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

CFX Natural Convection, please help.

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 13, 2010, 18:24
Default CFX Natural Convection, please help.
  #1
New Member
 
D
Join Date: Oct 2010
Posts: 3
Rep Power: 15
dbecker is on a distinguished road
Hello,

I am tasked to perform a natural convection analysis on electrical components. I am using CFX 11.0 and ICEM.

Before I began the analysis I wanted to see if CFX is capable of performing this type of analysis. I made a simple cylinder in a box, the cylinder extends to the walls of the box and I put slip planes on the end walls to simulate infinitely long cylinder. I put no slip planes on the sides and top and bottom.

Cylinder is set at 500C and roof and floor and side walls are at 55C. I turned on bouyancy and I am using shear stress transport because ultimately I am looking for heat transfer coefficients on the cylinder.

I ran the solution and this is the result, heat transfer coefficients are about right according to my hand calcs for a cylinder in air with this deltaT, but the gas temperature distribution is almost all uniformly 55C. This is wrong, there should be a spread of temperature between 55 and 500. This does not make sense and I need to know how to validate and run natrual convection models in CFX.

I tried using the help file and there is about 2 paragraphs on the subject and that's it. Does anyone have any experience using CFX for natural convection? Please help!

Thanks,


- D
Attached Images
File Type: jpg ScreenShot339.jpg (54.2 KB, 34 views)
File Type: jpg ScreenShot341.jpg (45.7 KB, 38 views)
File Type: jpg ScreenShot337.jpg (46.3 KB, 30 views)
File Type: jpg ScreenShot340.jpg (46.9 KB, 27 views)
dbecker is offline   Reply With Quote

Old   October 13, 2010, 18:30
Default
  #2
Senior Member
 
Michael P. Owen
Join Date: Mar 2009
Posts: 196
Rep Power: 17
michael_owen is on a distinguished road
I'm not sure what model you are looking at, but there is a spread in temperature between 55 and 500C. Look at your second graph.
michael_owen is offline   Reply With Quote

Old   October 13, 2010, 18:42
Default
  #3
New Member
 
D
Join Date: Oct 2010
Posts: 3
Rep Power: 15
dbecker is on a distinguished road
It looks almost uniformly 55C to me, maybe a little more, but there isnt much effect on gas temperature due to the rod which is why I ask.
dbecker is offline   Reply With Quote

Old   October 13, 2010, 18:48
Default
  #4
Senior Member
 
Michael P. Owen
Join Date: Mar 2009
Posts: 196
Rep Power: 17
michael_owen is on a distinguished road
Probe it. The bottom two ticks on the legend are 55 and 166 C.

Do you have some expectation of what it should be that is different from what it is?
michael_owen is offline   Reply With Quote

Old   October 13, 2010, 19:11
Default
  #5
New Member
 
D
Join Date: Oct 2010
Posts: 3
Rep Power: 15
dbecker is on a distinguished road
Alright take a look, I think the variation is there but it's just very suttle. I think I was expecting to see larger gradient, gradient exists but it's almost as if this is all conduction with little convection. Something I wasnt expecting with a 500C rod in a 55C room.

Gas is moving up which is a good thing; at least bouyancy is in effect.

By the way gravity is -9.81 along X which is vertical in this case.

I dont know why but my gut is telling me something different.
Attached Images
File Type: jpg ScreenShot342.jpg (58.8 KB, 21 views)
File Type: jpg ScreenShot343.jpg (48.3 KB, 17 views)
File Type: jpg ScreenShot344.jpg (88.1 KB, 23 views)
dbecker is offline   Reply With Quote

Old   October 13, 2010, 20:07
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,655
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Is this a steady or transient model?

If transient then don't you just have to run it longer to get more hot gas in there so it is easier to see?

Another point is that over the range 50-500 deg C the boussinesq assumption for buoyancy is quite inaccurate. You will probably need to use an ideal gas approach, also maybe with variable properties (Cp, viscosity mainly).
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
natural convection in ANSYS CFX 10 cryo man ANSYS 0 June 9, 2009 08:53
natural convection problem in cfx NVSD BABU CFX 7 November 13, 2008 11:56
Natural convection problem in CFX 11 Willy CFX 2 May 24, 2008 00:12
Natural convection problem (CFX 11.0) Willy CFX 0 May 13, 2008 22:19
Coupled vs Seg - Natural vs. Forced Convection Alex Siemens 5 December 12, 2007 05:58


All times are GMT -4. The time now is 23:04.