[Stoves] Thoughts and questions about buoyancy

[kgharris]

Hi All,

Thoughts on buoyancy have been grinding around in my head, and I have had some good ideas for experiments.  I have not been able to work with these because of another situation.  I sent a TLUD with turn-down capability to Aprovecho for testing, but they were only able to get a turn down ratio of 1.7 to 1.  I was able to get 3.3 to1 before I sent it.  My assumption is that during shipment some of the sealant broke off and the stove is leaking too much air into the reactor.  Now I am building a stove which does not require so much sealing by moving the primary adjustment from the outside directly to the bottom of the reactor.  It is a plate of metal that can cover the bottom of the reactor and can be pivoted down to allow adjustment of the air flow, thus eliminating several potential leak points.  This is a major redisign and is taking some time.

My first thoughts to the responses to my earlier writing about buoyancy was to be sceptical of what was said.  I could not believe that anything smaller than .01 inches of water column could drive a fire the size of what I see comming out of our TLUDs.  Further thought. has given me a mechanism which might be able to do this.  Pushing a boulder takes a lot of force.  One must overcome both inertia and friction.  If the boulder was made of styrofoam it would take a lot less force because the lighter material has less inertia and friction with the ground.  The fire gasses are very hot and of VERY low density, so very little force is required to move them.  Also, unlike the boulder, the friction forces are close to zero.  These two things could allow a tiny push from the air to move the fire gasses, and buoy them up. 

Also I note that as the fire rises, it narrows and comes to a point, meaning that it is accererating.  If the fire could be kept slower in the combustor, then the combustor could be shorter, thus possibly getting a better burn with a shorter stove.  Dr. Larson has been talking about making shorter stoves for safety reasons.  The down side I see is that at Aprovecho I learned that heat transfer into the pot is improved by a fast fire which can penetrate the surface layers of air under the pot.  Slowing the fire down might counter this effect.  

Kirk H.
Santa Rosa, CA. USA
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