[Stoves] radient heat capture

ajheggie at gmail.com ajheggie at gmail.com
Sun Mar 4 15:32:26 CST 2012


On Sun, 4 Mar 2012 09:32:37 -0800, Richard Stanley wrote:

>How does one measure radient heat versus convective heat ? seems a bit of both emmanate from any fire but other than feel for heat "below the fire" how do you determine the ratio and what are the guidlines for enhancing the latter, and,  do you always want it ? 

I expect Steve Taylor can answer this better than I. Radiative heat is
a form of electro magnetic wave, like light, X-rays etc. It is just at
wavelengths not visible to our eyes but an infra red thermometer
measures it in the same way a light meter on an old fashioned camera
does. Crispin has pointed to some which work from low temperatures to
over 100C I think, the little one I use is limited to 250C.

The trouble is different surfaces give off different amounts of
radiation even if they are at the same temperature. So a transparent
flame, like Alex's rice husk gasifier, whilst probably being around
1500-2000C will not emit much of its heat as infra red radiation,
because of this low emissivity. A black body ( or near enough black
for this purpose like a steel plate) above the flame will quickly
reach an equilibrium with the flame, but a lower temperature say 1000C
when it will glow bright red and radiate a lot of infra red radiation,
hence to a bystander the naked flame will appear cool but the steel
will be hot.

Paul Anderson and I discussed the effect of putting a bucket filled
with insulation on top of a stove to act as a radiant heater some
while back.

As the flame is taller than it is wide from a simple burner the
emissivity is higher looking down onto the flame compared with through
it. A line of burners in a furnace, as in a steel mill will be deep
enough to have a high emissivity.

You can measure the flames heating effect on something in it and then
measure the infra red from that. Convection is more easy because you
can sample the gases via an insulated tube and onto a thermocouple.
Allowance must be made for the fact that the thermocouple is itself
only in equilibrium with the hot gases as as it gets hot itself its
heat is dissipated by radiation, which is why a hot thermocouple in a
flame in free space under  reads the flame temperature.

In fact all energy is transferred by radiation at the macro and micro
levels , conduction and convection are just subsets where there is
internal transfer in a fluid or solid.



>You start with wood mostly all convective it seems; the fire progresses to coals heat radiates in al directions (Am guessing that this is the radient heat end of the "energy continuum". heat ing distance drops off markedly but it emmanates in all directions. Big stove design problem. Shitft the fule load up lower the pot
been a hot topic for years now

>Until now on one has come up with a good ( meaning, beyond technical,  applicable/ reliable. easily replicable/ solution for application-in-the-developing nations) solution.
>I recall a short mention of radient heat effects and having it dismissed as insignificant  a pre Ethos meeting several years back. Many as well dismiss the value ofthe hollow core int eh briquette other than for more rapid drying, but there it is again... 

Too many ideas to respond to but I have discussed radiant heating on a
number of occasions, as you rightly point out convective heating
always rises but a hot pinpoint radiates through 2 Pi steradians (a
sphere), Think how a match burns, once all the volatiles flame off why
are you left with a char stick? It's because the hot char rapidly
radiates its heat away and falls below its auto ignition temperature,
it loses so much temperature that it no longer has enough energy to
dissociate an oxygen molecule hitting its surface. Do the same with a
bunch of matchsticks and whilst they all still radiate the same amount
of heat they also mutually receive radiant heat from neigbouring
matches and the char burns. Consider the hole of your briquette to be
made up of finite little wood particles, now they are constrained to
radiate only through a hemisphere, as the solid briquette is their
back, but the only radiant heat that escapes is through the ends of
the hole, all the rest is mutually radiated and re radiated to other
particles lining the hole, so it rapidly gets hot. A similar principle
is used in commercial boilers where high moisture wood is burned, the
flames are used to heat a refractory arch above the fire bed, this hot
arch radiates heat back onto the fire  bed to keep the temperature up,
this radiative feedback compensates for the heat lost in vapourising
water in the fuel.

Dry wood has burned near stoichiometrically has the potential to reach
1600C. A hob glowing dull red is about 700C. The latter is only
capable of transmitting 7% of the radiant power of the former.

My thought is that a flame in a tall insulated fire chamber can
radiate heat both to the pot and to the  fuel below and this heat
transfer is not concerned with the boundary layer where the pot is
only heated by convection/conduction of hot flue gases.

It does mean we must address Crispin's hobby horse of excess air,
which after high moisture content is the biggest effect on reducing
the combustion chamber temperature. It is less of a worry where there
is a big heat exchange surface, as in the tubes of a boiler but a pot
bottom and sides is a limited heat exchange surface.

AJH





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