On Tue, 22 Jul 2003, Randall Clague wrote:
there is a temperature at which it changes sign, the Joule-Thompson
inversion temperature. Above that temperature, the gas gets hotter, not
cooler, when expanded through an orifice.
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.
A little googling reveals:
http://js082.k12.sd.us/van%20der%20Walls%20forces.htm http://science.widener.edu/~svanbram/chem145/ch12/vanderwalls.pdf
Man, I have a mind like a packrat.
-- Doug Jones
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