Experiment Study Using Laser Induced Fluorescence Technique For Quantitative Visualization Of Concentration Distribution In Interfacial Convection

The difference of solute concentration between interface and bulk can lead to gradient of physical properties near the interface during gas-liquid mass transfer process,and induce interfacial convection.The interfacial convection driven by density gradient is called Rayleigh Convection;the one driven by surface tension gradient is called Marangoni Convection.The occurrence of interfacial convection would accelerate the renewal of liquid near interface and enhance the mass transfer greatly.So the study of interfacial convection has important significance for enhancement of unit operation such as absorption and distillation.In this article,a laser induced fluorescence measurement system and a gas-liquid mass transfer apparatus were established.By using fluorescein sodium as fluorescence agent,we investigated the interfacial convection phenomenon in water-CO₂ absorption process under point-contact or plane-contact condition.Meanwhile,the influence for absorption of third-component-addition（ethanol and propanol）was also obtained.The results of experiment indicate that water-CO₂ absorption process can be divided into molecular diffusion period and convective diffusion period.In the beginning of absorption,the change of gravity accompanied by density variation cannot overcome the viscous force due to low CO₂ absorption amount,so interface remains stable.When density gradient reaches the critical value,Rayleigh Convection occurs,and plume structure is observed.Also the penetration rate of CO₂ increases.Considering the addition of third-component in gas phase,surface tension gradient along horizontal direction appears due to the accumulation of ethanol or propanol in local region,thus leads to local Marangoni Convection.Because Rayleigh Convection appears in the same time,the structure observed in experiment is the coupling of two different convections.The concentration of ethanol and propanol in gas phase has much difference because of vapor pressure;this would result in the differences of local Marangoni intensity and the location of Rayleigh Convection.Measurement error was eliminated and the real-time CO₂ concentration distribution was obtained through calibration and mathematic process,and quantitative analysis was able to be carried out.The results reveal that Rayleigh convection accelerate the movement of liquid near interface,and absorption process deviates from penetration theory gradually and total amount of absorption/mass transfer flux/mass transfer coefficient increases,so Rayleigh convection has positive effect for mass transfer.As for the addition of ethanol in gas phase,we got opposite conclusion under different concentration.The total absorption amount firstly increases then decreases with the increase of ethanol in gas phase.Low concentration ethanol would induce local Marangoni effect and accelerate the appearance of interfacial convection,while high concentration ethanol would make density gradient lower then inhibit interfacial convection...