Numerical And Experimental Studies Of Flow And Heat Transfer Characteristic Of Gas-liquid Two-phase Flows In Mini Tubes

Gas-liquid two-phase flow plays an important role in the design of nuclear reactors,petrochemical industry,air conditioning and other industrial applications.In recent years,with the rapid development of science and technology,industrial equipment tends to be more and more miniaturized.Compared with the conventional channel,the specific surface area of the small channel is greatly increased,and the heat transfer efficiency is higher,so it is more suitable for the integrated equipment.As the pipe diameter decreases,the surface tension gradually dominates,and the viscous force,the shape of the channel section,the wetting characteristics of the wall and the roughness of the channel also have important effects on the hydrodynamic characteristics of the gas-liquid two-phase flow.Therefore,the gas-liquid two-phase flow characteristics in conventional pipelines cannot be fully applied to small pipelines.At present,the research of gas-liquid two-phase flow in small channels is still in the development stage.In this paper,the flow characteristics and heat transfer characteristics in small channels are studied by numerical simulation and experiment.In order to study the flow characteristics in a micro tube,a virtual binocular vision system with air as the dispersed phase and water as the continuous phase was experimentally and numerically simulated in a small horizontal channel.In the experiment,a prism was used to capture images synchronously from two orthogonal views,and then the three-dimensional gas-liquid interface captured in the tube was reconstructed.The numerical results are in good agreement with the experimental data in terms of flow pattern,section and volume gas holdup.The results show that three flow patterns(bubble flow,slug flow and annular flow)are observed,and the change of flow pattern is mainly due to the increase of bubble size and interface fluctuation.The probability density function diagram(PDF)for intermittent flow(bubble and slug)has two peaks,while annular flow has only one peak.When the gas mass is low,the volumetric gas holdup is in good agreement with the homogeneous flow model.With the increase of the gas flow rate,the velocity difference between the two phases increases significantly,resulting in the gas holdup lower than the predicted value of the homogeneous flow model.Taylor flow is a widely used flow state in most liquid-liquid and gas-liquid microchannels due to its unique flow characteristics and periodic flow properties.In this paper,the heat transfer characteristics of Taylor flow in a horizontal circular tube are investigated numerically.According to the volume and formation frequency of bubbles in the experiment,user-defined function(UDF)is used to patch the volume of bubbles.In this paper,the variation of wall shear stress,pressure distribution,velocity field and local Nusselt number are discussed in detail,and the effect of gas content on heat transfer under constant wall heat flux boundary condition is studied.The results show that the wall shear stress in the liquid phase remains basically unchanged,while the wall shear stress in the tail and near the head of the bubble increases sharply.The internal circulation of the liquid bomb can increase the heat transfer coefficient,but the average Nussel number decreases with the increase of the gas content,because of the decrease of the liquid phase.According to the pressure distribution characteristics,a structural unit is divided into four regions(liquid phase region,head region,tail region and bubble region),and the presence of air significantly increases the pressure gradient.Compared with the circular tube of the same circumference,the flat tube has a higher ratio of heat transfer area to flow cross-sectional area,and has the advantage of high heat transfer efficiency.This type of strengthened tube has a wide range of industrial applications.In this paper,the heat transfer characteristics of two-phase flow in a flat tube are numerically simulated.The circumferential distribution of liquid film thickness and local heat transfer coefficient,as well as the distribution of temperature field,pressure field and velocity vector are studied.The simulation results are compared with those of a circular tube with the same circumference.The results show that the bubbles in the flat pipe tend to move in the upper part of the pipe,and there is a large surface tension at the corner of the circular arc of the channel,so there will be liquid film to gather there.According to the distribution characteristics of the liquid film,the whole half circle can be divided into four parts,namely,the upper wall surface,the upper corner,the lower corner and the lower wall surface.The local heat transfer coefficient of the upper wall is larger than that of the lower wall.The flow inside the bubble has obvious vortices,and the vortices are concentrated near the corners of the flat tube.