[Stoves] ash removal a (f) of gas velocity and/or temperature

[Richard Stanley]

Kevin,

The ask is indeed scoured off the inner core and indeed remains along  
the outer perifery of the cylindrical briquette but it may well also  
be due to the higher temperature in/through  that inner hole as well.
Taller briquettes also reduce flow velocity through the core to thte  
point where ash builds up there too. Ash builds up where the height  
of the inner core hole is generally > 4X its diameter (this changes  
somewhat (say within+/- 0.5 inches) with the kind of biomass being  
used as the roughness coefficient will of course change accordingly.  
Temperature has prooved a highly effective  "ash removal" agent in  
specialized stoves  where we have managed complete gassification  
(quite successfully I might add). No ash was left at all and it did  
not even tend to accumilate during the burn. We went from pyrolsis to  
gassification,  to nothing remaining in the combustion chamber at all.

But all that required careful attention to the shape and material  
design of the CC itself,  air flow velocity location and mixing and  
radiant heat reflection back into the briquette--not the stuff the  
ordinary villager would be able to afford, much less sustain. While I  
have reverted back to good 'ole bottom fed primary air and insulation  
as it can be adapted to a more robust and afforable stove(s) by the  
rest of the 90% of us at least for now.. The R&D for an affordable  
gassifier cookstove using biomass, for the masses, continues through  
my colleague (and fellow member of this list), Kobus Venter, in South  
Africa and I have no doubt that he will eventually come upon a final  
design which is equally as widely applicable. The design details are,  
by agreement,  for him to share as he eventually unveils his  
commercial model of the stove there...

kind regards,

Richard Stanley

          consilation
On May 25, 2008, at 19:31, Kevin Chisholm wrote:

> Dear Richard
>
> Is it possible that the "... wispy covering of ash..." is not what  
> is causing the decreased radiant heat transfer rate, but rather, it  
> is simply slowing down the rate of combustion?
>
> What might be happening is that with the 3" height, the extra  
> "stack effect" is creating enough  extra velocity to scour off the  
> ash coating, to expose fresh fuel for combustion.
>
> Your very simple system might turn out to be wonderfully complex...
>
> If you aren't already familiar with "Reynolds Number", find a text  
> that will give you an understanding of it
> Nr = (gas density * gas viscosity* gas velocity) / gas viscosity
>
> Between Nr=1800 and Nr =2000, fluid flows change from laminar to  
> turbulent. However, even in the laminar region, there may be a  
> critical velocity where the scour effect is enough to blow away the  
> ask that shields the fuel from oxygen.  That little increase in  
> height might just have been enough to create sufficient velocity to  
> "clean teh ash off the fuel."
>
> With this concept in mind, it might help you to diagnose and solve  
> other combustion or performance problems.
>
> Best wishes,
>
> Kevin
>
> Richard Stanley wrote:
>> ref radient heat emissivity and reflectance,  I was wondering  
>> what  the radient heat reflectance is within a cylinder such as  
>> that of the  hollow core briquette. We just completed two short  
>> trainings here and  I am again reminded of the tremendous effect  
>> of that glowing white to  red core (1.25" dia x 3" height) inside  
>> what is otherwise leaves,  grass and junk mail here..No where near  
>> as much glow with shorter  heights of say 1 - 1.5 inches--or for  
>> that matter. It also remains  true that with all its power the  
>> slightest --whispy covering of ash  on the red surface nearly  
>> obliterates that effect.
>> Externally, that is, IR reflectance off stove walls of different   
>> colors  is well corroborated with what you are saying. Same   
>> briquettes in a copper colored stove walls reflect far less than  
>> same  bqs in a black enameled (weber) stove wall.
>>
>> Richard Stanley
>> State of Jefferson
>>
>>
>>
>>
>>
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>
>

Richard Stanley
Director, Legacy Foundation,
4886 Hwy 66,
Ashland Oregon, 97520
USA
541 488 1559
rstanley at legacyfound.org