Convergence
I have a problem with the convergence of a system, and I found out, that when I use the segregated solver and solve the equations for flow and energy independently, the system converges! I think I can do this because I have an incompressible fluid so the energy equation is independent from the contunity and Navier stokes equation. But is it also possible that I change the fluid properties to shift the flow temperature during a simulation (so stop the simulation change the properties to new values and restart the simulation at the same point). When I simulate my flow with a higher temperature I have problems with convergence, but if I start my simulation at a lower temperature and change then the temperature (which changes also the fluid properties) I get good results.
Any ideas Thank you for your help 
Re: Convergence
I have the same problem before. You may like to switch to transient run to get convergence even though you are only looking for steady state solution (ref. Fluent manual section 8.9).

Re: Convergence
Dear Whitaker
As far as my experience is concerned, I've met fairly difficulaty situation for the convergence problem of energy equation for every simulation. It may be dependent of case by case. But I could obtain converged solution by adjusting underrelaxation parameter for energy equation for most case. I usually use underrelaxation parameter, 0.70.8, for startup and increase step by step and eventually near 1.0. Then, it was O.K. for most cases. For natural convection, transition calculation may be very helpful to you, as K. H. Wu said. I hope it will be helpful to you. Sincerely, Jinwook 
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