[Stoves] RE radiant heat capture, total heat measurement

[Crispin Pemberton-Pigott]

Dear Andrew and All

 

“…either way you still need to have the convection do most of the work.”

 

That is the money quote. Radiative heat transfer is not all that efficient no matter what is claimed about it – not at the power level we are talking about. The great majority of the heat getting into a pot will be from convective transfer. Conduction is insignificant and radiation only works well when the temperature difference is high, which as you point out, ceases to be so immediately after losing heat initially.

 

Paul’s performance increase has been achieved by reducing the excess air level (partially blocking the secondary air flow in particular as we know the primary air is being consumed) and lowering the gas velocity under the pot (which increase the total % of heat transferred). His test result is also contingent on a low baseline performance figure.

 

The way the net heat transfer efficiency increase has been achieved (by using a bluff body to create a better burnout of CO and a lower EA) is one of several methods one might apply. The reason the dome is not commonly applied is a) the available materials don’t last at that temperature and b) there are other ways to achieve the same result, principally by controlling excess air and maintaining a hot final combustion zone away from the pot so as to remove its quenching effect.

 

A similar discussion of the effect of pot skirts was held here some years ago with the same misdirection: the effect of a small pot skirt is not so much the bringing of hot gases near the pot, but because it was being tested on a stove with high excess air. The skirt was tight enough to control (limit) the EA to the point where the system performance improved. The improvement was attributed to the gap and what happened in it, not the effect the gap had on limiting unneeded air flowing through the stove (which has a far greater beneficial effect than a skirt).

 

The pot skirt gap and radiant dome theories are both proofs that you can’t develop a stove without a combustion analyser and the knowledge of what to do with the numbers it gives.

 

Please don’t test the small effects of stove structure changes using a pot without a lid. Removing the lid introduces two large additional errors which may be larger than the beneficial effect you are trying to pin down: 1) radiation upwards from the water surface (which cools the water) and 2) the effect of low velocity transient breezes removing additional moisture from the super-saturated layer immediately above the water surface (this also cools the water).

 

If all you have for instrumentation is a water boiling test, blacken the pot bottoms, polish the sides until bright and keep the lid on. Then you can start editing the stove for improvement.

 

Regards
Crispin

 

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