Dynamic Characteristics Of Micro-particle Impact On A Flat Surface Under The Influences Of Humidity And Surface Roughness

Particle deposition has been commonly applied in industrial manufacturing such as fossilfuel power station,chemical industry,electromechanical industry,pharmaceutical industry,etc.In fossil-fuel power station,the particle is evaporated by the conventional electrostatic precipitators(ESPs).For the particle diameter larger than 10?m,the dust removing efficiency is larger than 99%.There is a penetration window for the micro/nano-sized particles.Therefore,the humidity of flue gas promotes by reducing the gas temperature,thus increasing the adhesion force between two surfaces.It can both improve the efficiency of dust removal and achieve ultra-low emission.The fly ash particles collided to the surface of collecting plate by electrostatic and drag forces.When the incident velocity smaller than the critical impact velocity,the particles adhere to the collecting plate.On the other hand,the surface of the collecting plate would become uneven due to the particle-collecting plate collision and the corrosion of flue gas,which could influence the contact force during the collision.In the thesis,micro-particle impact on a surface under different relative humidity and surface roughness has been studied by using experimental and numerical methods.The experimental facility was constructed to investigate the particle-wall collision.The fly ash size was about 7±1?m which was made by anthracite coal from XinlinGol,Neimeng Province,bituminous coal from Fushun,Liaoning Province and meager coal from Zhundong,Xinjiang Province.The relative humidity varied within the range of 35%-65%.The effects of relative humidity and particle properties on the process of particle-wall collision had been investigated.The results indicated that the normal restitution coefficient first increased,then gentled and at last decreased with the increasing velocity due to the elastic-plastic impact of micro-particles.Compared with particle properties,the decreasing density and Young's modulus increased as well as limiting velocity was promoted to particle deposition.With the increasing relative humidity,the liquid bridge between the contact surfaces increased the capillary force to reduce the normal restitution coefficient.Relative humidity and contact angle had a great influence on the capillary force compared to temperature.Van der Waals force was reduced under humid conditions.Based on the experimental research,the effect of relative humidity on particle-wall collision was researched to explore the variation of the normal restitution coefficient under different relative humidity.Based on the dynamic model,the variation of damping coefficient,contact time,contact displacement and critical impact velocity were obtained by adding capillary force.It was found that the damping coefficient was less affected in the elastic deformation range but more affected by relative humidity and particle properties.Specifically,the damping coefficient decreased with the increasing relative humidity,density and Young's modulus.With the increasing relative humidity,the contact time and critical impact velocity were larger for liquid bridge hinder the collision between particle and flat,which would be conducive to particle aggregation.Based on the nonlinear variations of normal restitution coefficient versus relative humidity,a prediction formula for the normal restitution coefficient of particle-wall collision had been proposed by modifying Stokes number and elasticity parameter.Under dry condition,the effect of surface roughness on the collision between silicon dioxide spherical particle and the surface was studied using the experimental method.The surface roughness ranged from 19.76nm to 236.92 nm root mean square(RMS).The surface roughness impact of the normal restitution coefficient was obtained.The results indicated that the adhesion energy loss and friction loss increased with the increasing surface roughness in this experimental range,then the normal restitution coefficient was reduced.The normal restitution coefficient was mainly affected by surface roughness while the incident velocity was near critical impact velocity.The particle diameter played an important role in the larger incident velocity.Under the relative humidity condition of 35%-80%,the dynamic characteristics of a silicon dioxide spherical particle dioxide particle impacted on a rough surface were experimentally and theoretically studied.The effect of surface roughness on normal restitution coefficient was obtained under different relative humidity.The results indicated that the capillary force decreased with the increasing surface roughness.It was difficult to form the continuous liquid bridge between the particle and the rough surface at low relative humidity.With the increasing relative humidity,the continuous liquid bridge would be formed between the particle and the rough surface.The impact of the capillary force on the particle-wall collision was increased gradually.By establishing models of capillary force at low relative humidity,the variation of capillary force decreased at first,then increased with the surface roughness was obtained.