[Stoves] Black radiating surfaces

[Crispin Pemberton-Pigott]

Dear John

Kevin raises an interesting point. What is the emissivity of a red hot black
body?

While agreeing with Philip, I feel it is also good to view this as a dynamic
problem. By that I mean it is not just the red metal that we observe. We
also see heat moving though the metal from one moment to the next.

Suppose the emissivity of the steel drops when it is red.

If you were able to increase the flow of air over the hot metal, the
temperature would drop, increasing the coefficient of radiation. Also, if
you were to use a metal with a lower thermal conduction coefficient, the
amount of heat reaching the outside surface would decrease, similarly
reducing the temperature and increasing the radiation coefficient. If the
internal temperature was constant, heat would be retained.

Conversely, given a fixed temperature inside the stove, the more heat you
can conduct to the outside surface the higher the temperature and, after it
goes red, the lower the radiation coefficient (emissivity). This means
swapping mild steel for 3CR12 would decrease the outside temperature
(presuming they were both the same colour).

I have often suggested using 3CR12 for two reasons. The above is one. The
other is that for any given amount of money, you can get a longer life from
3CR12 than from mild steel.  It is quite a bit stronger then mild steel so
it can be thinner. The price relationship is about 1.5 to 1.8 times that of
mild steel. For a 2.0mm piece of mild steel, you can substitute a 1.2mm
piece of 3CR12.

Its thermal conduction rate (metric numbers) is 0.23 against mild steel's
0.48. Slightly less heat will be conducted through the 1.2mm section, lower
its temperature, perhaps keeping it 'black!

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.

I was given a stand-alone Peltier effect fan by my wife and it greatly
increases the perception of heating because it blows hot air over the top of
a stove and circulates it in the room.

Regards
Crispin