| Due to improper filling method,initial liquid stratification will be formed in liquefied natural gas(LNG)tank.Under the ambient heat leakage,the stratification was unstable due to natural convection driven by heat and composition diffusion.Stratification instability developed to rollover that liquid layers turned over and mixed in a short time.In LNG tank,rollover caused a large amount of boil-off gas,which will lead to serious accident.At present,a unified understanding of the mechanism of stratification instability has not yet formed,and the research on the influence of external heat leakage and concentration stratification conditions on rollover process is still a hot issue.Based on this background,experimental and numerical studies on the rollover evolution of stratified liquids driven by double diffusion were carried out.The main work and conclusions were as follows:(1)A visualized rollover experiment platform was built,and the rollover experiment under different initial vacuum degree and initial buoyancy ratio(R?i)with NaCl solution were performed.The mixing characteristics of the rollover process for stratified liquids with different initial buoyancy ratio were studied.The results show that the rollover process presented W-type mixing characteristics and Hourglass-type mixing characteristics with the increase of R?i,but the effect of non-condensable gas content on mixing characteristics was small.(2)A numerical model describing the rollover process was established.Based on the experimental verification,the mechanism of the stratification instability was studied by numerical simulation.The temperature,concentration and density distribution at lateral wall boundary layer and main body of the tank was analized in detail,and the local buoyancy ratio was used to study the variation of stratification stability.The results show that buoyant flow-panetration effect and horizontal flow-entraimnent effect were the dominant factors for stratification instability at boundary layer and main flow area respectively,and the two effects had a synergistic mechanism:buoyant flow was hindered by the interface to transform into horizontal convection,and horizontal convection continuously weakened stratification stability and then promoted buoyant flow to penetrate the interface.After penetration,the horizontal flow-entrainment effect basically disappeared,and the interface began to migrate downward entirely.(3)Combining experiment and numerical calculation,the rule of the change of rollover mode with R?i and its influence on pressure response of the storage tank were studied.The results show that the critical point for the transition of rollover mode with R?i was about 1.50.When R?i<1.50,rollover presented strong-circulation mode,and the buoyant flow effect was dominant,and the interface was penetrated in a short time,and then the stratified system presented W-type mixing characteristic after rollover.When 50,rollover changed to weak-circulation mode,and the horizontal flow effect gradually dominated,meanwhile rollover occured even later,and hourglass type mixing characteristic was presented after rollover,which will lead to more intense tank pressurization.(4)By numerical simulation,the effect of the transition of rollover mode on rollover characteristic time was studied,as well as the prediction method was proposed.The results show that when R?<1.50,the time of rollover onset and the time of rollover duration increased linearly with Rpi;When R?i>1.50,the time of rollover onset increased linearly with R?i and in contrast to former,the linear growth rate was increased significantly,but the time of rollover duration decreased linearly with the increase of R?i.In both rollover modes,the time of rollover end increased exponentially with R?i.By comparing the numerical simulation results of NaCl solution stratification system and LNG stratification system,it was found that the rollover prediction method based on the fonner is feasible for predicting the rolling process of the latter. |
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