why do we get divergence error?
 

My problem is Natural convection in a vertical tube. domain is 0.2m by 1m. I've given pressure-inlet and pressure outlet boundary conditions. Ambient is at 293K and wall is at constant temperature of 298K. I'm using Boussinesq approximation. everything is Ok, but when i start iterating, i get error message "Divergence in AMG x-momentum solver". what shall i do? can anyone help me. if possible try to run my case, it'll take not even 10 mins.