[Stoves] New kind of Sawdust stove

[neiltm at uwclub.net]

Please ignore if this does not contribute anything useful:

On 25 Oct 2016 at 2:51, Crispin Pemberton-Pigott wrote:

> The 'draft' part of the terminology does not refer to the air flow it
> refers to the gas flow.

I can't believe I'm entering this definitional argument, but it seems to 
me that a very important principle is embodied in what until now most of 
us have probably thought of as the TLUD, and that is that efficient 
pyrolysis, releasing gas in a steady controlled way is optimally achieved 
by the fuel burning against the direction of primary air flow.  This 
principle was used by Paal Wendelbo's Norwegian resistance TLUD open fire 
which he went on to embody in his Peko Pe TLUD, by the three stone fire, 
origins lost in the mists of time in which the sticks placed radially 
burn down against the direction of the air feeding the fire, and of 
course the rocket stove which does the same. Candles, rush lights, wick 
oil lamps, also use this principle surely, albeit with a continuous 
liquid fuel feed. The BLDD also sounds as if it pyrolyses fuel in the 
same way that results in a more controlled release of gas.  From what 
I've learned of the history of the use of fire, it seems to me that at 
some point most regular users of fire must observe pyrolysis progressing 
against the air flow feeding it and realise this offers greater control 
and efficiency, and then consciously employ this principle is some way.

Clearly sawdust does not lend itself easily to passing primary air 
through it, and so cannot employ this principle.

When I make a smokey mess with my TLUD, losing the flame, I blast it from 
the top with a BBQ fan, but surely it isn't operating as a TLUD in that 
moment, rather a TLDD if not merely better described as turbulent?  The 
air comes at the fire downwards from the top, reversing the direction of 
the air temporarily, doubtless accelerating the downward direction of 
pyrolysis unevenly, as the flames are directed downwards into the fuel 
stack.   I get that in a tightly packed sawdust bed little or no air can 
pass through, yet what I've described as my TLUD rescue seems to be 
embodied in this excellent stove Crispin discovered, but I would 
speculate is merely turbulence.
I find myself wondering if it even makes sense to talk of the 'gasses' 
flowing upwards, despite the chimney opening being above the top of the 
sawdust bed.  Would not a fly on the wall of the combustion chamber 
(briefly, before getting cremated), not observe simple turbulence as a 
combustion zone?  Does it even make any sense to talk of primary and 
secondary air in such a stove, (assuming the two secondary air holes are 
irrelevant as Crispin speculates)?

Yes it is TL obviously, and yes the pyrolysis migrates downwards through 
the fuel stack, but I'm surprised by Crispin's assertion that it is only 
the direction of the gases, and not the direction of the primary air that 
defines the TLUD, when what makes the TLUD fire so relatively smokeless 
as open stick fires go, is sufficient air to successfully ignite a 
controlled slow release of gas. Radiation and conduction progress through 
the fuel against the primary air flow, which convection therefore works 
against. This principle, embodied in all the above types of fire or stove 
is not embodied in this sawdust stove, nor could it be or need it be when 
pyrolysis of an individual piece of sawdust can presumably be completed 
quickly and in a turbulent air flow which this design must surely 
produce, presumably without a shower of sparks emanating from the chimney 
top?  Only the impermeability of a compressed mass of sawdust permits, 
for different reasons, the downward migratory pyrolysis front, but it 
does so by the air hitting it downward from the top, not coming through 
from the bottom.  This stove would presumably not be able to maintain 
such a pyrolysis front with larger permeable fuel, the whole stack would 
tend to ignite?

Doesn't this combustion have more in common with the home made backyard 
waste oil burners utilised to fire kilns or smelt metals by amateur 
forgers, than what we normally think of as a TLUD?  eg. 
<http://www.backyardmetalcasting.com/oilburners.html>

Neil Taylor (still cooking, and learning, outside in England with 
uncovered rained on sticks in the Chinese TLUDND camping stoves).