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Dear Stovers,
Peter Scott who is presently teaching stove
design in South Africa commented on an interesting set of experiments performed
by Damon Ogle recently at the Aprovecho lab. We were playing around with Foseco
riser sleeves (Kalmin 64). We placed a type K thermocouple at one foot, two
foot, three foot and four foot intervals in a 4' high chimney made from the foot
high 5" in diameter riser sleeves placed on top of eachother. When we
started a fire at the bottom of the tube, there was about a 75 degree F
difference between all sensors, averaging around 1300F or so. But if the top
three riser sleeves were removed the temperature at the one foot level was
around 1600F. When we added the second sleeve at the two foot level the
temperature was around 1500. Adding the third decreased the exit temperature to
around 1400F and then down to 1300F or so when the fourth 12" high section
was added. The heat was not being absorbed by the riser sleeve but instead
excess air pulled into the fire by the increased draft of the taller chimneys
was dramatically lowering the exit temps. and internal chimney temps, as well.
Lower exit temperatures equals lower Delta T between heat and pot which results
in decreased efficiency.
Increasing internal chimney height above the fire is maybe the
easiest way to decrease emissions from the fire. A 24" high chimney
contains the fire for a longer time, no flames exit the top of the chimney. But
we have seen again and again how tall internal chimneys decrease heat transfer
efficiency, so the stove is less fuel efficient.
It would be nice to use the taller chimney but somehow use the
increased draft to a beneficial purpose. One approach is to decrease the gap
between pot skirt and pot when using taller internal chimneys. That seems perfectly reasonable to me but we are then using the gap as a
damper to reduce air flow. If there is an optimum rate of airflow which is
determined by highest temperatures hitting the pot combined with cleanest
emissions then the other common sense option to decrease air flow is to use a
smaller diameter fuel magazine.
But, there is perhaps a third best option. Larry Winiarski,
Damon Ogle and Jim Wilmes were not sure how exactly to do it but they'd like to
leave the increased airflow alone until it is slowed down by creating turbulence
in the fire. Creating a frenzied, jumpy fire like we see when using jets of air
from a fan, that used up the excess air at speed, would be a nice
trick.
Best,
Dean
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