[Stoves] burning wood gas

[Harris, Kirk]

Gordon,

I like that story about the fluid mechanics scientist.  There are indeed 
lots of corners in this stove.  He was probably referring to turbulence 
created by rounding the corner.  I have not thought of it in terms of 
going around corners for mixing, but if I did, the technique in this 
stove is to have the secondary air go around an interior corner, which 
will increase its pressure, and have the wood gas go around an exterior 
corner, which will decrease its pressure.  Having them do this right 
next to each other maximizes the pressure difference that injects the 
secondary air into the wood gas.  The two corners work together, like 
the two sails on a sloop.  Going around a corner is two dimensional, you 
are going one direction, and then you are going a second direction.  
TLUD stoves are three dimensional.  In the case of this stove, the wood 
gas is moving up, and than it is spread in all directions by the bluff body.

The shape of the flame-lets are like the petals of a tulip, and appear 
to be laminar, at least for two or so inches after being formed by the 
bluff body.  For a natural draft primary air supply most of the flame is 
here, so there must be mixing here.  Forced primary air makes big flame 
petals.  More experimenting is needed to determine the dynamics of these 
larger flames.

The system doesn't seem to have need for electronic controls.  I can't 
be certain until I can get to Aprovecho to test it with their 
equipment.  I will keep an eye on it to note any signs of improper 
amounts of secondary air.  So far it looks good.

Mostly I use soft wood pellets for fuel, but have tried found wood.  
Some worked well, others not so good.

The test stove seems to be sensitive to wind.  I am trying various 
combinations to remedy this.  Suggestions are welcome.

Thank you for sharing your insights,

Kirk H.


On 7/20/2021 6:14 AM, Gordon West wrote:
> A number of years ago a retired fluid mechanics scientist/engineer 
> visited the shop and saw some of our attempts to mix air with smoke, 
> most of them involving swirling. He explained to me that swirling is 
> not very effective because it doesn’t do much to disturb laminar flow 
> - unless you put some obstructions in the flow of the swirl. He said 
> that the basic principle of mixing fluids is to make them go around 90 
> degree corners.
>
> I see in Kirk’s designs a lot of going around corners, which seems to 
> be where the best mixing effects are showing up. Something like 
> putting tabs in a swirl would probably mix well and extend the time of 
> mixing as opposed to a shorter path of the simple direct rising draft 
> of gases. Other important refinements relate to controlling the shape 
> of the resulting flame, for which the cones and such come into play.
>
> Optimizing the air/fuel mixture in an adjustable power scenario 
> without combustion gas sensors and feedback loops and powered 
> combustion air will probably be a guessing game without a good deal of 
> expensive gear and real data collection. Variability from feedstock 
> type and quality is a constant, so there can be no high degree of 
> precision in control - the question is, what degree of control is 
> necessary? My guess is that for simple stove technology the test is: 
> "if it looks good, it’s probably pretty good."
>
> Gordon
> /
> /
> /
> /
> /
> /
>
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>
>> On Jul 20, 2021, at 6:37 AM, ajheggie at gmail.com 
>> <mailto:ajheggie at gmail.com> wrote:
>>
>>
>>
>> On Mon, 19 Jul 2021 at 23:11, Daniel Pidgeon 
>> <daniel.pidgeon at hotmail.com <mailto:daniel.pidgeon at hotmail.com>> wrote:
>>
>>      Or was the swirl discarded as a lesser form of mixing than the
>>     Venturi effect?
>>
>>
>> I used to use a swirl/vortex in my forced draught devices and see no 
>> reason to believe it is the most efficient means of mixing, the 
>> mixing tends to occur at the shear face between two gases but the 
>> energy is still used in creating the vortex and very little is 
>> available from the buoyancy effect of the flue gases with no fan.
>>
>> I would have liked to pursue other ways to create movement of gases, 
>> like the steam aspirator but my poor engineering ability limited me.
>>
>> Andrew
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