CFD Online Logo CFD Online URL
Home > Forums > CFX

CHT problem

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

LinkBack Thread Tools Display Modes
Old   July 9, 2003, 10:56
Default CHT problem
Posts: n/a
Hi all I am testing the case of a flow across a solid cylinder (Aluminium) to study the CHT in the solid. I imposed T=1000K at the inlet and at the exit (opening). I defined the cylinder as a solid subdomain with no heat source. As a initial guess I set T=1000K for the fluid and I intentionnaly imposed T=800K for the cylinder expecting at the calculation that it will evolve to 1000K. After 500 iterations the equations all converged to 1.e-8 except the temperature equation of the solid (stable at 1.e-4). At the post, the temperature at the solid remains around the initial guess (800K).

can anyone tell me what I made wrong? Thanks. Thanks a lot
  Reply With Quote

Old   July 10, 2003, 18:49
Default Re: CHT problem
Glenn Horrocks
Posts: n/a
Hi sch,

I have not got a lot of experience with CHT problems, but a major factor to consider is that in many configurations the timescale of conditions is slower in the solid than the flow timescale, often by orders of magnitude. To explain what I mean, image you suddenly changed the input flow conditions from 1000K to 500K. This temperature change would work through the fluid section quickly (in seconds), but you would imagine it to take much longer in the solid part before its temperature stabilised out (maybe minutes or hours).

What this means is that a timestep suitable for the fluid section of the flow is not suitable for the solid section. In convergence control, you have an option "Solid Timescale Factor". This sets the ratio of timesteps in the solid portion of the domain versus the fluid part. I have used a value of 10 for a geometry with the solid being long and thin, so variations are "fast", but I could see much higher values being suitable for your case where you are using a cylinder, and the lower surface area means variations react "slowly". Try a value of 20 to 100.

Also, set a monitor point in the solid so you can watch its temperature throughout the simulation.

  Reply With Quote

Old   July 11, 2003, 10:32
Default Re: CHT problem
Posts: n/a
Hi Glenn You are right. In my initial model I set the local timestep factor to 3 and the solid timestep Factor to 1. When I inscreased the solid timestep factor (up to 1.e4) nothing happened, then I set Auto Timestep for all equations. Only in this case the influence of the solid timestep factor (from 10 to 1.e4) plays. and the results are good.

Thanks a lot for your help

  Reply With Quote


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
Problem Importing Geometry ProE to CFX fatb0y CFX 3 January 14, 2012 20:42
tips to speedup the convergence of CHT problem with gravity derisala FLUENT 1 February 24, 2011 03:28
Adiabatic and Rotating wall (Convection problem) ParodDav CFX 5 April 29, 2007 19:13
CHT strange problem MatejK CFX 3 September 1, 2005 05:32
Is this problem well posed? Thomas P. Abraham Main CFD Forum 5 September 8, 1999 14:52

All times are GMT -4. The time now is 10:39.