Secondary atomisation of drop impactions onto heated surfaces

The work addresses an experimental study aimed at characterizing the effect of the impact angle on secondary atomization, at different boiling regimes. The study considers impacts of water and isooctane droplets, onto a heated stainless steel surface, with known roughness. Sizing of the larger droplets, within the range of 40 m up to a few millimeters is carried out by post processing of the images recorded with two CCD cameras (Image Analysis Technique – IAT), while the diameter of small droplets, within the range of 5.5 m up to 250 m, is evaluated based on phase Doppler anemometry measurements. The PDA measurements are integrated in time up to the impact instant t0, before comparison with the IAT probability distribution. Finally, a scaling of the IAT p.d.f. and of the integrated PDA p.d.f. is performed by equating the count values where the two size ranges overlap, obtaining an extended p.d.f., which is used to evaluate secondary droplet size diameters, over a wide range of droplet diameters, from 5.5 m up to a few millimeters. The results show that the measured secondary droplet diameter is faintly influenced by large impact angles (>45º) but becomes considerably smaller for small inclination angles (<15º), as the larger droplets are swept away due to gravitational forces. The effect of the gravitational forces in decreasing the measured mean droplet diameter is less significant for isooctane drop impacts, as the mean secondary diameters are globally smaller, when compared to those obtained from water drop impacts. Differences in secondary droplet size diameters between isooctane and water droplets are attributed to the much smaller surface tension of isooctane.