Simulation Study On Breakup And Coalescence Of Bubbles In Microchannel With Non-newtonian Fluids

Recently,with the development of microfluidic Lab-on-a-chip,the precise control within the microchannel bubbles(droplets)by microfluidic technology has received extensive attention from researchers.Compared with the Volume of Fluid method(VOF),coupled Level Set and Volume of Fluid method(CLSVOF)can more accurately capture the gas-liquid interface.In this work,CLSVOF method was used to simulate the bubble breakup and coalescence in microchannel.The results can provide theoretical basis for the design of microchannel reactor and the accurate operation of bubble flow.We first carried out a mesh independence check and compared with experimental results to verify the validity of the numerical methods.And the aqueous solution of sodium carboxymethylcellulose(CMC),which is widely used in industry,is selected as non-Newtonian fluid.Then we simulated the behavior of bubble breakup and coalescence in the fluid within the microchannel.First of all,we studied bubble breakup behavior in four parallel microchannels.The simulation results showed that,with the increase of ug/ul,the sub-bubbles in turns to present long slug flow?slug flow?bubble flow and no blocking bubble flow four kinds of flow pattern after the bubble breakup;Decreasing the width of middle two daughter microchannels,the bubble flow pattern changed from slug flow to bubble flow,and the flow pattern for both sides of the two daughter microchannels changed from slug flow to long slug flow;Increasing the bifurcation angle of the two middle daughter microchannels,the flow pattern of the two middle daughter microchannels varied from long slug flow to slug flow while that of two sides ones varied from slug flow to long slug flow;And there would be a tendency of long slug flow to slug flow when increasing the concentration in liquid phase.In addition,when ug/ul ? 1,bubbles breakup uniformly;When increasing the value of ug/ul the bubbles in the two middle daughter microchannels would be much bigger;With the middle two daughter microchannels' width decreasing,the sub-bubbles significantly reduced,while the sub-bubbles on both sides of the two daughter microchannels increased faster;Increasing the bifurcation angle,the sub-bubble in the middle daughter microchannels reduced,while the both sides ones increased;With the increase of the concentration of liquid phase,the bubble breakup would be more uniformly.Next,we did a predicted calculation on bubble coalescence behavior in the partial expanded microchannel.The simulation results showed that: the probability of bubble coalescence increased when raising the expansion section?liquid flow velocity?bubble sizes and liquid concentration,but decreased with the increase of bubble interval;The bubble coalescence time increased when raising the expansion section?liquid velocity and bubble interval,but decreased with the increase of liquid concentration and bubble sizes.In addition,the results of two unequal size bubbles coalescence showed that: coalescence behavior alwaysappeared when "small bubble chasing the big one" while for the opposite situation of "big bubble chasing small one",it could not coalesce and tended to reject each other.