Study On The Mechanism And Model Of Fluid Particle Breakage In Turbulent Flow

Fluid particles(bubbles or droplets)breakage phenomenon in turbulent dispersion system(such as gas-liquid,liquid-liquid etc)usually determines the dispersion state,size distribution,phase interface of fluid particles in the flow field,so the fluid particle breakage has an important effect on mass transfer,heat transfer and reaction properties.Understanding the mechanism of fluid particle breakage and building the corresponding theoretical model can provide a significant theoretical basis for dispersed phase size distribution and phase interface distribution for multiphase reactor design,optimization and industrial scale-up.In this paper,based on analysing previous breakage frequency models,the unreasonable assumptions used in the previous model are pointed out,i.e.,"fluid particles size always falls in the inertial subrange" and "only the fluid particles of sizes less than or equal to the size of the turbulent eddies can cause the breakage "(combination of the two assumptions,it is equivalent to consider that only turbulence eddies in inertial subrange contribute to the breakage).In addition,previous breakage frequency models are mostly based on the method of gas molecule reaction rate.Due to the influence of random turbulence and random of eddy's life-time,the collision between turbulent eddies and fluid particles is more complicated than the collision between gas molecules.Because the previous models directly adopted the gas molecular collision,it cannot reflect the influence of eddy's life-time on collision between turbulent eddies and fluid particles.Different from the previous models,the size distribution of turbulent eddies which contribute to fluid particle breakage has been extended to the whole turbulence spectrum(i.e.,including the energy-containing subrange,inertial subrange and dissipation subrange).Based on the entire spectrum distribution function and considering the lifetime of turbulent eddies,this work deduced the new model by constructing the collision probability model of eddies and fluid particles,and introducing critical breakage velocity.With the introduction of relative velocity,collision angle,collision free path,collision parameters reflecting the physical collision process between turbulent eddies and particles have been considered.This paper obtained the collision frequency which can lead to fluid particles of breakage and the corresponding breakage frequency model,daughter size distribution model.The breakage frequency model and daughter size distribution in this paper show that fluid particle breakage frequency increases with the increase of turbulent kinetic energy dissipation rate.But with the increase of fluid particle size,breakage frequency increases firstly and then decreases.The higher turbulent kinetic energy dissipation rate,the more the energy of the turbulent eddies is transferred to the fluidparticle,and thus the larger the particle breakage frequency.In addition,the turbulent kinetic energy dissipation rate is low,meanwhile,the equal-sized breakage probability is the highest.However,when the turbulent kinetic energy dissipation rate increases,the probability of unequal-sized breakage increases,the droplet daughter size distribution curve becomes flat.The smaller the surface tension is,the easier breakage of fluid particles,and the larger the particle breakage frequency is,the larger the probability of unequal-sized breakage.The predicted results of the model agree well with the experimental data and cumulative size distribution in liquid-liquid stirred tank.