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Contained Combustion.

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Contained Combustion.

Unread postAuthor: frankrede » Fri Nov 24, 2006 9:37 pm

If you had a lenght of pipe that had 2 end caps glued on and you injected propane into it and ignted it, would teh pressure be stored or would it just explode. Maybe you could store the pressure from the explosion? I know this doesn't sound right but I'm not sure.
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Unread postAuthor: djt » Fri Nov 24, 2006 10:07 pm

i think someone told me once that the pressure is caused by the heat and that its only there for a split second. dont hold me to it though
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Unread postAuthor: frankrede » Fri Nov 24, 2006 10:25 pm

Ok I knew I was forgetting something.
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Unread postAuthor: SpudStuff » Fri Nov 24, 2006 10:28 pm

Yes, you will gwt ~75 PSI for a very short time. Then it will go back down to zero. But the pipe might start melting from the heat.
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Unread postAuthor: Scope » Fri Nov 24, 2006 10:35 pm

Well I dont believe it wouldnt explode (correct me if im wrong) because the pressure caused by a combustion(isnt it like 100psi or something) is not enough to over come Sch 40 pvc (again isn't it like 160psi).

If it did not explode it would not hold its pressure. Matter cannot be created nor destroyed. This means that if you have something and you burn it the byproduct(what ever stays in ur chamber {carbon moxide or w.e}) will equal what ever you put into the chamber (say propane). So when you burn something you arent actualy creating or destroying anything(chemicaly) you are changing this something into different chemicals and energies.

Phew well that explains why you wouldnt have different pressures before and after minus the explosion.

As for why you have all that energy created. Moleculs (sp) that are hotter are more spread out then colder moleculs. This explains why frozen water is hard and why really hot water (steam) isn't mostly visable. When you create the reaction inside the chamber it causes all the temperatures inside the chamber to spike greatly thus superheating the moleculs and maknig them expand (increasing the psi in the chamber). When the temperature goes down the gasses inside the chamber(byproducts) become closer (molecularly) and the psi goes back down.

I think that covers it.
Im not 100% sure about that but im pretty sure so if anyone can back me up or argue what i say i appreciate it.
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Unread postAuthor: frankrede » Fri Nov 24, 2006 11:22 pm

Ok, wow, thats alot.
Thanks, I was wondering if anyone knows how long it would take to cool down? Maybe you could make a portable pneumatic that way.
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Unread postAuthor: Scope » Fri Nov 24, 2006 11:28 pm

Enough for the pressure to go down. Ive heard 1/4 a second. Its only a spike...
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Unread postAuthor: pyromanic13 » Fri Nov 24, 2006 11:40 pm

umm? once it combusts, don't the atoms/molecules or whatever speread out more?
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Unread postAuthor: SpudBlaster15 » Sat Nov 25, 2006 12:48 am

The pressure generated is caused mostly by the expansion of the combustion products as a result of heat. The pressure spike would be very short lived, as the heat of combustion would be absorbed by the products as they form. The pipe would not burst unless other variables were present such as improper solvent cementing.
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Unread postAuthor: rna_duelers » Sat Nov 25, 2006 1:54 am

Wouldnt a vacum form?
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Unread postAuthor: ProfessorAmadeus » Sat Nov 25, 2006 2:16 am

Maybe... Then it would implode. That would be cool.
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Unread postAuthor: frankrede » Sat Nov 25, 2006 2:40 am

Hmm, That would make an awesome video, hint hint people.Thanks for the replys!
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Unread postAuthor: mark.f » Sat Nov 25, 2006 7:28 am

Scope wrote:
If it did not explode it would not hold its pressure. Matter cannot be created nor destroyed. This means that if you have something and you burn it the byproduct(what ever stays in ur chamber {carbon moxide or w.e}) will equal what ever you put into the chamber (say propane). So when you burn something you arent actualy creating or destroying anything(chemicaly) you are changing this something into different chemicals and energies.


Yes, but Avogadro's Principle states that a mole of gas, doesn't matter which gas, occupies 22.4 liters of space at STP.

When you look at the chemical equation for the combustion of propane, you will see that there are 6 molecules of gaseous reactants, and 7 moles of gaseous products. Therefore, a tiny bit of pressure will stay as long as the water, (part of the reactants), stays gaseous.

I estimate you will get around 116 PSI peak pressure.
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Unread postAuthor: SpudBlaster15 » Sat Nov 25, 2006 1:50 pm

I believe a propane-air reaction produces a peak pressure closer to 87psi in a closed chamber that contains only a stoichiometric mix of the reactants. Much less in a combustion cannon, where the peak pressure spike occurs after extra volume has become available for the expanding gas to fill.
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Re: Contained Combustion.

Unread postAuthor: Flying_Salt » Sat Nov 25, 2006 2:26 pm

frankrede wrote: Maybe you could store the pressure from the explosion?


That would be awesome if you could. Then you could make pvc rockets, pressureize them, then shoot them for a satisfying explosion.
Anywhoo, what sparked this idea in the first place?
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