Study On The Multiphase Flows And Interfacial Phenomena At Multiscale

Multiphase flows and interfacial phenomena are widely involved in the natural world,our daily life,and numerous industrial processes such as chemical engineering and petroleum engineering.The fundamental researches on the multiphase flows and interfacial phenomena could deepen the understanding on the internal mechanisms and provide theoretical guidance for the practical applications.By employing three different techniques including a high-speed camera,an ultra-high-speed Direct Current(DC)electrical device,and a high-speed micro-Particle Image Velocimetry(micro-PIV),the multiphase flows and interfacial phenomena at multiscale were investigated experimentally by both passive and active methods.The multiscale indicates the length scale from micrometer to millimeter and the time scale from microsecond to second.The main contents are as follows:The flow behaviors of the elastic non-Newtonian liquid-liquid two-phase flow in both T-junction and flow-focusing devices were investigated.The fluid elasticity affected the dynamics of droplet formation,stretching and breakup.During the droplet formation,the peculiar beads-on-string flow was observed and the flow pattern map as well as the transition lines were also plotted.The influences of both the fluid elasticity and the two-phase flow rates on the minimum neck width,the maximum length of the dispersed thread and the droplet size were studied.The predicting models for the droplet size were optimized by introducing the dimensionless parameters to characterize the fluid elasticity.For the stretching and breakup of elastic droplets,the influences of elasticity on the transition between various flow patterns,the dynamics of droplet stretching and breakup as well as the size ratio of the daughter droplets were also investigated.The power-law models were proposed to predict the maximum stretching length.Subsequently,the initial coalescence of a pendant drop at a liquid surface and the initial spreading on a solid surface were investigated.The ultra-high-speed DC electrical device with the sampling speed of 1.25×10~6 Hz allows to monitoring the dynamics of coalescence and spreading within 10?s.The coalescing width expands linearly with time in the inertially limited viscous regime and follows a power law in the inertial regime.The evolutions of the velocity fields during the initial coalescence and spreading were measured and computed by the high-speed micro-PIV with a capturing rate up to 5000 velocity fields per second,revealing the transformation of surface energy to kinetic energy.Besides,the consecutive electrical peaks with a regular interval were observed during the filament thinning of the polymer liquid neck.In addition,the active manipulation of the ferrofluid drop was realized by introducing an external magnetic field.Evident deformations of both the pendant ferrofluid drop and the bulk surface were observed prior to the contact even in the absence of a magnetic field.The dynamics of ferrofluid drop coalescence were investigated at various magnetic fields,where the power-law relationships were proposed to predict the increasing coalescing width with time.A high-speed micro-PIV technique was employed with a transparent model fluid to reveal the flow fields during the ferrofluid drop coalescence.The self-sustained coalescence-breakup cycles of ferrofluid drops were observed for the first time.The exponential models were proposed to predict the increasing periodic frequency and the decreasing maximum coalescing width with the applied magnetic field.