Experimental Study On The Rollover Process Of Concentrated Stratified Liquid

The instability of stratified liquids is widespread in many fields,such as circulation distribution and heat transfer in the ocean,metal alloy solidification process,crystal growth processes,and internal rollover of liquefied gas storage tanks.Its essence is the double-diffusion natural convection of liquid under the action of heat/component buoyancy.Studying and revealing its evolution mechanism can effectively prevent the damage caused by instability in industrial production.Now,the influence of parameters such as the initial buoyancy ratio on the stratified system is mostly concentrated on the simulation study.In this paper,the small-scale stratification experiment was constructed to study the instability evolution process of the stratified system.By changing the experimental conditions such as the wall heat flux and the initial mass fraction difference between the layers,the instability flow pattern changes during the experiment,the temperature change and pressure change were analyzed,and the migration of the interface during the instability process,the energy accumulation of the lower layer solution and the pressure step law were observed.Furthermore,the important role of the initial buoyancy ratio R? of dimensionless parameters in the instability of the stratified system was proposed.The main conclusions are as follows:(1)A visual experimental platform is constructed to construct a layered system with different concentrations of NaCl solution.A heating sheet is installed on the wall of the visualization container to simulate the leakage of the external environment,and the heat flux is regulated by the AC transformer.Temperature and pressure data are collected with thermocouples and pressure sensors,and the flow pattern changes are captured by the camera.(2)The heat flux at the bottom is kept constant,and the heat flux of the sidewall and the initial mass fraction difference between the layers are changed.The variation of the turbulent flow pattern transition,the stratified interface migration and the stability of the stratified system under different initial conditions are analyzed.The results show that it is affected by the leakage of external heat and the difference of interlayer quality.The increase of the difference in mass fraction between layers enhances the stability of the stratified system.Under the influence of external leakage,the floating force destroies the stability of the stratified system.Under different heat fluxes and mass fractions of different sidewalls,the stability of the stratified system is different.When the instability occurs,the mixing zone appears in different positions,forming different unstable flow forms and affecting the migration velocity of the interface.(3)The temperature variation characteristics of the stratified system and the variation of the accumulation energy of the lower layer are studied by analyzing the temperature data of the solution under different heat fluxes and interlayer mass fractions.The results show that the layered interface blocks the energy transfer between the upper and lower layers of the solution,and the energy of the lower layer solution cannot be released after being heated,accumulating in the solution.When the instability occurs,the two layers of the solution are mixed,and the energy of the lower layer is released in a large amount.As the initial mass fraction difference between the layers increases,the accumulation energy of the lower liquid at the time of penetration occurs.As the heat flux of the sidewall increases,the accumulation energy of the lower liquid decreases when the penetration occurs.(4)By increasing the heat leakage rate of the lower layer solution,increasing the energy accumulated in the lower layer solution,changing the difference in interlayer mass fraction and the initial vacuum inside the container,the pressure step phenomenon in the instability process of the stratified system under different initial conditions was studied.The results show that when the stratification system is unstable,the energy of the lower layer solution is released,the liquid evaporates violently,and the pressure rise rate of the gas phase space in the vessel rises rapidly and reaches a peak,and a pressure step occurs.When the difference between the initial mass fractions of the interlayer is large,the pressure step phenomenon is more severe;the non-condensable gas component in the gas phase space increases,and the pressure step strength increases when the instability occurs.