15th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 05-08 July, 2010 - 1 - Scaling the effects of surface topography in the secondary atomization resulting from droplet/wall interactions Ana S. Moita 1 , António L. N. Moreira 1 1: Laboratory of Thermofluids, Combustion and Energy Systems at IN+, Technical University of Lisbon-Instituto Superior Técnico, Lisbon, Portugal, anamoita@dem.ist.utl.pt 2: Laboratory of Thermofluids, Combustion and Energy Systems at IN+, Technical University of Lisbon-Instituto Superior Técnico, Lisbon, Portugal, moreira@dem.ist.utl.pt Abstract The present paper addresses the scaling of the effects of surface topography in the size of the secondary droplets generated by thermal induced atomization of single droplets impinging onto heated solid surfaces. The size and velocity of the secondary droplets is characterized making use of simultaneous image analysis and phase Doppler measurements to evaluate extended size distributions from 5.5µm up to a few millimetres. This procedure assured a consistent comparison of the results reported here with those previously reported in the literature, in similar experimental conditions. Although the study covers the various heat transfer regimes, particular attention is put on the phenomena occurring within the nucleate boiling regime, up to the critical heat flux temperature of the working fluids, as this is the upper boundary for the safe working conditions of many practical systems of interest, in the context of droplet/spray cooling. The analysis focus on the physical description of the phenomena occurring at the liquid-solid interface to explain the triggering of the secondary atomization and leads to an empirical relation between the mean size of the secondary droplets and the dimensionless topographical parameter R a /λ R , which quantifies the relative magnitude of the high of the rough grooves with the distance between them. This relation seems to describe well the results that were uncorrelated in previous work, thus explaining the large discrepancies coming from experiments performed under apparently similar conditions, which are mainly introduced by surface topography. 1. Introduction Secondary atomization at droplet/wall interactions has been addressed by several authors for impacts onto cold (e.g. Stow and Steiner, 1977, Mundo et al., 1995, Gavaises et al., 1996, Van der Wall et al. 2006) and heated targets (e.g. Naber and Farrel, 1993, Cossali et al., 2005, Akhtar and Yule, 2001, 2007a,b). For impacts onto cold surfaces, droplet disintegration and particularly the prompt splash, (i.e. the breakup occurring immediately after impact, as defined in Moita and Moreira, 2007 and more recently revisited in Pan et al., 2010) is usually associated with the critical impact conditions (often the critical impact velocity) at which the inertial forces overcome surface tension and viscous forces: 2 0 2 0 / D h U L lv σ ρ > , where ρ and σ lv stand for the liquid specific mass and surface tension, respectively, while D 0 and U 0 are the diameter and impact velocity of the primary droplets. h L is the thickness of the lamella, which for an inertially dominated phenomenon is defined by the thickness of the viscous boundary layer, corresponding to the Stokes’ first problem. Following Yarin and Weiss (1995) and the scaling performed as in Pasandideh-Fard et al. (1996), h L ~ (νD 0 /U 0 ) 1/2 =D 0 Re -1/2 , where ν is the liquid kinematic viscosity. For impacts onto heated surfaces different atomization mechanisms occur, at much smaller impact velocities and within a larger temporal scale, which are induced by thermal effects. The heat transfer phenomena lead to substantial morphological modifications of the spreading lamella, as a consequence of the temperature dependence of liquid properties and, ultimately, the disintegration occurs by disruption of the lamella by vapour pressure forces during liquid phase change. This is the so-called thermal induced atomization, which has been characterized by few authors such as Richter et al. (2005), Moreira et al., (2007), Cossali et al., (2008) and Muller et al. (2008). The characterization of the secondary atomization resulting from droplet/wall interactions is frequently