Hello Rania,
It is hard for me to judge what is going one. However, my recommendation is to try one of the tutorial by building it from scratch and see if you miss anything. Also you could use one of the tutorial geometries and use your own flow operating conditions to see if the model converge. You may need to scale the geometry if needed to have a resonable matched conditions. By doing so, you may be able to get an idea what could be wrong.
Also you could try the transient solver with small time step and observe the change in the temperature. Make sure you have conformal mesh so the cell zones are connection. Additionally you may well under relax the potential to see if this help.
Best,
John
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