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Help with fueling setup

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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Help with fueling setup

Unread postAuthor: skyjive » Sun Dec 05, 2010 8:35 pm

Hi, I am thinking of doing some work with combustions, which I haven't done before, and am sort of screwing around with stuff to figure it out. To that effect I am trying to get a chamber of metered fuel/air mixture to explode (no barrel). My setup is a length of pipe which attaches to a small meter pipe at one end and has a spark plug at the other end. I've done my homework and have calculated that the following dimensions should work:

Air pipe volume: 14.5 cubic inches
Propane pipe volume: .22 cubic inches

The propane pipe is filled with PRESSURIZED propane at 30 psi, then dumped into the air pipe. This should produce a very close to stochiometric mix. After a pause to let it diffuse (hopefully), I spark the sparkplug (which is powered by an ignition coil and I know works just fine), and...

nothing happens.

Any wisdom on what I'm doing wrong?

Pic of setup: big galvanized nipple is air pipe, little brass guy w/2 BVs is propane pipe, spark plug in cap at end of air pipe:

Image
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Unread postAuthor: Lockednloaded » Sun Dec 05, 2010 8:50 pm

how can you tell if it goes off?
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Unread postAuthor: saefroch » Sun Dec 05, 2010 9:15 pm

skyjive wrote:I am trying to get a chamber of metered fuel/air mixture to explode (no barrel).
As said above, that might be your problem. The mix also doesn't explode, per se. It deflegrates, or burns. It does not explode. If you already knew that, I apologize, but your wording seemed to suggest otherwise.

The other problem might be that your chamber is very long, you're not mixing it, and the ignition is on the opposite side as the propane injection.

You should also know that what you have here is not a combustion, but a 1x hybrid. The injected propane isn't displacing any air. As such, your fueling may be a little off, but it should still be within the limits of combustion. Suggestion: Set it up, let it diffuse together, open the ball valves and spark it. If you don't hear anything then, let us know.
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Unread postAuthor: ilovefire » Sun Dec 05, 2010 9:16 pm

how big is your spark gap, plus you might need to use a chamber fan to actually mix it
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Unread postAuthor: Hotwired » Sun Dec 05, 2010 10:06 pm

Use a transparent plastic fizzy drinks bottle instead of a steel nipple.

It won't burst with such a low concentration of fuel/air but will give a flash when the mixture combusts.
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Unread postAuthor: Selador » Sun Dec 05, 2010 10:38 pm

Lockednloaded wrote:how can you tell if it goes off?


I'm with LockednLoaded here.

The only way I can see for this to work is if the two ball valves are opened again, just before ignition.

Otherwise, there is nowhere for the combustion to go.

And then, how can you tell if anything at all happened ?
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Unread postAuthor: DYI » Sun Dec 05, 2010 10:50 pm

Come on guys, don't assume he's an idiot immediately. It takes a special kind of stupid to research the topic and leave thinking that air/propane at 1X might rupture SCH40 steel. He's just taking a rather difficult approach to the problem. The steel pipe probably isn't sealed at all, and he probably unthreads the setup and checks the remaining gases after he pushes the ignition to see if combustion occurred. It's easy enough to tell whether a gas mix supports combustion with a match or something similar.

Achieving ignition under certain conditions is rather tricky, skyjive, and you've got one here - small diameter pipe, with ignition at one end and no supplementary oxidizer. The limits of combustion here are quite narrow, and they're obviously not being reached at the sparkplug. My first guess would be that the mix is lean near the ignition.

I'd suggest a shorter, larger diameter main pipe. Using clear tube, if possible, will obviously make the testing easier as you won't need to unthread and check the remaining gases after each test. Also, recheck your meter calculations, just to be sure.

It does not explode.

In the interests of accuracy, it should be noted that "explode" does not denote a detonation, or even a deflagration. In fact, the meaning of the word is so vague as to make it almost entirely useless. A balloon "explodes" when you stomp on it, a cloud of poorly mixed air/propane "explodes" when it is ignited (despite possibly taking a second or more to burn fully), and a barrel of ANFO "explodes" when initiated with a few kilos of secondary. When using the word "explode", you make no distinction whatsoever.
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Unread postAuthor: skyjive » Sun Dec 05, 2010 11:08 pm

Ok thanks for the advice everyone I guess I just need to redesign my setup.

And while I guess "explode" is a bit imprecise, I think everyone here knows what I'm talking about in the context of an atmospheric pressure mixture of propane and air being exposed to a spark.
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Unread postAuthor: saefroch » Sun Dec 05, 2010 11:13 pm

DYI wrote:It takes a special kind of stupid to research the topic and leave thinking that air/propane at 1X might rupture SCH40 steel.
Who said that?

saefroch wrote:your wording seemed to suggest otherwise.
This little addition was intended to avoid me getting into trouble with the ambiguity of "explode" and the fact that it implies (to me, at least) detonation. :? Guess it didn't work.

I've been taking apart threaded connections for maintenance, not re-taping them every time, and no leaks. That may or may not be an issue here. :?

Would a completely sealed system even make any sound if it combusts at 1x? One would probably be able to tell from the smell of propane if it combusted, so this is more a slightly related question than anything.
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Unread postAuthor: Technician1002 » Mon Dec 06, 2010 12:08 am

It is possible the long length of pipe is stratifying the charge. It may be too lean at the plug to ignite. A small paint can rattle type ball may help mix the contents. Drop a marble or a couple airsoft pellets in the chamber and shake it. The positive displacement agitation will mix the contents.
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Unread postAuthor: cfb_rolley » Mon Dec 06, 2010 12:30 am

I am a little confused, or I can't see the picture properly on my phone. Are you trying to use the force of the combustion to rupture the pipe?

In any case I would side with dyi and suggest that the mixture isn't mixed well enough and leads to being too lean at the spark. A spark gap in the middle or multiple gaps would be beneficial.
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 06, 2010 12:56 am

I've had hybrids go off without bursting the disk, you can tell because the chamber will warm up considerably.
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Unread postAuthor: skyjive » Mon Dec 06, 2010 1:47 am

Just to clarify, I am not trying to burst the pipe, and am just trying to get a feel for how combustion stuff works. Just out of curiosity, how high a mix is it safe to use sch40 PVC with? I assume its easily safe for 1X.
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Unread postAuthor: jackssmirkingrevenge » Mon Dec 06, 2010 2:53 am

The rough rule of thumb is ≈100 psi generated per mix
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Unread postAuthor: cfb_rolley » Mon Dec 06, 2010 2:33 pm

Sch 40 is generally considered fine for an advanced combustion, though pressure rated PVC is always better in my opinion because you know what it can take. Though for a hybrid most people discourage using plastics.
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