hankw said:
To All,
Thanks for all the input. Since my last post i have learned how to calculate bursting pressure. Basically, one using a combination of temperature, detonation velocity, and gammas of gasses and water. From this one can determine pressure at a distance. Then one can infer air volume. I am not yet clear about all this. Here is a snippet of the email i got:
"Similarly, remembering Cp/Cv equals gamma, and the gammas for CO2, N2 and H2O are 1.29, 1.40, and 1.30 respectively, and I guess an average of about 1.35. Assuming your initial temperature and pressure was 298K and 1 atmosphere, the bursting temperature will be as we just calculated and the bursting pressure will be (4886.6/298)^(1.35/0.35) to give 48,486 atmospheres or 712,754 psi.. If you know the burning rate of the TNT-AN mixture, and I'll guess it is maybe 7,000m/sec., you can profile the pressure gradient from the mensuration formulae of a sphere."
Some heavy thermodynamics, i know. And my knowledge of physics and chemistry can be held in cupped hands!
Does this bursting pressure sounds like that detonation velocity factor that Nerro mentioned? Or is this something else? Another question i have- of the four major factors i listed to calculate explosion air volume, does the bursting pressure/detonation velocity factor create its own air volume or does it include the air from factors 1 and 2?
-Volume of gas produced by explosives
-Volume of the produced gas heated up by explosives
-Volume of the original air heated by explosives
-Volume volume produced by burst pressure/denotation velocity
So i am guessing that these above factors are the "major" contributors of explosion air volume? I am not trying to make a detailed model here, not that i would succeed anyways.
I do realize that this demolition problem has grown a magnitude or two in difficulty. I may not be able to estimate the explosives used in the demoliton. But hey, the goal was learning. I am learning chemistry, physics, and behavior of explosives all in one problem. I am learning how bring together different science disciplines to solve things. That is a rich learning experience. I theoretically could take this knowledge and reverse engineer all sorts of explosions seen on TV- a trick i can do at parties for scooby snacks! If nothing else!
Hank
I'm not familiar with "bursting pressure" - it would be interesting if you could post the formula for it and some more details, - but nonetheless, I think I can give you some basic info that could be useful.
(Note: on reflection, it seems likely to me like the point of bursting temperature, and bursting pressure is to create an initial model of the gas - i.e. you want to model an initial sphere of gas generated by the explosion as having some temperature, volume, and pressure, to start with - then model what happens next via physics. But that's sort of a guess at this point. I don't know what the "bursting volume" would be, perhaps the volume of the explosives themselves?).
After your explosive has all burned, you will have a big ball of hot gas generated by the chemical process of burning. The manner in which this ball will expanded, _after the burning process is done_, is best approximated by adiabatic expansion.
You might try the wikipedia on this
http://en.wikipedia.org/wiki/Adiabatic_process
The idea behind adiabatic expansion is the conservation of enregy. This is where the ratio gamma, and the expressions Cp and Cv come in (the heat capacity of the gas at constant pressure, and at constant volume).
The adiabatic law , when you go through all the calcuations, winds up to be
P V^\gamma= constant, and it supplements the traditional ideal gas law PV=nRT.
Note that adiabaitc expansion will _not_ be a perfect model of the behavior of the gas. As others have noted, the gas will be doing work in the process of disassembling a building. The adiabatic law gives the pressure-temperature relationship
As I think about this, it seems to me that expanding of the cloud of gas against the pressure of the atmosphere itself implies that the gas is doing work, and that this work should be subtracted from the adaibatic model, creating a new, more complex model. I don't think I've ever read anything detailed about this though.
