Experimental investigation of size effects in colliding droplet

Abstract

The present work focuses on the size effect of binary collisions of PVP droplets (viscosity 5.5 mPas). The aim was the identification of the boundaries between bouncing, coalescence, stretching and reflexive separation. These boundaries are necessary for numerical simulations of droplet collisions in spray drying processes. Therefore, droplet chains were generated by droplet generators with oscillating membrane and directed towards each other at different angles for producing binary collisions. In the experiments two droplet properties (i.e. droplet size and size ratio) were varied. Two synchronised high-speed cameras were used to observe the collision process and outcome perpendicularly and parallelly to the collision plane. The variation of the impact parameter B was performed by a frequency offset for one droplet generator. The relative velocity (i. e. 0.5 to 4.7 m/s) was set by changing the collision angle. Keywords droplet collision, droplet size ratio, modelling separation IntroductionSpray drying processes constitute a very complex multiphase system due to the different elementary processes involved, i.e. atomization, drying, collision of droplets and formation of agglomerates. After atomization, droplet collisions with the different possible collision outcomes result in a modification on the product particle size distribution. Typical results of a collision of two droplets may be bouncing, coalescence and separation and were plotted in the well-known collision maps (see Figure 1). In these maps the non-dimensional impact parameter B (see Eq. 1) is plotted against collision Weber number (see Eq. 2). However, the collision map depends on droplet parameters, i.e. liquid and size properties of the droplets. For Euler/Lagrange calculation of sprays it is necessary to know the pattern of the collision map without running experiments.