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Heat and pressure ARE related

A place to ask general spud cannon related questions.
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Heat and pressure ARE related

Unread postAuthor: caspar97 » Tue Oct 12, 2010 3:04 pm

According to so some scientific law, I think its Charles's Law, if pressure is increased, the temperature also rises.
Now, theoretically say there is a certain material/substance that bursts into flame at a certain temp. In your hybrid is a tiny, tiny piece of that and your accelerant. You start pumping it up to pressurize the cannon, and according to the scientific law, the temperature would also rise. At a certain pressure (therefore being a certain temperature) the material/substance would ignite, bursting the burst cap and one would get one hell of a powerful hybrid.

In theory, would this work?

P.S. I know that there are materials out there with such properties, but actual names would be wondrous
:D :idea:
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 12, 2010 3:13 pm

Have quick a read ;)

I don't think pressurising it slowly would work as it would allow the heat to dissipate, you need to shock it like the Barracuda does with its spring piston.
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Unread postAuthor: POLAND_SPUD » Tue Oct 12, 2010 3:31 pm

I've notice on one of my pneumatics that the chamber gets pretty hot after a couple of shots. Which is acctually quite interesting given that it is first pressurised (and as a result the temp increases) and then depressurised (which lowers the temp.) when the gun fires.

My conclusion was that during and after filling the chamber heat had more time to get from air to the chamebr than the other way round upon firing. Don't ask me how fast you have to compress air to ignite mixed propane&air

I recall psycix's idea to fill the dead volume of pneumatic with acethylene. In theory it should ignite upon firing and increase the SOS as well as pressure, which will result in MOAR power. AFAIK it hasn't been tested yet
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Unread postAuthor: ramses » Tue Oct 12, 2010 3:32 pm

That's dieseling, and the compression must happen quickly. It can be induced in spring airguns by putting starting fluid, diesel fuel, or something in the base of the pellet. This is hard on the gun and inaccurate, but adds power.
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Unread postAuthor: SpudBlaster15 » Tue Oct 12, 2010 3:33 pm

You're thinking of Gay-Lussac's law, which will only apply if the system is adiabatic. At realistic pumping speeds, it will behave much more like an isothermal process, as heat conduction to the chamber walls will keep the temperature inside the vessel fairly constant.

To make this work, you would need to pressurize the chamber very rapidly, and find a material with a sufficiently low autoignition temperature to reliably ignite at low compression ratios.

It's impractical, and much easier to simply use an external energy source such as an electrical arc or a heating element.
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Unread postAuthor: Technician1002 » Tue Oct 12, 2010 3:44 pm

When you compress air into a chamber, all the heat couples to the chamber warming it. When you fire it at say 8 bar, the air expands and most of it leaves the chamber. Most of the cold is in the barrel and beyond. This is why the chamber remains warm. Less heat is removed by the smaller mass of cold air remaining in the chamber.

Be careful with acetylene. It is unstable and may detonate. Diesel oil needs about 12+ bar minimum for compression heat to ignite it. As mentioned above, the compression must raise the temperature faster than it radiates to the walls.
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Unread postAuthor: Fnord » Tue Oct 12, 2010 4:01 pm

Something I noticed once.
I made a simple copper piston cannon a long time ago. I would fill it from an air compressor tank, so the pressurized air was room temp. I attached a male QD coupler where the pilot valve was at one point, so it fired rapidly when you attached/detached the air line.

After about 30 shots the chamber was fairly hot.
Anyone know why? The air should not have experienced any net gain in temperature from just flowing from one chamber to another. Could air friction really be that significant?
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Unread postAuthor: POLAND_SPUD » Tue Oct 12, 2010 4:54 pm

@fnord
uhmm I guess it's the same effect that I mentioned in the post above.
I wonder how this can affect PVC cannons ??
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Unread postAuthor: DYI » Tue Oct 12, 2010 6:16 pm

Me and Larda were tossing around an idea a while ago using this idea, minus the heating effect. Although it was far too long ago for me to remember the specifics, it was essentially a solid fueled hybrid (the fuel being a flammable powder like flour or icing sugar, possibly mixed with volatile liquid hydrocarbons for ease of ignition, depending on initial results). The fuel would be held in the main chamber and a blast of high pressure oxygen (>2kpsi) dumped through it from a secondary chamber, distributing and igniting the fuel simultaneously. That was the theory at least. The amount of difficult and expensive testing required to make it work effectively and safely meant that neither of us got around to it.

It did, however, carry the potential to generate rather high pressures while doing away completely with the fuel metering system (very expensive on high mix hybrids). While not capable of the extreme velocities CLGGs are, it would certainly be worth a try. The major concern in such a system would be the valve to dump the oxygen; almost any conceivable seal would be destroyed either by chemical interaction with the O<sub>2</sub>, or the post-ignition conditions in the propellant gas. A BP actuated burst disk would likely be the most affordable option for initial testing (for certain totally unrealistic values of "affordable" :roll: ).

As to the construction of what was proposed initially: a moving piston to compress the gas rapidly enough would be a necessity. Modeling how the piston moves during the gun's operation is a nasty job indeed, and an excessively fast or rebounding piston can severely stress or completely destroy the launcher. This not to mention how the piston's seals would fare. The application of this design to anything other than small plinkers is getting into some very difficult design territory, which probably isn't tremendously beneficial. However, as always, if you've got the money and inclination to try it anyway, I'd be happy to help with the design.
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Unread postAuthor: saefroch » Tue Oct 12, 2010 6:20 pm

To Fnord: What you're observing happens to every pneumatic, when you put air in, the temperature raises and heats up the chamber, so some of the heat energy is lost to the chamber walls. Then you fire it, which results in a loss of heat, but of such a small margin, probably because of a density increase or sommat.

In a nutshell, your chamber material is stealing heat from the system.

I'm pretty sure PVC cannons can also heat up. And just as an afternote... I was looking at the spray 'n' pray a friend and I built two years ago... and the PVC on it is rated to "240psi at 70F" Is that correct? Can 2" PVC really take 240 psi?
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 12, 2010 11:08 pm

saefroch wrote:the PVC on it is rated to "240psi at 70F" Is that correct? Can 2" PVC really take 240 psi?


Given the liability culture expanding out of the US, would you really print that on your product if it weren't the case?
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Unread postAuthor: caspar97 » Wed Oct 13, 2010 6:31 am

Ahhh... well i see what would have to happen for something like that to take place and @SpudBlaster15, most all of my ideas are completely impractical and way overcomplicated :D

still, does anybody know a material with a really low temperature autoignition point??
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Unread postAuthor: Fnord » Wed Oct 13, 2010 2:52 pm

uhmm I guess it's the same effect that I mentioned in the post above.
I wonder how this can affect PVC cannons ??


I thought you were using a manual pump.

when you put air in, the temperature raises and heats up the chamber


The thing is, the air wasn't being pressuized. It was already pressurized in the compressor, then left to sit and cooled off. If anything, it was decompressing when filling and the walls of the chamber should have had the heat sucked out of them by the incoming gas.
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Unread postAuthor: Ragnarok » Wed Oct 13, 2010 3:16 pm

Fnord wrote:The air should not have experienced any net gain in temperature from just flowing from one chamber to another.

It's to do with the different chamber sizes. Taking or adding X moles of gas from/to a container will affect its temperature less the larger the container is (because it's a smaller overall pressure change), and vice versa (i.e more) if it's smaller.

Hence, the gas temperature falls less when being taken out of the bulk tank than it gains being re-pressurised in the smaller chamber. Consequently, the temperature of the smaller chamber increases.
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Unread postAuthor: maverik94 » Thu Oct 14, 2010 1:52 pm

According to so some scientific law, I think its Charles's Law, if pressure is increased, the temperature also rises.
Now, theoretically say there is a certain material/substance that bursts into flame at a certain temp. In your hybrid is a tiny, tiny piece of that and your accelerant. You start pumping it up to pressurize the cannon, and according to the scientific law, the temperature would also rise. At a certain pressure (therefore being a certain temperature) the material/substance would ignite, bursting the burst cap and one would get one hell of a powerful hybrid.

That's essentially how a diesel engine works!
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