Hi @marchywka,
this coupled equation ( momentum and energy) takes care of the continuity equation through the penalty FEM approach.
Also, I have put velocity components zero at each surface due to the no-slip boundary conditions. In my case, it is converging but not affected by the value of the Rayleigh number.
Also, changing boundary conditions show few noticeable changes in the iso-slice of temperature.
Could you please give me a few relevant portals to learn and implement such problems?