[Stoves] diy TLUD flame

Wed Feb 26 21:17:50 CST 2014

Dear Juanito

>Meanwhile, may I get the impression that this issue is not settled. What is
the critical ingredient: fan? chimney? concentrator lid? washers?

There is no magical answer - many things 'work'. What is perhaps a source of
distraction are claims that 'only this' or 'only that' works. I see that a
lot that works and a lot that doesn't. 

In order to build a decent stove it is important to first get the air flows
under control. By that I mean the primary and secondary and excess air. If
you put any more air into a fire than it really needs, there are negative
consequences. If you build a stove that already has far too much air passing
through the fire, then you will be really misled, or self-misleading, to
assume that anything that 'makes it work better' is really doing much if the
basics of the combustion are not right to begin with.  It is like putting
lipstick on a pig.

So let me say this conditionally: when the primary and secondary air flow
are right, it does not take much draft to get a fully mixed and clean
burning flame. A rule of thumb is that the diameter can be as large as the
draft vertically above the secondary air inlet. But this has to be qualified
further by saying that there is an upper limit which is  governed by the
physical properties of air and hot gases (which are very similar, actually).
It is hard to get a 300mm diameter gas path well mixed in a short vertical
distance.

You can use a concentrating ring to produce a smaller diameter gas flow into
which you can run secondary air. Unfortunately most advice on the internet
has the secondary air entering below that disc, so it is difficult to get
the secondary air flow into the centre of the gas column, even if there is a
relative small diameter hole in the centre. The result is a fire near the
edges and a long thin flame rising from the centre - a diffusion flame with
no O2 in it.

If the secondary air is admitted above the minimum diameter point, the
problem disappears, but nearly no one is advocating this. Jock is doing it
right, though perhaps not thinking of it in these terms but that is why his
layout works well.  Instead, I see people inserting a central air tube that
passes through the fuel and bleeds secondary air into the gas stream near
the top centre. This helps, but it is helping to solve a problem that should
not be there in the first place. It causes all sorts of other problems,
particularly relating to creating a controllable fire that can be turned
down, and accidentally creating a fire that progressively increases in
intensity with time. Many TLUDs suffer from these two problems but there are
others issues as well.

John Davies in South Africa worked with packed bed combustors (gasifiers)
which is a special case of TLUD where the airflow is controlled entirely by
the fuel. Chopped biomass is not suitable for such an approach. He worked
with chipped coal and it seals rather well providing true pyrolysation for 2
hours then char burning for 4 hrs. A lot of TLUD builders have complained
about material problems when burning char after pyrolysation is complete,
but they are doing this with far too much primary air. If they were going to
burn the char, and knew that in advance, they should have used a higher
superficial velocity throughout the whole burn.

It is complicated. The bottom line is that most things said are true part of
the time and most TLUD's suffer from one problem or another. One way to get
around this is to use a controllable fan as a substitute for auto-balancing
air. That is a good solution, but requires a fan and power.

One of the misleading claims for TLUD's is that they are automatically
'clean burning' because they are creating gases. This is simply not true. It
is quite possible that they can burn clean, but I have seen lots that only
do that under very particular circumstances. With a slightly different fuel
or fuel size, they burn with a lot of smoke and CO. That is why there is so
much discussion about fuel preparation. If the airflow through the fuel is a
major portion of the control of the fire, instead of the stove architecture,
it is fuel-dependent, not fuel independent. In consequence, the variability
of the firepower suffers because you would have to vary the fuel to get a
different firepower. Or blow air into it with a fan.

Thanks for your interest.

Regards

Crispin

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