>Crispin said: An implementation that most will find useful is to extend the >burner higher than the top deck as you have done, then have the larger gap, >then put a ring of similar height (a little higher) at about the OD of the >pot.
Crispin you are a mind reader or maybe you have already done this, because that is exactly what I plan to do with the next potholder prototype. The burner extends above the cooktop about 25 mm and the potholder will be about 12 to 18 mm or about 1/2 to 5/8 taller. (I am decreasing the gap from 25 mm to about 12 to18 mm to help the flow spread out around the pot) and then around the outside parameter of the potholder I will use a band about 25 mm, to trap the dead air. My cook top extends beyond the pot holder and pot so the "Pot Shell" will have a place to sit. The pot shell is the easiest way to improve a stoves efficiency. Without the pot shell, my stove would not be able to boil, and then simmer, 35 Kg of beans for 2 hours with only 3 Kg of wood. I am also working on a multiple pot, potholder that sits on a single burner and is able to direct the heat flow in 2 or 3 different directions. This pot holder will have a 250 mm/10" flat spot above the burner that could be used like a small griddle. Hinged flaps/doors will direct the flow. Lanny ----- Original Message ----- From: Crispin Pemberton-Pigott To: 'Discussion of biomass cooking stoves' Sent: Sunday, September 22, 2013 2:39 AM Subject: Re: [Stoves] A doctoral thesis on stoves Dear Lanny With the School Lunch Cooker I let the burner extend 1"/25mm above the cook top and used 2"/50mm tall pot holders. This puts the heat on the pot and shields the stove top with a layer of dead air. I have another interpretation that may be useful. You did the right thing, but the layer of air is not at all dead. What happens is that the buoyancy of the hot molecules is much stronger than the effect of turbulence. Even though the gap under the pot is large, it is not hot 'down there' on the lower surface. The heat rapidly rises into the pot. An implementation that most will find useful is to extend the burner higher than the top deck as you have done, then have the larger gap, then put a ring of similar height (a little higher) at about the OD of the pot. This creates a heat transfer zone that is not easily blown around by transient winds. The suggestion that the space should be small with a high velocity to bring the heat close to the pot is based on the presumption that the hottest molecules do not rise with more force than the turbulent force. In fact the buoyancy force is about 30 times the turbulent force. The heat is always at the top when the flow is in the 200 mm/sec range. "Top" in this case means the top 2mm of the cross section. It is cool right at the surface where the heat flux is taking place. The suggestion that 'more heat' gets into the pot if the velocity is high confuses the heat transfer rate with the heat transfer efficiency. They are both efficiencies (ratios) but of different things. Regards Crispin ------------------------------------------------------------------------------ _______________________________________________ Stoves mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org for more Biomass Cooking Stoves, News and Information see our web site: http://stoves.bioenergylists.org/
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