Unsteady solver/natural convection
 

Hi, I am modelling bouyancy flow from a hot horizontal cylinder enclosed by pressure boundary conditions domain in 2D. As per fluent advanced lectures, natural convection around abstacles must be modeled using the unsteady solver. That is what I am trying to do but I noticed that whatever number of maximum iterations per time step I define, i never get convergence at each time step, i even used 10000 iterations per time step!
My time step size is 0.6, calculated from:
Time step={square root[characterestic length/(beta*delta T*g)]}/4
So I am wondering what is wrong or what should I do? Even when i disregard this problem and wait for overall convergence i never get it, the net heat transfer never becomes zero it is either less than or more than zero.
Any help will be appreciated.
Thank you.