Natural Draft Mixing in the Kirk Harris TLUD
Time, Temperature, and Turbulence are the three ”’T’s”’ of combustion. “A short period of time, high temperature, and very turbulent flame indicates rapid combustion. Turbulence is the key because fuel and air must be thoroughly mixed if the fuel is to be completely burned.” https://www.myodesie.com/wiki/index/returnEntry/id/3054 . The temperature in flame above the fuel is around 1,100C. https://sciencing.com/hot-bonfire-8770.html. The temperature inside burning charcoal is also around 1,100C. https://en.wikipedia.org/wiki/Charcoal
Time, Temperature, and Turbulence are the three ”’T’s”’ of combustion. “A short period of time, high temperature, and very turbulent flame indicates rapid combustion. Turbulence is the key because fuel and air must be thoroughly mixed if the fuel is to be completely burned.” https://www.myodesie.com/wiki/index/returnEntry/id/3054 . The temperature in flame above the fuel is around 1,100C. https://sciencing.com/hot-bonfire-8770.html. The temperature inside burning charcoal is also around 1,100C. https://en.wikipedia.org/wiki/Charcoal
In most stoves forced draft jets of primary or secondary are needed to achieve the thorough mixing of air, wood-gas, and flame required for close to complete combustion. Natural draft is usually not forceful enough to create adequate mixing in biomass cook stoves. The notable exception is the 0.7 mg/min PM 2.5 Kirk Harris natural draft Wonderwerk TLUD. *
Pre-heated secondary air meets and mixes with the wood gas at several locations, but mainly at the top edge of the fuel reactor chamber. A widened chamber topped with flutes gives the gas and air more surface contact for better mixing. The rising column of wood gas is forced outward to the fluted mixing edge by a round plate over the center of the fuel reactor chamber. The wood gas rises around the edge of the plate that has radial slits in it through which some of the wood gas comes up forming radial sheets of gas which push into the air and flame above. See Clean Burning Biomass Cookstoves at www.aprovecho.org
The flame and air from the edge of the chamber surround the wood gas sheets, forming ridges of flame. The idea is to increase the surface contact between the wood gas and air for more rapid mixing and burning.
The design burns the easy to burn gasses first, and then uses that heat to crack the hard to burn long chain hydrocarbons. More air is later mixed in to allow combustion of the short chain hydrocarbons. This clean-up burner is a group of 6 tubes with slits to inject air into the rising gases.
The secondary mixing technique for this stove uses a pressure difference created between the air and wood gas by the buoyant force and enhanced by the Venturi effect. The enhanced pressure difference, combined with a burner geometry that gives the gasses considerable surface contact, seems to mix the gases rapidly and effectively. The Wonderwerk stove has a small pressure difference but with a large surface contact coupled with the shallow wood gas depth that the air must penetrate.
*Testing at Lawrence Berkeley National Lab
