CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Negative Pressure in fan modelling

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

Reply
 
LinkBack Thread Tools Display Modes
Old   September 26, 2007, 03:21
Default Negative Pressure in fan modelling
  #1
Jenny
Guest
 
Posts: n/a
I have set up a model of an inline fan using the boundary conditions of an opening and a mass flow outlet, as advised by engineers at CFX. For some reason I seem to be getting negative pressure readings at a point downstream of the fan. I have used 1atm reference pressure and 0Pa relative pressure on the opening boundary and have my fan spinning in the correct direction. I am using GGIs between the fan inlet and opening and fan outlet and the main outlet test pipe and the frozen rotor approach.

I have tested different mass flow values and have tried different meshes and turbulence models (SST and k-e), but have the same problem, so it must be something basic I am missing in the setup.

Would anyone have any suggestions as to what I may be missing? I have put the question to staff at CFX over the weekend but as yet haven't had a reply, so would really appreciate any comments people may have.

Thanks in advance for your help.

Jenny

  Reply With Quote

Old   September 26, 2007, 10:24
Default Re: Negative Pressure in fan modelling
  #2
BB
Guest
 
Posts: n/a
Pressure is only a nominal value (relative value). Talking about pressure increase or decrease makes more sense than the single point value.

If you set every boundary conditions as test setup (For example, 0 Total Pressure at the entrance of inelt side duct and mass flow at exit of outlet duct), you should get correct pressure. In this case, the static pressure at duct inlet and wheel inlet is negative. The wheel makes the work and raises the pressure.
  Reply With Quote

Old   September 26, 2007, 11:52
Default Re: Negative Pressure in fan modelling
  #3
CycLone
Guest
 
Posts: n/a
If you set your Reference Pressure (Pref) to zero you may run into round-off issues (because you end up taking the difference between two large numbers). It's better to always set the Reference Pressure to be in the range of your calculation. A simple way to do this is to set Pref to the value at one of your boundaries and set that boundary condition to a relative pressure of zero, as Jenny has done.

The "Pressure" variable is then a Guage Pressure, relative to your domain reference pressure. A value of Pressure less than zero simply means it is lower than Pref. Note that "Total Pressure" is also reported relative to Pref. To see the Absolute Pressure, just plot or calculate using the "Absolute Pressure" variable or add your reference pressure in.

That said, it is still possible to get a negative Absolute Pressure if your fluid properties are not a function of Pressure. If the density is constant, the pressure level really has no meaning and it is only the relative pressures that matter. Physically this makes no sense (although it is possible to have a liquid in tension), but fluids which don't change with pressure don't exist!! So just take care when interpreting your results.

-CycLone
  Reply With Quote

Old   September 30, 2007, 08:47
Default Re: Negative Pressure in fan modelling
  #4
Jenny
Guest
 
Posts: n/a
Thank you for your replies. I've double checked that I wasn't getting the pressures mixed up and I'm looking at the absolute pressure still being a negative value of 10Pa, where in reality my experimental value was 30Pa, so somehow 40Pa has got lost somewhere. I am currently trying to refine the mesh, but having some difficulties with resolving some complicated parts of the geometry (see new question submitted on Sunday).

Thanks again for your help.

Best wishes, Jenny
  Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Inlet shapes for modelling a fan in an inlet duct buzzybee CFX 10 June 11, 2009 20:15
Fan dilemma Jenny CFX 7 February 11, 2008 13:17
Negative Pressure S Christopher CFX 2 October 6, 2005 18:14
Please Help ! Pressure Jump of Fan?? Boris FLUENT 0 January 28, 2003 21:26
what the result is negatif pressure at inlet chong chee nan FLUENT 0 December 29, 2001 06:13


All times are GMT -4. The time now is 01:24.