Radiation wrote:trigun wrote:Well propane is heavier than air.
So when you inject proane in any chamber it technically sinks to the bottom. The fan would begin circulating inside and move the propane and air together spreading said propane in the chamber.
Uh... No. Propane will not "sink to the bottom" it will disperse evenly over the course of several minutes possibly hours. The molecular weight doesn't matter with gases as they tend towards randomness. A fan simply speeds up this process.
It is true that a heavy gas won't sink to the bottom very effectively but if it is injected at the bottom it will tend to stay there. Indeed, it will tend to "stay" wherever it is injected especially if diffusion is the only driving force for mixing. You're right that for complete mixing it can take anywhere from several tens of seconds to several minutes, depending on the temperature, chamber geometry, velocity that the fuel was injected at etc.
The molecular weight won't affect the final distribution but it does affect how quickly the gas will equilibrate (diffuse) on it's own. The rate of diffusion scales as the square root of the molecular weight. For most spudgun fuels it doesn't make much difference, propane (MW=44) will diffuse SQRT(58 ) / SQRT(44)=1.15 times faster than butane (MW=58 ). If you used a very light fuel like hydrogen (MW=2), it will diffuse SQRT(44) / SQRT(2)=4.7 times faster than propane.
Diffusion is strongly dependent on the temperature. Fuels will diffuse noticably slower on a cold winter day then they will on a hot summer day.
EDIT: Stupid smileys