I have a conjugate heat transfer problem in FLUENT 12. The energy residuals do not converge. What could be the cause and how to converge it?

1. Poor mesh quality: high size change, and/or high skewness, and/or coarse mesh
2. high jump in thermal conductivity
3. invalid boundary conditions
4. improper mesh scaling

1. make sure mesh is scaled properly. Go to Mesh --> Scale

2. double-check model setup and boundary conditions

3. reduce explict-relaxation, using the following 3 lines:

(rpsetvar 'temperature/explicit-relax? #f)
(rpsetvar 'explicit-relaxation? #t)
(rpsetvar 'temperature/explicit-relax 0.1)

By default, explicit-relax is set to 1.0.

4. Keep implicit underrelaxation to 1.0, by using the following line:

(rpsetvar 'temperature/relax 1)

It can also be set in the GUI.

5. Use alternative wall formulation, which can be activated using the folloiwng TUI command:

/solve set expert , yes , , ,

6. Ignore secondary gradient on all the cells, but shell conduction walls (if you have them), by using the following rpvar:

(rpsetvar 'temperature/secondary-gradient? #f)

By default, this is #t (true). To obtain the value, type:

(rpgetvar 'temperature/secondary-gradient?)

which will return either #t or #f.

Warning: This may bring some measureable inaccuracy, depending on how skewed the cells are at the wall and how high is the magnitude of the wall heat flux.

7. If you have turned on shell conduction in any of the walls, ignore secondary gradient in shell conduction zones only using the following command:

(rpsetvar 'temperature/shell-secondary-gradient? #f)

This does not cause measurable inaccuracy.

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