Numerical Study On Motion Of Non-isothermal And Volatile Particles In A Channel

Non-isothermal and volatile particulate flows widely exist in natural and industrial pro-cesses,such as forest fires,volcano eruption,chemical engineering processes,coal combus-tion and pharmaceutical industry.On one hand,the temperatures of the particles and the fluid are different,which leads to thermal convection between the particles and the fluid;on the other hand,the particles are composed of inert components and volatile components,and the volatile components can diffuse into the ambient fluid.Thus,non-isothermal and volatile particulate flows are typical problems with multi-physical-field interaction,where the particle motion and distribution are affected simultaneously by fluid flow,thermal con-vection and mass transfer between the particles and the fluid.Due to its clear physical back-ground and convenience to treat the particle-fluid interaction,the lattice Boltzmann method is suitable to solve non-isothermal and volatile particulate flows.Though some studies on non-isothermal and volatile particulate flows have been conducted,it is insufficient to un-derstand the effects of thermal convection and mass transfer on the particle motion and dis-tribution.Thus,based on the lattice Boltzmann method,we studied several problems with non-isothermal and volatile particles,and analyzed the effects of thermal convection and mass transfer on the particle motion and distribution.The main works of the present thesis are listed as follows:(1)We developed the lattice Boltzmann method for non-isothermal and volatile partic-ulate flows by proposing the unified iterative scheme for moving boundaries.Different from the previous combination methods,where the boundary nodes and fresh fluid nodes around the particle are treated separately with different rules,in the unified iterative scheme,we treated the boundary nodes and fresh fluid nodes consistently.Compared with the previous combination methods,the inconsistency between the constructed distribution functions and those evolutionary ones is decreased,the numerical accuracy is increased and the fictitious force fluctuation is suppressed significantly,while the computational efficiency is compa-rable.Further,similar to the velocity boundary condition,the unified iterative scheme is extended to treat the temperature and concentration boundary conditions.(2)We adopted the lattice Boltzmann method and the unified iterative scheme for mov-ing boundaries to study the lateral migration of a non-isothermal particle in a Poiseuille flow,where the effect of thermal convection on the equilibrium position of the particle was ana-lyzed.Due to the effect of thermal convection,the equilibrium position of the non-isothermal particle is dependent on the critical Grashof number,Gr_c.When Gr is relatively small,ther-mal convection is weak,and the equilibrium position of the non-isothermal particle is rele-vant to its initial position.When Gr>Gr_c,the equilibrium position of the non-isothermal particle is the same,no matter where its initial position is.To analyze the lateral migration of the non-isothermal particle,five mechanisms were taken into account,i.e.,wall repul-sion,inertial lift related to shear slip,lift due to curvature of velocity profile,lift due to particle rotation and thermal convection.Further,we investigated the dependence of Gr_c on the channel Reynolds number,Re_c.With Re_cincreasing,the wall repulsion increases,as a result,Gr_cincreases monotonically,and a power-law dependence of Gr_con Re_cwas observed.(3)We studied mass transfer between the particle and the fluid during the sedimentation of a volatile particle.In the problem,fluid flow,particle motion and particle-fluid mass transfer affect each other,at the same time,the concentration at the particle surface and the particle properties vary due to mass transfer between the particle and the fluid.If we adopted the dimensionless time,the process of particle-fluid mass transfer is independent on the Schidmt number,Sc.Further,to investigate the respective effect of convection and diffusion on the rate of mass transfer,a comparative case of a stationary particle was conducted.For the stationary particle,mass transfer is merely affected by diffusion,and the scaling law of the rate of mass transfer is close to 0.5.While for the settling particle,mass transfer is affected by both convection and diffusion,and the scaling law is close to 1.5.(4)We investigated the sedimentation of a single and multiple non-isothermal and volatile particles,where the effects of thermal convection and mass transfer on the particle motion and distribution were analyzed.For the single particle,due to thermal convection and mass transfer,the particle motion and distribution are significantly different.For the multiple particles,to describe the particle motion and distribution quantitatively,the mean settling velocity,fluctuation velocity,mean ensemble concentration and ensemble disper-sion of the particle cluster were calculated.Due to the effects of thermal convection and mass transfer,the fluctuation velocity of the non-isothermal and volatile particle cluster is stronger,and the particle distribution is more inhomogeneous.Through these studies,a deeper understanding of the effects of thermal convection and mass transfer on the particle motion and distribution is achieved,at the same time,they can provide us with some new insights for further studies.