Pressure evolution over time

General discussion
1

Hello there,

I have been doing some more simulations on molflow+ simulating simple cylindrical vacuum chambers. Pump on one end, outgassing on other. I have been taking some time to do some benchmarking against published papers to make sure i’m setting up the simulations right. I came across this paper “New time-dependent simulations with the test particle monte carlo code MOLFLOW+” by Ady et al. and used the problem detailed in the time-dependent simulations section to check my setup.

It turns out my setup was correct, but i’ve come into some interesting behavior when using the equations attached (they came directly from the paper.) The graph bellow the equations is the exact simulation detailed in the paper and the MOLFLOW+ result checks out pretty much perfectly.

I was playing around with the same setup and increased only the length of the pipe, therefore increasing the conductance of the pipe. I was checking the simulation results with a calculator i setup using the equations and suddenly there is a large jump in pressure at the moment gas is turned off (t = 0.02 s). Are the equations not good for higher conductance pipes or is there something fundamental i’m missing here?

Thanks for the assistance ahead of time!

Equations

Small_conductance
Small_conductance746×473 10.1 KB

larger_conductance
larger_conductance1163×724 20.3 KB

2

Hello Skyler, and thanks for posting here:

I am not sure I understand the jump at 0.02 s on the bottom plot… but I can certainly tell you that a longer tube has a SMALLER conductance, not a bigger one. It has a bigger volume.
Please send the molflow+ file file, there may be something to fix in it, namely the time-dependent outgassing rate (looks to me, but I may be wrong).

R.

3

Also, when you attach the file, pleaase let us know which Molflow version you used. Thank you, Marton

4

Thank you Roberto for getting back to me so promptly.

I have learned that I was looking at the conductance incorrectly and that it is like you said that a longer pipe means a smaller conductance. Since I posted last I was able to figure out the problem I was having.

It was the time-dependent outgassing rate. The I was looking over the setup with the hint you gave about the possible problem and saw what was wrong. Thanks for your help.

Also Marton, I will be sure to include the version next time I post!

thanks for the help as always.