CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

How to set up the inlet boundary condition for a low pressure case?

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 10, 2012, 02:15
Default How to set up the inlet boundary condition for a low pressure case?
  #1
New Member
 
Join Date: Apr 2012
Posts: 17
Rep Power: 14
beastieboys6 is on a distinguished road
Hi, everyone. I'm new to Fluent. I'm currently trying to simulate the flow field of a vacuum chamber operated at 1 Torr. Since the inlet gas is controlled by a mass flow controller, I thought it might be better to set the inlet as a mass flow inlet. The following are the settings for my case:

Solver: pressure-based
Viscous: Laminar
Operating pressure: 133 pa (=1 Torr)
Inlet: mass flow inlet with mass flow rate of 2.728E-05 kg/s, corresponding to 1000 sccm of Ar
Outlet: pressure outlet with gauge pressure of 0 pa

I got a convergent result but it seems weird to me. First of all, the velocity is too low. According to the outlet area (38.465 cm2) and the volume flow rate calculated based on ideal gas law (=1000 sccm * 760), the velocity at outlet should be 3.29 m/s. However, the area-weighted average velocity I got from Fluent is just 0.009 m/s. I checked the absolute pressure. It's correct, which is still 133 pa. But I found that the density is 1.62 kg/m3, which is exactly the value at 1 atm. Since I’m running a case at 1 Torr, shouldn’t the density be 0.002 kg/m3?

So, I decided to change the inlet from a mass flow inlet to a velocity inlet. I calculated the inlet velocity based on the inlet area (0.636 cm2) and the volume flow rate based on ideal gas flow (=1000 sccm * 760). The inlet velocity is 199 m/s (Is this reasonable?). I didn't get a convergent result this time. But I found that the density still remains 1.62 kg/m3 and the highest pressure in the simulation domain has jumped to 1E+05 pa.

Can anybody tell me what I did wrong? Thanks for your kind help in advance.
beastieboys6 is offline   Reply With Quote

Old   April 10, 2012, 04:25
Default
  #2
New Member
 
Join Date: Apr 2012
Posts: 17
Rep Power: 14
beastieboys6 is on a distinguished road
I think I have found out what's wrong with my case. I didn't change the setting for density. The default is 1.62 kg/m3.

Here comes new questions. Should I change the density to 0.002 kg/m3 (=1.62/760 kg/m3) directly? Or should I select the imcompressible ideal gas law? Since there is no heating source in my case, is it more convenient to set the density as 0.002 kg/m3?
beastieboys6 is offline   Reply With Quote

Old   April 10, 2012, 17:22
Default
  #3
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,739
Rep Power: 66
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Quote:
Originally Posted by beastieboys6 View Post
I think I have found out what's wrong with my case. I didn't change the setting for density. The default is 1.62 kg/m3.

Here comes new questions. Should I change the density to 0.002 kg/m3 (=1.62/760 kg/m3) directly? Or should I select the imcompressible ideal gas law? Since there is no heating source in my case, is it more convenient to set the density as 0.002 kg/m3?
It is better to define the density as a global constant. Using ideal gas law will increase computational cost to solve for density everywhere at each iteration, whereas it is a simple look-up if it is defined explicitly.
LuckyTran is offline   Reply With Quote

Old   April 10, 2012, 22:46
Default
  #4
New Member
 
Join Date: Apr 2012
Posts: 17
Rep Power: 14
beastieboys6 is on a distinguished road
Thanks for your reply. I've run the case with a constant density (1.62/760 kg/m3). And yes, it's less computational expensive since it does not have to solve the energy equation. However, I found that the outlet velocity is overestimated because the pressure of a certain part of my simulation domain is not 133 pa. It jumps to as high as 1000 pa. I guess I have to try the ideal gas law and hope that I will get results. Thanks for your help again.
beastieboys6 is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Wind turbine simulation Saturn CFX 60 July 17, 2024 05:45
Help for setting 3D boundary condition in compressing water vapor sogolf FLUENT 0 September 27, 2009 15:05
Implications for heat transfer when outlet pressure boundary condition used Blob CFX 2 March 26, 2009 06:22
Convective Heat Transfer - Heat Exchanger Mark CFX 6 November 15, 2004 15:55
Please help with flow around car modelling! Tudor Miron CFX 17 March 19, 2004 19:23


All times are GMT -4. The time now is 19:27.