Fluent may produce a solution with small regions of extremely high or low temperatures. This is due to the instability or convergence issues in energy equation. Usually poor mesh quality is the cause. Below please find a list of actions to eliminate this problem:
1. Select Solution/Controls/Advanced under the Multigrid tab change energy cycle type to "Flexible".
2. Select Solution/Methods/Gradient as "Least Squares Cell Based". (this is the default)
3. If you have temperature dependent properties, start the solution with constant properties. Once, the energy solution settles, turn temperature dependent properties back on.
4. Turn off second gradient in energy calculations by typing in TUI: (rpsetvar 'temperature/second-gradient? #f)
5. Identify cells with extreme temperatures. Mark them (don't adapt) and improve poor cell numerics by typing in TUI: solve/set/poor-mesh-numerics/user-defined-on-register yes