Effects of surface roughness on water droplet impact history and heat transfer regimes

Abstract

Still and high speed photographic techniques and heat transfer measurements were used to study the impact of water droplets on heated surfaces with different roughness. The study encompassed droplet Weber numbers of 20, 60 and 220 and surface temperatures of 100–280°C, covering droplet stability and heat transfer regimes established previously by the authors for polished surfaces. Three different surface finishes, polished, particle blasted and rough sanded, with respective arithmetic average surface roughness values of 97, 970 and 2960 nm, were applied to the test surfaces. While the temperature corresponding to critical heat flux (CHF) was fairly independent of surface roughness, the Leidenfrost point (LFP) temperature was especially sensitive to surface finish. The parametric effects of Weber number and surface temperature were consolidated into droplet impact regime maps for each of the three surface finishes. Aside from depicting the commonly known boiling curve regimes of film, transition and nucleate boiling, and thin film evaporation, these maps illustrate the complex liquid-solid interactions which occur during the lifetime of the impacting droplet within each of the boiling curve regimes, thus serving as an effective reference for future modeling of droplet heat transfer. Copyright © 1996 Elsevier Science Ltd.