[Stoves] Thickness of flame front

[frank]

AJH wrote:

><snip>
>Tom Reed talked about patenting a glass container with a molecular
>thickness gold coating for his experiments.
>  
>
To study the speed of gases going through the small fuel (no flame) I 
used a Drierite tube ( http://www.drierite.com/ ) like the versical tube 
in the lower right. Control the gas flow of air and inject CO2 into the 
stream and determine the time it takes to get out the top using IR 
detection. If I was to design a system to study fuels it would be a 
larger diameter version of this using glass that would take the heat. 
The metal screw on top could have a hole drilled into it for a burner.  

>> <snip>
>>
>>>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).
>>    
>>
I think it is particle density (lets say Carbon density in the fuel 
particle) is the first thing to look at. But once it has reached a 
critical point we then change to look at percent void space (lets say 
carbon packing) and make sure that is enough voids to carry the air (O2) 
needed and there is sufficient carbon density in the packing to carry on 
low combustion for pyrolysis -then look at surface area if we can 
develop a procedure.


>
>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.
>
>
>  
>
><snip>
>  
>
>>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).
>  
>
Yes - conduction is what I meant. In dry wood with lots of air (very low 
density) I wonder if the carbon is not dense enough (close together) to 
produce the heat during oxidation to conduct enough heat to the distant 
one next to it? The two carbon molecules must have a required minimal 
distance to conduct enough heat to the other and keep combustion going 
without the help of radiant heat from the other fuel particles in the 
packing. I wonder what that distance is? OR As they (lignin) form chains 
of carbon all a fixed distance, and it still does not burn, it must mean 
for each carbon to oxidize it must require more than one other to 
provide the heat needed. Perhaps five carbons close by to provide the 
conduction heat required to oxidize every one carbon. If there is only 
four close by the combustion stops. Something like that?


Frank


-- 
Frank Shields
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