Study On The Dynamics Of Nanoparticle And The Flow Field Of Low Pressure Impactors

With the exposure that aerosol can have adverse effects on human health, and the aerosol application increase in the industrial production, the study of the nanoparticle dynamic properties is becoming more and more important. The dynamics evolution of nanoparticle and the single stage low pressure impactor, which can measure the aerosol size distribution, has been investigated.This study focuses on the following three aspects:(1) The study on the evolution of nanoparticle size distribution. Considering the aerosol system with particle production, diffusion, coagulation and deposion, the evolution of aerosol size distribution was investigated. Firstly, All the programs were written by C Programing language to solve the nanoparticle general dynamical equation by section method. It’s shown that because of the competitive effect of source strength and coagulation, two modes appear, and only the first peak is reserved finally. The nanoparticle size distribution takes a skewed distribution in the end. It is found that the ultimate diameter where the peak locates are the same value for all four cases, which have different total particle number concentration. And the particle number concentration intensity is found it requires less time to approach the steady state when the source is stronger. Secondly, Large eddy simulation(LES)and Taylor-series expansion moment method are employed to study the evolution of particle size distribution in different diesel fuel sulfur contents. It is shown that particles mainly form in the exhaust shear layer, while the maximum value of particle number concentration and particle diameter appear in the downstream 0.6 m from the tailpipe. It is obvious the fuel sulfur content has a significant on the formation of secondary nanoparticles. The total particle number concentration and total volume concentration are about four and six orders of magnitude in the lowest fuel sulfur standard smaller than that in the highest fuel sulfur standard, respectively.(2) According to the real single stage low pressure impactor, the flow field and particle field was simulated using Fluent 6.0 software. The flow field at different ratio of inlet pressure and outlet pressure has been shown from the simulation. With the decrease of pressure ratio,t he mass flow of inlet approach the maximum value first, then keep a steady state, the shape of particle impaction curve becomes better, and the resolution is increased. The influence of particle additional force, Saffman lift force and Brown force, on the particle impaction curve have been taken into account. The Saffman lift force is applied for the small Reynolds number. The influence of Saffman lift force on the larger particles is more obvious. It can make the aerodynamic beam stay closer to the axis. The influence of Brown force on the small particles whose diameters is less than 200 nm is obvious. Taking into account the Brown force can make the small particles escape from the inlet, and reduce the particle lost to the wall.(3) In order to design an impactor more reliable and more accurate, the particle collection efficiencies of different structure parameters were investigated using the DPM model of commercial software FLUENT. The study on the diameter of inlet orifice D1, shows that, increase D1 can make the shape of the collection curve more ideal, and the cut diameter smaller. It can get the most ideal shape of collection curve only changing the orifice diameter, when D1=4d1. Decreasing the diameter of impaction nozzle D2 can increase the impaction velocity, and get a better shape of collection curve, but make the resolution decreased after get the ideal collection curve. The best resolution can get when D2=0.5d2. The relaxation chamber length l1 is the maximum size of the single stage impactors. Decrease l1 can make the shape of collection curve worse, then get better. The cut diameter is 268 nm, and the resolution is 4.48, when l1=0.01L1. The study of the plate to nozzle distance l3 selected the nozzle diameter D2=0.5d2. When got the best distance l3 for 0.5mm, cut diameter is 214 nm, and the resolution is 6.8.