Study On The Behaviors Of The Mass Transfer, Breakup And Coalescence Of Fluid Particles In Multihphase Flows Reactors

The phenomena of multiphase flows in industry reactors have often been encountered where the accompanying behaviors of mass transfer, breakup and coalescence of fluid particles (droplets or bubbles) play a crucial role in determining the mixing and dispersion of fluid particles. It is very important for simulation, design and optimization of reactors to establish reasonable mechanisms models of mass transfer, breakup and coalescence of fluid particles. However, the studies in this field are still a challenge and the corresponding theoretical works are needed urgently since the multiphase flows are strongly random, nonlinear and very complicated.Based on the previous work, this thesis focuses on the intrinsic mechanism of mass transfer, breakup and coalescence, and established the corresponding improved or novel models through applying the turbulence, probability statistics theories and computational fluid dynamics. This work may provide a systemic model framework for the simulation of multiphase flow reactors. The contents are mainly as follows.The discrete particle model has been used to simulate the gas-liquid flows in stirred tank reactors, the effects of the number of the tracked bubbles; surface contamination and turbulence correction for the drag coefficient have been tested. An improved model of turbulent dispersion has been proposed and the prediction ability for the bubble velocity and gas holdup has been improved. The processes of mass transfer, breakup and coalescence of fluid particles can be simulated by the coupling of the discrete particle model and the PBM.Based on the above model framework, the mechanism of mass transfer of fluid particles has been studied. A novel multi-scale model for gas-liquid mass transfer based on the wide spectrum eddy contact concept has been proposed. This model considered the contribution of the eddies of different scales to the overall mass transfer coefficient and can give a good agreement with the experimental data in various turbulence conditions. It is not sensitive to the liquid film depth and can provide a proof for the simplification of the model form.The mechanism of fluid particle breakup has been studied and the issues of the existing breakup criteria have been analyzed in depth. A novel breakup criterion of surface energy density increase for droplets and an improved breakup criterion for bubble has been have been proposed. Two improved breakup models have been proposed for droplets and bubbles respectively. These models show a good agreement with the reported experimental data. In addition, the daughter size distribution for the multi-breakage has been studied from the breakup physical process. A novel distribution function has been proposed and shows a good agreement with the experimental data. This work can provide a basis and idea for the multi-breakage issue.The processes of film drainage and approach of two fluid particles have been studied. The models for parallel and curved fluid film have been developed. Based on the model of parallel fluid film, the improved approach model has been coupled with the fluid film drainage rate model. Using the criterion of critical coalescence velocity, the systemic initial interior kinetic energy of two fluid particles has been related with turbulent eddy kinetic energy, a novel model of coalescence rate has been proposed. This model does not use the exponential correlation of the coalescence time and the contact time and reflects effect of turbulent fluctuation on the coalescence.