Numerical Simulation Of Continuous Formation And Coalescence Of Bubbles From Double-nozzle

The collision,coalescence and interaction between bubbles exist widely in gas-liquid contact equipments like bubble column.These phenomena could remarkably change bubble size,gas-liquid contact aera and bubble residence time in the liquid phase,and thereby influence the heat and mass transfer efficiency.?n this work,the process of continuous bubble formation from double nozzles in Newtonian and non-Newtonian fluids were investigated with numerical simulation method.As be well known,the interaction mechanism between adjacent continuous bubbles is the basis and a prerequisite for modeling the mass and heat transfer of bubble cluster,this study could provide a lot of valuable information for the design and optimization of bubble column equipments.The formation process and subsequent evolution behaviors of continuous bubbles from submerged double-nozzle in Newtonian and power-law non-Newtonian fluids were numerically simulated by 2D-VOF method.The power-law and continuous surface force models were added to the VOF model to describe rheological properties and surface tension of non-Newtonian fluid.The grid independence check method was adopted to obtain the most suitable mesh interval,the effectiveness and accuracy of the simulation method were validated by experiment.Three different bubble flow patterns and two flow regime transition lines were obtained.There exist two critical nozzle intervals,i.e.,the critical nozzle interval L_c1 for bubble coalescence before pinch off and the critical nozzle interval L_c2 of bubble formation alternative then in-line coalescence under different conditions.When the nozzle interval L<L_c1,the bubble flow pattern is horizontal coalescence?type ??;When L_c1<L<L_c2,the bubble flow pattern is alternative formation and in-line longitudinal coalescence?type ???;When L>L_c2,the bubble flow pattern is rising side by side and non-coalescence?type ??.The coalescence time of continuous bubbles in Newtonian fluids was analyzed with Ca and BoL to gain the judgement rule of bubble flow patterns.Furthermore,the bubble flow patterns in non-Newtonian fluids were analyzed systematically by means of the dimensionless numbers Re and Bo,the empirical correlations for predicting dimensionless nozzle interval between bubble flow patterns were established by date fitting.The effects of nozzle interval and diameter,gas flow rate,viscosity of Newtonian fluid and rheological property of non-Newtonian fluid on continuous bubbles dynamics were studied.?t could be found from simulation that the bubble flow patterns depend mainly on bubble size and gas flow rate,the nozzle diameter could obviosly affect bubble formation size.Gas flow rate is closely related to the generation frequency of bubbles and liquid phase disturbance.The viscosity of Newtonian fluid could influence moving velocity of fluid in liquid film near the gas-fluid interface and accordingly change the bubble formation time and detachment volumn.The rheological property of non-Newtonian fluid could affect in-line interaction of bubbles.Under the same condition,the either increase in the nozzle diameter,gas flow rate,viscosity of Newtonian fluid and flow index of the non-Newtonian fluid could promote the bubble flow pattern evolution from type ?? to type ? or from type ? to type ??.For type ?? bubble flow pattern,increase nozzle diameter,gas flow rate or decrease viscosity of Newtonian fluid and flow index of the non-Newtonian fluid respectively,the in-line coalescence height of bubbles would decrease.