Effect of an external magnetic field on natural convection in a porous medium

Abstract

Heat transfer is one of the most fundamental and essential engineering principles. Heat transfer is used in some way in almost every sector. To either add or remove heat from a system, a heat transfer medium, including solids and fluids are employed. Since they are adaptable and versatile, fluids are employed extensively. However, the conventional heat transfer fluids (HTF) employed in industry have poor thermal properties. Thus, they have a limited ability to transport heat. In recent years, a new class of HTF called nanofluids (NFs) have been developed. These engineered fluids have enhanced thermal properties making them the future of HTF. In order to better comprehend the impact of different factors on nanofluid flow and heat transfer behavior, a detailed analysis of a nanofluid natural convection within a porous enclosure exposed to a magnetic field has been conducted. Additionally, specific research has been conducted to look at how entropy is generated in nanofluid natural convection flow. the mathematical model explaining this phenomenon has been constructed. The resulting non-dimensional equations have been discretized using the Galerkin weighted residual technique of finite element formulation. In this thesis, the steady-state incompressible free convection flow of nanofluid in two enclosures was investigated using computational fluid dynamics (CFD) software. The first enclosure is square with elliptical heated hole in the center and cold wavy sidewalls. The second one is an annulus space formed by a heated Koch-snowflake-shaped cylinder and a cooled circular cylinder. The numerical results are presented for the effects of Rayleigh numbers (Ra) from 103 to 106, Hartman number (Ha) from 0 to 100, Darcy number (Da) from 10-5 to 10-2, geometrical parameter (undulation number and inner cylinder position) solid volume fractions 0, 2%, 3%, 4%, 5% and 8% on the distributions of isotherms, streamlines, average Nusselt number (Nuavg) as well as on total entropy generation and Bejan number (Be). IV The findings suggest that the addition of nanofluid under certain conditions, increased heat transmission. Also, the maximum heat transfer increase was recorded in the conduction dominated flow regime, where the increased thermal characteristics of nanofluids play a key role. When convection is the major heat transfer mode, utilizing nanofluids offers a lesser improvement in heat transfer efficiency. The computational outputs reveal that raising the Ra number which is feasible by varying the temperature between the hot and cold sources, boosted the buoyant force. Thus, raising the value of Ra enhances the natural convection flow and improves the average Nusselt number. Furthermore, it is discovered that for large Ra numbers the average Nusselt number is more susceptible to the other factors. Implementing Lorentz force, if not in the direction of natural flow, forces the flow velocity to be depleted. As the Da number grows, the penetration of the flow cross-section in the cavity rises, and the flow circulates in the cavity with less depreciation.