Investigation On Gas-Liquid Interfacial Mass Transfer Behaviors In Confined Space

The process of gas-liquid interfacial mass transfer in confined space shows some differences from the process without confinement due to the space limitation of liquid flow and gas-liquid interface.Investigation on the mechanism of gas-liquid interfacial mass transfer in confined space is helpful for the application of this kind of equipment and deeper understanding of the interfacial mass transfer phenomenon.The process of gas-liquid interfacial mass transfer in confined space is roughly divided into two categories according to the liquid movement,i.e.the natural convection with vortex flow and the bubble rising with shear flow.The mass transfer behavior of above two processes were investigated based on fluid confined in thin gap.As to the first kind of process,the investigation was made by means of experiment and three-dimensional numerical simulation for the absorption of CO₂ in stagnant liquid.A coupled method between the particle image velocimetry(PIV)and the laser induced fluorescence(LIF)techniques were applied to simutaniously measure the liquid flow and the dissolved CO₂ concentration distributions.The experimental results showed that the liquid flow and mass transfer have strong coupling effect in form of Rayleigh convection,and the effect can greatly promote the gas-liquid interfacial mass transfer.In the three-dimensional numerical simulation,a nonequilibrium condition at the gas-liquid interface produced better results that are closer to the experimental results in comparison with a phase equilibrium boundary condition.It was revealed from the numerical results that some‘hollow regions'with low CO₂ concentration were presented in the middle part of the convective fingers in gap direction.The“hollow regions”were acturally induced by the fast replenishment of fresh liquid between downward convection fingers,which improve the gas-liquid interfacial mass transfer rate by reducing the thickness of mass boundary layers inside and outside of the convective fingers.The mass transfer rate of gas-liquid interface in stable convective period showed strong dependence on Rayleigh number.To investigate the second kind of process,an UV-induced fluorescence(UIF)technique for quantitative measurement of dissolved CO₂ was proposed for the movement of a single bubble and the interphase mass transfer from the bubble to surrounding liquid.Based on the technique,the characteristics of bubble movement and gas-liquid mass transfer for pure liquid and the liquid containing surfactant were investigated.For single bubbles in liquid without surfactant,we found that the aspect ratios of the bubbles in all gaps show three different periods as the bubble rising.The aspect ratio of single bubbles having same volume in different gaps was controlled by the gap width and the liquid hydrodynamics.For liquid containing surfactant that fully contaminate a bubble,the confined bubble in general had three types of shapes,i.e.,circle,ellipse,and oblate ellipse as the bubble size increases.The surfactant on bubble surface obviously decreased the terminal velocity of the fully contaminated bubble than that of bubble without surfactant.The decrease may be resulted from two aspects on the bubble influenced by the surfactant,i.e.,shear force between bubble and liquid films and drag force including Marangoni effect around the bubble exerted by free moving liquid in gap.The shear force gradually dominates the overall resistance forces with the increase of bubble size.The interphase mass transfer from confined bubble to surrounding liquid comes from two regions,i.e.,a region named liquid film region between the bubble and two liquid films formed by confinement of solid walls and a region named free-flowing region between the bubble and the surrounding free-moving liquid.The UIF technique can characterize the dissolved CO₂ mass and the mass transfer coefficient according to the fluorescence intensity change of liquid under the effect of CO₂.It was found from the experiment that,for a confined bubble without surfactant,the liquid-side mass transfer coefficient from the bubble having same volume to liquid increases as the gap width decreases.The increase of mass transfer coefficient shows that the interphase mass transfer for bubble in confined space has superior to the process in unconfined space by increasing the bubble's specific area and the mass transfer coefficient.The gas-liquid mass transfer flux from two regions were calculated by assuming both fluxes from corresponding region keep constant for mass transfer occurred in a gap,and the results showed that both fluxes increase as the gap width decreases.The increase in the liquid film region results from the decrease of thickness of mass boundary layer and the increase of mass trasnsfer area.The increase in the free-flowing region comes from the decrease of thickness of mass boundary layer and the reduce of attatched fluorencer on bubber surface.For bubbles that was fully contaminated by surfactant,the interphase mass transfer rates from two regions decrease compared with that in regions of bubble without surfactant.The decrease in the liquid film region is due to the attatched surfactant on bubble surface and the increase of thickness of mass boundary layer.The decrease in the free-flowing region is on account of some factors including the contamination of surfactant,increase of thickness of mass boundary layer,and the interface convection around a rising bubble.