13th Int. Symp. on Appl. Laser Techniques to Fluid Mechanics, Lisbon, Portugal, June 26-29, 2006 Considerations in Phase-Doppler Measurements of Spray/Wall Interaction Davood Kalantari 1 , Cameron Tropea 2 1: Institute of Fluid Mechanics and Aerodynamics, TU-Darmstadt, d.kalantari@sla.tu-darmstadt.de 2: Chair of Fluid Mechanics and Aerodynamics, TU-Darmstadt, ctropea@sla.tu-darmstadt.de Petersenstr. 30, 64287 Darmstadt, Germany Abstract In this work, an experimental study of spray impact onto a horizontal flat and rigid surface is presented to specify the suitable measurement volume height above the surface in which the Phase-Doppler measurements are valid. Furthermore the influence of the input laser power on the characteristics of the impinging and secondary spray is studied in detail. The phase Doppler technique has then been used to characterise both the impacting and the secondary spray in terms of number flux, size distribution and velocities of the droplets above the target. A high-speed CCD camera has been used to characterize the splashing or rebounding droplets and to measure the average film thickness formed due to spray impact. In this paper some new considerations in Phase-Doppler measurements of spray-wall interaction are presented. Keywords: spray; spray impact; secondary spray; Phase-Doppler; interaction. 1. Introduction The interest in a reliable modelling of spray/wall interaction is widespread, focussing on the prediction of the deposited mass fraction and the characteristics of the secondary spray. Important applications include fuel injection and spray cooling. For formulation and verification of suitable models, experiments under well-controlled conditions are essential. For precisely capturing the secondary spray, physical and morphological characteristics of the spray impact phenomena, including different sources for generation of the secondary spray, should be clearly understood and considered when employing a Phase-Doppler measurement of a spray/wall interaction. The spray impact onto a rigid wall consists of many individual droplest impacting onto the rigid wall or liquid film and different interactions. In spray impact phenomena, secondary droplets are generated from: splashing droplets, ejected wall films, and rebounded droplets from the wall (Fig. 1). Interactions in a spray take place between: two droplets (two in-going drops, in-going and ejecting drops or two secondary droplets); an uprising jet and a drop; and a splashing droplet and other droplet (ingoing or ejecting droplet). Impacting droplets are deposited on the wall at very low impact energy (i.e., 2 < nb We ), generating the wall film. Partial deposition occurs also at higher impact Weber numbers, i.e. 80 30 < < nb We , when the spreading droplet loses its kinetic energy due to dissipation at the boundary layer of the spreading droplet. At relative low energetic impact conditions, impacting droplets can rebound from the wall. A droplet rebounds from the wall if the surface energy of the droplet at the end of partial spreading is larger than the kinetic and surface energy of the impacting droplet minus the viscous energy dissipation during the spreading. Threshold criteria is given by Bai and Gosman (1995) for “Rebound-deposition” as 5 nb We = ( σ ρ / 2 d u We nb = , where u is the normal velocity component before impact) based on the result of an isolated single drop impact, or 20 nb We = for spray impact conditions based on the observation of Lee and Hanratty (1988) and Ching et al. (1984). Also the observations of Wang and Watkins (1993) show that rebound occurs only for 30 nb We < .