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-   -   Vacuum in FLUENT (http://www.cfd-online.com/Forums/fluent/44973-vacuum-fluent.html)

 Shanti June 8, 2007 13:43

Vacuum in FLUENT

Hi

I want to know how to create vacuum in FLUENT. I have given the operating pressure as 0 Pa. But I dont think it is creating a vacuum that way as I could see the recirculation of air. Can anyone help me with this?

Thanks,

Shanti

 A A S June 8, 2007 14:07

Re: Vacuum in FLUENT

maybe you could try turning off the flow equations in the solution control panel. Also, be sure you have zero gauge pressure in all your boundary conditions

 Shanti June 8, 2007 14:23

Re: Vacuum in FLUENT

Hi

Thanks for your reply. But I am solving for the interface tracking of a fluid. Won't turning off the flow equations affect that?

Thanks,

Shanti

 Vivek June 8, 2007 14:52

Re: Vacuum in FLUENT

Navier Stokes equations which are solved in Fluent are not valid in very low pressures.

 Shanti June 8, 2007 16:01

Re: Vacuum in FLUENT

Hi

When I deselected the flow equation, vacuum has been created. But I am not able to study the forces acting on the walls. Is there any way I can study that?

Thanks,

Shanti

 kk June 8, 2007 16:40

Re: Vacuum in FLUENT

can you define the 'low pressure' range. How low is that low?

 Will June 11, 2007 13:57

Re: Vacuum in FLUENT

If your Knudson number is greater than 1/10, then Navier-Stokes is not valid.

 Shanti June 11, 2007 14:03

Re: Vacuum in FLUENT

There is no specific range for pressure. I just want to create vacuum.

 mvee January 17, 2013 04:16

create vacuum

Hi

Did you create vacuum and able to solve the NS equations? How?

Thank you

 RodriguezFatz January 17, 2013 05:35

No, he wasn't. No one can do that. Fluid equations without any fluid just don't make any sense at all.

 mvee January 18, 2013 00:26

Hi RodriguezFatz

You are exactly correct. Perfact vacuum can not be modelled. I have following questions:

(1) Can we model low pressure 1e-5 Torr and solve it by compressible flow equations?
(2) If the pressure of 1e-5 Torr is not possible then what is the minimum pressure we can go?
(3) How Knudson number is related to compressible flow?

Thank you
Mvee

 RodriguezFatz January 18, 2013 03:18

Knudsen number is the mean free path for atom/molecule collisions divided by the size "L" of your domain. It means: How many collisions occur, when a particle travels through your domain (Kn is the inverse of that number). If the number of collisions is large enough the gas can be considered to be a continuum.

Thus, you can answer these questions only if you know L.

 sumeet kotak May 3, 2013 09:17

seeking help related to vacuum...!!!!

Quote:
 Originally Posted by RodriguezFatz (Post 402580) To all your questions: Knudsen number is the mean free path for atom/molecule collisions divided by the size "L" of your domain. It means: How many collisions occur, when a particle travels through your domain (Kn is the inverse of that number). If the number of collisions is large enough the gas can be considered to be a continuum. Thus, you can answer these questions only if you know L.
Hello RodriguezFatz sir,

I am simulating Electron beam physical vapor deposition system (EB-PVD) for coating of evaporant on substrate. For improvement of purity of coating, I required vacuum inside the chamber. The amount of vacuum required at least 3e-5 mbar using (0.005 Pascal) (practically measured value).

so can u help me how can i create vacuum inside the chamber...???
how to set operating pressure in operating condition dialogue box and pressure outlet pressure in boundary condition dialogue box....???

0.005 Pa is measured using vacuum gauge, practically, by performing experiment.

 RodriguezFatz May 3, 2013 09:23

Please read the posts of this thread and you will see that it is absolutely senseless to use Fluent for this case.

 sumeet kotak May 3, 2013 09:33

thanks for replay sir...
sir in knudsen no. how to calculate mean free path length.. ie. distance between two particle just before collision?

 RodriguezFatz May 3, 2013 09:55

Have a look at:
http://en.wikipedia.org/wiki/Mean_free_path
Take the first formula of section "mean free path in kincetic theory" and put in your gases density and collision cross section!
Although I don't expect that your particle distribution will be maxwellian...

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