Alright, DDT again...

Postby markfh11q » Thu Nov 23, 2006 10:38 pm

First off, I get the event, to the extent it's to be gotten.

I remember reading somewhere, though, that when a gas flame-front transitions from deflagration to detonation, the molecules completely dissasociate. This means that they separate into completely separate molecules of each element.

I'm guessing that this means they'll stay in their respective various forms. Like hydrogen will stay as H<sub>2</sub>, and oxygen will stay as O<sub>2</sub>, right? Reason I say this is because those bonds are very hard to break. Or do they separate into completely separate monoatomic particles?

Secondly, I'm guessing that this event is only noticeable in a detonation because in deflagration it occurs at a slower rate, allowing time for the molecules to recombine before other separate.

For a normal atmospheric compression combustion of propane/air, I'm calculating around 9.4 atm pressurization of a sealed and insulated vessel.

For a detonation at the same temperature, (and I think that the detonation temperature would actually be higher), with the molecules splitting into respective diatomic configurations, I'm calculating 16.1 atm pressurization in a sealed and insulated vessel!

For a detonation at the same temperature with the molecules all separating into monoatomic form, I'm calculating <B>28.2 atm</B> pressurization in a sealed and insulated vessel!

So, obviously, I'd like to know which occurs? Any thoughts from all you people who've been there, done that, got the T-shirt? It seems the Gas Explosion Handbook jibes with them separating into diatomic molecules.

Notice, all these pressures are calculated starting from atmospheric compression at STP. These aren't pre-compressed mixtures.
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Postby GalFisk » Fri Nov 24, 2006 5:47 am

Have you compared your results to those of GasEQ for detonations?
The amount of dissociation seems to be dependent on many factors, but if (I don't know how big of an if this is) you know the temperature and pressure in the shock, wouldn't it be possible to calculate to which degree this happens?
This PDF may be of interest, though it's not directly related (it's about detonation in refrigerants): ... 763241.pdf
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Postby markfh11q » Fri Nov 24, 2006 9:14 am

Looks good actually. And, yes, I tried GasEq later last night. All of em came at least close to what I predicted with diatomic dissasociation, which this article supported.
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