[Stoves] attempt at swirling secondary air

[David Young]

The other night I started the charcoal for our grill badly and we ended
up with a feeble charcoal fire for grilling chicken.  I have a BioLite
campstove and the portable grill for it, so I started the BioLite and
transferred our chicken to it.  We cooked the rest of our dinner on
American Sycamore twigs.

The BioLite campstove, as you know, forces air through the combustion
chamber using an electric blower.  If you feed the fuel carefully,
combustion occurs in a spiral of red-orange flame, and scarcely any
smoke is produced.

I'm not entirely sure how the spiral is produced, but it looks to me
like the forced air enters the stainless "jacket" around the combustion
chamber circumferentially, circles the chamber, and enters the chamber
through the air ports with some momentum parallel to the chamber sides.
So it is the momentum of the air, not the shape or arrangement of air
ports, that produces the spiral.

I was inspired by the spiral of flame in the BioLite campstove to try
to create a spiral of flame in my natural-draft TLUD that will help to
complete the fuel/air mixing and clean up the combustion.  To induce
the spiral, I cut some sheet-metal strakes from 26-gauge mild steel and
installed them between the inner & outer cans of my can stove.

I have attached some photos of my stove and strakes under construction.

My first experiment with the straked stove, using wood pellets as the
fuel, produced a central column of flame, blue at the bottom, yellow at
the top, that left soot on the stainless bowl of water that I topped the
chimney with.  I noticed a few qualitative differences from prior tests.
The diameter of the flame was greater than usual.  The stove seemed to
bring the water to a rolling boil much faster than usual.  The stove
also made a hissing sound, presumably because of increased turbulence.
I don't remember hearing that sound from this stove before.

I ran a couple of experiments with natural fuel (broken-up twigs) and
one with less pellets than the first.  Each of the tests produced less
flame than the first, and I had to restart each of the natural-fuel
burns at least once.  I think that I used too much wax paper to start
these tests, and the layer of char left by the paper blocked the draft.
There wasn't any hiss in any of these experiments.

I ran another experiment, tonight, using the same amount of fuel as the
other night (101 grams wood pellets, the top layer of which consisted
of pellets soaked in 91% isopropyl alcohol to aid starting), but with a
couple of changes to the stove.  I removed the steel wire loop from the
chimney.  I also removed the fan-shaped insert (shown in an attached
photo) from the bottom of the fuel chamber.  Conditions were also
different: windy, with a rainstorm starting during my test.  This was a
less vigorous burn than the first one.  I could not detect any hiss.

If the strakes induce any swirl, it is very subtle.  Perhaps more
strakes, or strakes at a different angle will produce a more powerful
effect.  (My strakes rise 5.8 cm in 9.8 cm, measured around the outer
can, diameter = 10.7 cm.)  The dramatic burn in the first experiment may
have been due to windless conditions.

Dave

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