[Stoves] Black radiating surfaces

[AJH]

On Sun, 25 May 2008 11:09:37 -0700, 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.

This all makes sense, mutual radiant heat transfer between burning
sticks is important to maintain pyrolysis and combustion, look how a
match self extinguishes leaving a charred stick without this.

So with your hole through the briquette the outside of the briquette
can radiate heat away to become cooler, the inside can only radiate
heat out through the top of the hole, so there is a limited cross
section through which the black hole emitter can lose heat, the longer
the hole is the proportionately less area through which to radiate
heat away (there is in fact a similar effect here with one type of
solar concentrator which nano-tubes give some new hope for, if they
don't give us mesothelioma first!) The sides of the hole are feeding
back heat to the opposite side and maintaining high pyrolysis and
combustion temperatures, I guess blocking some of this mutual
radiation with ash cuts the feedback loop.

AJH