Experimental studies on binary water droplet collisions considering size ratio effects

Abstract

The results of binary droplet collisions are usually bouncing, stretching/reflexing separation as well as coalescence and are usually summarized in so-called collision maps, where the impact parameter B is plotted versus the Weber number. The outcome of droplet collisions depends on the fluid properties, droplet size ratio, the kinematics, and kinetic conditions. Such collision maps are used to delineate the different outcome scenarios by appropriate boundary lines that can be applied, for example, in numerical simulations. The present work focuses on the study of binary droplet collisions with defined size ratios and water of different quality (tap water and distilled water). Although water has been studied by many researchers, only droplets of the same size have been considered. However, size ratio effects have a great influence on the collision outcomes associated with a shift of the boundary lines normally towards higher Weber number [1]. For example in a spray, collisions of small droplets with larger ones are more likely to occur due to their different response behaviour [2]. Generally, binary droplet collision experiments were conducted by using two interacting droplet chains. But for obtaining experimentally also very small size ratio below 0.3, an additional experimental arrangement was selected here. A droplet chain was injected into a spray. The spray droplets typically have a size distribution ranging up to about 200 µm and the droplet generator produces droplet sizes between 600 and 700 µm. With these novel experiments it is now possible to extend and validate the boundary lines also including size ratio effects.