The Lattice Boltzmann Simulation About The Phenomenon Of Enhanced Boiling Heat Transfer

With the rapid development of microelectronic integration technology,the power per unit volume of electronic devices is getting higher and higher,resulting in increasingly serious heat dissipation problems of electronic devices,which has gradually become a key issue restricting its technological development.In terms of heat dissipation of microelectronic devices,the optimization of micro-scale boiling heat transfer and its enhancement technology,as well as related heat transfer components,has received extensive attention from scholars in the heat transfer community in recent years.Experimental studies on boiling,condensation and other phase change heat transfer are emerging one after another,but theoretical and simulation studies are difficult to make breakthroughs.Although the theoretical and simulation studies of single phase heat transfer is very mature,once two-phase or phase change problems are involved,the transmission of quality,momentum and energy on the interface,as well as the structure of the solid wall must be considered,which lead to difficult applications in phase change heat transfer of many computational fluid dynamics methods.Based on previous studies,this dissertation has studied the enhanced boiling heat transfer problems on the structured surfaces,as well as the optimization of the heat transfer performance of the flat loop heat pipe evaporator and running performances of loop heat pipe in different initial state using numerical method at a micro scale.Experiments show that the boiling heat transfer can be enhanced on the micro-structured surfaces.In this dissertation,the three-dimensional two-particle distribution functions lattice Boltzmann method is used to simulate the nucleation,growth and departure of bubbles on the micro-pillar structured surface using Fortran programming technology,and the influence of the characteristic size of the micro-pillars on the bubble nucleation and departure period is mainly studied.The simulation results show that,compared with the smooth surface,the micro-pillar structured surface has lower boiling superheat and greater nucleation site density.The height of the micro-pillar has an obvious effect on the bubble growth.If the micro-pillar height is too low,it is hard to form an activated nucleation site,but if the micro-pillar height is too high,it will hinder the growth of the vapor bubble.The lower of the micro-pillar height,the shorter of the bubble departure period.The change of the micro-pillar spacing has no obvious effect on the bubble departure period,but it has an important influence on the formation of an activated nucleation site.When the micro-pillar spacing is too large,the high temperature at the nucleation site is hard to maintain,and the bubble can not periodic growth and departure from the surface.The density of the surface nucleation site usually has an important effect on the boiling heat transfer efficiency,so it is necessary to study conditions that needed for one single cavity nucleation.This dissertation uses the two-dimensional lattice Boltzmann method to study boiling phenomenon on four cavities,namely square cavity,circle cavity,trapezoid cavity,and inverted trapezoid cavity.Single bubble growth is simulated on the four cavity surfaces,and non-condensable gas inside the cavities are not considered.The results show that the onset boiling time of the square cavity and the inverted trapezoid cavity is longer than the circular cavity and the trapezoid cavity.The difference of the onset boiling time is due to the difference of the cavity structures,which lead to a different way for the temperature rise in the cavities.In addition,two conditions must be met for a cavity faster nucleated to form an activated nucleation site.One is that the cavity has the largest convection heat transfer resistance compared with other place on the surface,and the other is that the fluid in the cavity is not easy to take place convection heat transfer with the external fluid.The capillary wick is an important element inside the evaporator,and its performance plays an important role on the stable operation of the evaporator and the entire loop heat pipe.In this dissertation,the three-dimensional lattice Boltzmann method is used to study the effects of wick capillaries with different porosity distributions and different thermal conductivity distributions on the heat transfer performance of the evaporator.The three selected porosities are 70% porosity,80% & 60% porosity and 60% & 80% porosity,and the latter two represent a gradient distribution of the wick porosity from the side close to the compensation chamber to the side close to the heating surface.The simulation results show that the different porosity of the wick has little effect on the stable operation temperature of the evaporator,but it will lead to the difference of vapor-liquid flow and vapor-liquid distribution inside the wick.Therefore,it has a greater impact on the robustness of the evaporator running under different heat flux.The ratios of the thermal conductivity of the three capillary wicks to the thermal conductivity of the working fluid water were 80,0.08,and 0.08&80,respectively,and the latter means that the thermal conductivity has a gradient distribution from the side close to the compensation chamber to the side close to the heating surface.The results show that the thermal conductivity of the wick has a slight influence on the stable operating temperature of the evaporator,but it has a greater influence on the range of heat flux in which the evaporator can operate stably.Using a wick with a gradient distribution of thermal conductivity,that is,when the thermal conductivity near the heating surface is larger and the thermal conductivity near the compensation chamber is smaller,the heat flux range of the stable operation of the evaporator is wider,and the temperature of the stable operation of the evaporator is less affected.Lattice Boltzmann simulation is used to study the startup and operating characteristics of a two-dimensional flat-plate loop heat pipe,and the density field,temperature field and flow field at each time during the system from startup to stabilization can be given.The results show that under different initial vapor-liquid distributions,the startup process of the loop heat pipe is different.In the initial state,the closer the steam is to the steam channel,the more stable the startup of the loop heat pipe and the shorter the time required to reach stability,the lower the temperature of the evaporator during stable operation,and the wider the range of heat flux that the loop heat pipe can operate stably.When the steam is far from the steam channel in the initial state,the steam generation in the steam channel is different under different heat flux.At low heat flux,the vapor-liquid interface near the side of the heating surface gradually moves to the liquid side and enters the steam channel,while at high heat flux density,the liquid in the steam channel directly takes place phase change and translates to vapor.