Research On The Capture Behavior And Mechanism Of Fine Particles By A Single Droplet

Wet scrubbing and wet deposition are efficient methods to remove particles from industrial flue gas and the atmosphere.Particle capture by a single droplet is the foundation of the aforementioned processes;therefore,study on single droplet capture is significant for developing innovative industrial wet scrubbing technologies as well as understanding the capacity and mechanisms of wet deposition in the atmospheric environment.Firstly,three-dimensional direct numerical simulations were performed to study the migration behaviors and capture efficienciesof the particles dominated by inertia orthermophoretic mechanism at typical droplet Reynolds numbers(Re).In the vortex shedding regime,the flow field oscillation induced by vortex shedding resulted in oscillation of particle inertial deposition.In consideration of flow boundary layer evolution with Re,St was modified and a new formula for the inertial capture efficiency was derived.The thermophoretic motion of submicron particles around the droplet and the deposition distribution exhibit different characteristics in three typical flow regimes.The thermophoretic motion patterns of particles depend on their initial positions in the upstream and flow regimes.The patterns of particle thermophoretic motion and thermophoretic deposition are diversified as Re increases.The particlethermophoretic motion pattern,initial position of captured particles,andthermophoretic capture efficiency change periodically,especially during periodic vortex shedding.The thermophoretic capture efficiencies calculated by simulation and previous formula are compared.The relative deviation is analyzed and the formula is modified.Secondly,the effect of droplet deformation on the particle capture efficiency was studied.On the basis of studies on the deformation of droplets falling at terminal velocity with typical diameters,the flow and the temperature fields around deformed and spherical droplets were simulated by large eddy simulation.The deposition behavior of particles with varying diameters was studied in both flow processes.The effect of droplet deformation on the inertial capture efficiency and thermophoretic efficiency of particles was analyzed.Inertial capture efficiency is enhanced by droplet deformation when St> 0.4 and reduced when St< 0.4.Thermophoretic efficiency is reduced by droplet deformation,but the decrement is independent of temperature difference.A methodology to predict capture efficiency of deformed droplet is established based on comparing the capture efficiencies of deformed and spherical droplets.Lastly,the impact behavior of airborne particles onto liquid surface was studied.This process was simulated by a dynamic model and analyzed for hydrophobic micron particles.Based on the analysis of energy conversion,the criteria were developed and verified by dynamic simulation to ascertain three impaction modes by the submergence/rebound critical velocity and the rebound/oscillation critical velocity.The criteria indicated that surface tension coefficient,contact angle,and particle diameter are the key parameters to affect the critical velocities.The high-speed microscopy device was established to study the impaction behavior of micron particles experimentally.The experimerntal results validated the theoretical model developed in this thesis.