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Rate of propane diffusion in air

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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Rate of propane diffusion in air

Unread postAuthor: jimmy101 » Wed Feb 27, 2008 2:10 pm

For several years I've wondered how fast a shot of propane will diffuse in a combustion chamber. To get the fuel to ignite it has to be pretty close to fully mixed. To get it to burn optimally it probably needs to be very close to fully mixed.

So, off to the lab we go (well, actually, out into the cold garage) to measure the diffusion rate of propane in air.

First we need a suitable chamber. To make it easier to measure the diffusion rate I will use a long skinny one, a polycarbonate 4' fluorescent light bulb protector (actual dimensions 44.1" x 1.69", aspect ratio 27:1).
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We'll close the ends of the chamber off with plastic wrap held in place with rubber bands. This gives basically airtight seals that will blow off at fairly low pressure. We'll measure and inject the fuel into a small hole at one end of the chamber using a syringe. The other end of the chamber has a spark gap and vent hole. During fuel injection both holes are uncovered. During equilibration and firing both holes are covered with tape.
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So, inject, wait a while for the fuel to diffuse, then hit the trigger and see if there is a bang.

I tried many shots (for details see here.)

My original plan was to fuel the chamber and then attempt ignition every couple minutes. I discovered that multiple non-productive ignition events lead to the chamber never igniting. Apparently, unsuccessful ignitions deplete the fuel and/or oxygen in the chamber and drop the mixture below the lower combustibility limit.

Bottom line, the test chamber requires about 30 minutes of equilibration to get a combustable mix at the spark gap located 43 inches away from the injection point. Therefore, the diffusion rate of propane in air is about 1.5 minutes/inch of length. This gives a very well equilibrated mixture, ~97% equilibrated, at the spark gap.

The diffusion rate to a combustible mixture, ~75% equilibrated, is of course faster, about 0.6 minutes/inch.

These are worst-case numbers. I was purposely trying to get the fuel to diffuse as slowly as possible. No mechanical mixing (e.g., a fan), the chamber was held still during fueling and equilibration, the fuel was injected slowly at one end of the chamber, the spark gap was at the other end of the chamber, the chamber is long and skinny, it was pretty cool in the garage (40~50F), etc.

YMMV
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Unread postAuthor: psycix » Wed Feb 27, 2008 4:43 pm

30 minutes??
Thats alot!

That proves that chamber fans are really good things to have since they mix 100% perfect within seconds.
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Unread postAuthor: D_Hall » Thu Feb 28, 2008 9:14 pm

Any chance of repeating the test using more "realistic" conditions? By that, I mean nothing more than injecting the fuel at 15 psi or so (ie, something more typical and obviously turbulence producing)?
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Unread postAuthor: Velocity » Thu Feb 28, 2008 9:38 pm

psycix wrote:30 minutes??
Thats alot!

That proves that chamber fans are really good things to have since they mix 100% perfect within seconds.


Questionable...
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Unread postAuthor: jimmy101 » Fri Feb 29, 2008 1:00 pm

D_Hall wrote:Any chance of repeating the test using more "realistic" conditions? By that, I mean nothing more than injecting the fuel at 15 psi or so (ie, something more typical and obviously turbulence producing)?

Not sure what you mean by more "realistic". The injection method is basically what happens when you inject with a syringe, which is how I fuel my guns. The test posted above were fueled pretty slowly, perhaps ten seconds total to inject the 68cc. So it is probably more realistic to inject the fuel in just a couple seconds.

Injecting the fuel faster and not trying to more or less keep it all at one end certainly will mix faster. So would injecting near the middle of the chamber.

Injecting at higher pressure probably will help but I'm not sure of how much. Doesn't really matter what the injection pressure is, it is still just 4% of the chamber volume, which I would think would be the more important factor.

I can certainly push the syringe harder/faster. IIRC my needle is 20G, the ID is pretty small, so the gas velocity and pressure is pretty high. Rough estimate would be I can get pressures in the syringe of at least 1ATM, and probably 2 or more, above ambient.

I'm willing to give it a try, what exactly would you like to see?

BTW, the chamber is venting as it is being filled, which probably slows down mixing more than it would in a typical meter setup where the chamber isn't allowed to vent.
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Unread postAuthor: psycix » Mon Mar 03, 2008 4:38 pm

Velocity wrote:
psycix wrote:30 minutes??
Thats alot!

That proves that chamber fans are really good things to have since they mix 100% perfect within seconds.


Questionable...


okay... Near 100%
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Unread postAuthor: D_Hall » Mon Mar 03, 2008 5:09 pm

jimmy101 wrote:Not sure what you mean by more "realistic". The injection method is basically what happens when you inject with a syringe, which is how I fuel my guns.

Fair enough. I was just thinking in terms of the more "normal" metering pipe. If syringes are your SOP, never mind. :)
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Unread postAuthor: jimmy101 » Mon Mar 03, 2008 6:38 pm

D_Hall wrote:
jimmy101 wrote:Not sure what you mean by more "realistic". The injection method is basically what happens when you inject with a syringe, which is how I fuel my guns.

Fair enough. I was just thinking in terms of the more "normal" metering pipe. If syringes are your SOP, never mind. :)

How much velocity would you get with a normal meter pipe setup? Even with the pipe at say 60 PSIG it's venting through a 1/8" or 1/4" hole isn't it? I wouldn't think the velocity would be all that great.


Following up on the diffusion studies.

I tried to inject fuel as fast as I could to see how that changes the time required to get a combustable mix at the other end of the tube.

I can inject the 68cc in about 1 second. The OD of the syringe needle is 0.049", and I estimate the ID at about 0.043". Simple minded calculation says 68cc through that ID in 1 second is a gas velocity of about 240 FPS.

I was worried that the high injection rate would cause problems with the small vent hole on the far end of the tube so I added an 0.101"D hole (in addition to the original 0.05"D hole) at the spark end.

Same routine as beffore except the needle was pointed towards the chambers far end and injected in about 1 sec. Wait some minutes, hit the sparker.

Chamber fired at 22, 19 and 14 minutes. The chamber didn't ignite at 10 minutes (this loading also didn't ignite at 11 but did ignite at 14 minutes).

The next test failed to ignite at 12 and 13 but did ignite at 15 minutes.

Assuming that only the first attempt at ignition is a valid measurement, it appears the diffusion time to a combustible mixture over the 44" distance is more than 12 and less than 14 minutes. So, the t<sub>1/2</sub> is about 0.15min/inch and 5t<sub>1/2</sub> is 0.75 min/inch.

Fast injection of the fuel increased the apparent diffusion rate by a factor of two for this combustion chamber. Of course, this number will scale very poorly and is dependent on the injection velocity, chamber diameter ...
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