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Propane fueling question

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: jimmy101 » Thu Feb 05, 2009 4:21 pm

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.

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Unread postAuthor: Combustion Monkey » Fri Feb 06, 2009 7:14 am

Thanks for all the solid science that has been posted up. Makes me glad I started the topic! Now its time to find or build a tank warmer that will hold the tank at a steady 75 deg or so. I believe that I have seen such a thing for other applications, but can't remember were at the moment, need more coffee.
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Unread postAuthor: inonickname » Fri Feb 06, 2009 7:20 am

Combustion Monkey wrote:Thanks for all the solid science that has been posted up. Makes me glad I started the topic! Now its time to find or build a tank warmer that will hold the tank at a steady 75 deg or so. I believe that I have seen such a thing for other applications, but can't remember were at the moment, need more coffee.


You can get power bands used to heat homebrews. (beer) Also consider cutting up a heated rug then modifying the system in it. I was trying to build something similar for CO2 paintball tanks but it lacked portability. Good luck.
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