Heat transfer and fluid flow of nanofluids in enclosures with entropy generation and inner solid

Abstract

Understanding the heat transfer and fluid flow characteristics is crucial in many engineering applications, including designing efficient heat exchangers, thermal management in electronic devices, and optimizing fluid dynamics in aerodynamics and hydrodynamics. Various mathematical models and computational techniques, such as computational fluid dynamics (CFD), are used to analyze and predict heat transfer and fluid flow behavior in complex systems. By analyzing fluid flow and heat transfer phenomena, CFD enables engineers and scientists to design energy systems that optimize heat transfer rates. This thesis focuses on investigating heat transfer performance and entropy generation using both natural and mixed convection within different types of enclosures. The flow conditions considered are laminar, Newtonian, and two-dimensional, while nanofluid is employed as the working fluid. To mathematically model the problem, the boundary and governing equations are nondimensionalized, resulting in a set of nonlinear partial differential equations (PDEs). The numerical model equations are solved using the finite difference and element method, which is implemented in COMSOL Multiphysics software. To validate the computational solutions, the results are compared with previous studies. The findings of this thesis are presented graphically through visualizations such as streamlines, isotherms, isentropic plots, local Nusselt number distributions, average Nusselt numbers, global entropy generation, and Bejan numbers. The first case focuses on a square cavity with a solid insert placed on the center of the enclosure, while isothermal heat sources are located along the bottom wall and partially cold at the top and side walls. The results indicate that a larger solid insert and lower thermal conductivity hinder convective heat transfer within the square cavity. Next, a partially heated cavity with a nanofluid and a solid bottom wall is considered. The results show that convective heat transfer is enhanced with an increase in the heat source length on the bottom wall and the interface walls within the square cavity. Moving on, a trapezoidal enclosure with a localized solid cylinder positioned at the center of the cavity is studied. The top horizontal surface of the enclosure moves at a uniform velocity while being maintained at a cold temperature, and the bottom horizontal wall is thermally activated. The obtained results demonstrate an improvement in heat transfer performance when the solid cylinder is placed at the cavity’s center. Lastly, a triangular enclosure with three inner rotating solid cylinders is analyzed. The bottom wall of the enclosure is heated, while the side walls are maintained at a cold temperature. The results reveal that the presence of rotating solid cylinders in the cavity enhances entropy generation and improves the heat transfer analysis.