[Stoves] Changing rate of pyrolysis front in TLUDs

[Crispin Pemberton-Pigott]

Dear Friends

The draft change is not very significant between the top and bottom of the 
fuel (refering to an increase in draft when the fuel burns down.

Remembering that this is calculated for burning all the gases,not creating 
them, Nigel's Chimney draft calculator gives (for these numbers):

Type of FuelWood
AIR FACTOR, actual air flow / ideal air flow
110%
Diameter of chimney
100.00mm
Time to burn fuel
3600seconds
Mass of wood used in time above
300g
Temp inside chimney in this case a gasification chamber (average of entry 
and exit temperatures)
400C
% carbon in the wood
51%
% water contained in wood
12%
Height of chimney
initially 40mm and later 200mm as the fuel burns down
Heat content of wood/biomass
19.300MJ/Kg

The draft changes from 0.312 to 1.56 Pascals.   OK, so it appears to go up 
by a factor of 4, but as you can see in the field, the difference in so 
little that taking into account the heating of the air under the fuel in the 
column when it is full of fuel and the loss of that pre-heating when cold 
air is entering next to the last portion of the fuel, it seems the change 
will not be as big as first appears to be.

The radiant heat from the pyrolosis zone surely heats the fuel chunks 
underneath and the temperature of the inside air is not 'ambient'.  Heat 
conducts down the metal skin on the inside assisting this further.  It 
should be pretty easy to get some air temperatures from inside the 
combustion chamber.  Paul can you do this?  I can re-do the calc.

Regards
Crispin