transient conjugate heat transfer case diverged, any suggestion?

[pei-ying.hsieh]

Dear Simulation CFD experts,

I am solving a conjugate heat transfer case.  This is an enclosure with 2 openings.  There are a couple of fans inside the enclosure.  And I am using fan curves for these fans.  Each fan also generate 100 watts heat. 

So, I ran the flow only steady state calculations.  I did reduce the relaxation factor to 0.1 for v and ran 600 iteration steps.  Flow solution does not look too bad.  Then, I turned of flow and turned on heat transfer.  And changed the case to transient.  The temperature field looked fine up to time = 3000 seconds, then, the temperature field quickly diverged.

I am wondering anyone here has any suggestion on something like this.

For the 2 openings, I set pressure to gauge 0.  I am not too sure how to set the BC for temperature.  Ideally, I should have a big air volume that have the enclosure inside.  But, just want to get a quick feel of the performance.

Thanks!

Pei-Ying