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fuel air mixtures left for extended periods of time

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: jackssmirkingrevenge » Thu Nov 06, 2008 4:17 pm

Cheers Jimmy, just what I wanted to hear :D
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Unread postAuthor: psycix » Fri Nov 07, 2008 3:48 pm

So diffusion is more powerful then the weight of the molecules?
Does this also apply in extreme cases, like mixing hydrogen with a very very heavy gas?
And what if the chamber was a very long and tall column?
Instead of separating, I could surely imagine that a difference of concentration would occur.
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Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!

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Unread postAuthor: jimmy101 » Fri Nov 07, 2008 9:12 pm

psycix wrote:So diffusion is more powerful then the weight of the molecules?
Does this also apply in extreme cases, like mixing hydrogen with a very very heavy gas?
And what if the chamber was a very long and tall column?
Instead of separating, I could surely imagine that a difference of concentration would occur.

Yes diffusion is more powerful than earth's gravity. Diffusion is driven mostly by entropy (disorder). When gases mix their entropy increases (disorder increases) and the total internal energy of the gases decreases. To separate the gases you must increase their internal energy substantially since the separation involves a large increase in order (decrease in entropy). The entropic affects are much stronger than the enthalpic affects. In this case the enthalpic affects are basically due to gravity.

A very tall column still won't significantly separate the light and heavy gases. With a tall enough column, a large enough difference in molecular weights between the two gases and with a sensitive enough detector you might be able to just barely detect differences in concentration at the ends of the column. The difference would be of little practical use.

A long skinny column will equilibrate very slowly if you inject another gas at one end.

The earth's atmosphere is basically a column ~50 miles tall. There is only minimal separation of the light gases (like helium) from the heavier common gases like CO2. Of course, the atmosphere has winds that tend to mix things back up, but even so, there is very little separation.

There are a number of techniques to separate gasses based on their molecular weights (besides distillation). Off hand, they generally involve either extremely high G forces (as in centrifugal separators for the purification of U238 fluorides) or extremely high pressures, such as jet separators. Jet separators blow the gas mixture through a narrow gap between two pipes. The lighter molecules diffuse radially a bit faster than do the heavier molecules (IIRC, as the square root of their masses). All these techniques are extremely inefficient and are generally done in many repetitive stages.

As a routine matter it is safe to say that once a mixture of gases is fully mixed it will not unmix.

The same is true for miscible liquids. If you add pure water to salt water then mix it up well there is zero possibility that the water and salt water will spontaneously separate back into two layers. It is possible to layer pure water on top of salt water, particularly if the two are cold. If done carefully you can get distinct layers that will persist for a very long time, years if it isn't disturbed. But give the container a couple shakes and you have a mixture that will not separate back into two layers.
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Unread postAuthor: starman » Sat Nov 08, 2008 1:15 am

Thanks for that bit of illumination Jimmy.
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Unread postAuthor: jimmy101 » Sat Nov 08, 2008 2:08 am

waffles again.
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Unread postAuthor: starman » Sat Nov 08, 2008 2:41 am

jimmy101 wrote:waffles again.


:? :?: Uuhhh, Belgian or Eggo?
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