Study Of Bubble Dynamic Characteristics Based On Lattice Boltzmann Method

There are exist widely in nuclear reactor systems of boiling heat transfer and gas-liquid two-phase flow.Boiling heat transfer is based on bubble dynamics when pool boiling occurs in reactor systems.The dynamic behavior of bubbles,including the generation,growth,separation,rise,deformation,splitting,collision,coalescence and burst process,is the basics for studying the boiling heat transfer of gas-liquid two-phase flow.The small droplets carried by the bubble burst at the surface of the liquid pool affect the leakage of radioactive aerosol and the efficiency of the steam generator steam separator.Therefore,it is necessary to study the bubble dynamics of gas-liquid two-phase flow.Firstly,the free surface model is introduced into the lattice Boltzmann method to develop the free surface lattice Boltzmann method,which is suitable for the study of bubble dynamics of two-phase flow.The numerical examples of square cavity flow and droplet deformation are given to verify the results.Then,the numerical analysis model of bubble’s rising,deforming,colliding and merging behavior in the flow field is established,the shape and velocity change process of a single bubble’s rising process is analyzed,and the shape,velocity field distribution,rising velocity and other characteristic parameters of multiple bubbles in the coaxial and non-axial merging process are studied.The shape change process of the rising bubble under different Eotvos numbers and Morton numbers are obtained,and the Reynolds number changes with time during the rising bubble are obtained.The final shape and the final Reynolds number of the rising bubble under different Eotvos numbers and Morton numbers are compared with the experimental results,and the simulation results are in good agreement with the experimental results.The influence of the number and relative position of bubbles on the shape and velocity field of bubbles in the merging process is analyzed.Under the influence of the wake of leading bubbles,the trailing bubbles accelerate to the leading bubbles and merge.Finally,a numerical analysis model of the process of bubble rupture at the free liquid level is established to simulate the process of bubble rupture at the free liquid level.The effects of bubble size,fluid properties and the interaction of multiple bubbles on membrane droplets produced by bubble rupture were studied.The process of bubble breaking at free liquid level and producing film droplets is closely related to bubble size and fluid physical parameters.The rate of rupture and the number and distribution of droplets were different under different Eotvos numbers and Morton Numbers.The fluctuation of the liquid level caused by the rupture of the upper bubble can reduce the severity of the rupture of the lower bubble.In the case of large number of Eotvos numbers and Morton numbers,decreasing the distance between adjacent bubbles will advance the bubble breaking time.For the small number of Eotvos numbers and Morton numbers,the adjacent bubble spacing has almost no effect on bubble rupture.Based on the free surface model and lattice Boltzmann method,the two-phase bubble dynamics program FSLBM under adiabatic condition is developed.It can be used to study the process of bubble rising,deformation,coalescence and bubble breaking at free liquid level in flow field.In this paper,the kinetic behavior of bubbles under different flow conditions under adiabatic conditions is obtained.The related research results can provide basic support for the study of two-phase flow heat transfer mechanism and safe operation of reactor.