Fabrication And Analysis Of Heat Transfer And Flow Characteristic On The Bi-layer Composite Porous Wick

With the development of science and technology,the higher power,integration,and miniaturization trend of electronic components has exhibited increasingly prominent cooling problems.The retention of heat due to poor heat dissipation has negative impact on the electronic components in terms of the performance,service life and safety,etc.Loop heat pipe(LHP)has many advantages such as high heat transfer efficiency,flexible layout,anti-gravity heat transfer and so on,which is very competitive in the field of electronic equipment.As a core component of LHP,the porous wick has decisive effect to the performance of LHP.It is worth noting that the composite porous wick is an important research direction to optimize the performance of LHP,which can satisfy the requirement of high capillary force,high permeability,low thermal conduction in the direction of the flow and high evaporation efficiency at the same time.The research work in this paper focuses on the bi-layer composite porous wicks.In this paper,with copper powders and nickel powders as raw materials,the loose powder sintering technology was employed to prepare the bi-layer composite porous wicks.Besides,the method of adding the pore former in the nickel layer was proposed to optimize the performance of composite porous wicks.The effect of sintering temperature,sintering atmosphere,thickness ratio and additive proportion of pore formers on their performance were characterized and analyzed by SEM,Hot Disk,the self-designed testing device and other testing ways.Afterwards,accorrding to Darcy’s law and one-dimensional energy equation,the mathematical model of the bi-layer composite wick was established.The effects of different physical parameters on flow characteristics and heat transfer characteristics of bi-layer composite wicks were analyzed with examples,and hereby proposed some optimized ideas on the relevent physical property parameters.The flux of capillary pumping of bi-layer composite porous wicks dropped with the increase of sintering temperatures.When the sintering temperature was 800℃ and the holding time was 60 min,the porous wick had the best overall performance.When the sintering temperature was 850℃ and the holding time was 60 min,compared with the samples sintered in vacuum atmosphere,the argon atmosphere had higher flux of capillary pumping.Combined experiment results with numerical analysis,the thickness ratio(Cu: Ni)can greatly increase the capillary pumping performance of the bi-layer composite porous wicks,and thus reducing the flow pressure loss,while the overall thermal conductance will enhance to some extent.The proper thickness ratio will lead to lower flowing pressure loss and thermal conduction.In addition,the sample with 15%wt of the pore former was superior to the sample without the addition of the pore former,with a porosity of 61.6%,a capillary suction flux of 0.76 kg /(m2 · s),an increase of nearly 6 times,and the effective thermal conductivity of 3.147 W /(m · K),a decrease of 70%.When the effective aperture was constant,the proper porosity should be adjusted in the range of 40% to 78%.Increasing the porosity could effectively reduce the flowing pressure loss,as well as effectively decrease the thermal conductivity,which is of great significance to increase the capillary limit power and reduction of the heat leak of LHP.The results of this paper have some positive and guiding signifcance for improvement of performance of LHP by optimizing the properties of porous wicks.