[Stoves] Controlling the Primary / Secondary Air Split in ND-TLUDs

[Crispin Pemberton-Pigott]

Dear Julien

There is a draft calculator on the Stoves website written by Nigel which is used for the simulation of what you describe below.

Your sketch is correct. The right hand side is the part I plan to make a pull-out metal sheet. If you change the fuel it will be exchanged for one of the appropriate relative dimension.

Regarding your question about the change is draft: because the central column temperature increases as the fuel burns down, it has an increasing ability to pull primary air in, if it enters the bottom. Ideally the primary air is being preheated by a downdrafting entry way, hopefully less that the full height of the chamber. As it gets hotter, it will oppose the pull up the centre more. That is how the primary air is preheated in a Vesto. No air is allowed to enter the bottom.

By controlling the heights and heat leaking through the walls, you can get the flow you want, restricted by the entry plate.

Meanwhile back at the secondary air entrance, that can be preheated by updrafting air because the draft above the fuel doesn't change unless the power changes. The Vesto does this by sending it along the outside of the combustion chamber, which is based on the Tsotso Stove from David Hancock. The Vesto has a Tsotso stove inside the centre, basically. The Tsotso's problem was that during turndown it smokes a lot from retained heat and the secondary was starved. The Vesto was an attempt to overcome that limitation.

David may be remembered as the inventor of the Rocket Stove. Following that he made the first Tsotso in 1984. It was because he gave me one in '87 that my interest turned to stove manufacturing in the 90's. He really should be credited with more of what he accomplished. He headed ProBEC for a number of years. In terms of genuinely improved stoves with modern design aspects, his Tsotso has been in continuous production longer than anything else save perhaps the American fan stove promoted to the Boy Scouts since the '70's - ZZ Stove I think, isn't it?

Regards
Crispin


Hi Crispin;

If I understand your splitter, it looks schematically something like this.


    ===== stove turntown control valve
      |
      |
      | _______________
      |                |
 ———>                     ———> primary air
 ———> variable         |
 ———> aperture            ———>
 ———> for incoming        ———> secondary air
 ———> air                 ———>
 ———>                  |
      | _______________|


Given that formulas for air flow through an orifice are not hard to find,
if one knows the buoyancy force in a TLUD reactor, and the buoyancy force
in the gas burner, as well as air flow rates, it should be possible to
simulate this, and see if it is possible for this system to create the
change in secondary/primary air from 6:1 to 3:1 as primary air increases
from 0.017 to 0.062 m/s over an increasing gasification rate increases
(Reed et al., 2000).  Even if we don't have good data, it may be possible
to make a theoretical prediction.  However, ...

For anyone interested in a quick illustration on the effect of orifice size
on gas flow, one can be found here:
http://www.engineeringtoolbox.com/orifice-air-volume-leakage-d_1191.html

One can see from the chart that the resistance to air flow increases
logarithmically.  As one would expect, the resistance to air flow will
increase faster for smaller holes than larger holes as air velocity
increases.

Does that raise a problem if we want the proportion of primary air vs
secondary air to increase as the stove's power is turned up?


Cheers,
Julien.



--
Julien Winter
Cobourg, ON, CANADA
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