Turbulent Forced Convection Air Cooling of Electronics with Heat Sinks Under Flow Bypass Conditions

University dissertation from Stockholm : Energiteknik

Abstract: Air cooling is still the dominant method for dissipating theheat produced by electronic components. In a typical electronicdevice, the components are situated on a Printed Circuit Board(PCB). For most applications natural convection is sufficientto cool the components. However in some cases forced convectionair cooling, which provides more effective cooling, has to beused. In cases with high heat fluxes the switch from natural toforced convection is not enough and in these cases the surfacearea of the components must also be increased by means of aheat sink, i.e. a finned surface. The size of the components isoften considerably smaller than the PCB, and in cases whereheat sinks are used, the heat sink seldom covers the air flowduct formed by two parallel PCBs. Since the air flow is seekingthe path of least resistance, some of it will bypass the heatsink. This air flow bypass has a major impact on theperformance of the heat sink, why knowledge of the amount ofbypass flow is of vital importance.The aim of this work has been to investigate the influenceof flow bypass on the performance of heat sinks used forelectronics cooling. The focus has been set on determining theparameters that influence the bypass, and to quantify theirrelative importance using experimental and numericaltechniques.Experimentaland numerical parametric studies have beenperformed. The influence of duct dimensions, air velocity, fintype, array arrangement, fin height, and fin-to-fin distance onthe thermal and hydraulic performance of plate fin, strip fin,and pin fin heat sinks have been investigated. The flow wasturbulent, and the Reynolds number was varied between 1 700 and16 000. Measurements have been performed in a wind tunnel. Intotal 42 different heat sinks were tested in seven differentduct configurations. For the plate fin heat sinks and thecircular pin fin heat sinks, numerical predictions have beenperformed by using the finite element code FIDAP and the finitevolume code PHOENICS, respectively. For turbulence modeling therevised RNG k-? model and the Chen-Kim k-? model wereused.The agreement between the results from the experimental andnumerical investigations was very good, suggesting that acombination of the two methods can be a powerful tool forpredicting the thermal and hydraulic performance of heat sinks.The experimental data can be used to validate the numericalmodel, which in turn can be used for parametric studies.The results of the investigations showed that the thermalperformance was most influenced by the duct height, and the airvelocity while the hydraulic performance was dependent on allthe investigated parameters.From the experimental and numerical results severalcorrelations have been developed to describe the relativeimportance of the investigated parameters. The agreementbetween the source data and the correlations is very good, aswell as the agreement between the correlations developed withdata from the experimental and the numerical investigationsrespectively.Keywords: forced convection, heat transfer, pressuredrop, turbulent, air cooling, bypass, plate, strip, pin, fin,heat sink, electronics, experimental,numerical, CFD

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