| Multiphase flow widely exists in nature,industrial production and engineering applications.However,due to the complex phase boundary movement of two-phase flow and the occurrence of phase change heat and mass transfer in a single component two-phase flow,which makes it always a complex scientific problem.In recent years,with the development of computational technology and advanced measurement methods,it also provides a better experimental method for the study of two-phase flow.Therefore,the two-phase flow problem has attracted more and more attention,and has rapidly developed into an important interdisciplinary subject of fluid mechanics and heat transfer.In order to study the motion and heat and mass transfer characteristics of two-phase flow,it is necessary to study the two-phase flow more carefully by means of numerical simulation and visualization experiments.Therefore,numerical simulation and visualization experiments are used to study the two-phase flow.It can be divided into three main aspects:(1)Based on the diffuse interface method,numerical simulation of bubble rising in still water is carried out.The feasibility of the application of diffuse interface method in two-phase flow is verified,and the computational efficiency of different size bubble under different meshing modes is compared.(2)Based on the diffuse interface method,the growth process and influencing factors of a single bubble in nuclear boiling were studied by introducing the source term of heat and mass transfer caused by phase change.(3)Visualization experiments of bubble growth on hydrophobic surface were carried out by high-speed camera.Firstly,the basic idea and control equation of diffuse interface method are introduced,and the existing two-phase interface capture methods are compared.The change of physical quantity of diffuse interface method in the treatment of interface is smoother The description and calculation method of interface are consistent with the description of interface in molecular chemistry.It has more reasonable physical meaning,can deal with the effect of surface tension on the interface and avoid the calculation error caused by the physical singularity on the wall,showing its unique advantages.Then,the bubble rising process in still water is calculated based on the diffuse interface method.The comparison between the calculated results and the experimental literature shows that the diffuse interface method can simulate the velocity and morphological change of the bubble in the bubble rising process.Furthermore,the rising characteristics of large bubble in still water and the influence of narrow channels on the rising process of bubble are further studied.Subsequently,in order to improve the computational efficiency of two-phase flow on the premise of ensuring the accuracy of calculation,the rising process of bubble in different sizes in still water were calculated by using the Arbitrary Lagrange Euler(ALE)and the Adaptive Mesh Refinement(AMR),respectively.The results and calculation efficiency of two different grids were compared.The results show that the ALE method has a faster calculation efficiency when the bubble is small.However,for the large bubble,the AMR method can better reflect the deformation of bubble and has a higher calculation efficiency.The bubble growth process in the nucleate boiling is further numerically calculated after verifying the diffuse interface method can accurately calculate the motion of the phase interface.The continuity equation,phase field control equation and energy equation are modified by adding heat and mass transfer source term to the phase interface.The growth process of bubble at different contact angles were simulated.The change of bubble shape,detachment radius,detachment time,contact line motion and temperature distribution on heating wall during the growth process were calculated and analyzed.The results show that the detachment radius and growth time of bubble increase with the increase of contact angle under wetting conditions.The central temperature of bubble is the highest and the temperature of the three-phase contact position of vapor,liquid and solid is the lowest in the wall temperature distribution.However,the shape of the bubble is obviously different if the wall is no-wettability.The upper part of the bubble was linked with the vapor attached to the wall through the neck.When the neck is thinned and broken,the bubble rise upward from the wall,but some of the vapor still attached to the wall.The temperature distribution on the heating wall is similar to that on the wetting surface,but higher at the center of the bubble.In order to further explore the growth mechanism of the single bubble on the heating wall under nuclear boiling,a microlayer heat and mass transfer model between the bubble and the heating wall was added to the bubble base.By establishing one-dimensional partial differential equation,the heat and mass transfer model of microlayer in the bubble growth process is introduced into numerical calculation.The motion characteristics of the microlayer and its influence on the growth process of the bubble and the temperature distribution on the wall were studied.The results show that the coverage area of microlayer increases with the increase of base line of bubble during the bubble growth process.But at the same time the central area becomes dry region due to steam evaporate.Thus,the region of the microlayer increases first and then decreases during the bubble growth process.In the final stage of the bubble growth,the bottom of the bubble is completely transformed into dry region.The temperature distribution at the bottom of the bubble is annular due to the influence of the microlayer.Finally,a pool-type nucleate boiling test bench was built,and the process of bubble growth,detachment and rise on hydrophobic surface was visualized by temperature control system and particle velocity imaging system.The growth shape,growth period and detachment radius of bubble were studied under different heating power and supercooling The fluorescent particles are used to disappear the motion of the surrounding fluid during the growth of the bubble.The results are of great significance to further study the flow and heat transfer characteristics of bubble growth under the nucleate boiling. |
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