Study On The Theory And Application Of The Pseudopotential Multiphase Lattice Boltzmann Model

Multiphase flows and gas-liquid phase change heat transfer are widely encountered in the power plants of energy and environment,catalytic industry,materials processing,oil exploitation and so on,which are associated with the coalescence and breakup of droplets,the growth,merging and breakup of bubbles and the flow and heat transfer between solid and fluid.Therefore,investigating the physical laws and influence mechanisms of multiphase flow and phase-change has attracted a lot of attention of researchers.Traditional numerical methods most require sophisticated interface tracking or capture technology for solving the evolution of multiphase interface,and there are some artificial factors for the realization of phase change process.In the past decades,the lattice Boltzmann method(LBM)based on mesoscopic scale and gas dynamics theory has been extensively applied in the numerical simulation of multiphase flow and phase change.Among them,the pseudopotential LBM is widely utilized to simulate the droplet impacting and phase change due to its simplicity,stability and self-capturing the interface.However,the original pseudopotential model suffers from some drawbacks,such as spurious velocity,density ratio and thermodynamic consistency.For those reasons,this work concentrates on the theory and development of the pseudopotential LBM and developing two-dimensional(2D)and three-dimensional(3D)pseudopotential multiphase and phase-change LBM models.Some engineering applications of droplet impacting and phase change are carried out based on those pseudopotential models.The main works and conclusions of thesis are given by as follows:(1)Comparison of the three main forcing schemes of pseudopotential model in the recovery of macroscopic Navier-stokes equation are firstly investigated in this thesis,and the schemes of adjusting surface tension,gas equation of state and contact angle are investigated in detail.After detailed discussions,the corresponding 2D and 3D models are finally established.The results show that: the forcing scheme proposed by Li et al.based on the Guo et al.'s model can satisfy the thermodynamic consistency and mechanical stability under the condition of large density ratio.The pseudopotential-based forcing scheme for adjusting contact angle performs well on the dynamic behaviors of wetting and has smaller spurious velocity(2)Based on the developed 2D and 3D pseudopotential models,we systematically study the dynamic behaviors of binary droplet collision,droplet impacting on solid particle in microchannel and 3D single droplet and double droplet impacting on the dry surface.Regarding the 2D impact process,effects of Weber(We)number,Ohnesorge number,droplet physical properties and geometry on the interface evolution are mainly discussed.The main results show that: with the increase of We number,the outcomes of double droplet collision are from coalescence to separation;decreasing the initial distance of double droplet along the y-axis will yield to a slower rotational speed.Regarding droplet impacting solid,as the We number increases,the liquid film formed by the single droplet impacting the solid particles is more elongated;decreasing the solid particle will result in a shorter time of breakup;for the hydrophilic wall channels,the liquid film will infiltrate the upstream wall.For the cases of the 3D droplet impact process,the impacting droplet will bounce off the hydrophobic surface;the continuous impacting of double droplets has more complicated dynamic behaviors and a longer spreading time.(3)The thesis further systematically discusses the single-relaxation-time(SRT)scheme,multiple-relaxation-time(MRT)scheme and finite difference method schemes for the phase-change pseudopotential LBM.2D and 3D hybrid phase-change LBM models are established and verified by simulating the evaporation droplet.The results show that hybrid phase-change model can agree well with D2 law for the droplet's evaporation,which further prove the accuracy and stability of the model.(4)Based on the 2D and 3D hybrid phase-change LBM models,condensation process of saturated steam on homogeneous and heterogeneous subcooled walls under 2D and 3D conditions are comprehensively studied.Furthermore,the pool boiling is also presented in this thesis.The results show that: the condensation process mainly occurs near the three-phase contact area and has a quite higher heat flux;the hydrophilic wall surface is much easier to grow condensed droplets,but many hydrophilic spots are not conducive to droplet's drop.For the 3D condensation,the substrate is hydrophobic mixed with hydrophilic solid particle is more conducive to droplet's drop under vertical gravity.In addition,regarding the pool boiling,the three main boiling schemes are correctly reproduced by the proposed model.The growth,coalescence and detaching of bubble evolution and the transient heat flux of nucleate boiling,transition boiling and film boiling are performed.Finally,the average heat flux of boiling cure under different superheat walls are presented,which agrees well with experimental result.In summary,the basic theory and development of the pseudopotential multiphase LBM are discussed in detail,and double droplets collision,droplet impacting on solid particle and the gas-liquid phase-change of condensation and boiling are comprehensively investigated.This thesis further deepens the understandings of multiphase law and influence mechanism in the process of droplet impact and gas-liquid phase transition and provides some valuable attempts to promote the engineering application of pseudopotential LBM in multiphase flows and gas-liquid phase change.