[Stoves] Black radiating surfaces

[Kevin Chisholm]

Dear John

John Davies wrote:
> Gentlemen, ( not forgetting the Ladies ),
>   

OK... how about "Gentlepersons"? ;-)

There are two issues here:
1: What happens to the Emissivity when a body goes from "Black Body 
Conditions" at room temperature to red heat?
2: What happens to Radiant Heat Transfer capability as a body goes from  
room temperature to red heat?

To answer #1, it is my guess that it tends to fall off somewhat.

To answer #2, it doesn't matter about how the emissivity is reduced, as 
long as the temperature is raised a few degrees higher.

I don't mean to be flippant, but if one looks at the formula for 
calculating Radiant Heat Transfer:
Qr = A*K*e*(T2^4-T1^4),
we see that at "red heat temperatures", the "4th power effect" 
enormously dominates over even a significant linear change in the 
emissivity factor.

One could easily make up a simple spread sheet digitize the above 
generality and to show  visually how an incremental decrease in the 
Emissivity Factor would decrease Radiant heat transfer rate, and the 
extent of the incremental increase in body temperature that would be 
required to overcome the loss because of decrease in the e-factor.

Best wishes,

Kevin

 
> 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. 
>
>
>
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