Numerical Simulation And Experimental Study On Heat And Mass Transfer Of Microchannel Gas-liqud Two-phase Flow Based On Diffuse Interface Method

Two-phase flow in microchannel is a common phenomenon in reactor thermohydraulics,industry and engineering applications.Because of the existence of interface,two-phase flow has complex flow phenomena.The reducing of the channel size to micron results in a high specific surface area,which is different from the conventional scale two-phase flow and it also improves the heat transfer efficiency by phase transformation at the wall.Because of its wide application prospect and uncertain mechanism,microchannel two-phase flow has attracted more and more attention.In recent years,with the development of numerical simulation and advanced micro-measurement technology,there have been better research methods for microfluidity problem,and more flow phenomena and capillary heat transfer mechanism have been discovered,providing an important basis for the improvement of the theory.In order to study the flow characteristics and heat and mass transfer characteristics of micro-channel two-phase flow and improve the theoretical model,numerical simulation and experimental study were carried out for the formation and fracture of dispersed phase of twophase flow and evaporation at capillary interface.This paper can be divided into four aspects:(1)The diffuse interface equation is derived from the thermodynamic equilibrium point of view,and the dynamic contact Angle model suitable for the diffuse interface equation is developed.(2)Based on the diffuse interface method,the flow characteristics of two-phase flow in typical microchannel structures are simulated.(3)The diffuse interface method was improved by adding the heat and mass transfer source term caused by the phase transition,and the evaporation process and influencing factors of the capillary bending liquid level were studied.(4)The evaporation process of the capillary bend is visualized by using a high-speed camera and a particle velocity imaging system.The details are as follows:In this paper,based on the diffusion interface theory of van der Waals,the diffuse interface equation was derived from thermodynamic equilibrium.The equilibrium equation of two-phase flow was obtained by describing the Gibbs free energy and the mixed energy of the flow field by the mean field approximation.Based on the equilibrium equation,the steady-state phase interface was reconstructed,and then the transient diffuse interface equation was derived according to the influence of the concentration gradient of the local mixture on the mass,momentum and energy balance equations.Then a dynamic contact Angle model suitable for the diffuse interface equation was established,and the diffuse interface method and the contact Angle model are verified by a capillary rise base example.Then,based on the diffuse interface method,the formation process of dispersed phase in microchannel two-phase flow was simulated,and the morphological changes and motion characteristics of dispersed phase were studied.The accuracy of the model was verified by comparison with the experiment.The influence of different flow rates,flow ratios and main channel entrance structures on its formation process were revealed.The influence of surface tension and viscosity on the formation of polydispersal phase was analyzed,and the influence of inlet structure on the bubble size was simulated.Then the fracture process of the formed dispersed phase droplet at the T-shaped microchannel was simulated,three kinds of droplet fracture modes are obtained,and the stable fracture process of the blocked droplet was selected for study.By analyzing the sensitivity of capillary number and contact Angle,the mathematical model of the relationship between the critical droplet length and the Ca number and contact Angle of the critical droplet was presented,which had a good agreement with experimental data.According to the principle of minimum surface energy at the interface,the critical neck width was studied theoretically and the formula of critical neck width is obtained.Then by adding heat and mass transfer source term and vapor diffusion reaction force into the phase interface equation,the continuity equation,the diffuse interface equation and the energy equation were modified,and the mass transfer model of the interface evaporation rate was established.The simulation results were in good agreement with the experimental results in chapter 6.The distribution characteristics of evaporation rate,flow field and temperature field at the interface were studied.The evaporation process at capillary interface with different contact angles and superheat as well as pipe diameter was numerically simulated,and the law affecting the evaporation rate at capillary interface was obtained.In the end,the visualization experiments are carried out to study the morphology change of the interface and the fluid movement around the interface during evaporation.The capillary evaporation at the bending liquid level under different pipe diameters and different heating power was observed with high-speed camera.The interfacial sag of capillary evaporation,the movement of contact line and the change of contact Angle were analyzed.The particle velocity imaging system was used to track the flow field near the bend and the central movement and vorticity variation of the vortex were studied.Based on the evaporation rate formula of static contact Angle capillary interface obtained by simulation,the variation trend of contact Angle obtained by experiment is used to supplement the evaporation rate formula of simulation.In this paper,the formation and fracture of microchannel dispersed phase and capillary evaporation at capillary interface were studied theoretically and experimentally.The research results can provide experimental data and theoretical reference for the optimization of microfluidic industry and capillary tube.