On The PIV/LIF Measurement And Analysis Of Interfacial Convection Phenomenon In Gas-liquid Mass Transfer Processes

In interphase mass transfer processes,concentration gradient near the interface may lead to gradient of density or surface tension,and thus induce instability of the interface,and natural convection may occur.The interfacial convection driven by density gradient is called Rayleigh effect and that driven by surface tension gradient is called Marangoni effect.Interfacial convection can greatly enhance the interfacial mass transfer,and thus the study on such phenomenon is of great importance for intensification of unit operations like absorption,distillation,reaction,etc.In this work,an experimental quantitative observation technique by simultaneous applications of the particle image velocimetry(PIV)and the laser induced fluorescence(LIF)was established to study the interfacial convection phenomenon.Liquid velocity fields and concentration distributions in mass transfer processes were obtained simultaneously.The evolution of interfacial convections and the extent of turbulence,the intensification of mass transfer due to such phenomenon were investigated quantitatively.By the experiments,new visual insight of the relationship between momentum transfer and mass transfer in interfacial convective mass transfer processes was achieved.The research indicated that mass transfer process under Rayleigh effect was a kind of convective mass transfer process driven by vortex flow near interface.In such process,vorticity had a dominant effect on the rate of mass transfer.A simple correlation between mass transfer coefficient and vorticity distribution was established through experimental data fitting.Such correlation can be used as an efficient tool to estimate mass transfer coefficient under Rayleigh effect by measuring velocity field.The measurement and analysis of the Rayleigh convective mass transfer showed that the irreversibility due to mass transfer and viscous dissipation increased greatly because of the onset of convection and,however,decreased with the decrease of the value of the Rayleigh number with less energy loss.The irreversibility of the process is mainly due to mass transfer.Non-equilibrium areas inside the fluid were mainly distributed near interface and on the edges of finger-like currents of high concentration.The effects of liquid bulk flow rate on convective mass transfer with the two interfacial convection phenomenon were then investigated respectively.Under slow bulk liquid flow,triangular currents of high concentration were shaped when the solute permeated deeper under the effect of Rayleigh convection.The convective mass transfer processes were shown to be controlled by Rayleigh effect rather than liquid bulk flow,therefore,there will be a negative correlation between Sherwood number and Reynolds number in this situation.Banded currents can be observed in convective mass transfer processes under slow bulk liquid flow with Marangoni effect near the interface.The processes were shown to be controlled by liquid bulk flow so there will be a positive correlation between Sherwood number and Reynolds number.