Magneto laminar mixed convection and entropy generation analyses of an impinging slot jet of Al2O3-water and Novec-649

Abstract

In this study, the cooling capability and heat transfer characteristics in the mixed convective regime of a hot surface by a confined impinging slot jet in the presence of a uniform magnetic field have been investigated numerically. Besides, the second law of thermodynamics is applied to analyze the entropy generation inside the simulation medium. The simulations are for a two-dimensional slot jet that is directed vertically downward and impinges on a hot isothermal surface at the bottom. The set of non-dimensional governing equations is solved using ANSYS Fluent, which employs a finite volume approach in combination with a non-dimensionalization scheme. Flow patterns, isotherms, and entropy generation rates were calculated for various fluids (Air, Water, Novec-649, Al2O3-Water (3 %) nanofluid), that cover a wide range of Prandtl number values (Pr = 0.71–12.64), in the presence of a uniform magnetic field with Hartmann number values ranging from 0 (no magnetic field) to 100 (strong magnetic field). We prove that the Prandtl number plays a key role in determining heat transfer efficiency and entropy generation. Fluids with a high Pr number are more efficient for surface cooling, although they generate more entropy than low Pr fluids. Nanofluid with a high-volume fraction can be a better choice rather than air and water. On the other hand, we show that applying the magnetic field result in a very efficient method to reduce entropy generation although they degrade heat transfer efficiency. It is interesting that at a higher magnetic field, a high Prandtl number plays a better role. The percentage of reduction of Nuave by applying magnetic field is 28.5 % for air (Pr = 0.71), 45.2 % for water (Pr = 6.2), 43.4 % for Al2O3-water nanofluid (Pr = 4.91) and 48.3 % for Novec-649 (Pr = 12.64).We postulate that applying magnetic fields combined with high Pr number fluids, could provide an excellent balance between heat transfer efficiency and exergy destruction, thus providing efficient control options for thermal management and heat transfer.