I’m a new user of Molflow+ and I’m encountering a discrepancy between the pressure drop calculated from my simulation and the pressure drop calculated using the Hagen-Poiseuille equation. I’m trying to understand why this difference is occurring.
My model is a <test case with L/R = 100> (R = 1 cm, L = 100 cm).
Using the Hagen-Poiseuille equation, I calculated an inlet pressure of approximately “3.5 mbar”. However, the Molflow+ simulation result shows an inlet pressure of “55.5 mbar”. This is a difference of about a factor of 15.
I’m not sure why there is such a large discrepancy between the two values. Could anyone offer some insight into the possible reasons for this difference? Are there specific settings in Molflow+ that I should be checking?
Thanks for the quick reply. I was expecting the two results to be at least somewhat similar within a reasonable range, so your answer helps me accept the large difference.
Actually, when I calculate the Knudsen number with the above conditions, I get a value of around 0.001, which indicates a “viscous flow” regime. I wanted to run a simulation in a lower pressure range (near the transition flow) to compare the results, but I couldn’t set the pumping speed at the outlet higher than 35.66 L/s. (This is another question I have. According to my understanding of vacuum theory, the pumping speed can be set to freely high values, not limited. Why does Molflow+ have a maximum value for the pumping speed?)
Molflow works for Kn>>1, in practice below 1E-3 mbar. You’re trying to simulate a process with at least 3500x pressure of the highest limit (taking 3.5mbar). In free molecular flow, the maximum theoretical pumping speed is the conductance of the pumping orifice. At room temp and N2 gas, it’s 11.7l/s/cm2. Molflow correctly calculates the 35.6l/s for the test pipe at sticking=1, which is the highest theoretically possible (100% of molecules are pumped). I think you’re trying to use Molflow as a fluid dynamics simulator, whereas it’s for MOLecular FLOW (hence the name)
Your answers have been extremely helpful. My boss will be pleased too.
Actually, I’m trying to analyze the wall collisions of radicals contained within a gas flow in semiconductor equipment. It seems I need to consider the usage methods of Molflow+ more carefully.
I have one last question. According to the throughput equation Q = PS, if Q and S are defined, the outlet pressure P should also be defined. Using the conditions described above, the theoretical value of P should be 1.4 mbar(=50.66/35.66). However, Molflow+ is showing a different value. Is this also due to the issues with the analysis region you mentioned, or is there another problem I should be considering?
P is not a scalar value, it is a tensor, it depends on the orientation of the facet used to compute it.
When you read the pressure of facets (or texture elements) placed in the immediate vicinity of one or more facets with sticking coefficient “close” to 1 then the anysotropy of the pressure field becomes evident, and the pressure value depends on the orientation of the facet/texture element. In that case discrepancies like the one you have noted arise, it is perfectly normal. One should use the DENSITY rather than pressure, and eventually convert to pressure via PV=n**k_B**T.
I think to remember that this is well explained in Marton Ady’s PhD thesis, here…
Just one more comment to complement @rkerseva ‘s answer: The pressure is a mechanical quantity calculated from the impulse change of rebounding particles. If Molflow has sticking=1, it means that the particle doesn’t rebound but sticks (stops) on the surface. This effect alone explains why the pressure is half of what you’d expect: the rebounding component (usually present on a non-sticking wall) is missing since 100% of the particles stay on the pumping surface.