Study On The Micro Dynamics Of Particulate Matters Subjected To Standing-wave Acoustic Field

PM_2.5 pollution has become an important atmospheric environmental problem in China.The increase of PM_2.5 in the atomosphere is due maily to the emissions from coal-fired power plants and industrial sources.Therefore,how to control PM_2.5emissions from the sources has become an urgent concern.Acoustic agglomeration is recoginazed as a promising preconditioning technology in PM_2.5 removal.The principle of acoustic agglomeration is to induce relative motion and thus agglomeration between the particles by the effect of acoustic field,as a result the particle size increases,and the particle number decreases.Therefore,the efficiency of conventional particle removal device can be improved.It is important to discover the micor dynamics of particles in the acoustic field to understand the acoustic aggloemeration mechanisms and processes.Furthermore,the acoustic entrainment method can be used for particle diameter measurement.It is helpful to investigate the micro dynamics of particles for improving the particle measurement theory.Based on the application of particle dynamics in acoustic coagulation and particle size measurement,the numerical simulation method was used to study the effect of temperature and particle density on spherical particle motion in the acoustic field,to examine the error and the measurement range of acoustic entrainment method used for particle size measurement,and to explore the motion of particle aggregate in acoustic field.Single particle dynamics model in the acoustic field was established,considering the viscous drag force and unsteady forces,including the Basset force,the pressure gradient force and the virtual mass force.The influence of temperature and particle density on particle dynamics in the acoustic field was studied.The results show that the viscous force dominates the particle motion.As the gas temperature increases,the phase difference between the pressure gradient force and the viscous force decreases,while those between the Basset force as well as virtual mass force and the viscous force increase.It is also found that when the gas temperature is low,the particle density affects significantly on particle motion.In this case,as the particle density increases,the entrainment coefficient decreases rapidly.Less dependence of the particle motion on the particle density is observed when the gas temperature is higher.Both the particle displacement amplitude and the entrainment coefficient increase obviously in case of a high temperature relative to a low temperature.Based on the dynamic model considering the unsteady forces,the influence of acoustic frequency and particle densityon particle size measurement error and range by the acoustic entrainment method was examined.The results show that the measurement error decreases first and then increases with the increase in the acoustic frequency,thus there exists a frequency value at which the theoretical error for a certain particle size decreases to 0.If the frequency is lower than the value,the measurement value is greater than the true value.If the frequency is higher than the value,the measurement value is smaller.As the particle density increases,the measurement error decreases,and finally the error is almost zero.For the same particle density,the larger the particle size is,the smaller the measurement error.As the acoustic frequency increases,the particle size measurement range decreases.As the acoustic intensity increases,the particle size measurement range increases.Dynamical model for the particle aggregate in the standing-wave acoustic field was established.The translational and rotational motion of chain-like aggregates with the same primary particle diameter and the aggregates with two primary particles of different sizes were investigated.For the chain-like aggregates,the results show that the translational motion is not affected by the primary particle number,whereas the velocity of rotational velocity increases with the increase of primary particle number.As the primary particle diameter increases,the translational amplitude decreases and the rotational velocity decreases.The translational motion is independent of the initial angle between the aggregate and the wave motion direction,whereas the rotational range is determined by the initial angle.In the process of the initial position of aggregate mass center moving from the velocity wave node to the velocity wave loop,both the translational amplitude and the rotational velocity increase.For the aggregate with two primary particles of different diameters,it is found that the larger the primary particle diameter difference is and the greater the aggregate volume is,the larger the translational amplitude of the aggregate are underestimated if the aggregate is taken as a spherical particle.As the primary particle size difference increases,the rotational velocity firstly increases and then decreases,when the ratio of the particle diameters is approximately 1:2,the rotational approaches the maximum.The translational motion of the aggregate is not affected by the initial angle,whereas in the process of the initial angle increasing from 0 to p/2,the rotational velocity increases firstly,discreases later,and finally increases.As the initial position of aggregate mass center moves from the velocity wave loop to the velocity wave node,both the translational amplitude and the rotational velocity of rotation decrease.