Numerical Simulation Of Micro-particle Deposition And Diffusion Characteristics In Gas-solid Two-phase Flow

Gas-solid turbulent flow is a relatively common phenomenon. As economic and environmental needs, it is more and more important that the gas-solid flow is understood deeply and comprehensively. The gas-solid two-phase research becomes an active field of the fluid dynamics. The micro-particle diffuses in the space or deposits on the wall by the effect of various factors when gas carrying the micro-particles flows in a limited or unlimited space. This phenomenon has important applications in industrial production and environmental protection, and it may also harm. The research target of the micro-particles depositing on the wall and diffusing around the square cylinder is to understand the factors which affect the micro-particles depositing on the wall, reveal the micro-particle diffusion mechanism in the space and find the approach that improves the micro-particle deposition on the wall. The research has an important academic significance and industrial application value.For the gas-solid two-phase flow in the straight square tube, square bend and around a square cylinder, the characteristics of the micro-particles depositing on the wall and diffusing around the square cylinder are studied by numerical analysis. Reynolds stress transport model is used to predict the flow field, and two-layer model is used to resolve the wall-bounded turbulent flows in the channel. The flow around the square cylinder is studied by large eddy simulation method. The micro-particle trajectories are tracked by the micro-particle stochastic model. The effect of various factors on the micro-particle deposition and diffusion are discussed in detail. The following conclusions are obtained.Firstly, it is studied deeply that the gas-phase of the gas-solid flow affects the micro-particle deposition in the straight square channel. The results show that the secondary flow in the cross-section can be simulated accurately by the Reynolds stress model which based on the anisotropy. The results are closed the DNS simulation results. The gravity is the main reason that causes the difference of the micro-particle deposition velocity on the different wall in the square channel. As the micro-particle size range is from 1 to 30μm, the velocity of micro-particle deposition on the floor is the largest, and the velocity of micro-particle deposition on the wall is the second place. With the micro-particle size increasing, the micro-particle deposition velocities on the floor and the wall increase, but the ceiling deposition velocity increases at first and then decreases. The micro-particle deposition velocity on the wall is greatly affected by gas velocity. With gas velocity increasing, the micro-particle deposition velocity increases. The turbulent fluctuation has a significant effect on the micro-particle deposition velocity. At the same gas velocity, the micro-particle deposition velocity in the developing flow area is greater than that in the developed flow area. For the vertical flow, the micro-particle deposition velocity increases when the micro-particle size increases. And the micro-particle deposition velocity in the vertical downward flow is larger than that in the vertical upward flow.Secondly, the micro-particle depositing on the square bend walls is studied. The results show that Reynolds stress model can accurately predict the secondary flow in the square bend. With the bending angle and gas velocity increasing, the intensity of secondary flow increases, and the vortex center of the secondary flow gradually shift to the direction of the inner wall. The micro-particle penetration rate decreases as the micro-particle Stokes number and the bend radius of curvature increase. With Stokes number increasing, the micro-particles deposition velocities on the outer wall, ceiling and floor increase, and the micro-particles deposition velocities on the inner wall increases at first and then decreases. The centrifugal force has a significant effect on the micro-particle trajectories which causes that micro-particle deposition velocity on the outer wall is the largest.Finally, the characteristics of the micro-particle deposition and diffusion are studied when gas-solid two-phase flows around a square cylinder. The results show that the vortex shedding frequency and the drag coefficient are affected by the block rate. The micro-particle diffusion along the vertical direction and the spanwise direction is affected by the spanwise vortex structure and stream-wise vortex structure. When Stokes number is 1, the particle distribution is non-uniform, and the particle diffusion forms the quasi-coherent structure. The micro-particles gather in the outer region of the large vortex. When Stokes number is 0.01, the particle distribution shows the same quasi-coherent structure as flow structure. The particle concentration is more evenly distributed,and particle has a considerable diffusion along the horizontal direction. With the Stokes number of the particles increasing, the particle deposition rate on the front wall of the square cylinder increases, but the deposition rate on the back wall decreases. The blocking rate has little effect on the micro-particle deposition rate.