Numerical Simulations And Investigation On Mechanism Of Two Phase Flows In Meso-scale Channels

Multiphase flows in meso-scale channels involve complicated dynamic behaviors.Analytical theories with simplified assumptions and experimental observations withsemi-empirical empirical formula are usually difficult to portray the forces acting onparticles, the energy and momentum transfer of the fluid-solid system and the changesof interphase boundary. Therefore they significantly influence the understanding theinherent true mechanism of meso-scale multiphase flows.Based on the study of isothermal inertial particle sedimentation, we use afinite-element method to solve the initial value problem for the sedimentation ofparticles in a vertical channel. The algorithm is based on an Arbitrary Lagrangian-Eulerian technique(ALE). The fluid motion is computed from the conservation laws.Momentum balance are governed by the Navier-Stokes(N-S) equations, while theenergy conservation is controled by a convection-diffusion equation, in which thenatural convection is taken into account through thermal expansion. Particles moveaccording to the equations of motion of a rigid body under gravity and hydrodynamicforces arising from the motion of the fluid. An unstructured mesh of triangularelements is generated by the Delaunay-Voronoi method. As a particle moves, the meshalso moves and deforms according to a mesh velocity which is determined by aLaplace equation. When the elements become severely distorted, a re-mesh procedureis carried out to restore mesh quality. The equations are discretized using Galerkinformulation. The solid and fluid momentum equations are combined into one weakform. In this way, the forces and torques acting on the particles are balanced, and thereis no need to compute the forces and torques explicitly. The positions of the particlesare updated according to their velocities, and the time step is automatically adjustedaccording to their velocities and accelerations. The nonlinear part of the governingequation is solved by Newton iteration and the linear parts are solved using thepreconditioned GMRES algorithm. This paper aims to simulate the sedimentation andthe interphase deformation by melting, to investigate forces, trajectory, and to abtainflow field of one or more particles, obtain mechanism of particle-particle interaction with different structures and flow patterns. The main conclusions are as follows:(1) Thermal convection induces asymmetric of the flow field. For cold particles,downward thermal convection accelerates up the sedimentation, The vortex sheddingalso contributes to the movement style of the particles; During the sedimentation of hotparticle in the cold fluid, the warm wake forms a strong upward thermal plume, the hotlayer of fluid next to the particles carry upward momentum, therefore, the hot particleshave smaller vertical velocities, lateral and angular velocity; The interface deformationof melting particles makes the particle oscillate and change the sedimentation velocity.(2) The drafting, kissing and tumbling (DKT) scenario is found during thesedimentation of two isothermal particles. The two particles will settle steadily near thewall finally. For the sedimentation of two cold particles settling, periodic DKTscenario appears and the cold particles tend to disperse; The drafting, kissing andtumbling scenario was not found in the sedimentation of two hot particles and hotparticles tend to aggregate; Compared with isothermal sedimentation, the vortexshedding, mass losing by melting and morphology change the sedimentation velocityand trajectory.(3) The isothermal elliptical particle rotates from parallel to vertical to the x-axisduring the process, and displays weak and somewhat irregular lateral oscillations aboutthe centerline. Elliptical particle in hot fluid develops a regular lateral oscillation alongthe centerline finally. The elliptical particle in cold fluid moves away from centerline,and then develops a regular lateral oscillation about an off-center equilibrium positionwith a periodic velocity and angular velocity.Besides, Grid-based numerical methods within the frame of continuum mechanicsare usually difficult in capturing inherent flow physics such as boundary slip andthermal disturbance. They are also not valid to problems with multiple scale physics.In contrast, molecular dynamics (MD) is practical only on extremely small time scales(nanoseconds) and length scales (nanometers) even if the most advancedhigh-performance computers are used. In this paper, a modified dissipative particledynamics is used, which employs an interaction potential with short-range repulsion and long-distance attraction, and enables the simulation of multiphase fluid flowprocesses. Further more, we studied and improved numerical techniques to calculateforce and torque on solid particles and the governing equation of motion as well as therelated parameters. The sedimentation of a particle is later investigated with DPDmethod, In this way, we demonstrated the effectiveness of the DPD method inmodeling particle-fluid two-phase systems.The multiphase flow through a Y shape mesoscopic channel is simulated bydissipative particle dynamics with this new potential function with differenta_w/a_f afratios of interaction strength coefficients of fluid-fluid and fluid-wall particles,rate of particles injection, external force. The results show that the new method iscapable of simulating the flow process and flow pattern. The flow past athree-dimensional sphere within two parallel plates is also studied with comparisons toclassical results. The results show that the DPD method can predict drag coefficientaccurately while Re is less than 100. When Re is bigger than 100, the results deviatefrom analytical values, mainly due to the fluid compressibility. The sedimentation of asolid sphere ball is studied and compared with the result of DNS, The results show thatit is feasible to simulate particle-fluid two-phase systems using DPD method.