| With the rapid development the integrated circuits towards high integration,high power and micro miniaturization,the heat flux of electronic components increases significantly,and hot spots occur frequently,which brings a series of serious challenges to the stable operation of electromechanical equipment.At present,approximately 55%of the failure problems of electronic components are caused by high temperature.When the temperature of chip surface exceeds 348 K,the reliability of electronic components will decrease by 5% as the temperature increases for every 1 K.Therefore,it is necessary to develop a cooling system which can effectively solve the problem of high heat flux and realize the adaptive cooling of hot spots.Based on this,we propose a smart fractal heat sinks which embedd with thermosensitive hydrogels,and develop a flow redistribution adaptive microfluidic cooling system incorporating the "switch" characteristics of the hydrogel.That is,based on the higher temperature in the vicinity of the hot spots,the hydrogel will deform,and more fluid will be allocated to the channel near the hot spot to achieve efficient adaptive cooling of the hotspot.Therefore,based on the structural characteristics of fractal heat sink,the genetic algorithm optimization,multi factor analysis,CFD numerical simulation and experimental research are employed.The fractal heat sinks structure and its hydraulic-thermal performance,thermo-responsive hydrogel synthesis,the experimental study of automatically adaptive cooling performance based on fractal branch with thermo-responsive hydrogel,the adaptive flow and heat transfer characteristics of fractal flow channel heat sink with different fractal series for hot spots at different locations,as well as the adaptive flow and heat transfer characteristics of different area hot spots with different branch numbers,are studied in detail.The automatically adaptive regulation of flow rate based on the swelling-shrinkage characteristics of thermo-responsive hydrogels is coupled with the bionic fractal micro-channel with efficient heat and mass transfer characteristics,so as to achieve the purpose of efficient and fast adaptive cooling of the local hotspots.The regulation mechanism of fractal channel heat sink embedded with thermo-responsive hydrogels on the automatically adaptive cooling performance of the local hotspot under different hot spot heat flux is expounded.The automatically adaptive cooling mechanism of fractal series and branching number on the cooling performance of hotspots by fractal heat sink embedded with thermo-responsive hydrogels is revealed,which provides theoretical guidance and engineering design experience for solving the problem of frequent local hot spots in electronic components.Firstly,the theoretical flow resistance model of generalized fractal heat sink is constructed,and it is found that the fractal network structure with symmetrical width and asymmetric length plays a positive role in reducing the flow resistance of the heat sink.By employing the combination of GA optimization and CFD numerical simulation,the optimal geometric structure parameters of symmetric and asymmetric bionic Y-shaped fractal network heat sink under different thermal resistance constraints are obtained.The pumping power of bionic Y-shaped fractal network heat sink increases with the decrease of thermal resistance,and can be divided into five regions,namely linear growth region(I region),transition region from linear growth to parabolic growth(II region),parabolic growth region(III region),transition region from parabola to exponential growth(IV region)and exponential growth region(V region).Further CFD studies show that under the same number heat flux and fractal braching neumber,the temperature uniformity of symmetric fractal network heat sink is better than that of asymmetric bionic Y fractal network heat sink.Secondly,by employing the combination of GA optimization,CFD numerical simulation and multi-factor analysis,the optimal geometric structure parameters of different fractal series of fractal heat sinks under different constrained thermal resistance are obtained.Under the same constrained thermal resistance,the heat sink with higher fractal series has less pump power;considering the applicable scope of the empirical formula used in this paper and the difficulty of actual processing,the best fractal series is 4.In addition,the effects of different influence factors on heat transfer,flow and comprehensive performance(considering heat transfer and flow characteristics)are obtained by multi-factor analysis.Thirdly,in order to prepare high strength and fast response hydrogel and make it suitable for valves used as flow control in fractal channel,a two-step method for synthesizing PSG hydrogel is proposed,and the effects of AM content,SDS content,synthesis time and temperature of NIPAM-co-AM crosslinker on the properties of PSG hydrogel are revealed.Furthermore,the cooling characteristics of the fractal channel with and without embedded thermal-responsive hydrogel are compared by experimental method,and the mechanism of hydrogel deformation on hotspot automatically adaptive cooling is revealed.Additionally,the flow and cooling characteristics of the embedded thermal-responsive hydrogels fractal heat sink with different fractal series on the uniform heat flux and hotspots at different locations are investigated.The embedded thermal-responsive hydrogels fractal heat sink with higher fractal series can handle higher heat flux.The introduction of hydrogels can not only regulate the heat transfer performance,but also increase the pumping power and pressure drop.The temperature sensitivity of the hydrogels in the first series channel is lower than that of the hydrogels in the secondary series channel,but the regulating effect of the hydrogels in the first series channel on flow rate is stronger than that of the hydrogels in the secondary series channel.As the position of the hot spot is far away from the center of the disk heat sink,the ability of induction and adaptive regulation of the hot spot by the fractal heat sink with three different fractal series is gradually weakened.When the hydrogels in the first series channel are deformed,the heat sink has a strong effect on regulating the flow rate.Considering the heat dissipation capacity of different fractal series heat sinks and the adaptive cooling characteristics of hydrogels to the hotspots,the comprehensive cooling performance of the embedded thermal-responsive hydrogels fractal heat sink with three fractal series and four fractal series is better.Finally,the flow and cooling characteristics of the embedded thermal-responsive hydrogels fractal heat sink with different branching numbers on uniform heat flux and hotspots with different areas are studied.Increasing the branching number of the fractal heat sink can effectively enhance the heat transfer capacity.When the hydrogel has no deformation or complete deformation,the pumping power of the fractal heat sink with higher branching number is greater.For the small area hotspot,the adaptive cooling ability of the embedded thermal-responsive hydrogels fractal channel heat sink with different branching number is poor,while the heat transfer performance of the fourth-series fractal heat sink with higher branching number is better.The fractal heat sink with higher branching number is more suitable for the adaptive cooling of higher heat flux and larger area hotspots. |
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