[Stoves] radiant heat capture, total heat measurement

[ajheggie at gmail.com]

On Fri, 9 Mar 2012 12:25:49 +0200, Crispin Pemberton-Pigott wrote:

>>...and passes visible light and higher frequency infra red from hot bodies,
>like the sun, 
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> 
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>Most of it, the rest makes it warmer (absorbs).

Yes that which it absorbs make it warmer but equally as a warm body at
equilibrium it will be re emitting longer wavelength IR.
>
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>
>>but absorbs the lower energy infra red from cooler bodies like earth and
>our bodies.
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>Yes, and because it is warm, and active in the IR, it also emits IR but with
>a low emissivity. In other words if you know the emissivity, you can read
>the temperature with an IR gun.

 it is emitting a different (lower) frequency from which it has
absorbed.
>
> 
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>But more to the point I was saying that at a lower (non 90°) angle, it
>starts to reflect radiation from the top of the surface. Look at glass at a
>low angle and it looks like a mirror.

Agreed
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> 
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>The point is that when reflecting heat, if the incident angle is past a
>critical value, it reflects pretty much all of it so the issues Kevin
>mentioned about the mirroring on the back don’t come into play.

This depends on the refractive index of the two materials, going from
a lower to higher there is always some light transmission, the other
way round and you have total reflection, this is how a light fibre
works.
>
> 
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>Paul’s question was about reflecting the heat. So the principles are the
>reflectivity, surface finish, incident angle and emissivity. While a stove
>may be good at sending IR radiation towards the pot, pots are not all that
>good at picking it up, actually. Stainless steel pots are quite reflective
>and do better picking up heat by convection.

The major salient point is that radiation from heating something by
flame/flue gases is only ever going to extract a minor portion of the
heat in the gas stream. Consider a black body in the gas flow, it can
never be hotter than the gas flow downstream of it, as you make it
hotter it radiates more heat but the rejected gas flow is also hotter,
either way you still need to have the convection do most of the work.
AJH