Thanks Dr amine. first, i don't understand 'time averaged solution' because for me i notices the problem by monitoring the area weighted average of temperature curve in fuction of flow time. second,For the CFL number, i don't have any idea about the velocity because it is a problem of a solidification of a phase chabge material based on natural convection. What can i do in tis case. Should i run any case till te end even it converges or not and see the velocity contours or dtreamlines or vectors and then calculate the CFL ?