My energy residuals are increasing rapidly after steadily decreasing for about 40 iterations. Temperatures for convergence criteria jump to 5000 C, way higher than what would make sense. The mesh seems to be fine. I have tried different solution methods, including SIMPLE and Coupled Pseudo-Transient. I have decreased the under-relaxation factor for energy to .4, and I get the same result. Any suggestions?
What is the courant number for your simulation?
Try simulating it with a smaller time step.
You have to look for the results. Check contours of temperature all over the domain and identify the location where you are getting maximum spurious temperature values. There can be number of reasons.
Can you explain more details about your case?
Actually there are many reasons. It can be you select wrong y+ for first cell height so wall function to obtain wrong calculation of heat transfer.
Based on your residual graph, I think 1e-8 is very good. So you can stop them between 30 and 40 iterations if your simulation is same compared to experiment, it's ok. Energy Equation URF is at least 0.9.
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