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

Simulation of air flow in a chamber

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 22, 2009, 11:08
Default Simulation of air flow in a chamber
  #1
New Member
 
Join Date: Nov 2009
Posts: 12
Rep Power: 16
Issa is on a distinguished road
Hi,

I would like to inform you that I'm using CFX to simulate an air flow in a chamber, the air inters the chamber through a (0.5 m X 0.5 m) flat faced displacement ventilation (DV) diffuser (photo attached). The flat faced DV diffuser is consist of 3944 holes, each hole is 2 mm diameter and these holes represent only 5% of the total area of DV diffuser. The air velocity measured at the inlet is 0.1 m/s, (so the flow rate = 0.1 X0.5X0.5X5%). I think it is very difficult to draw this DV diffuser with 3944 holes. So, instead I reduced the air velocity which comes out from the diffuser from 0.1 m/s to (0.1*5%=0.005 m/s). But unfortunately the predicted air velocity and air temperature is different than the measured ones. Then I tried to keep the air velocity at 0.1 m/s but in this case I reduced the inlet area from (0.5 m X 0.5 m) to (0.1118 X 0.1118 m) which is 5% of the total inlet area but still there is discrepancy between the predicted and measured air velocity and air tempertature. Hence, I will be very grateful if you advice me what to do to get an accurate prediction of the air flow ? Thank you

Regards,
Issa
Issa is offline   Reply With Quote

Old   November 22, 2009, 16:53
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,830
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
By the way: The image was not attached.

It is a tricky thing to model. On a micro-scale, each little hole creates an air jet at a high velocity which then enters the main room. These jets will travel out for a little way, then they will start to diffuse and turbulent action will chop them up. After sufficient diffusion adjacent jets will start to merge and it will become just a bulk air flow.

I would recommend you consider putting the inlet at the point where the little jets have merged and become a simpler bulk air flow. Your simple approach fails to capture the details of this jetting and merging effects and that is why it is not matching results. So best not even attempt to model it and move the boundary to somewhere where the flow is better known.
ghorrocks is offline   Reply With Quote

Old   November 23, 2009, 08:40
Default
  #3
New Member
 
Join Date: Nov 2009
Posts: 12
Rep Power: 16
Issa is on a distinguished road
Dear Horrocks,

Thank you for your reply. However, actually I don't know where is the point where the little jets have merged and become a simpler bulk air flow. I think there is another way to simulate the displacement ventilation unit with many holes but I don't know how. So, if possible would you please recommend another solution? Thank you

Regards,
Issa
Issa is offline   Reply With Quote

Old   November 23, 2009, 17:16
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,830
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
My first suggestion is not good enough? You are the one doing the analysis so you must decide.

But in general the concept is that you MUST put the boundary somewhere you know enough about the flow to describe it with the functions available. If you don't know the details of the flow at the grill then you have to go somewhere else to somewhere you do know.
ghorrocks 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
Two-phase air water flow problems by activating Wall Lubrication Force challenger85 CFX 5 November 5, 2009 05:44
calculate volume flow from a 2D simulation SimonH. OpenFOAM 0 October 27, 2009 04:39
Internal flow simulation boundary conditions Kishore FLUENT 1 July 10, 2007 11:42
moving water from a chamber using air Matt Mason CFX 0 December 2, 2003 19:46
simulation of mixing flow willams Main CFD Forum 6 October 18, 1999 00:46


All times are GMT -4. The time now is 03:42.