OK... What's the physical mechanism? Adiabatic cooling is straightforward: apply Boyle's Law. But why does passing through an orifice have any effect on gas temperature? Is it a sonic thing?
Alas, you've hit the limits of my background knowledge on this one -- I'd
have to go digging to sort out the theory. If dim memory serves, the J-T
coefficient is exactly zero for an ideal gas, so we're out in the messy
realm of non-ideal behavior.
As I vaguely recall, it has something to do with the attractive Van der Walls forces- which are what make gases non-ideal. Expansion through an orifice, even when it does no macroscopic work, does work against the van der walls force.
Moreover, this also explains why there's an inversion temperature. Some fraction of the molecules at any time will be very close to others, experiencing strong short range repulsion. As the temperature increases the energy stored in these short range interactions is larger, since more molecules have enough energy to come close to other molecules. The heat released on expansion through an orifice is this short range repulsive potential energy.
Paul
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