Mechanism Research On Inertial Impact Process Between Ash Particle And Planar Surface At Normal Temperature State

During coal-fired boiler operation process, ash deposition occurs on the heat exchange surface, resulting in severe corrosion of heat transfer surface and reducing boiler thermal efficiency. In the process of ash formation, mass transfer mechanism is composed of inertial impaction, turbulent diffusion, thermophoresis, condensation, and chemical reaction, among the five ash deposition mechanisms, inertial impaction plays major contribution to mass accumulation of deposition. In the paper, the formation of ash deposition on heat transfer tubes in a boiler was taken as the research background, adhesion mechanism of inertial impact process between ash particle and planar surface at normal temperature state was investigated based on soft sphere physical model, experimental and numerical calculation method. The main purpose was to build foundation and provide research ideas and methods for subsequent thermal state and surface coating of inertial impact research.Firstly, particle dynamics theory was discussed in details. Fluid drag force and effective gravity of particle in flow field were introduced to provide theoretical basis for verification flow field effect on particle movement. In the respect of particle collision dynamics, several static/quasi-static contact mechanics models were introduced, which consist of linear, Hertz, DMT and BD models. Material viscoelasticity and plastic deformation were the main causes of energy dissipation, the size of energy dissipation caused by material viscoelasticity was proportional to the impacting cut-off time, larger plastic deformation could also cause larger energy dissipation, and the degree of energy dissipation can be measured by the damping coefficient. The soft sphere physical model which could describe the process of inertial impact was introduced after combining static/quasi-static contact mechanics model with energy dissipation theory.Secondly, experiment research was carried out to investigate the inertial impact of ash particle with planar surface. The experimental results show that, the normal coefficient of restitution first increases, then maintains gradual change, finally decreases as the normal incident velocity increases, in other words, experimental curve can be divided into up and down segments, this is due to the variation of the ratio of energy dissipation caused by material viscoelasticity or plastic deformation to the total energy dissipation in the different ranges of normal impact velocity; the critical capture velocity changes with the particle diameter:the larger the size, the smaller the capture velocity.Finally, numerical calculation was also carried out to investigate the inertial impact process. Based on dynamics of particles in the flow field, the effect of flow field on particle movement can be ignored. On the strength of soft sphere physical model, combined with the experimental data, the impacting cut-off time and the damping coefficient of different normal incident velocity were obtained by simulation calculation of Hertz, DMT and BD model, which can judge the causes of energy dissipation and provide evidence to explain variation tendency of the experimental curve. The calculation results show that, variation tendency of calculation results for three models is consistent, in other words, the impacting cut-off time decreases as the normal incident velocity increases, and at smaller normal incident velocity, the impacting cut-off time is relatively sensitive to the normal incident velocity, but gentle variation tendency is appeared at larger velocity; in the up segment of experimental curve, the damping coefficient decreases as the normal incident velocity increases, however, the damping coefficient increases as the normal incident velocity increases in the down segment of experimental curve; the damping coefficient can be characterized as the function of normal coefficient of restitution, particle diameter and impacting cut-off time. In addition, compared calculated values of different models with experimental values of the critical capture velocity, the calculated values of BD model are relatively close to the experimental values, it indicates that the force introduced by BD model between the particle and the planar surface is most reasonable, BD model can reflect the characteristics of the actual inertial impaction process.