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Measuring how fast propane mixes without a fan

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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How long until there is a combustible mixture at the spark gap?

1 minute or less
1
25%
a few minutes
2
50%
an hour or so
1
25%
a day or longer
0
No votes
this is a stupid poll because there is no way jimmy has the patience to let the thing sit long enough.
0
No votes
 
Total votes : 4
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Measuring how fast propane mixes without a fan

Unread postAuthor: jimmy101 » Tue Aug 21, 2007 11:09 am

I've setup a 12"L x 3"ID combustion chamber. The chamber does not have a fan (well actually it does but it is not turned on).

The barrel has been replaced with a threaded plug so this is a closed chamber. The gun has a spark gap 10" from the breech end of the gun, almost all the way to where the barrel should be.

The spark source is a "100KV" stungun.

I injected the appropriate amount of propane (55cc, measured with a syringe) into a port located about 1" from the breech end of the chamber (roughly 9" from the spark gap).

The chambers is lying horizontally on my work bench. It has not been, and will not be, moved. It is well sealed. The ambient temperature is about 85F.

Question: How long will it take to get a combustible mixture at the spark gap?
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 21, 2007 11:17 am

When I inject butane into my combustion cartridge with a syringe, it's ready for firing almost immediately - does it take that long to disperse?
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Unread postAuthor: paaiyan » Tue Aug 21, 2007 11:25 am

jackssmirkingrevenge wrote:When I inject butane into my combustion cartridge with a syringe, it's ready for firing almost immediately - does it take that long to disperse?


Diffusion of gasses is not instantaneous, so it does take some amount of time to disperse. It won't take hours, but you would probably notice better results if you waited a minute rather than firing immediately, however you also may not, due to the tendency of both propane and butane to sink to the bottom. This can be remedied by the all-important chamber fan.
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Unread postAuthor: noname » Tue Aug 21, 2007 11:26 am

My small combustion (3" diameter x 5-6" long) uses 32-40 cc of propane injected into the center of the chamber. I don't have a chamber fan (but will get one) and the gun can shoot as soon as I screw the schrader valve cap on again.
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Unread postAuthor: mark.f » Tue Aug 21, 2007 6:08 pm

I believe, from my ventures in last year's chemistry class, that the rate of <B>effusion</B>, (which is similar to, but not identical to <B>diffusion</B>, which is what you are asking about), is something like inversely proportional to the square root of the molar mass, or something like that. I <I>believe</I>, that's it, but you might wanna check it. You might also find the law for diffusion, which I completely forgot. :wink:

It's proportions, so you couldn't input your variables and just get an answer, but you could find more readily available diffusion rates for more common gases like nitrogen, and then use the proportion to solve for the rate of diffusion for propane.

Good luck!
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