[Stoves] ash layer beneath fire

[Richard Stanley]

Ron,
 My answer is  "yes" 
Not to be facetious but the idea is to offer both solutions and let the user decide which is best.  Crispin's elucidation on char production in relation to the grill design was entirely new to me but  combined with my own experience in observing the bridging effect in briquette combustion and ash accumilation ( and a few years as a hydrologist in looking at the Johnson company's water well screen design), led me to the flat top-taper down conclusion. 
The idea appears to have merit for char producer but not for optimizing heat transfer, so I proposed both options for the stove builder according to what outputs they are intending.
The insights of shared, civil assessment on this list, never ceases to amaze !

Richard Stanley
Nicaragua


On Jan 22, 2014, at 9:13 PM, Ronal W. Larson wrote:

Richard:

   1.  A question:  by “optimize char production”, did you mean maximize or minimize?  My experience in watching third world cooks is that they would love to get more, not less.  Those wanting more are going to want large surface contact area.

  2.  I hope anyone trying these ideas out report both how to minimize and to maximize char.

Ron



On Jan 22, 2014, at 7:34 PM, Richard Stanley <rstanley at legacyfound.org> wrote:

> So there you have it Vincent and Crispin
>  
> Seems that a compromise would be for a smaller surface contact area to optimize char production but still one which tapers to the bottom of the stove to limit bridging would be the ideal. 
> viz;
> <Screen shot 2014-01-22 at 8.32.12 PM.png>
> 
> Richard 
> 
> 
> 
> On Jan 22, 2014, at 4:51 PM, Crispin Pemberton-Pigott wrote:
> 
> Dear Richard and Vincent
>  
> Jambo.
>  
> I see just about everyone found that adding a grate to a flat-bottomed stove improves the performance.
>  
> RS>…I would advise use of round bars as Crispin but I would suggest you grind any round bars down to result in effectively a series of had cylinders along their length with the flat side up.
>  
> I would advise the opposite and the metric to use in evaluating the difference is the mass fraction of charcoal produced by the fire. We found (for reasons that are not yet known) having a flat top definitely produces more char thus losing more heat. It may have something to do with the thermal contact the char piece makes with the metal, conducting the heat away. This lowers the temperature of the ‘coal’ and causes it to burn poorly or go out. There was a large mass difference for flat and round bars – more than double.  The thermal contact between a coal and the round bar is much smaller.
>  
> That is the only ‘guess’ I have so far. We will look into this further at YDD this year. The two things tried were round bars from mild steel (multiple welded pieces) and cast iron with a flat top (single piece). Cast iron conducts less heat than mild steel but it still consistently produced much more char. That is not useful as fuel in most homes and it is tossed with the ash. Indar reported the round bar grate produced virtually no char at all, finishing its burn at virtually the same time as the wood. That was amazing.
>  
> You may need to restrict the air entry area to limit the air level below the combustion chamber. If it is ‘right’ you will see large flames wandering around in a low velocity environment below the pot – no flapping arms of flame.  You should limit the pot gap (by shaping those tongues of clay) to about 7mm for all pot sizes. This means trimming the slanted ones and lowering the top ones (in most stoves).
>  
> The combination gives you the right balance of primary and secondary air.
>  
> Nzuri kuhusu
> Crispin
>  
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