Instability and heat transfer in mixed convection flow in a horizontal duct with application to cooling of electronic systems

Two- and Three-dimensional numerical models have been developed to investigate the laminar, oscillatory and transitional flow through a horizontal rectangular duct, with significant buoyancy effects. Multiple discrete heat sources are mounted on the bottom surface of the duct, modeling integrated circuit (IC) chips on printed circuit boards (PCBs). The main governing parameters, besides the geometry, are the Reynolds number (Re) and the Grashof number (Gr). The objective of the present research is to study the flow instabilities, conjugate heat transfer, and the resulting effects on thermal transport, leading to cooling enhancement in electronic systems.; In the 2D model, the heat sources are two isolated protruding blocks. At a given Grashof number, when Re exceeds a critical value, vortex is shed from the second block, and rolls over the bottom wall with its size decaying, while one or two recirculating cells are trapped in the groove. The base vortex shedding frequency is dependent on the buoyancy level Gr/Re 2. The critical value of Re is much lower when perturbation is introduced by a rectangular vortex promoter in the channel. The frequency and amplitude of perturbation are changed by adjusting the geometry of the promoter. An improved fluid exchange between the main flow and the recirculating cell occurs when the promoter is employed. The effects of frequency and perturbation amplitude on the flow and on the heat transfer are investigated.; In the 3D simulations, the flow stability of Poiseuille-Benard flows with discontinuously heating from below is studied, and four different flow patterns, besides the basic flow, are found. The discontinuous thermal boundary condition on the bottom makes the longitudinal rolls expand and shrink periodically, and also helps in the generation of transverse rolls. Steady transverse rolls are found even at subcritical Rayleigh numbers for relative low Reynolds numbers. A variable-property model is also developed to validate the Boussinesq approximations. Numerical solutions, given by the constant and the variable property models, are presented and compared. The limitation and advantages of both models are discussed.; This study finally considers the conjugate heat transfer at the bottom wall. The effects of the size and spatial arrangement of heat sources, the thermal conductivity ratio of the bottom plate material to air, and the Reynolds numbers on the overall and local heat transfer in the duct are evaluated. The magnitudes of the conduction and the convection transport were compared in different parametric combinations. It is shown that conjugate transport effects are particularly important in three-dimensional configurations and can strongly affect the optimal location of the heat sources.