Lattice Boltzmann Model And Its Applications For Multiphase Heat Transfer Problems

The problem of multiphase heat transfer exists widely in the fields of nature,industrial production and energy environment,and it has also become a common scientific problem in many fields.This problem generally involves the coupling of multiple physical fields,the interaction between complex fluids,and the interaction between fluids and solid walls,and thus the transport mechanism is very complex.Existing theoretical and experimental methods have some certain limitations in dealing with such a complex problems.With the rapid development of computer technology and numerical methods,the numerical simulation has been gradually developed into an effective method to study this complex problem because of its repeatability and high efficiency.Due to the fact that the mesoscopic lattice Boltzmann(LB)method has the advantages of both the macroscopic numerical method and the microscopic particle method,it has some great advantages in the study of multiphase heat transfer problems,such as simplicity in describing the interaction between complex fluids and efficiency in handling complex boundaries.Up to now,some scholars have carried out some researches on the multiphase heat transfer problems,and have made some important progress,but there are still some shortcomings.For example,the hybrid LB method cannot guarantee the mass conservation of the whole system,and these available works are limited to the case with the same density.(1)For a general form of temperature equation,we construct an improved LB model through coupling the information of flow field.Different from the previous LB models,the present LB is more suitable for the problems of conjugate heat transfer in the inhomogeneous media.Theoretical analysis shows that the improved LB model can not only recover the temperature equation correctly,but also be free of the non-local calculation of the gradient term,which ensures the locality of the collision process.In addition,our numerical results show that the model has a second-order convergence rate in space and good numerical stability.(2)Based on the idea of local calculation of gradient term in above LB model above,we develop a new LB model for multiphase heat transfer problems.We first rewrite the convective-diffusion equation for temperature as a diffusion equation with a source term,and then design the LB model for the diffusion equation.In the LB model,the gradient terms of temperature and order parameter can be calculated locally through the corresponding non-equilibrium distribution functions,which ensures the locality of the collision process.Finally,we test the efficiency and accuracy of the LB model by some numerical of the thermocapillary flow problems.(3)Based on above proposed LB model for the multiphase heat transfer problems,we study the thermocapillary migration behavior of a droplets on the solid wall,and analyze the effects of viscosity ratio,density ratio and thermal conductivity ratio on the thermocapillary migration of the droplet.Finally,we also consider the dynamic behavior of the droplet on non-uniform solid walls,and investigate the effect of wettability.(4)With the developed LB model for multiphase heat transfer problems,we further investigate the thermocapillary migration mechanism of a self-wetting fluid droplets on solid walls.Firstly,we consider the thermocapillary migration process of the self-wetting fluid droplet,under different coefficients related to the surface tension,and obtain the critical coefficients.Secondly,we also analyze the effects of thermophysical parameter and the initial position of the self-wetting fluid droplet on the thermocapillary migration,and compare the present results with the existing work.In conclusion,this thesis first develops an improved LB model for the general form of temperature equation,and further proposes LB model for multiphase heat transfer problems.Finally,based on LB model,we also systematically study the thermocapillary migration behavior of a droplet.These works not only further enrich the theory of the lattice Boltzmann method,but also help to deeply understand the mechanism of thermocapillary migration of the droplet.