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another DDT question.

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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another DDT question.

Unread postAuthor: SpudFarm » Sat Apr 26, 2008 6:24 pm

when i am going to build my hybrid i start thinking about DDT hazard.

the chamber is going to be 24" of 5" ID pipe..
i am going to operate it at 11x propane/air
any thoughts of DDT in this chamber?
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Unread postAuthor: MrCrowley » Sat Apr 26, 2008 6:40 pm

Not a chance i'd say. As you know DDT prefers long, thin chambers. 24" of 5" pipe should be fine at 11x.
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Unread postAuthor: rp181 » Sat Apr 26, 2008 6:40 pm

if your worried about it, then use a spark strip of 2 gaps, dont even need to drill another hole.
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Unread postAuthor: SpudFarm » Sat Apr 26, 2008 6:45 pm

i am planning at 3gaps..
so you think it is fine?
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Unread postAuthor: MrCrowley » Sat Apr 26, 2008 6:56 pm

Yeah, you'd probably need it several meters long for DDT. And I can't remember, but does DDT have a higher chance of occurring at a lower mix then higher?
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Unread postAuthor: rp181 » Sat Apr 26, 2008 6:57 pm

with a higher mix, more fuel combusts at once, creating a denser/ faster shock wave

EDIT: yes,your fine with 3.
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Unread postAuthor: MrCrowley » Sat Apr 26, 2008 7:12 pm

rp181 wrote:higher mix, with a higher mix, more fuel combusts at once, creating a denser/ faster shock wave

Hmm must of mis-read something. For some reason it just pops to mind.
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Unread postAuthor: willarddaniels » Sat Apr 26, 2008 8:34 pm

MrCrowley wrote:Not a chance i'd say. As you know DDT prefers long, thin chambers. 24" of 5" pipe should be fine at 11x.

Amen.
rp181 wrote:if your worried about it, then use a spark strip of 2 gaps, dont even need to drill another hole.

Amen.

I created a thread about this over a year ago and I've been meaning to say this again for some time as more people have been talking about over 8X mixes... At 70 degrees F, propane will liquify at 125 PSI. If your total mix is above 125 PSI, you do not have a stoichiometric combustion. This brings in the use of oxygen in place of compressed air in mixes over 8X. I am not talking about using O2 as an oxidizer, but as a replacement to air. Air is about 21% O2, most of the rest is nitrogen (it doesn't burn.) So, if you take out the Nitrogen and just use O2, you can theoretically go up to about 30X.

My point? Consider using O2 instead of compressed air at very high mixes. This will cause you to add yet another metering system to your gas influent- but it is necessary and worth it.

On another note: I think your project is a complete waste of time and money. I am jealous and wish that my similar project could be at the point yours is at right now.
Cheers
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Unread postAuthor: DYI » Sat Apr 26, 2008 9:42 pm

Propane liquefies at around 120 psi, depending on temperature.
A stoichiometric mix is 4.02% propane, 95.98% air, so the partial pressure of the propane in the mix will always be 4.02% of the absolute pressure of the mix. So to find the maximum possible mix with air, we simply divide the liquification pressure of propane by 4.02, and multiply that by 100. Assuming liquification at 120 psi, this means that we can use air/propane mixes up to pre-ignition pressures of 2,900 psi. When we account for fuel pressure, this means that we can use air/propane up to a theoretical maximum of 194x.

The highest mix gun I can think of that's actually in construction will use 100x mixes, so I can't see any problems yet. One must also remember that propane makes up over 16% of an oxy/propane mix, which means that it can't be used at pre-ignition pressures higher than 720 psi, which means... you get the exact same amount of fuel independent of what oxidiser you use. Makes sense really.

If you want to go higher than 194x, you'll have to use a different fuel. Hydrogen comes to mind. With hydrogen mixtures, you're only really limited by the liquification pressure of the oxidiser, and considering that a stoichiometric oxy/hydrogen mix is 66% hydrogen by volume, you could, in theory, use pre-ignition pressures in excess of 20 000 psi.
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Unread postAuthor: willarddaniels » Sat Apr 26, 2008 10:04 pm

I do not claim to know of a surety, but it makes sense to me that the total pressure affects all gases inside the chamber. As soon as the gas mixture reaches 125 PSI, the propane will begin to liquify and will continue to liquify if the pressure continues to increase.
It is my guess that the total pressure of the chamber mixture will not exceed 125 PSI untill all propane has liquified, because the pressure affects all gasses in the chamber equally.

Although I have not researched partial pressure, it seems to be a plausible reality; however, I still feel that the pressure affects all elements equally.

I have never argued something I am not sure about, that is why I am saying I don not knoww 100%- I'm just throwing it out there... maybe I'll learn something new. For now, I need to get back to studying for a final.
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Unread postAuthor: DYI » Sat Apr 26, 2008 10:18 pm

As illogical as it may seem initially, partial pressure is a reality. Ever wondered why there is water vapor in the air? This is why. 100% humidity is the point where the water concentration in the air is high enough that no more water can evaporate at the current temperature. The extremely low partial pressure of water in the air means that it can evaporate at room temperature.

Also, on a more spudgun related aspect, _Fnord achieved an 11x propane/air mix in his testing of malleable iron fittings.
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Unread postAuthor: SpudFarm » Sun Apr 27, 2008 7:59 am

i was about to point out the _Fnord test at 11x.

i have a thread about this problem in the hybrid discussion category.. feel free to find it
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Unread postAuthor: Lentamentalisk » Tue May 06, 2008 2:27 am

Ok, I realize the topic is a little dead, but I thought I might impart a little of my DDT knowledge. DDT is more likely to happen at higher pressures, due to the fact that there are more little fuel and oxidizer molecules bouncing into each other. It also prefers longer, rougher run up distances. The shorter and smoother (causes less turbulence) your barrel the better your chances of survival. Also multiple spark gaps should do their job of preventing long run up distances.
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Unread postAuthor: SpudFarm » Wed May 07, 2008 8:32 am

ok. i just realized that with 3gaps at 11x there would have to be one heck of an ignition.. i am going to make two gaps. one back and one almost at the barrel side.
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Unread postAuthor: DYI » Wed May 07, 2008 5:47 pm

Come to think of it, what kind of ignition device are you using anyway? You're going to need some impressive voltage here. Are you just ordering a stungun from BCARMS, or making a camera cap system? If you are, you're going to need REALLY short gaps.
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