[Stoves] attempt at swirling secondary air

Tue Apr 29 18:26:31 CDT 2014

Dear Dave

That is a nice effort and well-reported. Thanks for the small size pictures.

In general, I feel you will get better results and at less cost by using
incoming secondary air to achieve the effect. 

To get better penetration into the rising column of gas, make holes as
follows:

Drill the size you want 1.5mm undersize, maybe 2mm.

Make/find a metal rod the diameter of hole you want.

Sharpen it enough to go into the drilled hole.

Place the sheet metal over a hole drilled into a thick piece of steel - say,
10mm flat bar which can reach inside the stove. The hole should be 3 or 4 mm
bigger than the sharpened rod.

Punch the bar through the drilled hole so the rod enters the centre of the
hole in the thick bar.

Withdraw it.

Now you have a hole with a radiused edge and a small 'blurted' channel that
will direct the air.

If you really really really want to optimise this, the outlet hole should be
1/3 of the area of the inlet and the taper constant, and the length 6 times
the inlet diameter. This is hard to make. It has been known since the
1880's.  It applies to all fluids (air is a fluid). I used it in Malawian
tobacco barns and it tripled the effectiveness of the 'educting effect' of
the combustion gas energy in the chimney. (It means using the chimney to
pull air through the barn like a fan would. The reason is higher energy, the
effect of the efficient gain in velocity, remembering that momentum is
½mv2).

The blurted hole is far better at getting air into a combustion chamber with
velocity and direction. The tools are home made. You may have to reduce the
hole size, it is so much more effective than a drilled, sharp-edged hole.
You can ‘point’ the hole by tilting the rod as it is hammered through.

For those with a technical bent, look up the ‘elongation’ rating for the
material you are using – say, 0.20. Calculate the final diameter of the
outlet. Multiply the diameter by (1-elongation) i.e. Ø*0.8.  That is the
smallest hole you can start with to avoid splitting the end of the material.

For artisanal stoves, all secondary air entry holes should be shaped in this
manner. Full stop.

Regards

Crispin roaming

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

 <mailto:dyoung at pobox.com> dyoung at pobox.com    Urbana, IL    (217) 721-9981

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