Simulation Of Deposition Of Aerosol Particles On Fibres Surface

Aerosol particles depositing in fibrous media form chainlike agglomerates(dendrites) on the fibers.These dendritic structures grow relatively unhindered for an initial period of time,but eventually they begin to interfere with each other's growth and to intermesh.This pattern of deposition has profound effects on the filtration efficiency and pressure drop, both of which increase rapidly with time.Using surface filters,the layer of dust,also known as the filter cake,has to be removed periodically from the surface of the filter medium to reduce the resistance to flow and the pressure drop.Filter cake removal happens by using short intermittent bursts of compressed air.The pressure drop across the filter cake,which is additional to that across the filter media itself,is strongly dependent on the cake's structure,which includes its porosity and the filtration conditions.Therefore, rational design,optimization,operation,troubleshooting and innovation require intimate understanding and accurate analysis of the dendritic deposition process and the formation of filter cake.To achieve these goals,two practical three-dimensional simulations were carried out.The main research work and finding are as follows:(1)A stochastic simulation method based on the concept of control window and particle trajectory calculations for expressing the growing processes of particle dendrites on a fiber in Kuwabara's cell.The effects of the particle size distribution and the twist deformation of the fiber on the deposition morphology of particles onto a collector were considered.The simulation results showed that morphology of particle dendritic and the distribution of deposits around the fibre depend on Stokes number,the particle size distribution and the twist deformation of the fiber.The morphology of deposit obtained in the simulated results was found to agree well with the existing experimental results.Single fiber collection efficiency was estimated from the simulation results and it was further correlated with the number of deposited particle in filter.For monodispersed Spherical particles,through these calculations for Stokes number,interception parameter and parking density,the single fiber collection efficiency was approximated to a linear function of deposition amount.For poly-disperse particles,the single fiber collection efficiency wasn't approximated to a linear function of deposition amount.Filters parking density and particle size distribution have a greater effect on single fiber efficiency.(2)A new developed simulation algorithm based on diffusion limited aggregation simulation techniques to simulate the cake formation and growth in surface filters at an individual particle level.In the model,the modeled aerosol particles were introduced from control window far away from fibres surface.Convective diffusion was assumed to be the main mechanism of particle transport and deposition.The Peclet number,which defines the contribution of convective and diffusional effects on deposition,was used for analysis of an influence of both effects on the structure of deposited particles.In addition,since the particle size in the suspension is likely to be no uniform in practical applications,its effect of size distribution on the structure of deposited particles were investigated.The fractal dimension of deposited structure and its local porosity were calculated for the Peclet number ranging from 0.25 to 1600.The simulation results showed that the Peclet number and particle size distribution have a greater effect on the cake's structure,which includes its local porosity and fractal dimensions.When the diffusional deposition of particles has stronger influence over a convective effect,cake has more loose structure(lower fractal dimension) than that obtained at the higher value of the Peclet number.The cake's local porosity descreased with increasing the standard deviation in normal distribution function.