Numerical Simulation For Flow And Force Characteristics Of Cross-flow Around Particles With Lattice Boltzmann Method

The particle-fluid system is widespread in the industry, such as fluid flows through astatic bed filled with particles, the fluidization of solid particles, and pneumatic conveying inclosed pipe. The process of two-phase flow in particle-fluid system is often accompaniedwith heat transfer, mass transfer, and chemical reactions. As the complexity of kineticscharacteristics between particles and particle-fluid, it is necessary to research on the localenvironment between particles and the mechanism between particles and fluid, which willhelp to facilitate the design, improvement and control of the production process.In this paper, cross-flow over the basic unit of particle-fluid system―double particlesand four particles were researched. Lattice Boltzmann method with D2Q9model wasemployed as the mathematical model. The non-equilibrium extrapolation was used toprocess the boundary of the computational domain at the entrance, the export, the upper andthe lower wall. The granular convoluted surface was disposed by a combination method ofnon-equilibrium extrapolation and spatial interpolation. The force statistics of the solidsurface was calculated by the momentum exchange method. The realization of the modelwas based on C++platform. The flow patterns and force characteristics under differentparticle spacing, particle arrangement and Reynolds numbers were researched.When Re=200, under particles in side-by-side arrangement, according to the spacingratio, the flow pattern is classified into three classes: Single-bluff-body mode, Gap-flowmode, and Parallel vortex streets mode. The critical spacing ratios are1.2and2.2. Thechange of time-averaged drag coefficient (CD) is corresponding to the flow pattern. In eachflow pattern, the time-averaged drag coefficient decreases with the spacing ratio increases.However, the change of time-averaged lift coefficient (CL) decreases with the spacing ratioincreases but nothing on flow pattern, which means that the larger the spacing ratio is, theweaker the mutual exclusion between particles. Under particle in tandem arrangement, theflow pattern is classified into three classes: Single-bluff-body mode, Shear layerreattachment pattern, and Co-shedding pattern. The critical spacing ratios are2.0and4.1. In the single-bluff-body mode and shear layer reattachment pattern, the time-averaged dragcoefficients of the upstream particle decreases with the spacing ratio increases, and the CDvalue of downstream particle is negative.Under this condition, these two particles areattracted to each other. In the co-shedding pattern, both CDslowly increases with thespacing ratio increases that indicate the attraction is weaker.The flow pattern and force characteristics of cross-flow over four particles aredepend on Reynolds numbers, particle spacing, and particle arrangement. Under particles inan in-line square configuration, the flow pattern is classified into four classes: Single bluff-body flow pattern, Stable shielding flow pattern, Wiggling shielding flow pattern, andVortex shedding flow pattern. The critical spacing ratios are affected by Re. Under smallRe, there is larger spacing ratio to transform the flow pattern. When Re=200, the criticalspacing ratios are1.2and2.5. However, when Re=100, the critical spacing ratios are1.2and3.0. The flow pattern transformation is the main cause for the change in time-averaged dragcoefficient, the root-mean-square drag coefficient and the root-mean-square lift coefficient.As spacing ratio increases, the CDof the upstream particles is smaller and smaller whilethe CDof downstream particles increases gradually. This indicates that part of the dragforce of the upstream particles is assigned to the downstream particles. Under particles indiamond arrangement, the flow pattern is classified into three classes: Single bluff-bodyflow pattern, Narrow gap flow pattern, and Vortex impingement flow pattern. Similarly,the critical spacing ratios are affected by Re. When Re=200, the critical spacing ratios are1.2and2.5. However, when Re=100, the critical spacing ratios are1.2and3.5. The flow patterntransformation will lead to a significant jump of force characteristics on different particles.The results of this study provide a reference to understand the transport properties ofparticle-fluid system under the conditions with large-scale, complex shape, chaoticarrangement and high Reynolds numbers.