Design principles for optimizing cooking stoves – Dean Still

Improving heat transfer 
The temperature of the hot gas is increased.
Expose as much of the pot to the hot gas as possible.
In convective heat transfer, the primary resistance is in the surface boundary layer of very slowly moving gas immediately adjacent to a wall. Increase the velocity of the hot gas as it flows past the pot without reducing the temperature. Reduce the thermal resistance with appropriately sized channel gaps under and at the sides of the pot. Match the firepower to the channel gap size/size of pot (“Biomass Stoves:” Sam Baldwin).

Reduce losses 
High mass stoves frequently use more fuel than a three-stone fire because of losses due to conduction. Heat passes through materials by exciting molecules that move faster. The heat that is absorbed by un-insulated walls diverts heat that could have gone into cooking food. The open fire is surrounded by air. The molecules in air are further apart and losses from conduction are diminished. The stove loses the least energy when the entire flow path of the hot gases is insulated. Reducing losses (resulting in increased temperature) is an important part of heat transfer efficiency.

Metering Fuel
Metering results in more complete combustion. Clean combustion occurs when the gas, air, and smoke (mixed together) enter the flame for a long enough period of time.
When too much wood is burning too much gas is made but not enough flame. The un-combusted pollution flows into the air.
Burning more than the tip of a stick can result in too much gas and too little flame.

Turn Down Ratio
Fuel economy is dependent on three factors, namely, the efficiency (as a function of power output), the maximum power output, and the ratio of maximum to minimum power outputs.  (Prassad)

One of the most important factors determining field performance of a stove is the firepower it is run at during the simmering phase. Because simmering times tend to be long, quite modest increases in firepower above the minimum needed can greatly increase fuel consumption. (Baldwin)

Using more or less sticks of wood, burning at the tips, creates high and low power. Adjust the amount of burning wood to suit the cooking task. (Winiarski)

Excess Air
Most cook stoves do not have doors covering the combustion chamber. A lot of ‘excess air’ is available in the fire. Decreasing the amount of air entering the fire generally increases combustion temperatures. However, decreasing ‘excess air’ can result in less mixing and combustion efficiency is reduced. Increasing the amount of mixing is effective even when there is “too much” air. Mixing seems (to us) to be more effective than raising temperatures in decreasing the emissions of PM.
Red flame can be clean. Blue is often caused by CO. Yellow makes black carbon.

Complete combustion occurs when well mixed gas, air, flame, and smoke enter flame for a long enough period of time. In a Rocket stove only forced air mixing results in almost complete combustion. Strong jets of air are needed to fully mix the air, flame, smoke, and gas. Adding a chimney to a Rocket stove doesn’t result in the forceful jets that create adequate mixing. Adding height to the Rocket combustion chamber, while giving more time for combustion to occur, also draws in more cold air that reduces the heat transfer efficiency. In a TLUD stove the symmetrical flow of the gas/air/smoke into the flame makes natural draft almost complete combustion possible. Dr. Winiarski believes that turbulence is useful when it results in mixing.

Automobiles, industrial applications, and many heating stoves sold in the United States use scrubbing to reduce harmful emissions. Scrubbers take many forms such as water baths, small water droplets through which smoke passes, catalytic converters, cloth bags, etc. The catalyst lowers the temperature at which smoke and CO combust to 500-550 degrees Fahrenheit. The catalytic converter works at temperatures between 500F to 1200F and cannot be contacted by the flames from the fire. It must become hot as quickly as possible to prevent clogging. Although catalytic converters originally were expensive, their widespread use has reduced prices considerably.