Dr. Larry Winiarski would remind me to imagine the languid rising of smoke from a cigarette when thinking about the velocity of natural draft gases in the Rocket stove. I remember Larry saying that rising smoke is sexy, contemplative, and slow.
Sam Bentson, General Manager of ARC, and Chenkai Wang, Division Business Manager of SSM, spent months designing an inexpensive 2 Watt fan that developed a pressure of 0.75 inches of water column to blow high enough velocity air jets into a Rocket stove fire to increase mixing and combustion efficiency.
When Sam measures the dynamic pressure in the chimneys of household natural draft rocket cooking stoves he finds less than 0.01 inches of water column. Sam estimated, using the Archimedes principle, that a 10” inch in diameter chimney pipe at 700°C for its entire length would have to be 15 meters tall to generate 0.50 inches of water column. It’s amazing how powerful a little electric fan can be!
Kirk Harris writes that he has wondered how an exceedingly small pressure variance could drive the tall flames that we see in some stoves. He envisions fire gas as having exceptionally low density, very light weight, with very low inertia. Kirk thinks of fire as like a “hole in the atmosphere”, easy to push around. His stoves use static mixers and small velocity induced natural draft pressure differences to mix flame that has been divided into thin sheets. Using these approaches, the Harris TLUD stove achieves less than 1mg/min for PM2.5 when burning pellets.
The Jet-Flame, on the other hand, uses very high-pressure jets of air that blast up into charcoal and then mix wood gas and air as the jets pass through sticks on fire. The bottom air technique requires the equivalent pressure of a 15-meter-high, extremely hot chimney to lower emissions to about the same degree as the Harris stove. It is interesting to think of these two stoves side by side, representing quite different approaches to clean burning. The Harris stove is gently manipulating flame as easy to move around as a hole in the atmosphere while the Jet-Flame is dynamic, a bit loud, creating hot jets of air that drill holes in burning wood.