Study On Dynamic Light Scattering Measurement Of Aerosol Particles

Particle size distribution(PSD)is an important index to characterize aerosol properties and also essential for determining the effect of aerosols on air quality.From combustion processes,nuclear fuel cycles,microelectronics,new materials,to pollution prevention,the need to obtain aerosol PSD in industry is increasing.However,due to the characteristics of aerosol particles,such as high diffusion rate,rapid change of size and shape,it is difficult to achieve accurate on-line measurement of PSD by conventional measurement methods.Based on the basic theory of dynamic light scattering(DLS),an aerosol particle measurement platform using DLS is developed.By analyzing the characteristic time in the autocorrelation function(ACF),the mechanism of Brownian motion and translational motion in DLS measurement of flowing aerosol particles is studied,and the DLS online measurement of aerosol PSD is realized,The main contents of this paper are as follows:1.Analysis of the characteristics of the light intensity ACF of the flowing polydisperse aerosol particles.It is found that the accuracy of aerosol PSD measurement is still affected by the increase of flow velocity,under the laminar flow velocity which does not interfere with Brownian motion,by using the light intensity ACF with diffusion and translation contributions.This effect limits the application of DLS in aerosol measurement.2.The influence of flow velocity on the aerosol PSD inversion in laminar flow was studied.Simulated and experimental data were used to investigate the PSD recovery from a flowing aerosol via DLS.It is found that,for a unimodal PSD,increasing the flow velocity leads to an increase in the error of the peak position,a decrease in the peak height and a broadening of the PSD.For a bimodal PSD,the separation of the two peaks decreases and results in a single-peak PSD as the velocity increases.The effect of velocity on the measurement is similar to adding signal noise,which increases with the velocity.This effect is related to the measured aerosol particle size.The larger the particle,the lower signal-to-noise ratio.3.To explore the restriction of flow velocity on aerosol PSD measurement.The results,used to retrieve the simulated and measured DLS data of flowing aerosols,show that the strong influence of the increase of velocity on the aerosol PSD retrieval results can not be expressed in the ACF model through the contribution of flow velocity.The reason for the difficulty of particle size inversion is that the increase of velocity aggravates the ill condition of ACF equation,which is manifested by the increase of condition number of the kernel matrix.From the perspective of signal analysis,the increase of flow velocity reduces the amplitude of particle size information in ACF.4.The mechanism of the flow velocity limitation of flowing aerosol particles is analyzed in DLS measurement.By calculating the proportion of particles exchanged in the scatter volume due to translational flow in the correlation time,it is found that the number of particles flow DLS measurements of aerosol velocity restrictions related to the number of particles input and output of scattering volume.With the increase of the number proportion of particles input and output of the scattering volume in the total number of particles,the particle size obtained by inversion gradually decreases,the distribution is broadened and the peak value decreases.Meanwhile,the influence of the particles input and output of the scattering volume is also related to the measured particle size.This is exacerbated for larger aerosol particles.The velocity suitable for on-line measurement of DLS of moving aerosol particles is not only limited by the Reynolds number of the fluid,but also by the ratio of the input-output particles number to the total particles number in the scattering volume.This ratio greatly limits the velocity selection range of on-line DLS measurement of aerosol particles,and is usually ignored,which leads to the failure of dynamic light scattering technology in the measurement of mobile aerosol particles.In this paper,the DLS measurement technology of aerosol particles is studied,and the restriction mechanism of flow velocity on measurement is explored,which provides theoretical basis and experimental verification for the industrial application of aerosol particles DLS measurement.