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Author: Subject: Final volume of gas expansion in (for example) RDX detonation
Rocinante
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[*] posted on 3-5-2018 at 14:33
Final volume of gas expansion in (for example) RDX detonation


Does anyone have any data on the final volume of the detonation products of high explosives when their detonation products reach p = 1 bar?

It can't be ~ 900 l/kg for RDX, since this is STP and the products are heated

but it can't be 9 000 l/kg, since the the temperature of gaseous products does not equal the detonation temperature (since the temperature of detonation is theoretical adiabatic construct inside a closed volume).

CHEETAH has such a feauture but I don't have it.

[Edited on 3-5-2018 by Rocinante]
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LearnedAmateur
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[*] posted on 4-5-2018 at 01:30


Use the equation pV=nRT to figure out the final volume from the decomposition reaction, e.g:
2 TNT -> 12 CO + 5 H2 + 3 N2

P is pressure in Pascals
V is volume in metres cubed
N is the number of gas moles
R is the gas constant (=8.314)
T is temperature in Kelvin

Hope that helps you out for an approximation.




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Rocinante
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[*] posted on 4-5-2018 at 08:03


This is basic stuff and completely useless since the term T is not known hence my OP.. we don't have the temperature at p = 1 bar.

All I know that CHEETAH produces 12 bars at 160× expansion for RDX but the relationship is not linear.
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LearnedAmateur
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[*] posted on 4-5-2018 at 10:38


I was just trying to answer your question, which was the volume of products at a given pressure, of which you’d need to use that equation to figure out anyway. I had assumed you would’ve known temperatures because you didn’t mention it when you asked, and said what value would be used as an ideal temperature approximation so I thought you already had that data on hand.

[Edited on 4-5-2018 by LearnedAmateur]




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Dornier 335A
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[*] posted on 12-5-2018 at 13:56


It's a surprisingly difficult question to answer since the gases cool very rapidly during expansion. The rate of cooling depends on the detonation pressure and composition, and therefore loading density. It's a few years since I simulated this but I think the gases were still > 1000 K at atmospheric pressure for low density explosives like nitromethane but reached room temperature at more than 1000 bar for HMX. In other words, the gases would be far below room temperature when they reached atmospheric pressure!

If you want to do some simple calculations, look up the JWL equation and integrate over the pressure-volume curve until you reach atmospheric pressure.
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