Units, Validation, Theoretical Background

Bugs & Issues
1

Hi

I have been playing with MolFlow for some time and I have to admit that I do not understand the results. I do have a set of simulation results that were obtained using the Monte Carlo solver in ANSYS and were benchmarked against the
results published by F. Sharipov, but I do have a problem with interpretation of the Molflow results.

To limit this enquiry to a simple pipe test case:

  • It looks like that the pressure distribution is independent of the volume flow rate. As there are no units in the pressure distribution Molflow is presenting, my suspicion is that some normalisation takes place. Can you please explain?

  • For the inlet, it seems that the outgassing values do not affect the results. What is the mathematical expression behind?

  • The volumetric pumping speed produces a sticking factor value that is too low. This may be an incorrect statement, but in absence of any mathematical formulations, I can only
    compare the values with the values from other sources.

  • Can you please explain, for the test cases that are available with the package, what are anticipates results and more importantly, how they are defined.

Thank you very much for your help.

Kind regards, Andrej

2

Hello Andrej,

  • Both outgassing rate and volumetric pumping speed should change the results. (Higher pumping results in lower pressure and higher outgassing means higher pressure).

  • The formula for the pumping speed / sticking factor conversion is in the “facet parameters” section of the documentation. If the sticking factor is too low, that’s because the dimensions of your system are incorrectly large. To verify it, keep in mind that all distance units in Molflow are in cm.

  • In the same section the interpretation of the pressure units is explained: they are arbitrary. As UHV systems are linear, you can decide your pressure units - if you define outgassing in mbar*l/s, then you should read pressure results in mbar.

  • For the downloadable geometries, I have no information at the moment (but can look it up). However, the easiest way to verify Molflow’s results is by creating a test pipe (Test menu), and verifying the P=Q/S formula next to the pump. I will get back to this test case and contact you on Thursday when I’ll be back at CERN.