Dear Andrew
I think your very last comment is very important, and that its importance is
being overlooked.
"Yes the glowing dome will radiate more power than the blue flame."
A "blue flame" has very poor flame radiation characteristics. A yellow or
red flame has very superior flame radiation characteristics. "Flame
Luminosity" basically equals better radiation characteristics.
One way to look at it is that "Blue Flames are too smart by half." :-) There
is a gain in completeness of combustion, and probably higher temperature for
"products of combustion", and reduced sooting of pots, BUT there is a
significant loss in luminosity, and heat transfer to the pot by radiation.
The Mesh Dome, heated by direct convection of the "blue flame" then is in a
position to add a "radiation contribution" to heat transfer capability.
It would be a very interesting experiment to take a "flame temperature" of
Paul Oliver's system, with, and without the dome. My guess is that the
"flame temperature" of the "Blue Flame", and the gases leaving the dome
would be similar, and relatively low, even though the actual thermocouple
measured temperature of the gases would be high. In very simplistic terms
(for illustrative purposes only)
1: Blue Flame temperature by radiation measurement: 1000
2: Gas temperature above dome (radiation) 1000
3: Thermometer temperature of both gases 1500
4: Flame Emissivity of 1: and 2: .2
5: Flame emissivity for luminous flame: .8
Thus, if some of the pyrolysis gases were diverted away from the "blue flame
conditions", and were allowed to increase the flame luminosity, there could
potentially be an opportunity to "have your cake and eat it." More
specifically, the addition of some "luminous fractions" to the flame could,
for example increase flame luminosity to say .6. but with a drop in actual
gas temperature to say 1300 because of incompleteness of combustion.
In other words, "Heat Transfer capability to the Pot" would be:
1: Blue flame: 1500 x .2 = 300
2: Blue + Yellow" flame conditions = 1300 x .6 = 780
For a "Blue Flame + Radiation Dome, conditions might be
1300 x .4 = 510
(NOTE: All these numbers are arbitrary and purely illustrative, to explain
the concepts)
So.... if the concepts are correct, then it would seem that the best way to
make a stove would be to make one with a burner that produced BOTH yellow
flame(for luminosity) and a blue flame (for completeness of combustion).
Does that "hang together for you?
Best wishes,
Kevin
----- Original Message -----
From: <[email protected]>
To: "Discussion of biomass cooking stoves" <[email protected]>
Sent: Monday, March 12, 2012 6:58 PM
Subject: Re: [Stoves] radiant heat capture, total heat measurement
On Mon, 12 Mar 2012 06:18:14 +0700, Paul Olivier wrote:
More primary air means more gas.
Yes
It does not necessarily mean more excess or secondary air.
No but it the primary gas stream acts as an ejector or aspirator to
entrain the secondary air.
My simple understanding of what is happening here is that
wind does not disturb what is happening within the dome.
I think so too.
Wind does not dissipate heat so easily.
I cannot see this once outside the dome.
This gives a better transfer of convective heat,
Nor this.
The only single thing that increases the transfer to the pot from the
rising gases is if they are at a higher temperature.
and it also gives more thermal radiation.
Yes the glowing dome will radiate more power than the blue flame.
AJH
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