Evolution Behaviors Of A Drop In Ambient Liquid Under An Impact

A mixed flow of two immiscible liquids may form drops of one liquid in another.Deformation and breakup of drops occur when there is sufficiently severe interaction between the drop and ambient liquid,which is a common phenomenon in a variety of engineering practices.To study the behaviors of a single drop surrounded by liquid flow not only helps to grasp or predict the engineering phenomena,but is also an effective supplement to the mechanism of evolution of the drop.However,there are still many shortcomings in the research of drop evolution process in the existing liquid-liquid sys-tem due to the defects of the experimental devices,the not-in depth quantitative analysis,and the lack of systematic research.Therefore,in the present work,a combination of the experimental methods,theoretical analysis and numerical simulation is carried out to systematically investigate the evolution process of a drop in ambient liquid under an impact.The main work of this paper can be summarized as follows.The drop evolution process was experimentally studied using a free-falling drop tower facility which similar to the shock tube.With high-speed photography,four typi-cal drop evolution modes were captured,namely oscillatory mode,bag mode,hat mode and mushroom mode,sorting in order of evolution severity.The transition process be-tween above evolution modes was studied and the corresponding transition modes were obtained.Three dimensionless numbers,i.e.Weber(We)number,Ohnesorge(Oh)number and density ratio(?d/?a)were used to quantitatively analyze the influence of the experimental parameters(such as vertical fall height h,initial drop diameter do,density ratio ?d/?a and interface tension coefficient ?)on the drop evolution.Consid-ering the density characteristics of the liquid-liquid system,force analysis of the initial relative movement between the drop and ambient liquid was conducted and a theoreti-cal model based on momentum theorem was proposed to re-calculate the We number.Furthermore,the dependence of the drop evolution pattern on the dimensionless num-bers was established.It is found that,in the test range of present study(1<We<800,0.0018<Oh<0.005,1.129<?d/?a<1.563),the evolution pattern is barely affected by the Oh number,which is mainly determined by the We number;with the increase of Oh number,the maximum We number of corresponding conditions shows a decrease trend.Secondly,the drop evolution process in ambient liquid under an impact was studied based on an experimental method combined with computational fluid dynamics(CFD).A numerical analysis was carried out for the flow field analysis of the five types of drop evolution modes before the breakup of liquid film,for which the mechanism that dom-inate the drop evolution process were also discussed.It is found that the drop evolution is strongly affected by vortex strength,namely the greater the vortex strength is,the more severe the evolution mode will be,during which an annular jet will be generated and continues to turn over;on the contrary,there is no annular jet formation or even drop deformation recovery phenomenon occurs.Based on the Front-tracking interface tracking mothod,a grid tracer method is proposed to capture the trajectory of the gen-eration process of the annular jet inside the drop.The results show that the formation of the annular jet is caused by the continuous accumulation of fluid inside the drop to near the equator,while the vortex is the dominant factor in the drop gathering near the equator and forming the annular jet.After a further examination on the generation and development of the annular jet inside the drop,it is found that there is a competition mechanism between vortex evolution and interfacial constrain,in which deformation of the interface is promoted by vortex evolution,while interface constrain plays an in-hibitory role.The competitive mechanism has been further revealed by the numerical results of dropt deformation under the conditions with/without interfacial tension.Finally,the key factors of drop evolution were revealed mainly by numerical sim-ulation.The similarity of drop deformation morphology shows that there is a certain equivalence between the vertical fall height h,the initial drop diameter do,the density ratio ?d/?a,and the interfacial tension coefficient ?.Correspondingly,adjusting these experimental parameters can obtain similar drop evolution patterns.Furthermore,nu-merical results show that under a wide range of initial diameter do,interfacial tension coefficient ? and vertical fall height h,the similar drop deformation patterns can still be obtained.The influence of viscosity(Oh number)and experimental condition(impact time)on the drop evolution were numerically studied.It is found that,under the condi-tions of relatively high viscosity(especially Oh>0.1),the severity of drop deformation showed a significant ease trend,and the We number needed for the same evolution mode shows a significant increase trend.Compared with the ideal condition of relatively short impact time,there is no significant change in the drop evolution process under the im-pact time of 6ms.Therefore,with the verification,the experimental condition of our experiments(impact time of?5ms)is acceptable.