| Particle multiphase systems widely exist in nature and industrial production.It is of great significance to deepen the understanding of multiphase motion,reveal the interaction mechanism between different phases and conduct industrial production and engineering application after reasonable and effective multiphase coupling models were established to describe different multiphase systems.However,there are a large number of particles with different sizes in such systems.The characterization of massive particles and multiphase information with different scales has been major factors restricting the development of coupling methods for particle multiphase flow.In order to decrease the computational cost while characterizing the dynamics behaviors of different particle multiphase systems,we developed and established different coupling models for particle multiphase systems based on micropolar fluid model.After numerical simulation programs were compiled in open source software Open FOAM,LIGGGTS and coupling module CFDEMCoupling,the calculations of common particle multiphase systems are carried out and the feasibility and advantages of the proposed multiphase coupling models are verified.Firstly,for the description of sparse particle two-phase system,we proposed micropolar fluid model,and in the calculation of flow past a cylinder with fine particles,the quantitative relationship between the volume concentration of fine particles and micropolar parameter N/L was established based on the micropolar fluid and CFD-DEM model.In the calculation of the flow past a cylinder based on the micropolar fluid,we defined micropolar parameter N/L and micropolar Reynolds number according to the evolution of micropolar viscosities.In one-dimensional micropolar Poiseuille flow,the correctness of the numerical program of micropolar fluid model was verified.Then,we analyzed the effects of the micropolar parameter N/L and micropolar Reynolds number on the frequency of vortex generation and shedding,the velocity of fluid,the rotation velocity of the microstructure and the drag force on the cylinder.In the calculation of flow past a cylinder based on the CFD-DEM model,the influence of the volume concentration of fine particles on particle motion behaviors and the drag force on the cylinder was investigated.The results show that the increase of the micropolar parameter N/L and the volume concentration of fine particles,the drag force on the cylinder is decreased.Based on this result,we fit the quadratic function relationship between the volume concentration of fine particles and the micropolar parameter N/L,realized the purpose of investigating the influence of the concentration of fine particles on the hydrodynamic behaviors through the micropolar parameter N/L and reduced the computing cost of a large number of particles.Secondly,for the description of gas-liquid-solid three-phase system with multi-size particles,we established a coupling model of computational micropolar fluid dynamics and discrete element method,namely CMFD-DEM model,and implemented the numerical simulation program based on the open source platform.In this model,the different sizes of particles are described jointly by the micropolar fluid model and the discrete element method,and VOF method is used to track the gas-micropolar fluid interface.That is,micropolar fluid model is employed to characterize the mixture of the fluid with fine particles,while the motion of larger particles is characterized by the discrete element method.In addition,the influence of fine particles on the force between fluid and particles is reflected in the modified drag force based on the micropolar viscosities.In the case of sedimentation of single particle and particle assembly,the correctness of CMFD-DEM model in the calculation of the interaction forces between phases,guaranteeing the volume conservation of three phases and implementation of the numerical program was verified.Then,based on CMFD-DEM model,we carried out numerical calculation and parameter analysis of the common geological phenomena.In the calculation of debris flow,we explored the variation of collapsed ratio,runout distance and motion patterns of debris flow under different micropolar parameter N/Ls and rolling friction coefficients.The results show that with the increase of micropolar parameter N/L,the degree of collapse and the diversity of motion patterns of debris flow increase,and the overall velocity and runout distance of debris flow decrease.The increase of rolling friction coefficient will also reduce the overall velocity and runout distance of debris flow,but the rolling friction coefficient has a greater influence compared to the micropolar parameter N/L.In the calculation of bank collapse in the river with fine sediment,we analyzed the bank collapse behaviors under different micropolar parameter N/Ls and different configuration inclinations formed by river erosion.The results show that the collapse of the bank under different configuration inclinations goes through two stages.Compared to the bank collapse in the river described by classical fluid model,the bank collapse degree based on the CMFD-DEM model is reduced,and the introduction of microstructure decreases the fluctuation frequency of the river surface caused by collapsed bank.In the calculation of seepage,we analyzed the influence of the micropolar parameter N/L,the thickness of permeable layer and the particle size ratio on the seepage pressure drop.The results show that with the increase of micropolar parameter N/L,the thickness of permeable layer and the degree of grain grading,the pressure drop of seepage gradually increases,and the influence of microstructure is more significant in the thicker and better grain grading permeable layer.Finally,for the description of dense particle two-phase system,we established a classical-micropolar two-fluid model,namely CMTFM.In this model,the rotation of particle and its influence on particle collision are characterized by the rotational degree of freedom of microstructure and different micropolar viscosities.After the sensitivity analysis of mesh size and time step,we carried out numerical calculation to investigate the behaviors of the fluidized bed under different micropolar parameter N/Ls,particle collision restitution coefficients and inlet velocities,and compared with the experimental results.The results show that the increase of micropolar parameter N/L increase the pressure drop and expansion height of the fluidized bed,making them approach to the experiment values.The increase of collision restitution coefficient has no obvious effect on the expansion height of the fluidized bed but will decrease pressure drop and make the particle distribution more dispersed.In addition,the fluidized particle phase considering the microstructure effect can better reflect its non-uniformity distributed in the fluidization process.In the investigation of different inlet velocities,the expansion heights of the fluidized bed under the CMTFM model are all closer to the experiment values,but there is a certain difference between the average pressure drop and the experiment values at the inlet velocity lower than the particle fluidization velocity.In general,to consider the massive fine particles,our works based on the micropolar fluid model can effectively reproduce the dynamic behaviors of different particle multiphase systems at the low computational cost.However,the study on the particle multiphase system is still full of challenges,the consideration of irregular particle boundary,particle fragmentation and chemical reaction between phases will still be the frontier and hotspot in the particle multiphase field. |
