Design and Analysis of Hybrid Fin Heat Sinks in Natural Convection and Impinging Flows

Abstract

This thesis reports the hybrid fin heat sinks (HFHS) research under natural convection and impinging airflows. The proposed HFHSs are the hollow hybrid fin heat sink (HHFHS), and the solid hybrid fin heat sink (SHFHS). A hollow hybrid fin (HHF) represents a hollow pin fin integrated with radially-oriented plate fins and perforation near the fin base. A solid hybrid fin (SHF) constitutes of a solid pin fin with extruded radially-oriented plate fins. First, the study shows the thermal performance of natural convective hybrid fin heat sinks (HFHSs) under various parametric conditions which are explored numerically and experimentally. Generated CFD models of heat sinks are verified by the measurements and utilized to investigate the effects of fin spacing and internal channel diameter on the thermal performance of the HFHSs at various heat dissipations. The result shows that the increase of the internal channel diameter mitigates the mass-multiplied thermal resistance of the HFHS while it increases the thermal resistance of the HFHS. It is also found that the mass-multiplied thermal resistance of the HHFHS and the thermal resistance of the SHFHS are 32% and 13% smaller compared with the pin fin heat sink (PFHS). These interesting results can be explained by coupled effects of surface area enhancement, heat pumping via the internal channel, and mass reduction. Second, the study investigates the orientation effects on natural convective performance of hybrid fin heat sinks. CFD thermal models of the HHFHS, the SHFHS, and the PFHS are generated, experimentally-verified, and employed to investigate the orientation effects, ranging from 0 to 180 o, on their thermal performances. The results show the lowest thermal resistance, Rth, values for the HHFHS, the SHFHS, and the PFHS occurred at an orientation angle of 45o, less orientation dependence of the HHFHS compared with the PFHS, and consistently smaller mass-multiplied thermal resistance, RthM, values of the HHFHS, even up to 32%, than those of the PFHS despite various orientations. Third, the study shows the prediction methods and optimum design for natural convection around the HHFHSs. 3-D computational thermal models have been generated using a commercial CFD software package and used to develop correlations to predict Nusselt numbers around HHFHSs. Nearly hundred cases under various parametric conditions have been calculated to obtain a broad range of thermal data. Correlations for Nusselt numbers have been obtained by considering their dependence on Rayleigh number, internal and external diameters, and fin height. The correlations are seen to reasonably agree with the numerical results despite the complicated parametric dependence of the HHFHS. The optimum analysis was performed by utilizing the Nusselt number correlation to find the lowest Rth and Rth.M values of HHFHS under natural convection with various geometrical variances. Fourth, the study investigates the thermal performance of various HHFHS and SHFHS configurations under impinging airflow. In this study, various parametric conditions, such as fin height, fin spacing, and the internal diameter of HHFHS and SHFHS are numerically investigated under several impinging flow parameters. The result shows that the HHFHS offers superior mass-based thermal performance compared to the SHFHS, especially at low-profile or short fin height setting. Subsequently, additional low-profile HHFHS with bigger internal diameters are designed and compared to the similarly built SHFHS to have a better understanding of its thermal behavior. The result indicates that the low-profile HHFHS have much better thermal and mass-based thermal performance compared to the SHFHS. Finally, the study develops a Nusselt number correlation of the HHFHS under impinging airflow. The Nusselt number correlation is also utilized to find optimum HHFHS design from various HHFHS configurations. In this study, more than two hundred HHFHS CFD cases with various geometric and flow conditions have been calculated to develop a Nusselt number correlation with a broad prediction compatibility. The Nusselt number correlation was determined by considering Reynolds and Prandtl number, heat sink area, outer fin diameter, plate fin width, fin height, and the base length. The correlation and the numerical results show a high degree of conformity at 7% discrepancy on average. The optimum design of the HHFHS utilizes Nusselt number correlation to predict the thermal and mass-multiplied thermal resistances for hundreds of HHFHS geometric variations. The optimum methodology is mainly to minimize the thermal resistance and the mass-multiplied thermal resistance under several ranges of mass.