Numerical Study On Flow And Heat Transfer And Structural Optimization Of Microchannel With Drop-shaped Longitudinal Vortex Generator

With the high integration of electronic devices and the improvement of heat dissipation,greater challenges have been put forward for the technology of heat transfer enhancement.Microchannels have gradually attracted attention due to their high heat transfer flux and small size.How to meet the higher heat dissipation demand through structural optimization and layout optimization is the focus of microchannel flow and heat transfer research.In this paper,a drop-shaped longitudinal vortex generator(DLVG)is proposed.The effects of DLVG structure,symmetrical arrangement and asymmetric arrangement on the flow and heat transfer performance and comprehensive heat transfer performance of microchannels are numerically simulated.It has important reference significance for microchannel design.Firstly,the microchannel with DLVG is numerically simulated and compared with the microchannel with rectangular winglet(RWVG).It is found that the DLVG structure can produce higher local Nusselt number and stronger heat transfer effect.The front and rear ends of the DLVG have a semi-circular structure and are arranged at a certain angle of attack.When the fluid passes through the DLVG,it will experience multiple contractions and expansions at different interfaces.The number of longitudinal vortices generated is significantly more than that of the rectangular winglet structure,and the fluid disturbance is strengthened.Within the scope of the study,the Nusselt number Nu of the DLVG channel is 4.64%～7.62%higher than that of the RWVG channel.The Performance Evaluation Criteria(PEC)is increased by 5.3%,and the PEC reaches 1.41.Moreover,the field synergy analysis of flow and heat transfer in the channel shows that the average field synergy angle of the DLVG channel is low,and the better synergy improves the heat transfer performance.Then,the effects of DLVG structure and symmetrical arrangement on the flow and heat transfer performance of microchannels were studied,including the front radius,angle of attack and gradient longitudinal spacing of DLVG.The results show that when the front-end radius and longitudinal spacing are constant,the larger the angle of attack,the higher the Nu,the smaller the f,and the better the PEC.The PEC of the channel with an angle of attack of 75 ° reaches a maximum of 1.47 at Re=1200.When the front-end radius and angle of attack are fixed,the channel with longitudinal spacing of 5.4 mm and 4.5 mm has the largest Nu and the highest PEC.Compared with the channel with equal longitudinal spacing,PEC increases by 1.41%～4.55%.Within the scope of the study,the gradient ratio of the two gradient longitudinal spacings should be appropriately close to enhance the heat transfer.The response surface analysis method is used to optimize the three variables.The optimization goal is that the PEC value is the highest when Re=600.The optimization results are front radius R1=0.03 mm,angle of attack β=75 °,longitudinal spacing d1=5.99 mm,d2=3.32 mm.Finally,the parameters are dimensionless,and the criterion relationship between Nu and f is obtained by fitting,which provides a basis for the subsequent design of microchannels and the calculation of flow and heat transfer characteristics.The effects of the gradient angle of attack,longitudinal spacing and transverse relative position of the asymmetric arrangement on the flow and heat transfer performance of the microchannel were studied.The results show that the fluid in the asymmetric channel flows through the DLVG,and the fluid disturbance leads to the enhancement of the mixing degree between the fluids.When Re=1200,the combined channel configuration with the angle of attack decreasing first and then increasing has the largest Nu number and PEC value,which is 65.06%higher than that of the smooth channel,and the PEC is up to 1.5.The channel with a closer difference between the angle of attack of the first and second pairs of DLVGs in the three pairs of DLVGs is conducive to enhancing heat transfer.The longitudinal spacing should not be too large.The channel with a longitudinal spacing of 2 mm has the best heat transfer performance,and the maximum PEC value can reach 1.44.When changing the transverse relative position of DLVG,the asymmetric winglet with transverse symmetrical arrangement has better comprehensive heat transfer performance,and the offset arrangement has the worst effect.Compared with the channel with offset arrangement distance of 0.3mm,the Nu number of the channel with transverse symmetrical arrangement spacing of 0.5mm is increased by 7.5%～33.4%,and the PEC value is increased by 27.9%.Finally,the differences of flow and heat transfer characteristics,mean field synergy angle and vorticity between symmetric and asymmetric arrangement are compared and analyzed.In the range of 400～1150,the results show that the Nu number of the asymmetric arrangement channel with the front radius of 0.04 mm and the longitudinal spacing of 1.5 mm is 1.8%～5.5%higher than that of the symmetrical arrangement channel with the front radius of 0.04 mm and the longitudinal spacing of 3 mm,and the comprehensive heat transfer performance is increased by 5.3%.The friction resistance of the asymmetric arrangement channel is 4.9%lower than that of the symmetrical arrangement channel.Moreover,compared with the symmetrical arrangement channel,the asymmetric arrangement of the channel with a gradient angle of attack of 75°,45°,60° has a lower mean field synergy angle,a larger vorticity of the longitudinal center line of the channel,a larger fluctuation range,and a better fluid mixing degree.