An Investigation On Heat Transfer Augmentation Of Large Power LED Heat Pipe Radiators

In recent years, with the high-frequency and rapid development of high-power LEDlight pieces and other electronic components as well as the intensive and miniaturizeddevelopment of integrated circuits, the heat quantity of components and the chiptemperature are increasing, which puts forward a higher requirements on electroniccooling. Finned tube heat radiator is considered one of the advanced chip coolingtechnology and has broad application prospects with its characteristic such as highthermal conductivity, compact structure and does not require additional powerconsumption, etc. Therefore, the development and research of finned tube heatradiators and its heat transfer characteristics has important practical significance, andalso has important theoretical and academic value.On the other hand, the performanceof thermal interface material (TIMs) is a major factor of chip cooling, which is a keypart that connected with the external heat sink, and the performance of thermalinterface material restricts the performance of the entire cooling system. Inconsideration of the above reasons, the theoretical and experimental studies of finnedheat pipe heat radiator and thermal interface material were accomplished, in addition,the high-performance finned tube heat radiator and thermal interface materials whichuse nano-materials as conductive additive were developed. The main contents include:A series of heat pipe heat radiators with novel fins structure included diamond,perforated and slit outwards-radiating fins were designed by heat transferaugmentation theory and operational principle of the heat pipe heat radiator.Experimental results show that the performance of the three heat pipe radiatorstructure is significantly higher than conventional rectangular fin structure heat piperadiator, most of all, the slit outwards-radiating fins structure has the best thermalperformance, and the corresponding reason were analyzed by air flow, boundary layerand field synergy theory, and the temperature of fin were measured and contrastedwith simulation results. In addition, the performance of heat pipe heat radiator wasstudied systematically from inclination, heat pipe number, fin material, ambienttemperature and input power.Study on the heat transfer augmentation of thermal interface material. Good heat conduction performance of thermal interface materials is one of the key factors toreduce thermal contact resistance. In order to improve the performance of TIMs, thecopper nano-particles, nano-carbon powder, carbon nano-tubes, and graphite sheetwere selected as the thermally conductive additive that added to the grease and theirimproving effect on the same bases were compared to find the most suitable thermaladditive in the thermal interface material. The effect of additive volume fraction onthe heat transfer performance of silicone grease was also studied, and the resultesshowed that the equivalent thermal conductivity of the composite layer increased withthe increase of additive content when the volume fraction was below60%, whilebeyond the certain propo rtion, the equivalent thermal conductivity decreased with theincreasing additive amount. For carbon nanotubes and grapheme the turn volumefractions were lowest among the five nano-materials and the valid ranges were mostnarrow. However, when the volume fraction is lower, carbon nano-tubes and graphiteadditives can get a higher thermal conductivity than that of copper and carbon powder,which is favorable to improve the performance of grease. Low additive fraction isvery important for grease modification, which can reduce material consumption andcost. The experimental results are compared with those of the existing heat transfermodel prediction and theoretical analysis was given for the deviation.Study on the the influence of carbon nano-tubes on the thermal conductivityperformance of grease. A systematic study on the influence of carbon nano-tubes onthe thermal conductivity performance of grease was carried out from different wallstructures, carbon tube diameter, tube length and surface characteristics of carbonnano-tube. The equivalent thermal conductivity decreases as the number of carbonnano-tubes walls and the nano-tubes diameter increase and the nano-tubes lengthdecreases. Surface treatment on MWCNTs can enhance the thermal performance ofthe silicone grease. The modification of the MWCNTs by strong acid and baseintroducing some functional groups onto surfaces leads to a homogeneous distributionof the MWCNTs in the mother silicone grease. The enhanced homogeneity andweaker entanglement of the MWCNTs are confirmed by TEM observation. The effectof contact pressure on composites thermal property is also experimentally studied.The result shows that, although the thermal resistance of the composite layer formedwith different TIMs all decreases with the applied contact pressure, the TIMs with theaddition of the short and chemically treated MWCNTs are less sensitive to the contactpressure.