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My introduction to propane

Built a combustion cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: DYI » Mon May 28, 2012 12:26 pm

Propylene actually has a higher energy content than propane, so it's not quite as simple a comparison as that. In both cases it is possible to keep the pressure below a certain limiting value. It's just that using propylene would allow a somewhat longer barrel, owing to its higher n<sub>product</sub> : n<sub>reactant</sub> ratio (the deflagration temperatures are similar when air is the oxidizer).

Alterations in projectile mass (and friction), chamber, barrel, and transition geometry, and fan and ignition point placement can all contribute to altering the projectile base pressure profile

There is a lot of improvement to be had as regards flattening out the projectile base pressure over the barrel length, but actually getting it without simple luck or extensive experimentation depends on the use of modeling software which is very difficult (and time consuming) to write and prohibitively expensive to buy.
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Unread postAuthor: Bowman » Mon May 28, 2012 8:43 pm

jackssmirkingrevenge wrote:
Bowman wrote:I like how everyone doesn't seem to have a problem with it except you.......


I also object, that golf ball should have punched through the plywood like it wasn't even there.


It was because of the barrel size, he wasn't utilizing the full power because the barrel was too short. Look above my post :D
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Unread postAuthor: boyce123 » Mon May 28, 2012 9:15 pm

DYI, could you elaborate on your depiction of propylene as more suited for a longer barrel. I looked at the chemical equations for a complete combustion, propane has 6n yeilding 7n, where as propylene has 11n yeilding 12n. based off this and the fact that propylene combusion is a reaction that occurs at a faster rate under normal conditions (allowing it to release more of its energy, making it hotter for its intended use in plumbing). I may be wrong, but i had been tought to make the assumption that the slower fuel with more work output was more suited for a longer barrel, but its always a possibility to change the chamber geometry to ensure the desired rate, temp, and burn pattern... but i'm not sure how many people, as you said, would go through that amount of engineering on a relatively cheap cannon,
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Unread postAuthor: DYI » Tue May 29, 2012 8:35 am

Boyce, you're looking at an idealized case here, with only CO<sub>2</sub> and H<sub>2</sub>O as reaction products. This is an unrealistic treatment, especially for a high temperature reaction. Complete combustion isn't really what you should be aiming for either - in any hydrocarbon combustion reaction, slightly rich mixes generate higher final pressure for a given initial pressure. Depending on just how rich the mix is and what the fuel is, they may either increase or decrease flame propagation speed (some fuels have their maximum flame speed in slightly lean mixes, others in slightly rich).

Specifically, for "ideal" mixes (those which produce the highest combustion pressure for a set pre-ignition pressure), the ratios are 1.067 for propane/air, and 1.08 for propylene/air.

For the flame propagation speed you'll have to look at research papers on your fuel of choice (combustion research is big nowadays, you'll not have any trouble finding them for "common" fuels. If you find anything you need behind a paywall just PM me and I'll get it for you), but to get a better idea of the performance of various mixes, GasEq is a powerful (and easy to use) tool. GasEq allows the inclusion of many more product species than you would care to work with by hand, and already contains all the necessary rate data.
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Unread postAuthor: boyce123 » Tue May 29, 2012 11:39 pm

I know that i have research to do... but as i said in my original post, the cannon is still in progress, and once it is entirely completed, i will experiment and research fuel composition/mixtures that will best suit my situation. On a slightly different note, i was thinking of encasing the chamber with bedliner, do you think this would help in case of DDT, or even increase the chamber's resiliance. I know its alot to ask from bedliner, but after seeing the mythbusters episode exploring its capabilities, i hold faith in its potential.
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Unread postAuthor: jackssmirkingrevenge » Wed May 30, 2012 12:00 am

boyce123 wrote:On a slightly different note, i was thinking of encasing the chamber with bedliner, do you think this would help in case of DDT, or even increase the chamber's resiliance. I know its alot to ask from bedliner, but after seeing the mythbusters episode exploring its capabilities, i hold faith in its potential.


I doubt that would be a useful route to take. If you are genuinely concerned that your PVC cannon might explode, discard it and make a metal one. If you want to wrap it in something, carbon fibre would seem like a better idea.
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Unread postAuthor: boyce123 » Wed May 30, 2012 7:57 am

i'm not concerned that the cannon will explode, but ddt always has a potential of occuring, i might do some tests with bedliner and other materials to see how strong they are under the type of stress i would be concerned with..
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