The Mechanism Of Aggregation Of Fine Particles In Sound Field And Its Effect On Agglomeration

PM2.5,PM10-based fine particulates are the major air pollutants in most areas at the present stage.Acoustic agglomeration techniques can agglomerate fine particles to grow into coarser particles,which is advantageous for the conventional filter dust collectors in filtering PM2.5.Acoustic wave can promote the particles to produce a local dense phase area,where particles interact strongly.The study of the mechanical of the aggregation of particles in the acoustic field and the interaction between the particles facilitates to perfect the mechanical mechanism of acoustic wave agglomeration and provide a reference for the application of acoustic wave agglomeration technology.Based on the theory of thermal viscous acoustics,N-S equations and particle motion equations,a simulation model of the fine particle aggregation in the sound field was established to study the pre-collision aggregation of fine particles.The acoustic streaming and acoustic radiation forces were studied by the model.The influence factors of two kinds of aggregations,which are acoustic streaming and acoustic radiation force,were researched,and the relative strength of these two kinds of aggregations were compared.As a result,it was found that the sound pressure level of the standing wave sound field was significantly higher than that of the non-standing wave sound field,and the logarithm of the velocity of the acoustic streaming was linearly related to the sound pressure level.Increasing the frequency of acoustic waves can increase the quantity of acoustic vorticities.Increasing the acoustic intensity of space can increase the intensity of vorticities.The vorticities induced by acoustic streaming are mainly concentrated on the wall parallel to the propagation direction of the sound waves,and reducing the distance between the wall surfaces is beneficial to improving the sound intensity of the entire space.The acoustic radiation force and its aggregation locations,which are the lowest sound pressure level positions,whose quantity is greatly influenced by the acoustical frequency.When the Reynolds number is low(Re<1),the relative strength of two different aggregations is related to the particle size and acoustical frequency but unrelated to SPL The relative strength of two different aggregations is related to the particle size and acoustical frequency but unrelated to SPL.Frequency,particle size distribution and agglomerated geometry are the key factors in adjustment controlling the relative strength of acoustic radiation and acoustic streaming.The influencing factors of acoustic wakes effect and mutual radiation pressure effect are studied in theory,and their relative strengths are analyzed.The acoustic wakes effect and mutual radiation pressure effect are compared with acoustic streaming and acoustic radiation force.It is found that the effect of mutual radiation pressure effect is stronger than that of acoustic wake effect when the distance between particles is small.However,when the distance between particles is large,the mutual radiation pressure effect is weaker than acoustic wake effect.After the distance between the particles increases to a certain value,the acoustic wakes effect,mutual radiation pressure effect,the drag induced by the acoustic wake effect and the acoustic radiation force will cancel each other out in the direction that parallel to the propagation direction of the acoustic wave and reach an equilibrium state.Based on the simulation results,set up an experimental system to investigate some experiments,though which the “particles wall” phenomenon was observed.The experimental results verified the equilibrium state of the particles obtained from the simulation results.The effect of frequency and particle size on the agglomeration efficiency was studied by acoustic agglomeration experiments of particles.The results showed that the particle agglomerating efficiency in the standing-wave acoustic field has a “double-peak” characteristic,that is,there are two optimal acoustic frequency frequencies.In addition,the larger the par ticle size,the higher the agglomeration efficiency of the particles,which indicates that the aggregation of particles is significant to acoustic agglomeration.