[Stoves] Powering a TLUD Fan

[Crispin Pemberton-Pigott]

Dear Andrew

First I am impressed with your power calculation and second, I agree with
your conclusion.

>Axial fans are particularly good at thrashing air into heat with no useful
work as they stall, they are good at moving large volumes with little drop
in pressure across them, laptop fans seem to be better as they tend to be
centrifugal ones that push air along narrow ducts.

Where the flow resistance is worth checking out. As Paul Anderson has been
suggesting for some time, the flow resistance can be reduced by putting a
screen of sorts under the whole fuel bed because the flow resistance may be
where the air enters the space under the fuel.

Having been wrong several times about how much flow resistance there would
be offered be consolidation ash, I think it may not be the ash. It is
surprising how much air will move through ash.

I offer in support three things: downdraft coal ash, John Davies' coal
stove, and the structure of rice hull that may rather keep gaps open. The
downdraft stove known as BLDD6 and (5 actually) depends on air getting
through the coal ash even though it is stacked quite thickly in the
combustion chamber. I was surprised how much air can easily pass through
unhindered.

John Davies has quite a tall pile of small coal that is top lit, then after
reaching the bottom, the coke burns upwards with the ash underneath. All the
primary air has to pass through the ash to reach the coke and it works quite
well. In this case the unburned coke is sitting on top, compressing the ash
at that critical time.

The third is that of rice hull which as you point out, has a lot of silica
structure to it. This should prop open airways better than coal ash. For
these reasons I think the resistance is either where the air enters the fuel
store (later, the combustion chamber) or the exit which seem less likely.

In contra to the last point, the pressure requirement rises with time. If it
is an ait entry point issue, there should be a sudden rise in the need for
air pressure as the fuel over that point is pyrolysed. If the stove was
operated on a sale, the relatively sudden change in the mass loss rate
(which is the burn rate) would indicate the point during the burn that the
air is choked (just plot the mass on a chart and look for a change in
slope).

There should be a distinct deflection of that line when the burn goes from
pyrolysing to char burning. An excellent example of this is what is now the
Oorja Stove from India. The change in the mass burn rate is so clearly
visible that it is possible to assign a different heat content per g burned
for each phase to get a very accurate thermal efficiency number.

Regards
Crispin