Numerical Simulation Of Vapor-liquid Two-phase Flow And Condensation Heat Transfer In A Multi-channel Tube

With the continuous development of science and technology,the requirement of condensation heat transfer is also increasing.Facing the limitation of the traditional single-phase heat transfer mode,the research of multiphase flow is gradually deepening.In practical engineering,condensation in two-phase flow tube is a common way of heat transfer.It is of great significance to study the factors affecting condensation in tube to improve the efficiency of condensation heat transfer.At present,most of the researches on two-phase flow in horizontal tubes focus on the two-phase convective heat transfer in single channel tubes,while the researches on condensation heat transfer in multi-channel tubes are less.In this case,the vapor-liquid two-phase flow and condensation heat transfer in multi-channel tube are studied.In this thesis,CFD software is used to simulate the vapor-liquid two-phase flow and condensation heat transfer process in a horizontal multi-channel tube.The volume distribution nephogram of vapor-liquid two-phase in the multi-channel tube is obtained.The variation of heat transfer characteristics of two-phase in the tube with steam velocity,temperature,pipe inclination angle and other influencing factors,as well as the corresponding pressure field and temperature field are obtained.The specific research contents and conclusions are as follows:(1)In this thesis,a three-dimensional geometric model of multichannel tube is established.Workbench software is used to establish the model,divide the grid,set the boundary conditions,and capture the vapor-liquid interface by VOF method.Considering the heat and mass transfer process of the fluid,Lee model is selected as the evaporation condensation model,and the turbulence model is realizable K-? Model.(2)In this thesis,Lee model is used as the phase transition model and the appropriate phase transition factor is selected.As for the phase change factor(r value)of Lee model,the selection of different R values will have a great impact on the condensation heat transfer process,and the optimal R value under different working conditions is also different.The change of R value will affect the volume fraction of two phases and liquid supercooling degree,so it is necessary to analyze and select it.If R value is too small,the deviation between phase interface temperature and saturation temperature will be too large.If R value is too large,the calculation divergence will be difficult to converge.After reasonable and detailed analysis and comparison,the R value is set to 100 in this thesis.(3)In this thesis,the flow characteristics of fluid in multi-channel pipe are analyzed.The condensation flow pattern of steam in the multi-channel tube,the influence of steam velocity and wall temperature on the convection pattern were studied,and the pressure drop in the tube was analyzed.The results show that the main flow pattern of steam condensation in horizontal multi-channel pipe is stratified flow.When the steam inlet velocity is increased,the disturbance to the liquid film will increase,the interfacial shear force will increase,and the liquid film in the pipe will become thinner,but the flow pattern is stratified flow;Changing the wall temperature will not affect the flow pattern in the pipe;The change trend of dynamic pressure is consistent with that of velocity,which gradually decreases in the axial direction and also presents a stepped distribution in the radial direction.(4)This thesis analyzes the heat transfer characteristics of condensation in multi-channel tubes.The effects of steam velocity,inclination angle and superheat on the heat transfer characteristics in multi-channel tube were studied.The temperature field,pressure field,velocity field and integral number cloud diagram of vapor-liquid two-phase were analyzed.The results are as follows: the faster the inlet speed of steam,the higher the condensation heat transfer coefficient in the pipe,when the inlet velocity is increased to 4m/s and 8m/s under the simulated conditions,the condensation heat transfer coefficient in the pipe is increased by 10.8% and 21.4% respectively;When other conditions are constant,the larger the superheat of steam inlet is,the smaller the condensation heat transfer coefficient is;Increasing the inclination angle properly is beneficial to reduce the thickness of the liquid film and increase the heat transfer coefficient.However,if the inclination angle is too large,the distribution of the liquid film will be more uniform and the heat transfer coefficient will become smaller.In this thesis,the condensation heat transfer mechanism of vapor-liquid twophase flow in multi-channel tube is deeply studied to enhance the heat transfer performance.The research results can provide theoretical basis and reference for the design and performance optimization of new high efficiency condenser.