Dear Ron and All
>Are you aware of anyone who has theoretically and/or experimentally determined >an optimum spacing? Presumably the optimum is around 1 or 2 cm - and depends >on the ratio of pot and flame diameters? There is one useful piece of information that I can share, gleaned from Bejan’s convective heat transfer textbook. It is that the highest efficiency of heat transfer from a pot (inside surface) to the water contents is achieved when it is 20 C higher than the boiling temperature of the water. Above or below this figure, the efficiency of heat transfer drops. If the temperature rises significantly, like 100 C more, the heat transfer efficiency drops considerably as a lot of bubbling takes place, and that forms an insulating layer of steam between the water and the pot. As for the gap between pot and the stove body, there is a general rule that unless the velocity is above 3 metres per second, there is little effect on flow, thus one can consider the ‘residence’ time that hot gases have to transfer heat to the pot. If the flow rate is slow and the regime around it insulating, then there is a gain to be had. Remember that Prof Snow (London University) calculated that there is 30 times more buoyancy raising the hot molecules to the pot bottom than there is a tendency to stir that layered gas flow through turbulence. Unless a gas flow was at a very high speed, the buoyancy always overcomes the stirring action. This means that the depth of the gas path is not very critical at all. The tests done by Dale Andreatta showed that the gap on the side of a pot created with a skirt was not very critical either unless the gap was reduced to the point that it started to effect the excess air flow through the stove in which case it could help control the air flow into the fire. With a stove that has too much air flowing into the fire, the skirt or pot-stove gap can be used to bring that excess air rate under control. This is often mis-interpreted as a heat transfer enhancement, when in fact it is an excess air control mechanism. In the latter case the temperature of the gas is raised which increases the heat transfer efficiency no matter what the gap is. All this shows that without a combustion analyser it is not possible to have a clear idea what to change to get an improvement. My son Nigel and Arend Ten Hove and I were treated to a day of combustion at Alex English’s place on Saturday. We were able to get some very clear measurements on two of his combustion systems, both of which were developed using a combustion analyser. They are two of the cleanest burning devices I have seen. One is his home heating stove which burns pellets in the ‘dribble feeder’. It had a steady CO/CO2 ratio of 0.01% a great deal of the time. The excess air was running at about 40-80% and the system efficiency as a heater in the low 80’s. The cooking efficiency was of course much lower (about 12%) but it is not optimised for that. When optimising the heat transfer efficiency, the first thing to do it to make sure the excess air level is low enough to allow for really good combustion and no litre of air admitted that it does not need. The simplest change in the architecture of a stove that will help real world performance is to prevent side drafts crossing under the pot. A lot of heat is wasted. A skirt even 50mm high will do a lot to control that. Stoves with a large gap under the pot are very susceptible to cross drafts with large losses being seen even in modest breezes. Making the gap smaller helps reduce this loss. Regards Crispin
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