Research On Thermal Performance Optimization Of Microchannel Based On Response Surface Method

In 2018,China's auto market experienced its first negative growth in 28 years.Among them,the traditional fuel vehicle market was the first to bear the brunt.After the promulgation of the "Double Points" policy and the increasing acceptance of new energy vehicles,new energy vehicles,especially electric vehicles,are becoming more and more popular in the automotive market.The battery is the core of the electric vehicle,which guarantees the endurance of the electric vehicle and the normal operation of the electrical appliances in the car.When the battery is working,a large amount of heat is generated by the internal chemical reaction.If this heat is not discharged in time,it will cause the heat concentration inside the battery pack.Excessive battery temperature will affect battery life and may even cause accidents such as spontaneous combustion or explosion.The battery cold plate is a new type of battery cooling structure,which can provide efficient and uniform heat dissipation for the battery.In recent years,more and more scholars have paid attention to the research of cold plate channel structure.Among them,the wavy channel structure is considered to be an efficient heat dissipation structure due to its excellent heat dissipation ability without causing a significant increase in pump power.The battery cooling medium is a carrier for transmitting heat.The heat transfer performance of the cooling medium determines the heat dissipation capacity of the entire cooling system.The battery cooling medium is generally pure water or liquids such as ethylene glycol.Nanofluids,as an efficient heat transfer medium,can greatly improve the performance of battery cooling systems.First,the basic governing equations,turbulence model,and field synergy theory are introduced,laying a theoretical foundation for subsequent numerical simulation and mechanism analysis.Secondly,in order to explore the influence of the overall structure and size parameters of the cold plate of the straight channel on the overall heat dissipation performance of the cold plate,an orthogonal test was designed and completed.The evaluation index of the orthogonal test is the maximum temperature of the cold plate surface,and the factors are the channel height,the channel width,the clapboard thickness and the channel slope.Finally,through the matrix analysis method of simulation processing and orthogonal experiment,considering the influence of various factors on the heat dissipation performance and the manufacturing cost,the three factors affecting the overall heat dissipation performance of the cold plate are channel height,clapboard thickness and channel slope degree.In addition,the optimal level and optimal combination of the overall heat dissipation performance of the cold plate is a channel height of 0.5mm,a channel width of 10 mm,and a clapboard thickness of 0.8mm.Next,the application of the design method of response surface experiments,the design process of the experimental group of the center composite design of the surface center,and the construction of the multiple regression curve model are introduced.Due to its unique structure,the wavy channel structure causes the fluid to receive less shear stress during the flow process,which avoids a large increase in pump power.In order to facilitate the comparison of the comprehensive heat dissipation performance of the wavy channel and the straight channel,an overall thermal-hydraulic performance factor was proposed as the evaluation factor of the channel heat dissipation performance and the response of the response surface design.The response data comes from the simulation results.Before the response surface design,the interaction between the flow field and the temperature field of the wavy channel was analyzed based on the field synergy theory: the wavy channel caused the flowing cooling medium to be subjected to centrifugal force due to its curvature As a result,the flow will inevitably be generated.The vortex structure reduces the intersection angle between the velocity vector and the temperature vector,and improves the field synergy between the velocity field and the temperature field,thereby improving the overall heat dissipation performance of the channel structure.At the same time,the chaotic advection in the flow channel caused by the curvature of the wavy channel greatly enhances the convective fluid mixing.The lateral mixing disrupts the shear layer and this shear layer separates the main flow and the recirculating flow in the flow channel,resulting in a heat transfer enhancement.The consistency of the Nusselt number of the simulated structures under different Reynolds numbers with the field synergy angle curve once again verifies the correctness of the field synergy theory.Then,a second-order response surface equation of the ripple channel's comprehensive heat dissipation performance with respect to the wavy channel's amplitude and number of cycles(wavelength)is conducted.Analysis of variance shows that the obtained equation is sufficiently significant that it can be used to predict the results in the design interval.The predicted optimal heat dissipation performance is case 9(A = 1 mm,P = 4).The predicted value of the overall thermal-hydraulic performance factor is 0.5268.It is verified that the comprehensive heat dissipation performance of this scheme is still optimal under other Reynolds numbers conditions,which proves that the results of the experimental research on response surface are universal.Finally,the heat transfer enhancement mechanism of nanofluids in micro-wavy channels was explored.Firstly,the advantages and disadvantages of each nanofluid simulation model were comprehensively compared.Combined with the situation of this research,a single thermal diffusion model was finally selected and modified.It is found through simulation studies that the heat transfer enhancement capability of nanofluids applied to wavy channels is significantly greater than that applied to straight channels,because the presence of vortices in the wavy channels makes the Brownian motion of the nanoparticles more intense,thereby enhancing the thermal diffusion of nanoparticles.Second,the nanofluid can significantly enhance the heat transfer capacity of the cold plate without causing a large increase in pressure drop.At Re = 775,the heat transfer coefficient of the cold plate when the cooling medium is nanofluid is 117% higher than that when the cooling medium is pure water.The pressure drop has only increased by 14%.Finally,the application of nanofluids increases the temperature gradient in the channel and hence the heat transfer rate of the channel is obtained.