Flow And Coalescence Characteristics Of Bubbles/droplets In Expansion Microchannel

The highly monodisperse microbubbles/droplets in microchannels have attracted widespread attention in the fields of composite foams,atomic crystals,particle preparation and so on.In this paper,the flow and coalescence characteristics of bubbles and droplets in expansion microchannel were investigated by using a high-speed camera.The main contents are as follows:The flow and coalescence behavior of bubbles in the sudden expansion of microchannel were studied.Four stable flow patterns are observed: two-layer-bubble coalescence,hamburger-two-layer-bubble coalescence,hamburger coalescence,and hamburger flow without coalescence.With the increase of gas velocity,the flow pattern changes gradually from two-layer-bubble coalescence to hamburger-twolayer-bubble coalescence,hamburger coalescence and hamburger flow without coalescence.The liquid film draining time increases with the bubble length.The location of bubble coalescence is away from inlet with the increase of bubble length and bubble velocity,but move towards the inlet with the increase of liquid slug length.A prediction equation of bubble coalescence position was proposed,which had a good prediction performance.In order to accurately control the coalescence of microdroplets,the flow and coalescence behavior of droplets in the sudden expansion microchannel was studied.Six flow patterns were observed: one-layer droplet flow without coalescence,two-layer droplet flow without coalescence,three-layer droplet flow without coalescence,one-layer droplet flow with coalescence,two-layer droplet flow with coalescence and three-layer droplet flow with coalescence.The effects of the flow rates of dispersed and continuous phases,the viscosity of the dispersed phase and the width of the expansion structure on the coalescence efficiency of droplets were investigated.When the continuous phase flow rate is constant,the coalescence rate of droplets increases with the increase of the dispersed phase flow rate and the width of the expansion structure,and the decrease of the dispersed phase viscosity.The flow and coalescence characteristics of droplets in the T-shaped expansion microchannel were studied.Four types of coalescence were observed: sliding coalescence,colliding coalescence,squeezing coalescence,non-coalescence(including non-contact,sliding non-coalescence,squeezing non-coalescence).The influence of the two-phase flow rates and the viscosity of the dispersed phase on the coalescence type was investigated.It is found that the decrease of the flow rate of the dispersed phase,and the increase of the flow rate of the continuous phase and the viscosity of the dispersed phase could decrease the coalescence of droplets in expansion microchannel.In addition,the film drainage time for three coalescence flow patterns was investigated.With the increase of the dispersed phase flow rate and the decrease of the dispersed phase viscosity and continuous phase flow rate,the liquid film drainage time decreases accordingly.