Reaching 50% Thermal Efficiency

For good thermal efficiency, be sure that as much heat as possible is being transferred to the outside of the cooking pot. The temperature of the hot gas flowing past the surface of the pot is increased by 1.) Creating as much flame (1,100C) as possible in a low mass, insulated combustion chamber 2.) Decreasing the distance between the fire and the pot without making excess smoke 3.) Not allowing external air to cool the combustion gasses. 

In convective heat transfer, the primary resistance is in the surface boundary layer of very slowly moving gas immediately adjacent to a wall. Increasing the velocity of the hot gas as it flows past the pot without reducing the temperature is aided by a pot skirt. Reduce the thermal resistance with appropriately sized channel gaps under and at the sides of the pot. ( see “Biomass Stoves:” Sam Baldwin).

A 6mm channel gap in a 10cm or higher pot skirt has been shown to work well with up to 6kW firepower with a 24cm or larger diameter pot. 

Reducing thermal losses from the exterior of the pot skirt with 1cm of refractory ceramic fiber insulation increases thermal efficiency by approximately 8%.