From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: alt.engr.explosives
Subject: Re: ammonium nitrate detonation
Date: 28 Jan 1996 00:09:44 GMT

In article <4eds0a$g67@news.cc.utah.edu>, tb20a50@cc.utah.edu (T
Bereskin) says:

>Gerald L. Hurst (glhurst@onr.com) wrote:
>: In article <4ebehc$ppg@news.cc.utah.edu>, tb20a50@cc.utah.edu (T
>: Bereskin) says:
>:
>: >        I'm not sure if this is worth anything-but it strikes me as a 
>: >little unlikely that anything having to do with explosives would be 
>: >adiabatic.  Now, granted initiation generally is thought of as a quick 
>: >process, but to claim that it is so tremendously speedy that no heat is 
>: >able to leave the system, well, I'm just not sure how good an 
>: >approximation that is.  Also, it's pretty obvious that EM radiation is 
>: >going to go flying out all over the place, although that does not reflect 
>: >on the adiabatic assumption.
>:
>: Well, nothing is perfect. Would you believe "nearly adiabatic"?
>: How about "adiabatic enough for government work"?
>: 
>: Can you give us examples of a "real" adiabatic process involving
>: a temperature change and tell us why EM radiation (of, say, heat
>: waves) would not "reflect on the adiabatic assumption"?
>: 
>: Is it deltaH =q - W + deltaPV except when q is radiant heat?
>: 
>: Jerry (Ico)
>
>        I'll easily believe adiabatic enough for government work-a dewar 
>flask with a hole in the side is probably adiabatic enough for government 
>work.  I think the only processes which are really adiabatic are confined 
>to pieces of paper with names like Rudolf Clausius or James Prescott 
>Joule.  I think a concise definition of an adiabatic process is q=0, that 
>is, there is no net transfer of heat from the system in question and its 
>environment.  If by "heat waves" you mean IR and MW radiation, that would 
>certainly constitute a change in the enthalpy of the system, but I really 
>don't know how big a loss that is, calorie wise, in the time frame under 
>question.  I suppose the point to this is sort of a personal irratation 
>at people running about applying terms to things in the physical world 
>that were only meant to describe simple models; the distinction between 
>the two is not always large, but it is there.  A styrofoam cup may work 
>as a great calorimeter for some applications, but sometimes even big 
>steel bombs are too inaccurate and there's nothing left to do but throw 
>up your hands and say "We really can't measure delta-G for that reaction, 
>sir."  A blackbody is something for Planck to whack around with a lead 
>pencil, not a dirty furnace with a hole in it.  How important this 
>difference ends up being, in the situation that this started with or any 
>other, is sometimes as much a matter of philosophy as of engineering.

Your arguments are those of an idle ivory tower philosopher. 
There is absolutely nothing wrong with considering a detonation 
as an adiabatic process. The proof of this pudding lies in the 
usefulness of the outcome. The question is merely how well the 
map fits the territory. Obviously, no real process will attain
the rarified heights of theoretical thermodynamics. So what?
The same is true of Newtonian physics, but that does not diminish
the fact that the mathematical models have real utility in real
engineering.

The famous Hugoniot, which describes the PV state of explosion
products depends on the reaction being ESSENTIALLY adiabatic and
that is just what most explosions are. Radiation losses are 
generally trivial and the rapid, spherical expansion of the
gas cloud at ca 300 M/S or better hardly provides much time
for the exchange of heat at the air interface. Within that
expanding cloud and in the air shockwave there are no ideal
gases yet the ideal gas law gives us a reasonably accurate
model of the processes which is more than adequate to predict
most of the practical information we need for the application
of explosives.

For more precise (and usually more academic) work we can add
theoretical and/or empirical terms to the limit of our patience,
until or until we reach whatever number of decimal places our 
hearts desire.

If terms like "adiabatic" and "ideal" offend our academic and 
puritan sensibilities, we can always resort to the use of suitable 
limiting adverbs to advise the world that our map is merely a map 
and not the terrirory.

BTW, my use of "heat waves," in the clear context of EM was a 
troll. Why don't you explain to us why other EM radiation, e.g, 
"light waves," but not near IR or lower, would not also properly 
belong as a component of q in describing an explosion? Take your 
choice in the definition of "light waves," 0.4-0.7 microns, if you 
like - or shorter, if you prefer.

Jerry (Ico)