CFD-DEM Simulation Of Gas-solid Two Phase Flow Characteristics Of Ultra-light Expanded Graphite Particles

Ultra-light particles,which are widely used in modern industry,are extremely light in weight so that they are easy to scatter in open spaces.Therefore,pneumatic conveying with good sealing is often used as a means of transportation,whose characteristics greatly depend on the particle property.Expanded graphite has a particle density of only a few tenths of that of conventional materials and the particle shape is irregular wormlike,which is a typical ultra-light non-spherical granular material.This paper,taking the pneumatic conveying of EG particles as the research object,presents a fundamental study on the ultra-light particle gas-solid flow characteristics in terms of hopper discharging,horizontal conveying and gas-solid separation process based on computational fluid dynamics and discrete element method(CFD-DEM).The research aims to reveal the flow mechanism of ultra-light particle.A non-spherical particle model and a spherical particle model of the EG particle were established and corresponding DEM parameters were calibrated.Based on the multi-sphere element method and experimental measurements,the non-spherical particle model conforming to the characteristics of EG particles was firstly established;then the DEM parameters of the particle model were calibrated by the virtual calibration method with the design of experimental(DoE)method adopted and the experimental errors introduced;the effect of the micro DEM parameters on macro bulk responses was analyzed,following which the DEM parameters of the equivoluminal spherical particle model were adjusted;both of the calibration results were verified by the rotary cylinder experiment.The CFD-DEM model of ultra-light particle gas-solid two-phase flow was established and the key calculation parameters in the model are analyzed and determined.The effects of the shear modulus on EG particle collision,particle velocity distribution,particle concentration distribution and particle diffusion after bending were analyzed,thereby the minimum shear modulus with the largest calculation time step was confirmed under the premise of ensuring the calculation accuracy;the effect of the gas lift force on the ultra-light EG particle concentration distribution and force characteristics was also studied and the setting of lift force in the simulation was then confirmed.The discharging characteristics of non-spherical concave ultra-light EG particles were studied based on the non-spherical DEM particle model of expanded graphite,and the influence of the hopper structure on discharge performance was investigated.For the conical hopper,the effects of the cone apex angle,the outlet size and the eccentricity were studied;the curve hopper was designed and the influence of the outlet size was studied;for the negative-pressure-assisted side-opening rectangular hopper,the effects of the initial layer thickness and the box structure improvement were studied.The flow performance of the non-spherical ultra-light particles was studied,the compressive force distribution and particle velocity distribution in the hopper were analyzed,and finally the arching mechanism was revealed.The horizontal pneumatic conveying characteristics of ultra-light EG particles in straight pipe were studied based on the CFD-DEM model of turbulent conveying.The effects of pipe diameter and superficial gas velocity on pressure drop and particle flow pattern were studied;at the same time,the particle velocity vertical distribution,particle force and collision characteristics were revealed;the experimental pipeline system for ultra-light particle horizontal transport was built and the experimental research was carried out;by comparing the experimental results and simulation results,the effectiveness of the pipeline conveying CFD-DEM model was verified.The horizontal swirling flow pneumatic conveying characteristics of ultra-light EG particles were studied based on the CFD-DEM model of swirling conveying.The effects of the superficial gas velocity,the pitch and the length of the swirl pipe on the gas-solid flow field were investigated in terms of pressure drop,particle flow pattern,particle velocity axial distribution and collision characteristics;based on the above research,the optimal design parameters of the swirl pipe suitable for the ultra-light particle transport were obtained;furthermore,the flow performances of the ultra-light EG particles in the straight pipe and swirl pipe were compared,which proves the superiority of swirl conveying for low-speed transport.The cyclone separation characteristics of ultra-light EG particles were studied based on the gas-solid separation CFD-DEM model.The pressure and velocity of the gas phase at different inlet gas velocities were analyzed and the particle flow pattern,particle force and collision characteristics were revealed;the experimental cyclone system for EG particle separation was established and the experimental research was carried out;the experimental results were used to verify the effectiveness of the cyclone separation CFD-DEM model;in addition,the effects of the length of the cone zone and the cylinder zone on the ultra-light particle separation were studied,following which the cyclone separator was optimized.The research shows that: the ultra-light EG particle has unique gas-solid flow characteristics in the process of hopper discharge,pipeline transportation and gas-solid separation;the established particle model and the CFD-DEM method can effectively reveal the micro mechanism of particle hooking aggregation,particle force,particle acceleration and collision characteristics.The research provides theoretical references and technical supports for the process control and system design of ultra-light particle pneumatic conveying.