New to Molflow, going through the process of setting up my first simulation. I went through this 7 minute quick start video I found on YouTube without any problem, but now I’m having trouble figuring out how to model my given environment in Molflow.
I have a 25 cm diameter, 40 cm in length cylinder that’s capped at one end and has a 50-150 L/s pump at the other end of the face. The chamber is stainless steel, which from looking around online has an outgassing rate of roughly 10^-13 mbar*l/s. I want to determine: 1. What the final pressures are for given pump rates and 2. how long it’s going to take to pump down the vacuum from atmospheric pressure at given pump rates.
My thought is to set one end of the cylinder chamber to my pump rate, and then the other end to my outgassing rate. What else should I model? Perhaps there’s another tutorial you’d suggest that’s closer to my environment than the tutorial I went through?
Hello Colin and welcome to Molflow users. Also, sorry for the delay, usually we try to reply asap, but we were caught up releasing a new Molflow version with a convergence plotter tool.
First, let’s start with the final pressure question: yes, by putting the sticking equivalent to your pump’s speed on one side, you’re doing things correctly. One remark is that the manufacturer’s pumping speed (usually for N2) is at the orifice of the pump, so if your real-life system has a small distance (connected through a non-zero length tube), you need to account for that connection as well (it reduces “effective” pumping speed).
For outgassing, I would distribute the load over the whole surface, as opposed to outgassing at the other end (which distrots reality). In Molflow, you have two text fields, one for absolute outgassing, one for “specific” or area-wise. To be clear, your stainless steel outgassing value of 1E13 mbar.l/s is per cm2, the good news is that you acn enter this specific value directly in the second text field while selecting side facets.
For the pumpdown, Molflow has a time-dependent mode, but it works only in MOLecular FLOW (hence the name), in practice below approx. 1E-3mbar. I would use the analytic solution, if the volume of your system is known:
p(t) = Q/S * (1 + exp(-S/V*t))
Where S is the roughing pump’s speed, V is the volume, t is the time since start of roughing and Q is the outgassing, which for roughing can be estimated as 0.
