[Stoves] Time at Aprovecho

[kgharris]

All,

I have just spent 2 1/2 weeks at Aprovecho Wood Stove Research Center in Oregon.  Dean Still set for me the lofty goal of developing a TLUD-ND which is tier 4 in all catigories.  We probably missed this goal, but we did make progress.  We have a test TLUD-ND which can boil a 5L pot of water in around 20 minutes, and then turn down and simmer it between the required test temperatures of 93C and 98C.  At one point the water was in danger of droping below 93C, and the flame had to be turned up, so we also have some extra turn-down.  Additionally, the PM and CO levels were very low at all power levels for most of the time.  One exception was one test where the CO remained slightly high at low power.  The fire was below the wick (see attachment) and was burning char, creating an excess of CO.  We had the usual increases at the start up and flame out, and also small momentary increases in both PM and CO during the transition from high to low power levels.  

The turn-down method we used is a further development of the pilot flame method I discussed a couple of years ago on the list.  It now includes what I call a wick, a central disk with radial slits, attached to the combustor section.  It works with the pilot flames to stablize the low power flame.  The method requires only that the primary air be turned down, which makes it simple to make and use.  The control device has to be very sensitive at the low end since a small change makes a big difference in the low power flame.  Another problem we encountered was creosote which formed on the interior of the fuel/reaction chamber wall (see attachment).  Burning the char for a minute or two after flame out cleared this deposit.  Insulating the wall may keep it hot enough that the creosote will not deposit.

This improving of the pilot flame turn-down technique would not have been possible without the equipment at Aprovecho.  The real time sensing and computer graphing equipment enabled us to quickly see the effects of any changes we made.  At one point, when the stove was unstable and could not reach low power, I tried removing the stationary fan which swirls the high power flame thinking it might be causing a problem.  The stove was then able to reach the low power levels.  I made a small change in the bend of the fan blades to allow more opening between the blades, put it back into the stove, and the low power flame remained stable.  We now had both clean high power because of the swirl, and clean low power.  The low power flame needed a more open stove with less flow resistance than the high power flame.  This is counter intuitive at first, but makes sense because the high power flame has more buoyant force (draft) pushing the flame through the fan. I would not have found this without Aprovecho's real time readout equipment.

I hope to work at Aprovecho Research Center in the future and I encourage others who are interested in wood stove research to work at ARC or a similarly equipted lab.  The equipment very much helps with stove experimenting and designing!

Kirk
Santa Rosa, CA. USA

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