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A question/theory about DDT

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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A question/theory about DDT

Unread postAuthor: Fnord » Fri Aug 31, 2007 6:40 pm

Alright, most experienced people on here agree that DDT=Death.

As we know, DDT occurs when a flame front starts burning fast enough to break the local speed of sound, and the gasses in front of it are compressed and ignite all at once, thus the pressure spike.

That being said, I want to hear your opinions on an idea I had to avoid it:
Don't allow one flame front to exceed the SoS. More specifically, create as many flame fronts as possible and allow them to collide, so that no particular front accelerates too fast.

The short version is: can multiple spark gaps be used to avoid DDT?
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Unread postAuthor: 666lucky693 » Fri Aug 31, 2007 7:03 pm

whats does ddt mean
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Unread postAuthor: iPaintball » Fri Aug 31, 2007 7:07 pm

Fnord, I'm not that experienced with DDT, but your idea seems logical.

BTW, DDT means Deflagration to Detonation Transistion.
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Unread postAuthor: noname » Fri Aug 31, 2007 10:25 pm

Your idea does seem logical, to use iPaintball's exact wording. :P In my opinion, I don't think DDT=death, as long as you're using the right materials. Steel, even Sch 40, has a burst pressure in the tens of thousands of psi, and there's no way that any hybrid mixture below 10x, even if DDT occurs, will rupture the pipe. It may bulge a bit ( :lol: ), but then you just need to get a new piece of pipe for a chamber, and that shouldn't cost a huge amount.
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Unread postAuthor: Novacastrian » Fri Aug 31, 2007 10:56 pm

I would have thought multiple spark gaps increase the speed of the burn thus INCREASING the chance of ddt, although if it did occurr you would have a smaller effect because the relative area to ddt has been reduced by having 3 or more flame fronts. Does using a fan increase or decrease the chance of ddt? My guess is that it would increase it, but i'm no Einstein :joker:
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Unread postAuthor: SpudBlaster15 » Fri Aug 31, 2007 11:19 pm

A fan increases the likelihood of DDT by creating turbulence within the chamber, which increases the flame front speed.

Steel, even Sch 40, has a burst pressure in the tens of thousands of psi, and there's no way that any hybrid mixture below 10x, even if DDT occurs, will rupture the pipe.


2" Sch 40 hardware store grade steel pipe has burst pressure of less than 2,000psi, not tens of thousands of psi. Even though this is far out of the range of what a hybrid will produce, even if DDT occurs; steel does not have the impact strength to withstand DDT. Quite simply, if DDT is occurring, you can kiss your steel chamber goodbye after only a few firings, as it will fatigue and eventually fail due to the impact of the shock wave against the chamber walls.

Producing multiple, slower flame fronts could in theory reduce the danger of DDT, but it is not as simple as 'if the flame fronts have less run up distance, then DDT would be less likely to occur'. I don't feel like thinking about the physics involved right now, but I'm sure jimmy101 can give a good theoretical answer.
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Unread postAuthor: Novacastrian » Fri Aug 31, 2007 11:54 pm

What pressures could be expected at ddt in a smallish Hybrid? (say 500ml)
Lets assume a 4x mix, anyone know? :?
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Unread postAuthor: SpudBlaster15 » Sat Sep 01, 2007 12:00 am

According to GasEq, the detonation pressure of air/propane at an initial pressure of 4 atmospheres is ~1050psi.
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Unread postAuthor: Novacastrian » Sat Sep 01, 2007 12:03 am

That's quite a lot, now, what are chances of ddt occuring in said example?
I ask because i'm own a smallish Hybrid (500ml) :)
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Unread postAuthor: noname » Sat Sep 01, 2007 12:06 am

According to SpudTech members (written 3+ years ago), most Sch 40 steel pipe has a burst pressure of ~35,000 psi. He said knock off 5,000 because it's made in China, then cut it in half because of threaded joints. Give it a further safety factor of 10:1 and it's rated to 150 psi.

Novacastrian, there's almost no way you can incur DDT with just a 4x propane/ 4 bar compressed air mixture.

When I was messing around with DDT, I needed a 5 foot long chamber with a single spark at the very back, along wiht a 6x mixture, to get the gases to detonate.
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Unread postAuthor: SpudBlaster15 » Sat Sep 01, 2007 12:11 am

noname wrote:According to SpudTech members (written 3+ years ago), most Sch 40 steel pipe has a burst pressure of ~35,000 psi. He said knock off 5,000 because it's made in China, then cut it in half because of threaded joints. Give it a further safety factor of 10:1 and it's rated to 150 psi.


D_Hall was referring to the steel material having a yield strength of ~30,000psi. He then calculated the burst pressure using the burst pressure formula, and it was somewhere in the vicinity of 1,500psi.
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Unread postAuthor: noname » Sat Sep 01, 2007 12:15 am

What's the average pressure of a 6x propane/compressed air detonation?
I can see how close I was to blowing up the pipe that I was experimenting with.
I would download GasEq myself, and I have tried it, but my computer won't run anything I download, from GGDT and GasEq to Google Earth and other random programs.
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Unread postAuthor: Novacastrian » Sat Sep 01, 2007 12:20 am

Thanks heaps for the info, i have 6.5mm thick s/s tube as my chamber.
I feel alot safer now :D
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Unread postAuthor: SpudBlaster15 » Sat Sep 01, 2007 12:23 am

GasEq can calculate adiabatic combustion pressure, which is probably not reflective of the actual chamber pressure. That said, the adiabatic pressure is ~840psi.
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Unread postAuthor: noname » Sat Sep 01, 2007 12:29 am

SpudBlaster15 wrote:According to GasEq, the detonation pressure of air/propane at an initial pressure of 4 atmospheres is ~1050psi.


Maybe I'm just dumb, but what?
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