Simulation On The Growth Of Fine Particles By Heterogeneous Condensation

The problem of atmospheric particulate matter pollution is serious,which brings great harm to people's life and production.Fine particulate matter emitted from coal-fired power plants is one of the main sources of atmospheric particulate matter pollution.It is difficult for traditional dust removal equipment to effectively remove PM2.5 fine particles.A better solution is to make fine particles grow to larger size through pretreatment technology,and then use dust removal equipment to remove them.Vapor phase transition is a promising pretreatment technology with simple process and obvious effect.There have been many experimental studies on the effect of water vapor phase transition on the removal of fine particles,but there are relatively few studies on simulations.In this paper,the complete dynamics model of particle growth in water vapor environment is established,which consider three stages of particle growth:nucleation stage,transition stage and growth stage.In the nucleation stage,liquid embryos are formed on the surface of particle.In the transition stage,liquid embryo absorbs surrounding water vapor and grows up,which could be regarded as heterogeneous condensation.The mechanisms of obtaining water molecules include direct adsorption and surface diffusion.The starting of the growth stage is that the liquid embryo completely covers the surface of the particle,and the particle is surrounded by a liquid film.The droplet absorbs surrounding water vapor and grows up,which can be regarded as homogeneous condensation.Then the accuracy of the model is verified by experimental data.The results show that the air temperature around the particles has a important influence on particle growth.The predicted results of the model are in good agreement with the experimental results.In order to explore the influence of operating conditions and particle properties on particle growth,a two-dimensional heat and mass transfer model was used to obtain the supersaturation profiles in growth tube.The growth of single diameter particles in the growth tube was simulated.The results showed that:for the single particle,when the supersaturation level in the growth tube is higher,the final size of particle is larger.Initial size and wettability of particle have little effect on its final growth size.When particle quantity concentration is below 10⁸m^-3,it has almost no influence on particle growth.When particle quantity concentration of particles is large,with the increase of the particle concentrations,the vapor dissipation becomes obvious,and the final size of particle will decrease.The initial inlet flow velocity will affect the supersaturation profile in the growth tube.When the particle concentration is low,reducing the initial inlet flow velocity will increase the nucleation distance of the particles in the growth tube,so that the particles have better growth effect.When the particle concentration is high,the effect of changing initial inlet flow velocity on the growth of particles is not obvious.The supersaturation profiles in different radial directions are different.When the particle concentration is low,at r/R=0.5,the particles have a better growth effect.When particle concentration is high,particles have a better growth effect at the center line of growth tube.In practical studies,the initial particle size distribution is polydisperse,so it is of practical significance to simulate the growth of polydisperse particles in growth tubes.The results show that for polydisperse particles,the effect of particle concentration on particle growth is obvious,the larger the particle concentration is,the smaller that the particle will grow.With the increase of particle concentration,the peak of particle number proportion moves towards the direction of small particle size.With the increase of particle size at the initial peak,the dispersion of particle size distribution increases and the effect of particle growth become worse.When the particle concentration is small,the initial inlet flow velocity has a great influence on the growth effect of particles.With the increase of the initial inlet flow velocity,the growth effect of particles becomes worse.The effect of gas velocity on the growth of particles is not obvious when the concentration of particles is large.When the supersaturation level in the growth tube is constant,increasing the inlet flow temperature will lead to poor particle growth effect,which is not conducive to particle growth.When the supersaturation level in growth tube is low,improving the wettability of particles can promote the effect of particle growth.When the supersaturation level in growth tube is high,improving the wettability of particles has little effect on improving the effect of particle growth.