[milindlol]

I am simulating a thermosyphon in Ansys Fluent. It is a multi-phase simulation with water liquid and water vapor. A UDF is incorporated to help with the phase change. Lee phase change model is being used. VOF model is being used.
Time step of 0.0005 seconds for a total time of 60 seconds has to be simulated. The mesh details are as follows:
Aspect Ratio: 1.064(min) to 5.92(max)
Skewness: 1.30e-10(min) to 0.00026(max)
Orthogonal quality: 1(min/max)
The simulation fails at 0.0025 seconds i.e. time step 5 due to the global courant number error, velocity field diverging.
Temperature is constant throughout the thermosyphon at 300K.
Please find the attachments for your reference.
Any help would be appreciated, thank you.