0.75 Constant Cross Sectional Area: A Winiarski “Rule of Thumb”

The Uganda 2-pot stove that is described on page 26 in the EPA publication “Test Results of Cook Stove Performance” is a natural draft stove that also uses much less fuel to cook and protects health. The document can be found at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100EKU6.TXT.  Has it been a while since you looked at this book, a comparison of 18 cook stoves? The 2011 book started our surveys with the emissions hood and Test Kitchen, trying to quantify comparisons of fuel use and emissions from available stoves.

Dr. Nordica MacCarty’s paper with comparisons of 50 stoves is a much more complete survey. See: “Fuel use and emissions performance of fifty cooking stoves in the laboratory and related benchmarks of performance” (MacCarty, et al, 2010) https://www.sciencedirect.com/science/article/pii/S0973082610000311

The Uganda 2-pot stove has a Rocket combustion chamber. The hot gases made by the fire pass through narrow, insulated channels around the first pot, which is sunk into the stove. The gases then flow through an insulated tunnel and are forced into narrow channels around the second pot before exiting the chimney. The pots fit tightly into holes in the sheet metal top, preventing smoke from escaping into the kitchen. This stove is fast to boil and, because of the sunken pots, uses less wood than most stoves with chimneys. 

When we build and test stoves we often reflect on Larry Winiarski’s advice that helped to improve the Ugandan stove. Larry advised us that in a 2-pot horizontal stove, channel gaps around the pots that are 0.75 constant cross sectional area are a good compromise between maintaining needed draft and increasing heat transfer efficiency. The cross sectional area of the Ugandan fuel entrance in the Rocket combustion chamber was about 16 square inches so we made the channel gaps all the way to the chimney at 0.75 times 16 square inches. We use Larry’s “rule of thumb” and tests remind us how well Larry knew stoves. He had a good touch.