50% Thermal Efficiency Depends on Several Factors Including the Surface Area of the Pot

A boundary layer of still air on the bottom and sides of a pot keeps the hot gases from actually contacting the surface and is a dominant factor in heat transfer efficiency.

1. According to Newton’s Law, doubling the surface area doubles the heat transfer when the temperature and velocity of the gases are constant.
2. In a Rocket stove at high power, the gases can be around 800C and the velocity can be around 1.2 meters per second.
3. Keeping a constant cross-sectional area in the pathway the gasses take through the stove is important. Reducing the constant cross-sectional area channels under and around the sides of a pot to 0.75 of that area helps to keep the gases hot and flowing at highest velocity.
4. The 0.75 cross sectional channels encourage the gases to thin the boundary layer increasing heat transfer.
5. Pots have to have sufficient external area to achieve 50% thermal efficiency.
6. In recent tests of optimized Rocket stoves, a pot with an area of around 800cm2 scored 34% thermal efficiency. Increasing the area to around 1000cm2 increased thermal efficiency to about 40%. In the same stove, a pot with 1200cm2 can be expected to result in above 45%. We use 26cm to 30cm in diameter pots with at least 5 liters of water to get closer to 50% thermal efficiency.
7. Keep in mind that increasing the surface area of the water in a pot also increases the amount of steam, which makes bigger pots harder to bring to full boil without a pot lid.
8. Thermal efficiency, when burning biomass, tops out (so far) at around 55%. The gases in the channels at the bottom and sides of the pot loose temperature and velocity resulting in an upper limit to heat transfer efficiency. 
9. Raising the temperature and velocity of the gases will increase efficiency.