Experimental And Model Studies On The Mechanism Of Bubble Coalescence

Gas-liquid two-phase flow widely exists in petrochemical,food processing,environmental protection,bio-pharmaceutical and other fields.The coalescence and breakup behavior of bubbles usually determines the size distribution and dispersion of the dispersed phase in the flow field,so it has an important influence on the heat transfer,mass transfer and reaction performance of the whole system.Because the coalescence process has strong randomness,multi-scale and nonlinear characteristics,it is very complicated.How to deeply study the coalescence mechanism of bubbles and establish a reasonable theoretical model has always been the fundamental and difficult problem in the field of international multiphase flow.By analyzing the previous work,this paper holds that the lack of experimental data of coalescence(coalescence efficiency,critical collision velocity of coalescence,etc.)makes the existing coalescence model unable to be effectively verified,which is one of the main reasons for the relatively sluggish research on practical application of the model and coalescence mechanism.For this purpose,a set of experimental apparatus is designed which can be used to measure the relative collision velocity and coalescence efficiency of two bubbles.By recording the collision process between bubbles rising freely and stationary bubbles,the effect of relative collision velocity on the collision outcome(bounce or coalescence)was studied,and the experimental data of coalescence efficiency of bubbles of various sizes in pure water system was obtained.In terms of model research,this paper considers that the process of close contact between bubbles and the process of liquid film effusion and thinning are indivisible as a whole.In this process,the relative velocity of bubble,liquid film thickness and interphase force all change with time.Therefore,a new coalescence efficiency model based on the parallel film hypothesis is proposed.Because the model is a whole process of coupling the approach contact of two bubbles and the thinning of liquid film drainage,it is no longer necessary to adopt the decomposition and synthesis method like previous models.Previous models usually decompose the coalescence process into two non-interfering drainage thinning process and proximity process to study respectively and obtain the drainage time and contact time independent of each other.In addition,it is not necessary to assume that the ratio of contact time to drainage time obeying exponential distribution(this assumes that actually lack of sufficient theoretical basis).The convergence efficiency model established in this paper predicted the evolution trend consistent with the experimental data,but due to the use of the parallel film hypothesis,the predicted value of the model was somewhat different from the experimental data.Based on the experimental observation,this paper further analyzes that the shape of the liquid film formed by the two bubbles during their approach is closer to the surface,so it is necessary to modify the shape of the liquid film from the plane to the surface.In this paper,a curved film model is established and its stability method is explored.In terms of solving program and numerical stability,the curved surface film model is more complex than the parallel film model.The coalescence time predicted by the curved film model was close to the measured value,and the coalescence efficiency predicted by the model was better than that of the flat film model.These studies are of guiding significance for the construction of more perfect and reasonable model of coalescence mechanism in the future.