Experimental And Numerical Investigation On Bubble Bursting And Its Induced Droplet Release Processes

In the late phase of the severe accident of core meltdown,the decay heat,which is long-term but low heat density,will cause the reactor hit to boil.Bursting bubbles generated by boiling water will release a large quantity of radioactive substances in the form of jet droplets and film jet droplets.Therefore,research on bubble bursting and following droplets is one of important issue of nuclear safety analysis.For a quiet long time,the aerosol entrainment has been estimated by so called single-bubble generation model near the free surface.Nevertheless,results obtained by different scholars have great difference.Investigation on droplet generation process following bubble bursting is still lack.Therefore,research on bubble bursting process and jet droplet generation process are of great significance for improving the aerosol entrainment model and further understanding the release process of radioactive materials.In the study,experimental and numerical methods are used to investigate the process of single bubble burst and subsequent jet droplet generation.Initially,with the aid of high speed camera,the equivalent diameter and terminal velocity of single bubble rising in still water are measured,and it is verified the bubble has reached stable final state before its bursting.Subsequently,the high-speed camera is utilized to study bubble rupture process and following film droplet and jet droplet generation process.It demonstrate that the capillary wave after the bubble burst has an important effect on following jet droplet formation.It is found that the longer the bubble surface lifetime,the faster the velocity of ejected jet droplet generated by bubble bursting.And bubble surface lifetime not only affect the formation of film droplet,but also the formation of jet droplet indirectly.Finally,in view of the significance of the surface lifetime to bubble bursting droplet release process,research on bubble surface lifetime in distilled water and tap water is conducted through image processing methods.It is found that the surface lifetime of bubble floating on liquid surface could be divided into three different regimes,namely exponential decay region,transition region and rayleigh distribution region,which extends previous research results.Based on experimental determination of the equivalent bubble diameter,the VOF method is utilized to simulate bubble bursting process of four bubbles with different diameter in non-equilibrium state.It is found that VOF simulation could well simulate diameter and number of jet droplets following bubble bursting,and qualitatively describe the generation of bubble burst.With regard to medium sized bubble,numerical simulation using VOF could better predict collapse process,while it is difficult to predict fast-transmitting capillary waves.Numerical investigation on bubble burst dynamics with help of VOF method still needs further investigation.