Numerical studies on transient double-diffusive and mixed convection in cylindrical enclosures

Numerical studies on transient convection phenomena are introduced. The studies are related to computation and description of time dependent flow, temperature and concentration fields. Two topics related with convection phenomena in cylindrical enclosures are presented, the first being double diffusive natural convection in cylindrical vessels. The subject of research is concerned with the liquid-liquid interface stability and its relationship with the different flow and mixing characteristics observed in the system. Interface penetration phenomena during transient double diffusive convection in a vertical cylinder is analyzed through direct numerical simulation. The fluid is initially motionless with a pre-existent two-layers density stratification featuring a diffusion-like interface. The flow is initiated when a uniform heat flux is applied from the sidewall. The influence of relevant parameters on velocity of interface penetration and intensity of mixing are analyzed. Two regimes of interface penetration were identified and characterized namely: convective and diffusive-convective regimes of interface penetration.; The second topic is mixed natural and forced convection. In this case the investigation is focused on transient oscillatory convection and its suppression in the Czochralski growth melt. A numerical analysis assuming axisymmetric flow and temperature fields in a cylindrical enclosure was performed in which the swirl component of velocity is included in the computation to account for rotational effects. Temperature and flow field oscillations were computed for natural and forced convection dominant flow regimes. Numerical predictions are in excellent agreement with previously reported experimental results. Computational results are presented which disclose relevant features of oscillatory thermal and flow fields. Parametric studies were carried out to evaluate the effect of crystal radius and melt depth variations on the oscillatory flows. It was shown that non-oscillatory striation free conditions can be attained, for both natural convection dominant or forced convection dominant flow modes by reducing the radius fraction r_c/R_c, and increasing the crucible aspect ratio R_c/H. The effect of a circular flat plate immersed in the melt was also evaluated for different vertical positions of the baffle. It was found that inserting a baffle with a radius slightly larger than the crystal radius at an appropriate vertical position can effectively attenuate thermal fluctuations.