Mechanism of flash boiling bubble breakup based on rim-like structure

Abstract

Flash boiling is an important phenomenon in internal multiphase flow and could lead to unique spray structures. Many researchers have investigated this phenomenon using optical diagnostic methods, and progress has been made in spray morphology studies as well as bubble dynamics inside the nozzle. However, the detailed transition mechanism from in-nozzle bubble to out-nozzle spray droplets has not been experimentally observed, and present theories could not elaborate the morphology change in the micro-scale droplets with high velocities. To illustrate the transition procedure from bubble to droplets of flash boiling bubbles, high-speed imaging technique is adopted on a specially designed optical nozzle with a two-dimensional ambient cavity to investigate the transition from in-nozzle flash boiling bubbles to out-nozzle spray droplets. A unique rim-like structure has been observed, and quantitative and qualitative analyses are made to figure out the nature of this rim. Based on the study, the driving force, as well as the transition process of flash boiling bubbles, has been demonstrated in detail, it is found that under high superheated degrees, the bubble burst process is governed by joint effort of thermal phase change and mechanical deformation. Based on the observed phenomena and theoretical analysis, a new flash boiling spray breakup theory that coincides with the actual physical process has been established.