[Stoves] attempt at swirling secondary air

Tue Apr 29 13:32:52 CDT 2014

David,  cc List

	Thanks for a very interesting post.

	The subject of swirl is one we should be thinking a lot about.  I believe you didn’t observe much because you had a larger radius for the very hot gases than for the less hot, pre-combustion pyrolysis gases.  The fix I think is to have much wider strakes - maybe 3 or 4 inches, rather than what looks like about 1 inch.  Given the circular geometry, there is less cutting and not much more if any more material.

	But I also think it would be interesting to strive for a tighter swirl,  to get more twists for the hot gases before exiting.  The outer radius of the strakes can be cut down a little (in a second test, still keeping the new wider strake - of that works).

	Ron

On Apr 28, 2014, at 8:39 PM, David Young <dyoung at pobox.com> wrote:

> 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
> <2ndary-strake01.jpg><2ndary-strake02.jpg><2ndary-strake03.jpg><2ndary-strake04.jpg><2ndary-strake05.jpg><2ndary-strake06.jpg><2ndary-strake07.jpg><2ndary-strake08.jpg><2ndary-strake09.jpg><2ndary-strake10.jpg>_______________________________________________
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