Im using molflow+ 2.8.8 and was wondering how the facet temperature works with the gas. Lets say I set my desorption facet to 650 K, does this mean the gas exiting the facet is 650 degrees K, or just the facet itself has a temperature of 650 K and doesn’t effect the outgassing temperature? I was reading over the documentation and saw that it says that the temperature field effects sets the temp of the face but I was wondering if it effects the gas, or if the gas temperature is just determined by PV=nRT.
Thank you marton, that is good to know. I have a follow up question on how molflow deals with thermal energy transfer between a facet and a colliding particle. If there is a temperature difference between the facet and the particle, will thermal energy be transferred between the two upon collision?
Since Molflow simulations are not performed in chronological (time-driven) mode, the boundary conditions (such as sticking, opacity, outgassing, and in your concrete case: temperature) can not depend on the simulation results: they are either constant, or time-dependent in a way that is fixed by the user before the simulation.
It means that facets transfer thermal energy to the particles, but not the other way around.
By default, particles are immediately and fully thermalize upon collision, meaning that their velocities get updated to a random value following the Maxwell-Boltzmann distribution of the facet’s temperature.
There is a way to reduce this accomodation, in the “accomodation coefficient” textbox:
Wonderful, thank you for the reply. I have one more question to follow.
So the sticking factor(S_f) is a function of the pump speed, the facet area, and the average molecular velocity. The average molecular velocity is a function of the gas temperature (as well as R_o and M but those are constants) and that makes sense. However when you set up a facet to act as a pump, you specify the temperature of that Facet, not the temperature of the gas. What if the gas temperature is lower or higher than the temperature you set for the pump facet?
Is the temperature set for the pumping facet basically saying that “the gas will be this temperature when its pumped”? or how does that effect it. Because as I understand if the temperature of the gas is changing, that means the sticking factor should also be changing in step.
Hello Skyler: if the sticking of the facets representing pumps is not 1, i.e. molecules have a probability to be “reflected” by these facets, then the temperature of the reflected molecules will be set by the temperature of the facets and their accomodation coefficient, as explained by Marton earlier in this discussion.
What changes with the temperature of the incoming molecules, for a given sticking of the pumping facets, is the gas throughput Q (mbar*l/s), because Q depends on the molecular velocity, i.e. the temperature of the incoming gas. In this case it is up to the user to perform an educated guess, taking into account the type of pump. If you have a turbo pump, where the trapping efficiency of the rotor-stator stages depends on the molecular speed, then even a gas like H2 which is “badly” pumped by turbos due to its high molecular speed given by its light mass (1/M factor under square root in the mean molecular velocity, Boltzmann’s) then a turbo pump may pump it with a higher efficiency than it would have were the gas at room temperature. For ion pumps, on the other hand, the incoming gas temperature is not a big issue, since the pumping mechanism is based on molecular ionization and acceleration (plus sputtering rate on the cathodes and subsequent Ti burial) and these effects are very mildly dependent on the velocity (i.e. temperature) of the incoming gas.
For a cryopump it depends: if the incoming gas’ velocity (i.e. temperature) is too high, it may have a problem dissipating the energy of the molecules, in that case the pumping speed may be decreased… but by how much and if that happens depends on the type of cryopump, and its geometry. This may be a problem for instance in large panel cryopumps for thermonuclear fusion devices (tokamaks and most of all the so called neutral beam injectors).
So, you need to think a bit before deciding what sticking to use, depending on what system and what pumping configuration you’re simulating, there is no way for Molflow (as it is now) to “decide” or make a choice for you, sorry…
Hope it helps, if not write back, we can elaborate further.
Cheers.
