Numerical Analysis On Boiling Phase Change Heat Transfer Process In Micro-structures

With the characteristics of good temperature uniformity,large heat transfer rate,small equipment weight,and superior heat transfer performance,the micro-scale vaporliquid phase change heat transfer system has been widely used in the micro-electro industry.With the rapid development of micro-electro technology,electronic devices are developing in the direction of miniaturization and integration,which causes the occurrence of the vapor-liquid phase change in the micro-structure space.Revealing the boiling heat transfer mechanism in micro-structures and effectively enhancing the boiling heat transfer capacity is of great importance to scientific significance and application value for improving the reliability and stability of related mechanical and electronic equipment.The boiling phase change heat transfer process in micro-structures is widely involved in the bubble nucleation,growth and coalescence,and is also coupled with the interphase evolution,mass and heat transfer on the interphase and convective heat transfer process.The current experimental research is still limited to some bubble dynamics phenomena.Some important information such as temperature distribution and flow field in the boiling process is difficult to capture through experimental methods.The empirical theoretical model obtained through experimental researches can hardly reflect the detailed mechanism of nucleate boiling and its enhancement process.Therefore,a numerical simulation method suitable for the micro-scale vaporliquid phase change heat transfer process is urgently needed to further reveal the internal mechanism of nucleate boiling heat transfer.As an emerging mesoscopic method,the lattice Boltzmann method has shown many advantages in the numerical simulation of vapor-liquid phase change process.Therefore,a numerical simulation of vapor-liquid phase change heat transfer process in micro-structures is carried out based on the hybrid lattice Boltzmann method,and mainly consists of the pool boiling process in confined micro-structures and the flow boiling process in a micro channel.The influence on the heat transfer performance of surface wettability is analyzed.The specific research content is summarized as follows:(1)The Effect of Surface Wetting Modification of Thermal Conductivity Heterogeneous Structure on Pool Boiling Heat Transfer Process in MicrostructuresA lattice Boltzmann method programming framework suitable for vapor-liquid phase change simulation in micro-structures is constructed,and is applied to the simulation of nucleate boiling heat transfer on the thermal conductivity heterostructure surface.Comparing the vapor-liquid distribution under uniform wettability conditions and wettability heterogeneous conditions,it can be found that the nucleation site mainly depends on the wettability arrangement: the bubbles are preferentially generated on the hydrophobic area.Only when the evaporation surface shows uniform wettability,the nucleation site depends on the wall temperature distribution caused by the heterogeneous thermal conductivity.At this time,the bubbles tend to be generated in the substrate with higher temperature.In general,the hydrophilic substrates with hydrophobic inserts is conducive to nucleate boiling under low heat flux and reaches a higher critical heat flux,which is a relatively balanced surface wettability arrangement.(2)The Effect of Condensation Surface Wettability on Pool Boiling Heat Transfer Process in Confined Micro-structuresA theoretical model of the coupled boiling and condensation phase changer heat transfer in a confined chamber is developed and numerically analyzed based on the modified hybrid lattice Boltzmann method.The interaction of boiling and condensation behavior is investigated with particular attention focused on the influence of condensation surface wettability on the boiling heat transfer process.Besides,the temperature response and the detailed two-phase flow pattern inside the chamber with different condensation surface wettability are evaluated.The results indicated that an optimal contact angle of 85?for the condensation surface wettability promoted the heat transfer on the evaporation surface.Subsequently,two modified condensation surfaces(biphilic surface with alternative hydrophilic and hydrophobic regions and gradient surface with a gradual wettability transition from hydrophilic to hydrophobic)are proposed for further heat transfer enhancement of the confined chamber.The results of the boiling curves,the two-phase flow pattern and the local/averaged heat transfer coefficient on the evaporation surface indicated that,compared with the single wettability with the optimal contact angle,both modified condensation surfaces can achieve a higher critical heat flux and a higher boiling heat transfer coefficient.Among these,the biphilic surface can achieve relatively better heat transfer performance.(3)Research on Flow Boiling Characteristics in Micro-channelsThe flow boiling process in micro-channel is numerical analyzed based on the modified hybrid lattice Boltzmann method.With the increase of the heat load,the flow patterns in the micro-channels appears single-phase flow,bubble flow,slug flow and annular flow in order.From the view of the wall temperature response,boiling curve and two-phase distribution,the influence of mass flow rate,heating surface wettability and channel height on the flow boiling heat transfer behavior is explored.The simulation results indicate that a high mass flow may delay the onset nucleate boiling and reach a greater critical heat flux.In addition,the hydrophobic evaporation surface causes a vapor film covering the evaporation surface,and thus reduces the heat transfer performance.Therefore,the heating surface is recommended to be set as hydrophilic.A small channel height leads to a larger convective heat transfer coefficient of flow boiling when the heat load is small.However,as the heat load increases,the heat transfer coefficient decreases when the flow pattern reaches slug flow and annular flow.It’s relatively difficult for a channel with higher height to produce slug flow and annular flow,so the heat transfer performance is better when the heat load is higher.In this paper,a variety of vapor-liquid heat transfer processes in micro-structures are numerically simulated,and the relationship between the nucleate boiling heat transfer performance and surface wettability,characteristic length,mass flux is analyzed.The mechanism of the pool boiling heat transfer process and the enhancement technology along with the mechanism of the flow boiling heat transfer process are explored,which can provide guidance for the design of micro-scale vapor-liquid phase change heat transfer equipment.