Numerical Investigation On Flow And Heat Transfer In The Porous-wall Microchannel With Microencapsulated Phase Change Material Suspension

With the rapid development of science and technology,the heat load of microelectronic devices is increasing day by day,and the simple microchannel structure can no longer meet the heat dissipation requirements of modern electronic equipment,which makes the improved microchannel structure become a more effective method to enhance heat transfer.Microencapsulated phase change material(MPCM)suspension can absorb and release a large amount of heat through the solidification and melting of phase change materials in the shell during the flow process,and has developed into an advanced cooling working medium.In addition,because of its higher thermal conductivity and larger specific surface area,porous media can promote fluid disturbance and mixing to enhance heat transfer,which makes porous materials become a good heat transfer enhancement medium.In this paper,the minimum heat transfer unit in microchannel heat sink(MCHS)is selected as the research object,and a three-dimensional conjugated heat transfer model of double-layer and porous-wall MCHSs with MPCM suspension as coolant is established by numerical simulation.On this basis,the microchannel structure of gradient porous media is proposed and optimized,and its flow and heat transfer characteristics are studied and analyzed in depth.First,a three-dimensional heat transfer model of double-layer MCHS using MPCM slurry as working fluid is established in this work.The equivalent heat capacity method is used to deal with the phase transition process of microcapsules under turbulent conditions.The flow and heat transfer of MPCM slurries in the minichannel with constant heat flux at the bottom are investigated.The results show that compared with the single-layered MCHS and the water as working fluid,lower thermal resistance and more pressure drop occur double-layered minichannel with MPCM suspensions of different concentrations,thus,a comprehensive performance factor is defined to evaluated the overall behavior of minichannel,that is related to the thermal resistance and pressure drop.In the presented mode with MPCM suspensions,more than 13.8%higher performance factor occurs than that of water,and more effects on heat transfer happens in the lower channel.The optimized structures of double-layered minichannel are related to the operating conditions.Secondly,the combination of porous layer paved on the side wall of MCHS and MPCM suspension is beneficial to enhance heat transfer by increasing the surface area of channel matrix and the temperature difference between coolant and heating surface during phase change.The Brinkman-Forchheimer extended Darcy model together with energy equation in local thermal equilibrium account for heat transfer and flow in the porous layer as well as the equivalent heat capacity method for phase change of microcapsules under laminar flow.The effects of the materials of porous layer and substrate,heat sink design involving the porosity,thickness and height of porous layer as well as channel number under various inlet velocities and mass fractions of MPCM slurry on the thermo-hydraulic performance,which is evaluated with performance evaluation factor(PEF),in the porous-wall MCHS of MPCM slurry as coolant are analyzed in comparison to the non-porous mode with water passing through,and the 32.3%rise of performance evaluation factor can be obtained.The simulation results are validated with the experimental data.The lower thermal resistance occurs in the porous-wall mode of MPCM slurry as coolant with higher and thicker porous layer of higher porosity and thermal conductivity material as well as proper microchannel number in heat sink under higher inlet velocity than that in the non-porous MCHS with water flowing through,which is closely related to the phase change of MPCM in slurry,and influences PEF in consideration of flowing resistance.The MPCM mass fraction in coolant needs to be properly to avoid larger convection reduction and rise of flowing resistance,so as to obtain the better thermal and hydrodynamic performance of larger PEF.Finally,based on the previous studies,a novel configuration of variable porous-wall MCHS is proposed in this work.The feasibility in the novel design is validated by analyzing the effects of porosity(ε)arrangement,pore size(dp)arrangement as well as geometric parameters of porous-wall MCHS on the thermal and hydraulic characteristics under different inlet velocity,heating flux and mass fraction of MPCM suspension.The thermal resistance of the porosity gradient in porous media is less than that of linear variation,which is due to the more uneven velocity distribution in porous and fluid regions.The lower thermal resistance and more uniform temperature distribution occur in the graded porous media microchannel with higher height ratio,as well as the smaller and larger porosity in porous media respectively near the bottom and top walls.The size of microchannel limits the size of dp,making the thermo-hydraulic characteristics of gradient-d_p mode worse than that of gradient-εunder the same conditions,and the thermal resistance decreases as well as pressure drop increases with the augment thickness of porous media,so it is necessary to select the appropriate structural parameters and coolants according to the specific working conditions to obtain better comprehensive performance.The performance evaluation factor is introduced to evaluate the overall thermohydraulic performance of MCHS under different heat sink designs,and the stepwise increasing porosity mode with porosity varying along the y direction has larger velocity difference between the upper and lower layers and more reasonable MPCM particle distribution under the larger mass fraction of suspension,so as to obtain the larger rise ratio of PEF,up to 61%.