DDT

[microman171]

I was reading through the hybrids and the word/acornym 'DDT' came up. What is DDT? I hope this is the right forum for this kind of thing.

I gather that it is a bad thing (metal shards sticking out of my flesh yay!!).

Thanks

Microman171

[DYI]

Deflagration to Detonation Transition. The flame front experiences a sudden acceleration from turbulent flame front velocities to detonation velocities (over 3km/s in some cases). The mechanisms behind this are numerous, and not fully understood, although dieseling may be the problem in some cases.

[JDP12]

DDT=bad.. hahahaha... avoid long chambers at all costs.

[microman171]

So it is a sudden increase in the speed of the flames? I dont get it...

[D_Hall]

So it is a sudden increase in the speed of the flames? I dont get it...

What's not to get? The flame front accelerates to detonation velocities. That's what DDT is. That's ALL that it is. What part of that is escaping you?

[biggsauce]

Regardless of the reasons for it happening, as D-Hall said, its pretty simple. "Something" causes the speed of the flame front to surpass the speed of sound in the gases in the chamber.

Try google and this http://www.galcit.caltech.edu/EDL/publi ... _paper.pdf

[joestue]

Not completely off topic, but is the shock wave as a side affect of DDT the 'bad' thing, or is it the pressure generated?

As an example if you fill a 20 oz soda bottle with H2 O2 (styrometric) at STP and detonate the mix, the bottle will not explode, a 2 liter will though.
-basically >6inch dia. pvc can't contain the pressure either, I wouldn't recommend it, but am willing to test it.

It seems to me most of you are pushing this limit frequently with 4x or higher propane mixes in 4 inch PVC.

[biggsauce]

Not completely off topic, but is the shock wave as a side affect of DDT the 'bad' thing, or is it the pressure generated?

As an example if you fill a 20 oz soda bottle with H2 O2 (styrometric) at STP and detonate the mix, the bottle will not explode, a 2 liter will though.
-basically >6inch dia. pvc can't contain the pressure either, I wouldn't recommend it, but am willing to test it.

It seems to me most of you are pushing this limit frequently with 4x or higher propane mixes in 4 inch PVC.

Who is running a 4x in PVC? I didn't think anyone did that anymore...

As for the danger, it is the sudden pressure spike. The shock wave is what facilitates DDT. When the shock wave forms, IT causes the gasses to react instead of "burning" causing the reaction as in deflagration.

And how do you detonate the h2 o2 mix? Have you tried this?

[MrCrowley]

It seems to me most of you are pushing this limit frequently with 4x or higher propane mixes in 4 inch PVC.

No one 'frequently' builds PVC hybrids anymore, haven't seen one for months.

Edit: Beaten, didn't refresh the page.

[joestue]

I have done a number of experiments with hydrogen and oxygen, the largest was 200 gallons. *electric ignition via a light bulb filament.

However my question was, is it a shock wave (which could cause instantaneous pressures in excess of the average pressure) or is it the average pressure causing the "bad thing"

From my point of view it is neither a bad thing or a good thing.

I don't have any real examples of someone running 4X propane, but i just assumed that was standard. Has anyone measured combustion pressures?

[MrCrowley]

People still run 4x propane, even more. But not in PVC chambers.

[Pilgrimman]

To my knowledge, the flame front exceeds the speed of sound, thus classifying it as a detonation. The shock wave (this is a detonation, remember) produces a huge pressure spike at its point of origin, and this very quick shock load can compromise even very strong chambers. It's the same reason that acetylene is unsafe. The energy density is lower than propane, but it is more prone to detonation as opposed to deflagration (burning). In the case of hybrids, the detonation is classified as DDT, as it won't happen unless some conditions are met. As DYI said, not every aspect of DDT is understood, but that's the best (flawed) explanation I can give .

[microman171]

Ohhhh I get it now Thanks. So the flames themselve reach detonation speeds and forces? Sorry I havnt relly made and combustion spud guns let alone hybrids.

[biggsauce]

Scared of reading? If not, try these. (dont worry, they have a few pretty pictures and diagrams in there)

http://www.galcit.caltech.edu/EDL/publi ... _paper.pdf

http://arc.uta.edu/publications/pr_file ... Spiral.pdf

http://www.innssi.com/images/pde03/AIAA-2005-1343.pdf

[microman171]

got it now! thanks guys!