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SIMULATION OF CONTROL OF THERMAL CONVECTION USING EXTERNAL MAGNETIC FIELD WITH DIFFERENT ORIENTATION AND DISTRIBUTION

Sasa Kenjeres
Transport Phenomena Section, Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology and J.M. Burgerscentrum for Fluid Mechanics, Delft, The Netherlands

Kemo Hanjalic
Department of Applied Physics Delft University of Technology 2600 GA Delft, The Netherlands

Resumo

We report on numerical study of effects of orientation and distribution of an external magnetic field on the reorganization of convective structures and heat transfer in thermal convection in electrically conductive fluids. The simulations were performed using a transient RANS (T-RANS) approach in which the large-scale deterministic structures are numerically resolved in time and space and the unresolved contribution is modelled using an algebraic stress-flux three-equation subscale model. For low Prandtl (Pr) fluids the subscale model was extended by including Pr-dependent molecular dissipation of heat flux, which lead to excellent agreement with DNS results for low-Pr classical Rayleigh-Benard (RB) convection. The T-RANS approach, tested earlier in a number of cases of thermal and magnetic convection, was first applied to natural convection in a side-heated cubical enclosure subjected to magnetic field of different orientation, strength and depth, showing good agreement with previous benchmark studies. Then, a series of simulations was performed of turbulent RB convection subjected to different magnetic fields over a range of Rayleigh (Ra) and Hartmann (Ha) numbers. The computed Nusselt number showed good agreement with the available experimental results. Numerical visualization of instantaneous flow pattern reveals dramatic differences in the convective structures and local heat transfer for different orientation of the magnetic field with respect to the gravitation vector. It was found that a local magnetic field confined to the wall boundary layer along the thermally active walls provides almost equal effects as the homogeneous field over the whole flow, indicating an interesting possibility for controlling thermal convection and associated heat transfer.