First it is known that some bark is very fire 
>resistant like bark on redwood trees and I have wondered why.

I'll bet is because the air spaces (I remember using these giant
redwoods as punch bags) reduce the thermal conductivity, each outer
fibre that burns radiates its heat outward as well as inward, thus
keeping surface temperatures down. Thus the rate of burning is
reduced. From my experience to survive damage the cambium must be kept
below about 70C, so if the fire passes quickly and the rate of burning
is reduced by the air insulation in the fibres the tree can survive.

Bear in mind timber has quite a good fire rating when used as a
column. probable because there is no mutual radiation to give positive
feedback. light a match and see if you can burn out the char.


>>
>Radiant heat is absorbed by the water filling the pour space in the wood 
>and slows the downward process to a point the carbon above is depleted 
>before new carbon becomes available.

I'd suggest that conduction is likely to be a bigger player here and
the mechanism is simpler, all the time free water is near the
pyrolysis zone it will hold the temperature at 100C, in doing so it
will absorb energy, thus in fairly short order it will depress the
pyrolysis temperature below the auto ignition point of char (probably
around 300C).

AJH