I am trying to simulate particles entering a chamber at a velocity of 5 km/s, by increasing the temperature of the desorption facet. The idea is that the particles will then thermalize to room temperature when they come into contact with the chamber walls. I have a pressure profile and texture set up that runs the length of the chamber.
The resulting pressures I'm seeing do not make sense. I'm getting a higher pressure in the chamber when the desorption facet is set to room temperature, as opposed to an elevated temperature. I would expect the pressure to be higher when the particles start out at an elevated temperature. I get the same results using version 2.5.6 and version 2.6.25. I would appreciate any insight, or suggestions on how to simulate this.
You're right, by increasing the velocity of the particles, the pressure is expected to increase. However, if you're desorbing from the hot facet, when you increase the temperature with a fixed desorption rate, expressed in mbar.l/s, you decrease the particle flux:
d(pV)/dt=dN/dt*kT - if d(pV)/dt is fixed, increasing T decreases dN/dt (the flux). Then cooling down the reduced flux will indeed decrease the pressure in the volume.
You can either adjust the outgassing rate so that the generated particle flux (shown in Global Settings) remains constant, or desorb gas from a room temperature facet, then heat it up with a second one.
Alternatively you can use the Moving parts option (I can give more info) to set the particles' speed to a fixed 5km/s value, instead of accelerating them by the facet's temperature.
For checking if everything is correct, without the concrete geometry in sight, I can only give general advice:
- In the facet details window, you can see the average particle velocity of impinging particles. Check if it really increases.
- You can set up sepcial, "velocity" profiles that show you the speed distriubtion graphically
Thanks for the quick response Marton! Could you provide more info on the Moving Parts option? It seems like that might be a better approach than trying to "fake" the particles' entrance velocity.
I've added a description of Moving Parts to the front page. Basically it adds a selected velocity vector to all rebounding particles on select facets.
If I were you, I would set up my "particle gun" the following way:
The small facet generates particles. It is one-sided (so it generates particles only towards its normal, marked in blue), and transparent. You can put it closer to the bigger facet.
The large facet is opaque, doesn't generate any particles but turns the incoming particles from the small facets back, adding a large speed of 5 km/s towards its normal. Its temperature is low, therefore its "intrinsic" velocity will be negligible compared to the added 5 km/s vector. It is a moving part, with a 5km/s length velocity vector oriented towards its normal.
Adding the 5 km/s velocity vector will collimate the outgoing particles, so position this "setup" in a way that particles find their way to your system.
