Investigation Of Vapor-liquid Two Phase Flow And Boiling Heat Transfer In Microstructures By Using Lattice Boltzmann Method

For its capability of transferring the significant heat flux under a low superheat,nucleate boiling phase change heat transfer shows great potential in the application of heat dissipation devices for microelectronic chips that involved in the field of aeronautics and astronautics,information communication and so on.Boiling phase change heat transfer usually involves the bubble dynamics of nucleation,growth,and departure,and is accompanied by the behavior of two-phase flow,vapor-liquid phase change and the transfer of sensible and latent heat.Thus,it shows a complex heat and mass transfer process with coupled hydrodynamics,thermodynamics,heat transfer,and other theories.For the design and optimization of high-efficient heat dissipation devices,it is of great importance to have an insightful understanding of the flow pattern evolution and phase change heat transfer characteristics during the boiling process,as well as the mechanism of nuclear boiling heat transfer and enhancement.However,it should be noted that the behavior of two-phase flow and phase change heat transfer is involved and coupled in the boiling process,and thus the scientific understanding on it has not yet been completely established.The designed heat dissipation devices based on the boiling heat transfer process usually adopt the closed confined space.Thus,the boiling process inside it is not only different from that occurs in the open space,but also affected by the reflux of condensate from the condenser.Besides,the operation mode of the microelectronic chip devices may lead to some special heat flux conditions,like the pulse heat flux,and thus impose particular effects on the boiling process.Boiling heat transfer enhancement technologies represented by surface fabrication and modification significantly improve the surface boiling heat transfer performance.Nevertheless,the sustainability of these boiling enhancement methods is challenged after some examinations.The surfaces with bi-conductivities show attractive prospects for its advantages in both boiling enhancement and the maintenance of this enhancement.Thus,it deserves further investigation on the mechanism of this boiling enhancement and the optimization of this structure.Besides,it is noticed that most of the present study on boiling phase change heat transfer is still based on the experimental correlation.It is of particular significance to extend direct numeiral simulations for the realistic reproduction of the boiling process.In this context,the study of boiling phase change and two-phase flow are conducted based on theoretical analysis and numerical simulations in this work.A direct simulation model of the boiling process is developed by the pseudopotential lattice Boltzmann method.The vapor-liquid interface evolution characteristics of the boiling process inside the confined space is investigated,and the effect of pulse heat flux on it is explored.The boiling behavior on the biconductive surfaces is reproduced.Finally,an improved 3D pseudopotential lattice Boltzmann model is proposed for the simulation of a vapor-liquid two-phase flow system with a highdensity ratio.In general,the results and conclusions obtained in this study are summarized as follows.(1)Investigation on the evaporation/boiling regimes and heat transfer performance in a confined space.The multiphase lattice Boltzmann simulation is implemented to numerically analyze the vapor-liquid two-phase thermo-hydrodynamic behaviors in a confined space,with a focus on the evaporation regimes in an enclosed narrow space.The evaporation regimes are examined by the variations of the heat flux,chamber height,and filling ratio.The identified evaporation regimes are quantitatively evaluated in a regime diagram based on the Bond number and Jakob number.The thermal performance of a confined space is evaluated based on the thermal resistance.The results indicate that(a)with the heat flux increases(i.e.Jakob number increases),surface evaporation,intermittent nucleation boiling,fully-developed nucleation boiling,transition boiling,and film boiling are sequentially experienced in an enclosed narrow space which a local hot spot is imposed into the evaporator section.(b)The transient temperature evolution of the evaporator surface after the startup process is dependent on the boiling mode for a confined space.For the intermittent nucleation boiling,the periodic bubble generation with a long waiting time leads to the periodic large-amplitude temperature fluctuation of the evaporator surface.The random medium-amplitude temperature fluctuation is observed for the fully-developed nucleate boiling.(c)Space confinement affects the occurrence of different boiling regimes.When the enclosed narrow space is more confined(i.e.a smaller Bond number),the transition boiling and film boiling are more likely to occur inside a flat two-phase thermosiphon.Besides,for a given smaller Bond number,the interval of Jakob number is smaller for the nucleate boiling in which the thermal performance is superior.(d)Liquid filling ratio ? determines boiling and condensation phase change behaviors as well as their interactions,and thus play a role in the thermal performance inside the confined space.Low filling ratio results in the direct exposure of evaporator surface to vapor and high filling ratio leads to the emergence of a liquid bridge connecting the condensed liquid film with a liquid pool.Moderate filling ratio contributes to favorite vapor-liquid phase change as well as the desired interaction between the evaporator and condenser sections.The optimal liquid filling ratio obtained in this study is about 40% for the confined space.(2)Investigation on boiling heat transfer behaviors under pulse heating.The boiling phase change and bubble dynamics under pulse heating are investigated through the lattice Boltzmann method.Particularly,the onset of boiling,bubble dynamic behaviors(including nucleation,growth,collapse,or coalescence)are discussed.Besides,the effect of heat flux and surface wettability on the bubble dynamic behaviors during boiling under pulse heating is analyzed.The results indicate that three bubble growth regimes,including single bubble growth,coalescence growth,and film-boiling growth are observed with the increase of pulse heat flux.Higher heat flux contributes to an earlier boiling incipience and a higher surface temperature.The specific bubble coalescence phenomenon is observed under a moderate heat flux,which results in the maximum bubble radius,as well as the reduction of volume oscillation during bubble growth.The bubble is generated more frequently on the hydrophobic surface.The hydrophobicity contributes to the direct contact of the bubble and cooling surface after pulse heat is off.Thus,the bubble on the hydrophobic surface shows a faster collapse rate than that on the hydrophilic surface.(3)Investigation of boiling enhancement on bi-conductive surfaces.With the consideration of thermal conductivity contrast and the conjugate heat transfer,the boiling phase change heat transfer behavior on the bi-conductive surfaces is numerically investigated based on the improved hybrid lattice Boltzmann method.The effects of structure sizes and thermophysical parameters of the bi-conductive surfaces on boiling heat transfer are analyzed in this study.The results indicate that(a)the pinning effect on the contact lines of the bubbles induced by the bi-conductivity enhanced the boiling heat transfer.Restricting the movement of the three-phase contact lines not only prevents the lateral coalescence of multiple bubbles to form local vapor films but also ensures the growth of the bubbles in the vertical direction.Besides,the limited contact area between the individual bubbles and the heating surface contributes to the formation of the neck or the direct departure of bubbles.Thus,the frequency of bubble departure increases significantly,leading to the increase of CHF and HTC on the biconductive surfaces.(b)The boiling heat transfer performance gets optimized when the pitch of these low conductive inserts is comparable to the capillary length of the working fluid.Increasing the width of the inserts increases HTC under low superheat,but lead to the advance of CHF.The small insertion width does not constrict the lateral movement of the three-phase contact lines and thus leads to deterioration in boiling heat transfer performance.Changing the depth of the inserts has little effect on the heat transfer performance on the bi-conductive surfaces,while reducing the thermal conductivity of the inserts weakens the boiling heat transfer.(4)Three-dimensional pseudopotential lattice Boltzmann model for multiphase flows at high density ratio.The drawbacks of thermodynamic inconsistency and cannot adjust the surface tension independent of the density ratio limit the wide application of the original pseudopotential lattice Boltzmann model.In this study,an improved 3D pseudopotential lattice Boltzmann model with D3Q19 velocity sets is proposed for simulating multiphase flows with a high-density ratio.Besides,an improved geometric formulation is developed to model a wide range of contact angles,and an iteration process for the initialization is introduced to improve numerical stability.Through a high-order Chapman-Enskog analysis,it shows that the LB evolution function with an additional source term leads to the Navier-Stokes equation with a correct pressure tensor.Both theoretical analysis and numerical results demonstrate that the proposed MRT-LB model is thermodynamically consistent with a tunable surface tension independent of the density ratio.It has the advantage of describing a wide range of correct contact angles,particularly the accurate interfacial profile near the contact line.Finally,the applicability of the proposed 3D pseudopotential MRT-LB model is presented by the dynamic evolution process of a liquid droplet on a thin liquid film and a dry flat surface,demonstrating its capability of simulating 3D multiphase flows.In this work,the heat transfer characteristics of boiling phase change and the mechanism of nucleate boiling enhancement have been systematically studied.The obtained research results can not only provide a helpful guide for the design and optimization of high efficient heat dissipation devices based on vapor-liquid phase change heat transfer but also supplement and perfect the theory of vapor-liquid two-phase flow and phase change heat transfer.