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-   -   Modeling both radiation and convection on surfaces - Ansys Transient Thermal R13 (http://www.cfd-online.com/Forums/ansys/99308-modeling-both-radiation-convection-surfaces-ansys-transient-thermal-r13.html)

s.mishra March 31, 2012 05:12

Modeling both radiation and convection on surfaces - Ansys Transient Thermal R13
 
Hello everybody,

I am trying to model radiosity (default surface to surface radiation option in Ansys Thermal) and convection on surfaces of solid components in Ansys Thermal (R13, through Workbench). The geometry is of a closed container with a few components inside. I also have a fluid inside the container which I can assume to be non-participating in radiation.

I am modeling radiation using surf-to-surf radiosity option in GUI. For convection, I am creating Surf152 elements and applying convection loads using SFE command.

The exact process I am following is as follows:

1) Through GUI, insert Radiation BCs. I used default surface-to-surface radiation GUI settings and selected all surfaces under a single enclosure number. I am told that this method internally creates Surf252 elements to include radiation.

2) Create a layer of surface effect elements (Surf152) for all the internal wetted surfaces using ESURF command. These wet surfaces are also the surfaces internally exposed to each other for radiation. I am applying convection loads on these Surf152 elements.

3) Write cdb files for these Surf152 elements, basically it is the surface mesh on top of the solid components. As radiation is activated, this process also writes .rsm files corresponding to these elements.

4) Change Keyopt(8) = 4 for these surface effect elements (Surf152) to consider convection loads.

5) Create view factors - VFOPT, .., New..

6) Use FSI mapping panel in Fluent to create convection loads date on these surfaces using the cdb files created by Ansys in step 3. Fluent writes the convection loads using SFE command.

7) Apply loads step time in Ansys (TIME command).

8) Read the cdb convection load file written by Fluent for all the surfaces using /INPUT command. Also load the corresponding .rsm file (command RSOPT, LOAD..) written by Ansys in step 3 for all the wetted surfaces.

9) Use the existing view factors - VFOPT, ..OFF computed in step 5.

10) Solve for current load step.

Steps 6 to 10 are repeated in transient simulation for subsequent loads steps.


Now the problem:

If the convection loads are read (step 8), I do not see any effect of radiation even though the temperature difference is huge in my domain.

I checked this my applying very negligible convection loads (reduced HTC values in the cdb load files) and comparing the process with and without radiation. After suppressing radiation, I had to generate cdb files (step 3) again as the element numbers are changed.

I confimed this using 3 cases-
Case A: Radiation activated and negligible convection.
Case B: Radiation suppressed and negligible convection
Case C: Radiation suppressed and convection suppressed (I skipped step 8)

I am getting exactly same temperature results in all the 3 cases. Why is this so?

Perhaps may by I cannot have 2 layer - one of Surf252 for radiation and one of Surf152 for convection.

Any help ?

Many thanks,
Shreesh


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