I'm trying to setup and run a flow simulation that is basically a 'nozzle' discharging into a vacuum chamber. We maintain a vacuum chamber pressure of 100 mtorr and are trying to evaluate the pressure in the tube upstream from the nozzle, which in this case is an orifice, 1-2mm in diameter when we introduce an inlet mass flow of argon at 100 SCCM. Basic choked flow calculations are not matching the flow simulation results and I am struggling to understand why. Any guidance would be welcome. Thanks in advance.
If it were me I would add port of some realistic size the chamber and put the BC there and just leave the chamber walls as walls. In what you have they are sucking out at the pressure specified. That's a big area.