Kinetic Research On The Deposition Of Fine Particles On The Randomly Packed Granular Bed Filter

The granular filter has a great prospect in the field of high temperature flue gas purification and wastewater process due to its advantages of high filtration efficiency,high temperature and pressure resistance,wear and corrosion resistance,low cost,simple structure and etc.These filtration technologies are essentially the movement,deposition and separation process of fine particles in the surface of randomly packed granules.There have been many experimental studies on the dust removal of granular filtration.However,the systematic research on the macroscopic and mesoscopic dynamic behavior of fine particle,such as movement,deposition and blockage,is so poor,which limits the further improvement of granular filtration efficiency.Based on the above background,the gas-solid coupling granular filtration model is established to analyze and reveal the fine particle deposition characteristics on the granular surface of a randomly packed granular filter.The main research content of this paper includes five parts: the establishment of mathematical model of granular filtration model,the analysis of error and confidence degree of model,the study of the macroscopic gas-solid coupling CFD-DPM and mesoscopic gas-solid coupling LBM-IMBM under different particle size ratios,and the study of the motion characteristics of particle aggregates.Details are as follows:Firstly,considering that the ratio of granule diameter to fine particle diameter has an important influence on the gas-solid coupling and flow field,two models,CFD-DPM model based on traditional computational fluid dynamics and LBM-IMBM model based on lattice Boltzmann method,are built for large and small ratios respectively.Furthermore,based on CFD-DPM model method,the randomness of packed granules,the model size and the mesh size effect on the stability of the numerical results,which involved in the accumulation process of model,are analyzed in this paper.And the effective smallest model size and the corresponding model mesh size are obtained.In addition,the filtration performance difference between random pack and regular pack is compared and analyzed,which verifies the necessity of randomness of granular packing.In the research process,the self-built experimental platform and the existing empirical formula were used to verify the above two models,which ensured the accuracy of the models.Secondly,based on CFD-DPM model,the grade filtration efficiencies for different size fine particles in different gas velocity,granule diameter,packed height and fluid temperature are studied for large ratio.And based on the above operation parameters,a fitting prediction formula correlates dimensionless stokes number and filtration efficiency is obtained to calculate filtration efficiency under different working conditions of granular filter.The filtration efficiency keeps at a certain value when St1<0.0205 and linearly increase with St1 when St1>0.0205.Additionally,the detailed movement and deposition mechanism of fine particles in the granular filter are discussed,including the location and distribution characteristics of deposition of fine particles with different particle sizes.And the influence of airflow vortex in the flow channel on the movement and deposition distribution of fine particles is also analyzed.Thirdly,based on LBM-IMBM model,the movement,deposition and blockage of fine particles in the granular filter are discussed and divided into three stages: deposited particle chain development stage,blocking stage and deposited particle layer filtering stage.And the deposition location,form and development processes of fine particles in different stages are showed,which relates to the filtration efficiency and pressure drop.The results show that at the deposited particle chain development stage,the pressure drop increases at an accelerated rate at first and then at an approximately uniform rate.While the pressure drop increases at an decelerated rate at the blocking stage and increases uniformly at the deposited particle layer filtering stage.Besides,the influences of fine particle size,granule diameter,superficial gas velocity and dispersion of fine particle size on the filtration performance were studied,which helps to understand the relationship between the deposition characteristics of fine particles and the macroscopic operating conditions.Finally,aiming at the particle aggregate assembly with more general significance in production practice,the motion characteristics of aggregate and its deposition performance in the packed granular filter are studied.The interaction between aggregate and fluid is described by fully discretizing the spaces occupied by the internal channel of aggregation and itself.Then,the fitting equation among the terminal settling velocity,particle size and fractal dimension is derived and applied to the deposition of aggregate in the granular filter,which compared with the results of LBM-IMBM method and obtained good agreement.This work contributes to the mechanism of granular filtration from the view of morphology of fine particles.