Research And Application Of Multiphase Flow Model Based On Lattice Boltzmann Method

In the study of multiphase flow phenomena,the construction of accurate and stable multiphase flow models has always been a research hotspot in the field of computational fluid dynamics.Many multiphase flow models have been developed based on the lattice Boltzmann method,such as pseudo-potential models,free energy models,and Chemical-potential models.However,in this in-depth analysis and research,it is found that there are still some defects in Chemical-potential multiphase flow model:(1)it cannot be applied to the simulation of multi-phase flow phenomena with lower temperature and larger density ratio.The stability of the model needs to be improved;(2)the model ignores the computational errors caused by the numerical methods used in the calculation of density gradients and chemical-potential gradients,making it difficult to obtain accurate numerical results when simulating more complex fluid motions;(3)the spurious velocity produced by simulation is still too high.In this paper,we first consider using the critical temperature,critical pressure and critical density of the real fluid system,and the length and time under the grid unit as the reduced variables,and propose the reduced form of the Chemical-potential model.The obtained reduced form does not contain parameters related to specific substances,which makes the model more universal.Then,by simulating the density distribution of the gas-liquid two-phase transition zone,comparing the accuracy of the first-order derivative and second-order derivative of density in the original model and the modified model,by using high-order difference and the center difference,respectively.It is found that the calculation result using the high-order difference is closer to the exact values;finally,by simulating some examples and the force model uses the exact difference method,the Chemical-potential model using the high-order difference in the reduced form is tested,and the result show that:using the improved model,the two-phase density coexistence curve agrees well with the theoretical results of the Maxwell method.The simulated temperature range is wider,and the simulated two-phase density ratio reaches 10~8.The stability of the model is significantly improved.The numerical results obtained by the model are more accurate and reliable;in addition,the improved model satisfy Laplace's law and the spurious velocity is greatly reduced.Applying the improved model,we investigated the depinning behaviors of a droplet on a chemically heterogeneous and gradually inclined surface.Droplet of three different initial states with the same volume are placed on the same chemically isomeric substrate.As the substrate is gradually tilted,three different depinning behaviors occur:when the droplet is initially in equilibrium,the front and rear end contact lines occur depinning almost at the same time;when the droplet is initially in the contracted state,the front end contact angle first occur depinning and accompanied by the rapid sliding of the front contact line;when the droplet is initially elongated,the rear end contact angle first occur depinning and accompanied by the rapid sliding of the rear end contact line.And after finished partial adjustment of contact line,the drop naturally restored to similar critical state of stick-slip motion because the volume of droplet is same in three cases,and until the slope angle of the plate reach 15°,the droplets in all three cases near-simultaneously start to stick-slip sliding.In addition,the movement of the contact line can be clearly observed during the simulation,indicating that the instantaneous depinning behavior of the droplet is actually a dynamic process.The improved model has high stability and numerical accuracy,and is very suitable for simulating complex multiphase fluid motion systems such as electro-wetting of droplets,droplet evaporation and particle motion.In the next study,we will also apply this model to simulate the above phenomena and try to construct a 3D model to further improve the application value of the model.