[Stoves] radiant heat capture, total heat measurement

[rongretlarson at comcast.net]

Andrew, Paul, Crispin, list: 

1. The last several paragraphs below are coming out more negatively on radiant heat capture than I think are appropriate. Remember, the initial information provided by Paul Olivier on March 2, when he said: 

"When a wire mesh dome is placed on top of the burner and burner housing, 
this roughly doubles the amount of heat being transferred to a pot: " 

2. I think this doubling has little to do with the exchange below (and a few earlier that are similar). Radiant heaters are widely sold because of their efficiency in heat transfer - in many cases involving zero convection. Given we have only one (above) piece of data for one stove, the chances are that a doubling is not the maximum we can achieve. 

3. I would look at this as a conservation of energy problem. We know that we can transfer more energy to the pot if the radiator is hotter. There may be catalysts that we can employ. We know how to use reflectors and obtain high absorption (and sometimes simultaneously low emissivity ) 
In my reading on ceramic foam, I found that some foam is being employed so as to have gas combustion take place inside the foam!. 

4. I hope that others can perform some experiments along these lines. 

Ron 

----- Original Message -----
From: ajheggie at gmail.com 
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org> 
Sent: Saturday, March 10, 2012 3:31:17 AM 
Subject: Re: [Stoves] radiant heat capture, total heat measurement 

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, 
> 
> 
> 
>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. 
> 
> 
> 
>>but absorbs the lower energy infra red from cooler bodies like earth and 
>our bodies. 
> 
> 
> 
>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. 
> 
> 
> 
>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 
> 
> 
> 
>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. 
> 
> 
> 
>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 

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