Investigation Of Single Bubble Gas-liquid Mass Transfer In A Flow-circulating Chamber

Bubbles swarm widely exists in industrial applications,and it is common to quantify the mass transfer performance by correlating the volumetric gas-liquid mass transfer coefficient with different operational parameters.However,it is not easy to exhibit the gas-liquid interaction and mechanisms by simple correlations.Thus,it is significant to investigate the mechanism of single bubbles mass transfer in the liquid flow.However,the difficulties in designing experimental setups,controlling bubble size and motion,and analyzing data prohibit the research on single bubbles mass transfer in turbulent liquid flow.In this work,the mass transfer of single bubbles in Newtonian fluid,non-Newtonian fluid,and Newtonian fluid with surfactants was investigated,respectively.A flow-circulating chamber with rectangle cross-section,which can generate flow constraints and keep single bubbles circulating within the chamber,was used to investigate the mass transfer single bubbles.In order to obtain the mass transfer coefficient,the volume decrement of the bubbles in Newtonian fluid was photographed with high speed camera and analyzed with programme,while the flow field in the chamber was investigated with particle images velocimetry.Force balance analyses on bubbles were performed to calculate the bubble slip velocity.It was found that both slippage and turbulence contributed to gas-liquid mass transfer,and there is a more pronounced effect of turbulence on mass transfer than the slippage.The mass transfer increased with the increase of apparent liquid velocity in the chamber.Simulations were performed on a 4.0 mm bubble in the chamber.It was found that a region with strong mass transfer existed during both descending and ascending of bubble.Interface slippage happened due to the deflection of liquid flow around single bubbles.However,this region during bubble descending was larger than that during bubble ascending due to the turbulence.A combined model including two weighting coefficients on bubble rigidity and turbulent kinetic energy was proposed to predict the mass transfer and compare with the mass transfer coefficient kL from literature.Most of the predicted kL from the combined model vary within ± 25%in both quiescent and turbulent liquids.The surfactants,n-pentanol,n-hexanol,and n-heptanol,were used respectively in the liquid being the same as the previous fluid conditions.It was found that the mass transfer with the addition of fatty alcohols was enhanced before their concentration reached 1 ppm,and inhibited when their concentration was above 1 ppm.The dense configuration of the carbon chain of surfactants on the gas-liquid interface inhibited the diffusion of gas molecules from the interface to the liquid bulk.The inhibitation was also enhanced with the increase of the carbon chain length.In order to evaluate the relationship between the degree and surface coverage ratio of the absorption of surfactants on gas-liquid interface,an empirical modified model for the mass transfer with surfactants was proposed,by which both the deviations of the correlation and the combined model after modification vary within± 30%.Sodium polyacrylate(PAAs)aqueous solution was used as the non-Newtonian fluid,and particle images velocimetry method was used to investigate the flow field variation.It was found that the addition of PAAs would lead to an increase of liquid apparent viscosity,a decrease of liquid velocity and the deflection of velocity gradient.The circulation zone,the residence time,the turbulent kinetic energy around single bubbles,and mass transfer coefficient decreased as well,but the slip velocity of the bubbles was nearly the same as those in aqueous solutions.However,the mass transfer coefficient in pseudo-plastic fluids is lower than that in aqueous solutions given the same condition,but higher than that in Newtonian fluid with the similar apparent viscosity.Two models were used to predict the mass transfer in non-Newtonian fluid.It was shown that the model by Kishore et al.deviated from the results by 25%to 50%,whereas the model by Hughmark did by±30%.The gas-liquid mass transfer of single bubbles in turbulent flow was related to and varied with the degree of interface slippage and liquid turbulence.The mass transfer was dominated by turbulence in the turbulent Newtonian fluid;when surfactants existed in the fluid,the type and the concentration of surfactants could enhance or reduce the turbulence on the gas-liquid interface and thus the mass transfer.The mass transfer in turbulent pseudo-plastic fluids was prohibited and dominated by the interface slippage due to the increased liquid viscosity and following reduction of turbulent kinetic energy around single bubbles.Present investigation of single bubble mass transfer in various systems can contribute to the gas-liquid mass transfer of multiple bubbles and the design and the scale-up of the related gas-liquid reactors.