Studying the impacts of droplets depositing from the gas core onto a gas-sheared liquid film with stereoscopic BBLIF technique.

Abstract

In annular gas-liquid flow, the droplets are torn from the liquid film surface by the turbulent gas stream, accelerate due to the gas drag force and eventually deposit back onto the wavy film surface, hitting it with a high speed at a shallow angle. Here we investigate individual impacts of droplets in annular flow in a horizontal rectangular duct, in a wide range of gas and liquid flow rates for three different working liquids. A stereoscopic modification of the Brightness-Based Laser-Induced Fluorescence technique is developed to reconstruct the spatiotemporal evolution of waves on the film surface in three-dimensional space, simultaneously with capturing instantaneous position, size, and three velocity components of the individual droplets and bubbles. Droplet impacts create furrows (with bubbles entrapment) and craters (with secondary droplets) on film surface. Propagation of the furrowing droplet may be facilitated by gliding on an air layer but hampered by the ripples on the film surface. The number of entrapped bubbles is mainly defined by the droplet diameter and by liquid properties. The saturation of the number of bubbles entrapped due to furrow impacts is confirmed experimentally. The secondary droplets are rarely created due to break-up of the crater crown. In most cases, the main role in secondary entrainment is played by longitudinal movement of the impacting droplet, which helps the droplet to escape the impact-created crater and detach from the film, creating various transient liquid structures, with different ways to create secondary droplets.