[Stoves] attempt at swirling secondary air

Tue May 6 11:14:56 CDT 2014

David,

I did some experiments with swirl or vortex in a natural draft
TLUD stove. My experience was that a small amount helps mix the gasses some, 
but
when I added too much swirl the smoke level increased. I assumed the fast 
swirling gasses
acted as a centrifuge separating the cooler heavier air from the hotter
lighter weight bio-gasses, the heavier air migrating to the outer edge and
the lighter bio-gasses migrating to the center, thus reducing mixing. A
small amount of swirl did seem to avoid this problem and to help some,
producing less smoke and soot. Another problem I ran into with too much
swirl is that the swirling secondary air can descend into the stove along
the reactor wall. Buoyancy would be the driving mechanism, cooler heavier
air descending and displacing hotter lighter flame gasses which the air
buoys upward. Fast swirling air seemed to be especially affected by this. 
When the
air reaches the fuel it can cause an out of control fire that does not
respond to closing the primary air off. Both of these problems might
possibly be helped by bringing the temperatures of the air and gas closer
together, and thus reducing buoyancy forces. One good thing about the
descending air is that when it is controlled, it gives a mechanism for
enabling substantial turn-down in a TLUD stove by keeping the low power
flame near the char. The supporting hot char keeps the small flame hot and 
alive
when the primary air is reduced.
I never tried the strakes idea to make swirl, but you are right that
it looks work intensive. I used vertical slits made with a knife which I
twisted with a screwdriver for the secondary air inlets. I tried different
angles and combinations, but directing the air horizontally and tangentially
worked best. Once I added to this a set of fan like blades over the fuel and 
below
the secondary to add spin to the rising wood gas. The combination of the
two spins made the flame spin so fast that when it emerged from the stove
the flame was shaped like a round bee hive. This was a smoky, smelly flame,
which was interesting but probably useless.
Your stove is quite different from mine so I expect it will respond
differently. I'm looking forward to learning more from your work because
this is an area of interest for me too.

Kirk H
Santa Rosa, CA. USA

----- Original Message ----- 
From: "David Young" <dyoung at pobox.com>
To: <stoves at lists.bioenergylists.org>
Sent: Monday, April 28, 2014 7:39 PM
Subject: [Stoves] attempt at swirling secondary air

> 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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