[LJA_Koers]

Hi all,

I'm trying to make a transient simulation of the choked flow of methane in a piping system. I want the flow to become choked when the outlet pressure is reduced to atmospheric pressure (1atm). With a heat capacity ratio of 1.32, the critical pressure ratio of methane becomes 0.542. Hence the inlet pressure is set to 1 atm / 0.542 = 1.845 atm. And the speed of sound of methane at atmospheric pressure is 453.6 m/s.

To see if the flow is indeed choked at these conditions. I ran a steady state simulation and used the ideal-gas equation of state and an inlet and backflow temperature of 26.85C (isothermal throttling). Running the pressure based solver.

The result in the velocity contour shows a maximum velocity magnitude of 484 m/s, exceeding the critical velocity. To find the cause of this, I plotted the density and subsequently temperature plots to find that the temperature in the jet exiting the orifice drops to -26C. A 53C drop in temperature!

This setup is supposed to simulate the throttling of an ideal gas and thus be isothermal, yet the result is not.

Does anyone know what causes this?
And in addition, are there any other tips for modeling throttling/choked flow?

Thanks in advance.