[Stoves] mixing of gasses of different pressures

[Gordon West]

I should have asked this question earlier in the string - how are there gases at different pressures to be mixed in a system that is open to the atmosphere? My sense is that they could certainly be at different densities due to temperature differentials, but still all at atmospheric pressure.

On the subject of mixing gases, we have devised an assortment of swirl inducers and injection tubes and ports in our forced air batch stoves (small computer fans). The stoves are fairly tall with the smoke combustion zone >24” from the bottom of the pyrolysis cylinder, so they will work with natural draft but are much more controllable with the variable speed primary air fan. 

The secondary air supply (smoke combustion) does not seem to be very sensitive to fan control and probably could always work with natural draft only - we need to do some testing, though, as this is only a visual assessment of the completeness of smoke combustion. Bill had intended to go to the Aprovecho gathering last month for that purpose but events prevented it. 

A couple of years ago we were visited by a fluid dynamics scientist who was of the opinion that our swirl and injection strategies were not a very good approach. His explanation was that fluid mixing is best accomplished by turbulence and that turbulence was best caused by requiring the air flow to turn sharp corners. Our current design causes the secondary air to enter the combustion area after making a sharp turn over the top of the cylinder where it begins mixing with the smoke - and a second sharp turn around a central plate in the lower part of the burn chamber further turbulating the mixture of air and smoke. 

The principle of air causing turbulence by bending around a sharp corner is visibly demonstrated in a flame-cap pyrolyzer that is being operated correctly. You can see the flames curling back to the center of the kiln and rolling back under before rising at the edges again (this also creates the “flame-cap” that keeps oxygen away from the wood) - it is important to not over-fill the kiln with wood, or this dynamic will be over-ridded by the flame wanting to form a column going straight up and not achieving complete combustion. I have noticed that it is very common for inexperienced operators to fill the kiln up completely (sometimes higher than the top edges) and there is always visible smoke in the air.

Gordon West
The Trollworks
503 N. “E” Street
Silver City, NM 88061
575-537-3689






> On Mar 5, 2019, at 1:09 AM, Andrew Heggie <aj.heggie at gmail.com> wrote:
> 
> On Mon, 4 Mar 2019 at 22:42, Kirk H. <gkharris316 at comcast.net> wrote:
>> 
>> 
>> 
>> 
>> I’m headed back to Aprovecho this month to test some new concepts that emerged as a result of  the TLUD summit in early February.
> 
> 
> Good luck and please let us know how you get on.
> 
> I presume you will persevere with natural draught until you get "good enough".
> 
> I'd still like views on what "good enough" is.
> 
> Andrew
> 
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