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Pressures generated during combustion

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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Pressures generated during combustion

Unread postAuthor: pumpkin2themax » Mon Oct 16, 2006 8:29 pm

What kind of pressure is generated when propane combusts in an 1800 cc chamber? Has anyone ever had a chamber fail explosively using propane? just curious what im getting into.

nate

*im using sch 40 and 80 pvc
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Unread postAuthor: GalFisk » Tue Oct 17, 2006 6:38 am

The pressure is in the range of 100-120 psi for a closed chamber explosion, much less when launching a projectile. Sch40 will not fail under these conditions. The most common failures are of the cleanout plug, if the plug is unrated, damaged, heavily modfied or not screwed in properly it can fail or come loose.
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Unread postAuthor: sgort87 » Tue Oct 17, 2006 11:50 am

It's not an explosion. It's a deflagation.

A standard combustion launcher will generate a fairly constant 40-50 psi during the deflagation while a pneumatic launcher would start at say 60 and be down to say 30 by the end of the shot with a 1:1 ratio.
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Unread postAuthor: Pete Zaria » Tue Oct 17, 2006 12:47 pm

In theory, an ideal (stoichiometric) propane/air mixture would produce 102 psi at 75 degrees F at sea level, according to my (not so amazing) math.

In practice, since nothing is ever perfectly ideal, I'd guess more like 80 psi, 90 max.

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Unread postAuthor: boilingleadbath » Tue Oct 17, 2006 3:10 pm

...mind sharing your math, pete?
I've been getting ~86 psig, starting at STP.

However, using latke test data, I've figured out that the max pressure in the bore is (if you believe it) >70 PSI.
This looks almost like a fluke, because in the next "frame" (based on the speed increase between a 25:1 and a 15:1 ratio), the pressure is >38 PSI, nevermind that the pressure then <i>goes up</i> slightly over the next few frames in a likely-looking hood shape.
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Attachments
EVBEC v1.2-PressureAn.zip
Pressure analysis. Look at the bottom.
Ignore the EVBEC utility above it. Feel free to question me about my math.
(9.4 KiB) Downloaded 139 times
Last edited by boilingleadbath on Wed Oct 18, 2006 3:47 pm, edited 1 time in total.
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Unread postAuthor: Pete Zaria » Tue Oct 17, 2006 3:48 pm

I just realized, I forgot to subtract 14.7 for starting pressure. 102 - 14.7 = 87.3 . Close enough to your ~86.

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Unread postAuthor: joannaardway » Tue Oct 17, 2006 4:59 pm

The "pressure differences" for the high ratios in the latke data can be explained by the considerable muzzle blast, which will have affected the speed of the projectile noticably.

Those 25:1 and 15:1 ratios need to be taken with a pinch of salt.
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Unread postAuthor: brumby » Wed Oct 18, 2006 1:11 am

what would be the pressure for arosol can combustion
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Unread postAuthor: boilingleadbath » Wed Oct 18, 2006 3:50 pm

The fuels in most aerosols are [very similar to] propane, at least as far as preformance goes; the max pressure you'll get will be about the same, but will only happen when you get the fuel mixture just right.

Of course, because the fuel mixture isn't consistent with hand-metering, <i>most of the time</i> you'll get lower pressures than a propane system.

(by the way, I've updated my spreadsheet; it now includes a graph)
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