[Stoves] Cyclonic sorting/cooling

[AJH]

On Wed, 2 Aug 2006 20:24:45 -0700, Tom Miles wrote:

>The French company Pillard once recycled combustion (stack) gases with the
>combustion air to achieve the mass flow necessary to maintain the vortex
>while controlling excess air. It appeared to work but there must have been
>other complications because they never developed a commercial product using
>the concept.

I like the idea, it seems to me that the exhaust gas re circulation
would have to be added as an outer vortex and not combined with the
combustion air. This is not in the realms of cooking stoves. I was
only initially drawn to the vortex idea as a means of achieving good
mixing ( to aid clean burning of poorer fuels, I was trying to burn
horse dung and succeeded up to 50%mc)and flattening the flame, to
reduce overall height of the simple oven vented stoves we were
discussing at the time, I demonstrated an early prototype to Ronal
when he visited. This was before I knew about TEGs and we largely
discounted the concept because the need for an open vented stove and
the requirement for fan power were mutually incompatible. Also typical
industrial designs deal with the cyclonic effect by having tangential
air entry and tangential flue outlet, again not suitable for a stove
but it does break the vortex which remains persistent in my little
stoves. As I have mentioned before I suspect the persistence of the
vortex badly effects heat transfer to the pot. Plainly the vortex I
was developing was much stronger than in anything using natural
draught so the effects will be less is Lanny's device.

In the absence of proper evidence of excess air leaving the flue
without entering the combustion I tend to infer it from flue gas
temperature. Also the ODA cyclonic rice hull burner, which entrained
the hulls with the combustion air, reported a very high excess air
(IIRC300%) yet the rice hulls were very dry.

My guess, from looking downward at the flame is that the radial
stratification of the vortex has the effect of maintaining a low
temperature central pyrolysis zone in the fuel bed, offgas from this
spins and rises to mix with air spinning in from the outside.
Turbulent mixing occurs at the interface. The flame is then entrained
within the incoming air and outgoing flue gas but there is an annulus
of air just outside this flame that simply rises and mixes with the
flue gas, reducing temperature. This is unlike other premixed burners
in which the flame remains attached to part of the burner and where
all combustion products have to pass through the flame.

Maybe with some deflectors or choking down of the top of the
combustion area this escape of air could be countered whilst keeping
the axial outlet.

AJH