Numerical Simulation On The Interaction,Coalescence And Breakup Behaviors Of Multiple Bubbles Rising In Non-Newtonian Fluids

In typical gas-liquid contact equipment such as bubble column andbiochemical reactor, the interaction, coalescence and breakup between bubblescould remarkably change bubble size distribution and gas-liquid contact area,and accordingly change the heat/mass transfer performances and chemicalreaction rate of equipment. So far, the mechanisms of bubble interaction andcoalescence in non-Newtonian fluids remains still far from clear. In this study,the interaction and coalescence dynamics of multiple bubbles with variousarrangements rising in non-Newtonian fluids were numerical investigated toobtain some useful basic information for the design and optimization ofgas-liquid contact equipment.Both2-D and3-D VOF (Volume of fluid) methods were employed to studycoalescence and freely rising processes of multiple bubbles with variousarrangements. The power-law model and the continuous surface force model(CSF) were adopted to represent rheological property and surface tension ofnon-Newtonian fluid, respectively. The optimal mesh interval size was selectedby mesh independence check, and the accuracy and reliability of simulationmethods were previously validated by experiment.2-D VOF method was applied to study the coalescence and rising behaviorsof two, three and four parallel bubbles. The effects of initial bubble diameter,bubble interval and flow index of non-Newtonian fluid on lateral coalescenceand breakup processes were investigated. The complex bubble coalescence andbreakup behaviors were analyzed by means of dimensionless Reynolds numberand E tv s number. The dimensionless critical horizontal intervals of bubblecoalescence for two, three, and four bubbles with different diameters wereattained respectively by simulation under different flow indexes of power-lawfluid, which is the maximum initial bubble interval for coalescence, and thecritical bubble interval decreases with the increase of bubble diameter and flowindex of non-Newtonian fluid. When the initial bubble interval exceeds thecritical values, the effects of initial bubble diameter, bubble interval and flowindex of non-Newtonian fluid on the freely rising process were investigated systematically, it was found that the rising trajectory and variation of bubbleinterval depended on the flow structure of bubbles with different shapes.3-D VOF method was applied to study the coalescence and rising behaviorsof multiple bubbles with three different arrangements: equilateral triangle,rhombus and square. The influences of various factors on multiple bubblescoalescence and breakup were analyzed from the perspectives of the filmdrainage, relative approach velocity and energy conversion combining with theflow field distribution during coalescence process, and the mechanisms ofbubble coalescence and breakup were fully discussed. The effects of bubble size,flow index and bubble arrangements on critical bubble interval for coalescencewere studied. Based on the critical bubble interval for coalescence, thehorizontal approach velocity and vertical rising velocity for coalescence wereobtained. The horizontal approach velocity was a function of bubble diameterand rheological property of the fluid, and it was found that there was a suddendrop in the rising velocity of bubble in vertical direction during the coalescenceprocess. The effects of bubble arrangements, bubble size, bubble interval andflow index of the fluid on the rising velocities of multiple bubbles were alsoinvestigated systematically. A quantitative criterion was developed to predict thecritical bubble interval for bubble coalescence with various bubble arrangementsbased on film drainage model combining with approach velocity duringcoalescence process, and the prediction accords well with the simulation result.The prediction was useful for the design and optimation of poriferous gas-liquidcontact equipment.