Investigation On Flow Dynamics And Mass Transfer Of Film Flow Over Topology

In the process industry,such as distillation,absorption and heat exchange unit operation,film flow is widely used because of its high specific surface area and high mass transfer efficiency.The efficiency mass transfer of film flow depends on many factors,so it is very important to study the flow characteristics and mass transfer mechanism of film flow.The gas-liquid two-phase heat and mass transfer on the packing surface in structured packing tower is a typical film flow phenomenon.The general model of the film flow on the corrugated plate microstructure is extracted,which is fit for the film flow process in different equipment,and its flow characteristics and mass transfer performance are studied.This research has important theoretical significance and broad application value,which can guide the packing selection,optimization design of structure,and choosing operation parameters,such as film evaporator and structured packing tower.Among the film flow in different equipment and operation,the two-phase film flow over corrugated plate plays a decisive role in mass transfer,which is abstracted to study its mass transfer and flow characteristics in this research.The open source CFD software Open FOAM is used to compile the mass transfer solver and setup the simulation model.The influences of substrate topological structure parameters,operation parameters of mass transfer process,dynamic characteristics and stability of flim flow,fluid resonance phenomenon and vortex on mass transfer characteristics are considered comprehensively.Then the mass transfer mechanism of two-phase flow on topological structure surface is studied,and film fluid mass transfer characteristics and flow characteristics are obtained.The three-dimensional model can provide theoretical basis and design criteria for the structural parameters of packing and the mass transfer system design of two-phase flow.The main research contents and achievements of this thesis are as follows:1.The open source software Open FOAM is used to establish a three-dimensional model for simulating the mass transfer process of two-phase flow on a three-dimensional corrugated plate,in which the effects of gravity,surface tension and shear force between gas and liquid phases are considered.In order to study the accuracy of multiphase flow simulation model in Open FOAM,the simulation results are compared with the analytical solution,particle image velocimetry(PIV)results and the experimental data of mass transfer on the surface of corrugated plate to verify the accuracy of the simulation model and to realize the accurate prediction of mass transfer coefficient.2.Using the mass transfer simulation model,the mass transfer characteristics of film flow over two different scale triangle packing structures were studied.By analyzing the whole and local mass transfer phenomena,the effects of gas inlet velocity and liquid loading on mass transfer parameters are mainly researched.It is found that the small triangular corrugated structure can significantly improve the local mass transfer coefficient in liquid phase,while the large triangular corrugated structure can significantly improve the mass transfer efficiency of gas phase.In addition,when the inlet velocity of gas and liquid are the same or similar,the effective contact time of gas-liquid phase last longer,and the mass transfer efficiency is bigger.The larger the velocity difference of gas and liquid is,the larger the inlet liquid Reynold numbers corresponding to the maximum mass transfer coefficient is.The results show that the bigger the normal velocity of liquid phase is,the better the mass transfer performance of liquid phase is.Especially,the small vortices at the interface in gas phase play a significant role in improving the mass transfer coefficient.The bottom vortex in the liquid concentration boundary layer has little effect on the mass transfer efficiency.3.The relationship between the mass transfer characteristics and the dynamic characteristics of the film flow on the corrugated plate surface was studied innovatively.The results show that the horizontal component of velocity at the interface directly determines the surface renewal speed and the contact time of the two phases,while the normal component of the velocity in liquid phase is the driving cause to promote the mass transfer process.The resonance phenomenon of the large triangle corrugated structure is not obvious,while the resonance phenomenon of the small triangle corrugated structure is obvious.The resonance point does not coincide with the maximum mass transfer efficiency point,and the maximum mass transfer efficiency point is basically within the resonance section.4.The mass transfer performance and flow characteristics of three-dimensional corrugated plate and complex three-dimensional corrugated plate are studied innovatively in simulations and experiments.The effects of liquid viscosity,liquid inlet velocity and gas inlet velocity on mass transfer performance are analyzed.The mass transfer efficiency of plate,three-dimensional corrugated plate and complex three-dimensional corrugated plate are compared.The results show that,compared with the plate,the mass transfer efficiency of the three-dimensional corrugated structure can be increased by about 80%,and the efficiency of the complex three-dimensional corrugated structure can be increased by about 140%.A three-dimensional experimental system of film flow mass transfer process has been set up,which could verify the numerical simulation results.