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.
- According to Newton’s Law, doubling the surface area doubles the heat transfer when the temperature and velocity of the gases are constant.
- In a Rocket stove at high power, the gases can be around 800C and the velocity can be around 1.2 meters per second.
- 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.
- The 0.75 cross sectional channels encourage the gases to thin the boundary layer increasing heat transfer.
- Pots have to have sufficient external area to achieve 50% thermal efficiency.
- 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.
- 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.
- 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.
- Raising the temperature and velocity of the gases will increase efficiency.