Appropriate Use of Time-Dependent Simulations

[JEickman (Joey Eickman)]

Hello,

I’ve just started using Molflow and am trying to understand what scenarios can be evaluated with time-dependent simulations. There is some conflicting information based on other forum posts I’ve read.

Some responses say Molflow should only be used to simulate phenomena that last a few seconds, whereas others say it can simulate hours-long problems. Is the issue with simulating longer problems concerning the accuracy of the solution and the required computational expense?

I’ll describe the problem I am looking to simulate and maybe someone can recommend the correct course of action.

I want to evaluate the evacuation time of a chamber which is approximately a 7m long cylinder with a diameter of about 2m. This simulation would consist of two phases. First, gas is puffed into the chamber by a time dependent offgassing profile up to about 3e-3 Torr. Then the pumps turn on and evacuate the system down to 1E-9 Torr. I want to evaluate how long it takes to reach the desired pressures for different pump speeds and offgassing rates. In general, the evacuation needs to happen within 5-10 minutes.

Is this type of problem and time scale appropriate for a time-dependent simulation, or should I use Molflow to inform my solution based on analytical calculations as described in this forum post?

Question: Calculate pump time

Dear Tobi, One of the core principles of Molflow is that in can only generate, reflect or absorb molecules on surfaces (and not in the volume). This is the reason that you can't set a starting pressure: that would require recognizing "valid" space between the facets and uniformly fill it with initial particles. In theory, you could create such an initial condition by injecting some particles for a very short burst (controlled by parameters, as described in the time-dependent mode), closing inj…

Thank you,
Joey

[maarton (Marton Ady)]

Hello Joey,

Molflow calculates all molecule bounces until a desired time. Because of this, it’s not the actual physical time that is limited, but the number of hits to calculate. For example, your 7m long, 1m radius geometry would need the same computing time to simulate 100s of physical time as a 7cm long, 1cm radius system would need for 1s (the number of hits would be the same).

I believe the 5-10 minutes is certainly feasible - you can increase statistics by choosing larger time windows (so more hits are captured for each moment).

My feeling is that a 7m/1m geometry has almost immediate dynamic response, since it has good conductance, so your pressure will be governed by the changing input and the changing pumping, not by molecules needing time to readjust.

Good luck and let me know if need help to set up the simulation,

Marton

[rkerseva (Roberto Kersevan)]

Hello Joey:
You can certainly simulate up to several hours, especially if your geometry is as simple as you’ve described it here.
You need to use variable-length time moments, so as not to lose the details of the exponentially-decaying pump down.
I’m travelling right now with very poor internet connection, couldn’t connect my laptop using the cell phone as hotspot, so if you can wait 2-3 days that I go back home I’ll be happy to show it to you here, posting the file.
Cheers, and good luck in case you go on your own.
R.