[Stoves] Black radiating surfaces

[John Davies]

Gentlemen, ( not forgetting the Ladies ),

IPC wrote:
> You cannot get a black body to be black at red heat.  It will be red, no
> matter what you do.

Kevin wrote:
 the point of interest to John
is maximizing the emissivity of the iron, so that it radiates the
maximum amount of heat energy.

Crispin wrote:
> Discussing this with Nigel for a minute, it seems that you might get more
> heat off the hot surface by putting an open ended vertical convection tube
> over the hot part to increase the air speed past the surface. This will 
> pull
> off more heat than the radiation will, though it will deliver it in the 
> form
> of hot air instead of radiation which may give the perception of 'not 
> being
> as hot' to someone sitting nearby.

The stove in question is designed to cook and do space heating by radiation. 
With charges of coal of up to 1 Kg it is primarily in cooking mode, with the 
combustion tube surrounded with an air preating jacket. But the main purpose 
is to radiate heat when loaded to maximum with about 4.5 Kg coal. Now we 
have 3.5 Kg in the exposed combustion tube. This is the area that we would 
like to keep black, when below red hot temperatures. This is particularly of 
interest when the combistion has reached the turn down stage, and one would 
wish to have maximum radiation at this stage.

" So how do we treat the surface so that it will return to black as it 
cools"?

A thin carbon layer does not work, as it burns off.  The rest of the stove 
and chimney does not suffer from this problem so one can choose between a 
black surface or a silver surface. Easily done, a smear of cooking oil in 
the winter, and a polished surface in the summer, when only cooking is done 
with a cup full of coal.

Now the question of radiation vs. air heating arises. Which is the most 
effective?

 Given the scenario of the informal settlement house, with litte or no 
insulation, and possibily air gaps where heated air will escape, My 
intuition favours heating by radiation, but am I correct?  Have any studies 
been done in this respect ? Also, what would be best for an insulated first 
world house ?

As Philip points out the nature of physics seems to be working against us. 
But how can we work with nature for the best outcome, your views will be 
welcome.

Thanking you,
John.