Investigation On Bubble Behaviors And Breakup Mechanisms In Small-scale Venturi Bubble Generators

Venturi-type bubble generator is a bubble generation system with high efficiency,high reliability and low power consumption.With strong liquid turbulence caused by its convergent-divergent structure,large bubbles are broken into a great number of fine bubbles.It produces smaller bubbles with more uniform distribution in size compared with other bubble generation systems with similar bubble generation principles.Previous work lacks of research with regard to flow field structure and interaction between gas and liquid,restricting the applications of Venturi bubble generator.A futher investigation is carried out on the bubble transportation and breakup processes in the small-scale venturi channel,aming to clarify the mechaisms of bubble breakup in the vortex region in the divergent section.The research provides references to the development of small-scale bubble generators and to clarification of the principle of gas-liquid interaction in the vortex flow.The main work and progress of this paper are as follows:Firstly,a visualized experimental system was designed and fabricated to record the bubble transportation process in Venturi channels.Three small venturi bubble generators with different throat sizes(W_th=1 mm,2 mm and 4 mm)were fabricated with high-transparent plexiglass.A visualized work was then carried out.With the high-speed video and subsequent processed image data,parameters of kinematics and morphologics of bubbles in the Venturi channels at different gas-liquid flow rates were obtained,including the velocity,surface area,volume,diameter,etc.,and the visulization data of bubble breakup as well.Based on the processed data of digital images,the performance of the Venturi bubble generator was studied with respect to the effects of throat size,liquid flow rate,void fraction and bubble size.The bubble size can be effectively reduced by increasing the liquid flow rate or decreasing the throat size;increasing the gas quality results in increasing the bubble diameter,especially for the case of viod fraction exceeding 10%,large bubbles may escape from from the vortex flow in the divergent section to survive from being broken up,which seriously weakens the performance of the Venturi bubble generator.By reducing the throat size,finer bubbles with uniform distribution in size were achieved,showing the advantages of small Venturi channels in fine bubble generation.The small-scale Venturi channels produces fine bubbles,with average diameter about half of that in the conventional Venturi channels.Therefore,the bubble size and distribution can be controlled to a certaion extent by changing the throat size.The bubble transport in small venturi channels is similar with that in conventional Venturi channels.When a bubble enters the divergent section,it will undergo severe deceleration,rotation and deformation,which are more violent.The magnitude of the acceleration of the bubble deceleration process reach about 1,000～2,000 times of the gravity.The rotation velocity is about 12,000～17,000 rad/s,much higher than that conventional Venturi channels.With reduction in throat size,particularly for the Venturi channel of W_th=1 mm,liquid jet from the throat is enhanced,leading to more intensified vortex flow.The jet flow splits large bubbles from their wake regions,on the other hand,the vortex induced by the jet move slowly and therefore hinders the movement of the bubbles with the mainstream,which results in a more rapid bubble deceleration,prolonging their residence time in the diverging section.The bubbles can obtain much more surface energy and it will be more violent for them once broken up occurs to them.Based on the visual invistigation,breakup of individual bubbles in the divergent section are analyzed.Overall,for the small-scale Venturi channels,more violent processes of deceleration,deformation and rotation for bubbles contributes to surface energy accumulation with high efficiency,about several times higher than that in straight pipes.For the Venturi channels of W_th=2 mm and 4 mm,the bubble breakup is mainly affected by the separation vortex in the divergent section.The separation vortex strongly influences the process of bubble breakup,as is the result of the shearing arising from put-off by the vortex.Meanwhile,bubble breakup induced by surface oscillation is also observed in the separation vortex.While for the Venturi channel of W_th=1 mm,besides the separation vortex,liquid jet also plays a key role in bubble breakup.It greatly enhances the vortex intensity in the divergent setion,which greatly prolongs the relative residence time of bubbles,leading to successive bubble breakup,and probability of multiple breakup is consequently significantly increased.Finally,the open source CFD software of Open FOAM is used to simulate the bubble transportation and breakup processes.It is showed that the numerical work well simulated the bubble deceleration and deformation in the divergent section,as is well for bubble breakup form deformation,splitting and collapse.Further analysis of the numerical results shows that the first large separated vortex in the divergent setion plays a vital role in bubble breakup,and the recirculation flow near the wall has a dominant effect on the bubble deceleration.The shear stress on the surface of a bubble under surface oscillation in the separated vortex is more than twice of that under deformation during deceleration.