Effect of the surface roughness and oxidation layer on the dynamic behavior of micrometric single water droplets impacting onto heated surfaces

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The present research study investigates the effects of surface roughness amplitude and surface oxide layer thickness on the dynamic behavior of micrometric single water droplets during collision with surfaces at high temperature. Stainless steel-grade 304 (SUS 304) surfaces of different amplitudes of surface roughness; Ra=0.04, 2, 4, 6, 8 and 10 μm, have been considered. Each heat transfer surface was heated up to different temperatures; 1108, 1158 and 1198K, to control the oxide layer thickness over the hot surface. An individual water droplet is ejected from a needle of the micro jet dispenser where the droplet's size and its velocity were controlled independently. The behavior of droplet during the collision with hot surface was observed with a high-speed camera. By analyzing the experimental results, the effects of the surface roughness amplitude, oxide layer thickness, droplet Weber number, and surface superheat on the hot solid–liquid contact time, and on the maximum droplet spread diameter were investigated. Empirical correlations have been deduced describing the relationship between the hydrodynamic characteristics of an individual droplet impinging on a heated surface and concealing the affecting parameters in such process. Also, the comparison between the current results and the results due to other investigators shows the effects of oxide layer thickness and surface roughness amplitude on the impact behavior of water droplet onto the heated surfaces.