High-Vacuum CFD Analysis Methodology

[dmech]

I'm trying to determine the flow bottlenecks and vacuum process time of a laser tube, using CFD software. The final pressure of the laser tube will be 2.5x10-7 torr.

A high-vacuum (2.5 X 10-7 torr) will be applied to a sealed laser tube, which starts at 1 atm, with an outlet diameter of 10 mm. I would like to do a transient CFD flow analysis of this situation to determine approximate process time. I know the CFD software can handle flow analysis and visualization up to the rough vacuum range (Ptube > .45 torr), where viscous flow equations are relevant. However, can CFD successfully predict the volumetric flow rates in the Knudsen (.45 torr > Ptube > 9.8x10-3 torr) and Molecular (Ptube < 9.8x10-3torr) flow regimes?

My questions are:

1) Will a standard pressure difference (1 atm in the laser tube minus 0 atm at outlet) analysis indicate the bottlenecks in the system?

2) What is the correct methodology to run the CFD transient analysis beyond the rough vacuum regime (i.e. boundary conditions, fluid properties, etc.)?

Thank you in advance for your time and help.

Regards,

dmech