[Stoves] Thickness of flame front

[Paul S. Anderson]

Andrew, Frank and all,

I am enjoying the discussion.  Good comments and questions.

Quoting AJH <list at sylva.icuklive.co.uk>:

> We'd need to devise some tests but if we can ignore heat losses from
> the sides of the tlud stove

It is reasonable to ignore those heat loses because :

a.  we are referring to the zone of the pyrolysis front, not to the 
entire sides
of the TLUD canister.  Heat loss from the sides that are above the pyrolysis
front are cooling the resultant pyrolysis gases.  So the side zone to be
assumed to be zero or very low heat loss is only about the same height as that
of the pyrolysis front.

b.  The radius of the TLUDs is significantly large in relation to the 
height of
the side zone of the pyrolysis front.  Even a 4 inch (10 cm) diameter TLUD
(Reed's, Anderson's , Belonio's, MIDGE) has a radius of 5 cm.  So the 
heat loss
to the sides is mainly from the closest 2 to 5 mm of char/fuel, having 
no impact
on the rest of the char/fuel toward the center.

c.  Heat conduction from the already hot (usually metal) sides that are above
the pyrolysis zone will proceed downward to and actually below the pyrolysis
zone.  It could be argued that the outermost fuel is actually 
pre-warmed by the
hot metal even before the pyrolyis zone moves downward into that area.

> so
> while [the up-coming primary air] 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.

All correct, but I have one question.  Is there also some use of the 
oxygen for
combustion of some species of the pyrolysis gases that are being 
released?  And
is this different at the bottom, middle, and top of the fuel/char materials in
the pyrolysis zone?  We are discussing a zone of about 25 mm (one 
inch). Certainly some of the oxygen moves upward into this zone.  So if 
in the upper
half of the pyrolysis zone there is found O2 and pyrolysis gases, will 
there be
combustion of some of those gases instead of the O2 combining with the char?

We know that by blowing in excess primary air, we can have some O2 arriving
above the pyrolysis zone into a already hot area with hot (but not glowing)
char plus pyrolysis gases passing upward with the air.  What reactions 
might be
expected in that situation, and does that relate to the previous paragraph?

If both CO and H2 plus hydrocarbons are present with the O2, what takes 
the O2?

And is that important or not??

> So I think this [the pyrolysis?] 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.

Paul


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