Research On Heat Transfer Enhancement Of Complex Structure Microchannels

In many engineering fields,the degree of integration of micro-circuit boards is getting higher and higher,and the huge transient heat flux generated by micro-circuit board lead to an excessive surface temperature,which will affect the equipment performance.Therefore,in order to solve the heat dissipation problem of microelectronic equipment at a low Reynolds number,this paper adopts the method of combining numerical simulation and theory,and comprehensively considers the three factors including channel structure,working fluid type and heat transfer mode,and establishes the thermodynamic model and structural optimization model to design a micro heat exchanger with compact structure and excellent heat dispersion.The work carried out mainly includes the following aspects:(1)First of all,in order to improve the overall heat transfer performance of the triangular cavity and internal rib combined microchannel(Tri.C-Tri.R complex microchannel for short),30 group of microchannels with different size of microstructures are numerically simulated to obtain the optimized objective function(thermal resistance R_th and pumping power PP),cavity height e₁ and rib height e₂ are used as design variables.Response plane approximation(RSM),non-dominated sorting genetic algorithm(NSGA-?)and k-means clustering method are used for multi-objective optimization,and combine the principle of field synergy and enhanced heat transfer factors for analysis.The results show that the four representative solutions obtained by the k-means clustering method divide the Pareto optimal solution set into five regions.Compared with the structure before optimization,the enhanced heat transfer factors after optimization are 1.08,1.12,1.15,and 1.23,respectively.Among them,channel 4 has the best overall heat transfer performance.(2)Secondly,in order to study the influence of microstructure distribution on the flow and heat transfer characteristics of complex microchannel,Al₂O₃/water nanofluid is used as the heat transfer medium,and the four groups of ribs arranged in different complex microchannels were numerically simulated.It is evaluated by improving the performance evaluation chart and strengthening the heat transfer factor.According to different working conditions,the performance evaluation chart is divided into four areas.Among them,area 1 indicates that the fluid can effectively enhance heat transfer but does not save energy;areas 2,3,and 4 indicate the fluid can effectively enhance heat transfer and save energy at the same pump power,same pressure drop,and same flow rate.The results show that all the strengthened methods studied can effectively save energy(the data points fall on areas 3 and 4),and the staggered structure(channel D)has the best overall heat transfer performance.When the volume fraction is 2 vol.%,the maximum enhanced heat transfer factor of channel D is 2.2517.(3)Thirdly,in order to further improve the heat transfer performance of the microchannel,the Al₂O₃-TiO₂/water mixed nanofluid was used as the heat transfer medium to study the porous medium microchannel with a porosity of 0.5 to 0.7.Through the analysis of the temperature field and the velocity field,it can be known that compared with the solid microchannel,the porous medium microchannel can effectively reduce the pressure drop in the tube when the heat transfer is enhanced.When Re=257,the average wall temperature of the porous medium microchannel with a porosity of 0.7 dropped by 3%,and the pressure drop in the tube dropped by 72%.In addition,the characteristics of two-phase flow and boiling heat transfer in microchannels are also studied.The results show that the microstructures such as cavities and internal ribs can promote rapid rupture of vapor bubbles;And high-quality flow rate and low heat flux can effectively inhibit unstable boiling.