DROPLET-WALL COLLISIONS - EXPERIMENTAL STUDIES OF THE DEFORMATION AND BREAKUP PROCESS

Experimental studies of liquid spray droplets impinging on a flat surface have been performed with the aim of formulating an empirical model describing the deposition and the splashing process. Monodisperse droplets with a known viscosity and surface tension, produced by a vibrating orifice generator, were directed towards a rotating disc and the impingement was visualized using an illumination synchronized with the droplet frequency. A rubber lip was used on the rotating disk to remove any Film from previous depositions. The test matrix involved different initial droplet diameters (60 < d(0) < 150 mu m), velocities (12 < w < 18 m/s), impingement angles (4 degrees < alpha < 65 degrees), viscosities (1.0 < mu < 2.9 mPas) and surface tensions (22 < sigma < 72 mN/m). The liquids used to establish the different viscosities and surface tensions were ethanol, water and a mixture of water-sucrose-ethanol. One major result from the visualization is a correlation of the deposition-splashing boundary in terms of Reynolds number and Ohnesorge number. Noteworthy is that a distinct correlation between the Re and Oh number, K = Oh . Re-125, is only achieved if the normal velocity component of the impinging droplets is used in these dimensionless numbers. For the case of a splashing droplet, a two-component phase Doppler anemometer was used to characterize the size and velocity of the secondary droplets. The obtained droplet size distributions and correlations between droplet size and velocity around the point of impingement constitute the basis of an empirical numerical model.