[Stoves] tuning a new stove

[David Young]

As I mentioned before, I'm interested in experimenting with coffee
grounds as ND-TLUD fuel, and I've been reading the conversation between
Jock and Crispin with great interest.  I've decided to set aside
my previous TLUD, which was a very primitive MIDGE-like stove, and
create a new one that has separate primary/secondary air supplies,
some preheating of secondary air, and (I hope) some opportunities
for improvement.  My goal with the new stove is to achieve cleaner
combustion and to take control of the rate of combustion.

After my old stove burned for a few minutes, red-orange flames would
start to lick the top of the chimney.  If I put a pot of water on the
stove, soot would cover its bottom.  If my understanding is correct,
this means that the supply of wood gas from the primary combustion
exceeds the air supply in the secondary combustion, or the secondary
combustion is too cold, or both.

My new stove is made from a stack of three 18-ounce steel beverage
cans[1], bottom, middle, and top, and a 15-ounce vegetable can.

Bottom can:  I cut up this can into there cylinders and made a primary
air control from it.  I took the top off the top cylinder with a can
opener, and then I cut a notch out of the bottom of the cylinder: this
is the primary air aperture.  The middle cylinder, I slit along its
height.  Then I folded out a tab for my fingers to grip.  I nestled the
middle cylinder inside of the top cylinder.  That gave me an adjustable
air aperture.  The bottom cylinder, I saved for a future project.

Middle can: holds the fuel.  I have drilled 9 1/8-inch holes into the
bottom of it to provide primary air.  I took the top off of the middle
can with a can opener.

Top can: serves as concentrating ring, secondary air supply, and
chimney.  I have drilled 32 1/8-inch holes around the diameter of the top
can, close to the bottom, and I cut a circular hole out of its bottom about
1 1/4-inch in diameter.

In order to retain some heat in the chimney and pre-heat secondary air,
I have cut a large hole into the bottom of a 15-ounce vegetable can and
press-fit the chimney into it.

I did not arrive at this arrangement all in one day, btw.  I started
with two 18-ounce beverage cans (fuel can, chimney can), experimented
some, and built up from there.

I have attached some pictures of the parts and of the entire assembly.

BTW, for consistency's sake, while I develop the stove, I'm burning
wood pellets exclusively.  In order to shorten burn times, sometimes I
fill my fuel canister most of the way with chunky char from previous
tests, and then I fill to the top with wood pellets.

In a test that I ran last night, the stove settled down a few minutes
after starting into a blue flame with yellow tips.  There was no
visible smoke at that stage.  Gradually the flame grew longer, and
the top turned orange.  Sometimes a little flame would reach over
the top of the secondary combustion chamber.  At that point, I tried
to turn down the primary air in order to keep the combustion in the
chamber.  I'm not sure if that upset the stove's equilibrium or if it
was a coincidence, but the flame kept going out.  Sometimes it would
spontaneously re-light with a pop, establishing a feeble blue flame
that way.  The re-established flame would go out again fairly soon.
Sometimes the stove did not re-light in a reasonable amount of time
(there is only so much smoke I will tolerate!), and I had to restart the
stove with a match.  Does this behavior sound familiar to anyone?

Ordinarily, I would assume that wind was blowing out the stove, but the
air outdoors was fairly still last night.  On the same table as the
stove, a candle burned for the entire time that I experimented with the
stove.

I have been thinking about the variables that I can adjust to improve
the stove's performance, and there are a lot: I can add more secondary
air holes (or try to plug some up), remove the secondary-air preheater,
shorten the preheater, create a gap between the primary chamber and the
secondary with a wire, extend the chimney by a few inches above the
preheater, etc.  Any advice what to try, first?

Dave

-- 
David Young
dyoung at pobox.com    Urbana, IL    (217) 721-9981
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