[Stoves] Natural draft TLUD turn-down

[Crispin Pemberton-Pigott]

Dear KG

 

That is a very nice exposition on the turning down of TLUD's and confirms
many observations regarding the source of the problem of keeping the flame
going (in many home-build devices).

 

In particular I appreciate you highlighting the combustion of the top char
as a way of overcoming that issue. It should I feel be kept in mind whether
the device is supposed to cook or make charcoal. Cooking stoves have a
pretty large turn-down potential. There are not many burn cycles that are
mostly at high power. If burning some of the char on top is required to get
an acceptable turn-down ratio, so be it. I suggest a target of 4:1 for a
practical cooking stove.

 

Having tried a number of devices I concluded that the very simple TLUD's
have a turn down ration of about 1.4:1. This is not adequate to meet most
cooking requirements. It is a good way 'to produce heat' and there are many
applications of reliable heat, but cooking is a lot more demanding.

 

The use of staged secondary air (which we called tertiary combustion from
time to time on this list) is a good solution. I also appreciate the use of
buoyancy to affect the amount of secondary air entering. 

 

I can suggest one additional effect to try which is to put a shell around
the outside that is closed to the body at the bottom and open at the top.
This is a down-drafting secondary air preheater. The height of the upper lip
strongly affects the performance so you need to find where to put it. The
effect of this is to create a negative draft on the secondary air supply
that balances the heat inside in such a way that you get the exact airflow
level you want at different power levels. It can take a couple of minutes to
stabilise when you make a change in the heating power, but it will settle
when the metal changes temperature. 

 

In general, this is a good way to vary the power (which is fairly easy) and
maintain the right amount of air into the upper chamber (which is a lot
harder) and get a very clean burn (which is harder still, at different power
levels).

 

John Davies achieved the combustion of a portion of the top material by
having the secondary air draft into the chamber from a conical upper lid.
The stoves Nurhuda is making in Indonesia have a similar shape. I have
preferred secondary air to enter just at or above the fuel and to keep all
air out of the upper chamber because there is no way to get the flames to
finish before they hit the pot (unless the stove is made taller).  These
options may or may not be better than the way you have drafted it vertically
down to the periphery to draw it across the top of the fuel deck. 

 

Very interesting.

 

Many thanks

Crispin

 

 

All,

 

Dr. Ron Larson, Dr. Paul Anderson and I have been working on the problem of
lack of turn-down in natural draft TLUD cook stoves.  We have found
considerable success.  Attached is a document introducing the results of our
labors.  All information is in the public domain.  

 

K Harris

Santa Rosa, CA. USA

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