[Stoves] Thickness of flame front

Mon Jan 14 12:49:45 CST 2008

Dear Andrew;

Please see below:

AJH wrote:

>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 glass burner I made from the light bulb 'shell' let you see the 
flame crosection. Problem is it is not wide enough for the fuels I would 
like to test. I am still looking. When I place the sticks verticle in 
the burner the flame front is several inches as the flam goes up between 
the wood (BLUD) as one would expect. In the TLUD using small fuel hard 
wood the zone was ~0.5" but not even (again I think because the burner 
diameter was too small).   The sides of the glass could be touched an 
inch above and below the pyrolyse zone so glass does not conduct the 
heat very well compared to metal (?). I think glass is the way to go to 
study pyrolysis with the lower temperatures if we can find the right 
container.

>  
>
>>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.
>
>
>  
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>>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, 
>
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.

>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). 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 tried 
burning some coco cubes that were very fiborus and light weight with a 
particle density of 0.28 g/cc with carbon at 59.7% dw.   Lots of surface 
area because it is fiborus but would only glow and then go out. Should 
have been lots of O2 'in' the particle too. So I think it is due to heat 
transfer to the inside of the particle was slowed due to all the 
additional air space. I have yet to test Redwood bark

>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.
>
>
>  
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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. If the pyralysis zone heigth is 
increased so there is reserve radiant heat to carry over the slow 
movement downwards would allow us to use wetter wood. Finding a way to 
structure the wood shape in the fuel container to control the heigth of 
the pyrolysis zone the answer. (?) More verticle placement? Something 
like that?

>>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
>
>
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>
>  
>
Thanks
Frank

-- 
Frank Shields
Soil Control Lab
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Watsonville, CA  95076
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