Proceedings of the 15th International Heat Transfer Conference, IHTC-15 August 10-15, 2014, Kyoto, Japan IHTC15-9823 *Corresponding Author: marco.marengo@unibg.it 1 EXPERIMENTAL ANALYSIS OF HIGH WEBER NUMBER DROP IMPACT ONTO SUPER-HYDROPHOBIC AND HYDROPHOBIC SURFACES F. Villa 1 , C. Antonini 1,2 , I.V. Roisman 2 , M. Marengo 1,4 1 Department of Engineering, University of Bergamo, viale Marconi 5, I-24044 Dalmine (BG), Italy. 2 Laboratory of Thermodynamics in Emerging Technologies, Mechanical and Process Engineering Department, ETH Zurich, 8092 Zürich, Switzerland 3 Technische Universität Darmstadt, Alarich-Weiss-Straße 10, 64287 Darmstadt, Germany 4 School of Computing, Engineering and Mathematics, University of Brighton, Brighton BN2 4GJ, UK ABSTRACT The present work is focused on the water drop impact at high impact Weber numbers (up to We=1100) on surfaces with a good (hydrophobic, 100°<θ r <120°) or extremely high (super-hydrophobic, θ r >135° and Δθ<10°) water repellency. The low wettability has a potential of being an effective parameter in the heat transfer mechanism, especially in cases of two-phase flow heat transfer, and to prevent adhesion of dirt. An Air Flow Accelerated Drop generator is used to investigate the phenomena at high impact Weber numbers. The impacts are recorded to evaluate the outcome of the impact and to study various characteristics of the drop-wall interaction. Drop impacts are studied to evaluate the effect of higher impact velocity on rebound time. It is found that the impact velocity does not have an influence on rebound time. The average rebound time at different impact velocities for the lowest drop diameter (D = 0.98 mm) ranges from 2 ms to 4 ms, as a function of the tested surfaces. Then the effect of different drop diameters (D = 0.98-1.78 mm), at fixed impact velocity, is studied. The rebound time increases when the diameter of the impacting drops increases. The tested super- hydrophobic surfaces (SHS) do not show any upper limit of rebound in the investigated range (up to We = 1100), i.e. the rebound is still occurring at the highest impact velocities, while for the hydrophobic surfaces an upper velocity limit exists, but only in a probabilistic manner, i.e. at a given velocity only for a percentage of the impacts a rebound occurs. KEYWORDS: Computational methods, Heat exchanger, Cooling turbine blade, Film cooling, High temperature, Nano / Micro, Heat transfer enhancement. 1. INTRODUCTION The fluid dynamics of a water drop impact onto a solid surface is of significant importance in a variety of industrial applications including combustible fuel injection in engines, surface cooling by liquid sprays, ink- jet printing and atomization of liquids. The reviews of drop impact phenomenon by [1] provide salient details associated with the impact process. The drop impact phenomenon comprises of several sub-processes dominantly identified as spreading, receding, splashing, and rebound [2]. An exhaustive classification of the possible outcomes resulting from a drop impacting onto a dry solid surface was carried out by [3]. Water drop impact studies are needed to investigate the mechanisms of drop rebound, to identify which are the controlling parameters for rebound (in terms of both surface wettability properties and fluid dynamics parameters). Wettability is an essential property of solid materials, which is determined by the surface geometry (the surface roughness) and the surface chemistry [4]. The surface roughness plays an important role in determining the wetting behaviour of solid surfaces. Early studies on the role of surface roughness on the drop impact process were restricted to the description of the relationship between the surface roughness and the outcome of the impact. A recent investigation of drop impacts onto dry solid surfaces under negligible ambient pressures [5] shows that surface irregularities are responsible for the droplet splashing