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

Conjugate heat transfer, thermal boundary conditions

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 7, 2010, 05:31
Default Conjugate heat transfer, thermal boundary conditions
  #1
New Member
 
Join Date: Apr 2010
Posts: 5
Rep Power: 7
Sarah is on a distinguished road
Dear all,

I am fairly new to CFD, I hope the description of my problem is clear. I am using Fluent (Ansys 12).

My model is a room with dimensions 3.9x4.75x2.7m. At opposite sides of the room there is one inlet (average velocity 0.075 m/s) and one oulet (average velocity 0.28 m/s). The air coming into the room has a temperature of 289 K, also used as backflow temperature at the outlet.
I want to model the air flow pattern in the room, and the convective heat transfer at the surfaces. I also want to know the resulting temperature gradients over the surfaces and take into account the influence of the thermal mass of the walls, ceiling and floor. For this, I use shell conduction for all the walls, ceiling and floor with user-defined material and thicknesses of 0.1 to 0.2 m. As an external boundary condition I have to take either constant temperature or constant heat flux (other boundary conditions assume zero wall thickness).

Now I have three questions:

- Is it correct to use shell conduction in this situation? The main references I find where shell conduction is used, are for very thin structures as the fins in heat exchangers, not for heavy walls.

- To choose a temperature boundary condition at the inner side of the shell (so the side away from the flow domain), ideally, I would like to use the wall temperatures calculated by Fluent in the following way, to approximate adjacent identical rooms:
-> to assign the surface temperature of the floor to the inner side of the shell of the ceiling
-> to assign the surface temperature of the ceiling to the inner side of the shell of the floor
-> for the walls, I can simply couple the two sides of the walls
-> one of the walls is facing the exterior environment, so a constant temperature of 289K can be assigned
Do I need a UDF for this, or can I do this in Fluent directly? Is this a sufficient boundary information to solve the flow?

- Is this a steady-state problem? I am including buoyancy in my model, and am having trouble to make it converge. I make a first approximation by reducing the gravity to 0.098 and first order scheme. This simulation runs smoothly to convergence and results are nice symmetric. If, as recommended by the Fluent manual, I continue the calculation with normal gravity, the residuals get stuck and start oscillating around a certain value. I wonder of I may need a transient simulation?

Thanks for any help!

Sarah
PhD-student, KULeuven
Sarah is offline   Reply With Quote

Old   April 7, 2010, 07:20
Default
  #2
New Member
 
Join Date: Dec 2009
Posts: 13
Rep Power: 7
puga is on a distinguished road
One quick idea as per convergence issues:

You could step up gravity to 0.981, then to 9.81. Also, try lowering the inlet flows (maybe 50%?) and stepping those up as you get converged solutions.
puga is offline   Reply With Quote

Reply

Tags
boundary conditions, boussinesq, conjugate heat transfer, transient

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
Implementation of boundary conditions for FVM Tom Main CFD Forum 7 August 26, 2014 05:58
RPM in Wind Turbine Pankaj CFX 9 November 23, 2009 05:05
adiabatic wall boundary of conjugate heat transfer tomosada CD-adapco 1 May 13, 2005 00:14
Heat Transfer & Boundary Conditions across 2D wall SAAD Main CFD Forum 6 March 13, 2005 13:42
Boundary conditions? Tom Main CFD Forum 0 November 5, 2002 02:54


All times are GMT -4. The time now is 16:55.