DROPLET-WALL COLLISIONS - EXPERIMENTAL STUDIES OF THE DEFORMATION ANDBREAKUP PROCESS

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