Does a Helium Tank get Hot or Cold when Discharged? The program that I have been working on is to assess how much inert gas our team needs to buy for a small amateur rocket.
The way I have constructed the problem is two isobaric control volumes for the propellants, and a constant volume tank, whose volume and associated gas mass at some pressure and temperature, we want to design for.  The outlet of the high pressure constant volume tank is a gas pressure regulator, which has constant enthalpy from inlet to outlet.

What I expected from the time-step simulation was for the pressure AND temperature to go down in the tank with concavity up shape to the curve.  However what I computed was the following:  
This graph was from a test I ran, where mass flow rate was constant, and specific enthalpy leaving the tank was used to find total enthalpy of the tank at each discrete time step.  
Both of these properties were then calculated with REFPROP from tank specific enthalpy and specific volume.  Pressure looks about how I expected, but from ideal gas law, Pv = RT, I expected temperature to go down dramatically as both pressure and specific volume decreased.  Does this make physical sense?  I know Helium is supercritical at this point, and behaves funny.
Any insight is greatly appreciated.
 A: I did a little more research and found this on the chemistry stack exchange which suggests that it has something to do with the compressiblity factor, and intermolecular forces.  A little beyond my current knowledge, but I think this suggests I am not terribly far off.
https://chemistry.stackexchange.com/questions/61517/reason-for-negative-joule-thomson-coefficient-of-helium-and-hydrogen-at-ntp-cond
EDIT:  As mentioned by @ChetMiller, the Joule-Thomson heating is not a good description of what is happening inside the tank, only what is happening in the flow as it moves through a throttle.  Instead the following equations can be used to describe the tank in adiabatic expansion:
$ P = P_0\frac{\rho}{\rho_0}^\gamma $ 
$ T= T_0 \frac{\rho}{\rho_0}^{\gamma - 1} $
in my case, if I know the mass flow out of the tank at any time, I will know the specific volume of the fixed volume tank, and therefore will know P and T at any time.  These equations assume the expansion is adiabatic and reversible.
A: However those adiabatic expansions that happen at the tanks involve mass transfer and work the the He performs on the pressuriser. So it is unclear if it would warm up or cool down.
