Direct Numerical Simulation Of Rectangular Particle-Liquid Two-phase Flow

The particle-fluid two-phase flow exists widely both in nature and industrial applications. Deeply and comprehensively understanding of the motion characteristics of particle-fluid two-phase flow has an important academic significance and industrial application value. Though this kind of flow field has long been studied, many fundamental problems still remain to be answered. At present, many researchers have studied the motion of circle (spherical) in fluid by using direct numerical simulation, such as distributed Lagrange multiplier method (DLM), lattice Boltzmann method (LBM), finite element arbitrary Lagrangian-Eulerian domain method (ALE) and so on, and experimental method. However, the motion of abnormal particles was rarely researched. As a matter of fact, particles in engineering applications are seldom spherical. Recently, abnormal particle multiphase flow already gradually arouses people's interest due to its universality and the complexity of the anisotropic. In order to enhance the understanding on the behavior of rectangular particles in fluid, the motion of rectangular particles in flows was simulated by using ALE method. The effects of different parameters on the movement characteristics of rectangular particles were analyzed and compared.Firstly, the sedimentation of a rectangular particle in a Newtonian fluid was simulated. The results show that there is a critical particle Reynolds number when a rectangular particle falls in a Newtonian fluid. When the particle Reynolds number is smaller than the critical value, the particle falls with its long side parallel to gravity, otherwise the particle falls with its long side perpendicular to gravity. When the particle Reynolds number is greater than the critical value, with the particle Reynolds number increases, the settling behavior of a rectangular particle can be classified into several different regimes: no overshoot, damped oscillation and persistent oscillation. The critical particle Reynolds number is a decreasing function of the blockage ratio and the aspect ratio. Wall confinement generally increases the drag coefficient, and this effect decreases as the Reynolds number increases. Drag coefficient increases with increasing aspect ratio. With the increasing initial orientation angle, the fluctuation of the orientation angle, setting velocity, rotation velocity and lateral drifting velocity all increase. The impacts of aspect ratio on the process of settlement have a great relationship with the sizes of length and width. However, no matter the size of length and width, when aspect ratio increased, final settling velocity particles will be reduced accordingly. An inverted T-structure forms due to the interaction between particles when many particles fall in the channel. As time goes on, particles will tend to horizontal position.Secondly, the sedimentation of a rectangular particle in a viscoelastic fluid was simulated. The results show that there is a critical elasticity number when a rectangular particle falls in a viscoelastic fluid. When the elasticity number is smaller than the critical value, the particle falls with its long side perpendicular to gravity, otherwise the particle falls with its long side parallel to gravity. The critical elasticity number is a decreasing function of aspect ratio, but is an increasing function of blockage ratio. The fluctuation of lateral drifting distance decreases with increasing initial orientation angle when the elasticity number is fixed. The terminal velocity decreases with increasing aspect ratio, but increases with increasing blockage ratio. The rectangular particles will undergo drafting and kissing, and chain with their long side vertical when the particles are dropped one above the other in a viscoelastic fluid.Finally, we systematically investigated the migration of a rectangular particle in a simple shear flow. The results show that the equilibrium position of the rectangular particle is regardless of its initial position and the fluid property. The rectangular particle reaches an equilibrium position at the centerline of the channel when the paritlce moves in Couette flow. The rectangular particle reaches an equilibrium position at Y/W=0.29 when the paritlce moves in Poiseuille flow. Inertia forces the rectangular particle to migrate to the centerline of the channel when the paritlce moves in Couette flow. The effect of elasticity on the migration of a rectangular particle is related to the flow form. When the paritlce moves in Couette flow, perhaps there is a critical elasticity number. When the paritlce moves in Poiseuille flow, it moves far away from the nearest wall with increasing elasticity number. The migration of the rectangular particle was affected significantly by the density ratio. The migration of the rectangular particle varies with density ratio in Couette flow is different from that in Poiseuille flow.