[Stoves] Black radiating surfaces

[John Davies]

USING A COMBINATION OF RADIATION AND CONVECTION FOR SPACE HEATING

Stovers,

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.

Observing the heat radiation of the air preheating jacket leads to the 
following perceptions:

1. With matt silver internal and external surfaces ( oxidized galvanizing ) 
a large amount of heat is radiated despite the air heating duty.

2. When the external surface is blackened the radiation is increased, and 
appears to be similar to that of the exposed red hot tube.

3. Are these just perceptions, or are we genuinely extracting as much 
radiated heat as in 2 ?

Now If we added a convection tube as suggested by Crispin, which had a black 
surface both internally and externally,the following may be true.

1. Additional convected hot air will enter the room, having a cooling 
effect, of the red hot surface, but not so much, as heat from the red hot 
coals will flow faster into the cooled surface, heating it to nearly the 
same temperature as before.

2. Radiated heat will be well absorbed by the convection tube and radiated 
outwards at nearly the same rate as the exposed red hot tube.

3. The air gap between the combustion and convection tube will determine the 
ratio of heat extracted by each energy form. I would start with a 25mm gap 
( 1" ) with a convection tube with a height of 240 mm ( 9.5 " ) and take it 
from there. This would add a safety feature in the event of the combustion 
tube failing. The combustion tube does become brittle and crack after a 
period of time.

4. IF maximum heat was required at the top of the stove, e.g.. powering a 
water heater, the air movement could be blocked off forming a sealed jacket, 
which if given  polished silver surfaces, would reduce radiation 
significantly. This would give the inverse of 1 and 2. , but might be 
detrimental to the metal of the combustion tube. In this case a silica based 
combustion tube might be required.

This brings us to the point where careful control of radiation and 
convection losses could be highly beneficial to the heat efficiency of a 
cooking stove or space heater. Do we want maximum or minimum heat loss from 
the unit. ?

Your discussion on the above will be greatly appreciated.

Thanking you,
 John Davies