Abstract

Many studies have been devoted to particle flattening and resulting splat cooling. However, if recent models allow to compute the particle flattening time evolution, very few experiments in spraying conditions have been achieved to back such calculations. The aim of this paper is to describe an imaging device allowing the visualization of particle impacts on cold and hot surfaces. This technique makes it possible : • to investigate the "impact mode" : splashing, deposition or rebound, • to link the particle parameters at impact and the substrate parameters to the observed impact mode, • and therefore, to have a better understanding of coating formation. It consists in a controlled atmosphere chamber where is followed the impact of a single particle on a substrate which can be inclined. The particle parameters prior to its impact are measured : its surface temperature by fast (100 ns) two-color pyrometry, its velocity and diameter by Phase Doppler Anenometry (PDA). The particle image during flattening, splashing or rebounding is given by a fast camera (exposure/delay time 100ns to 1ms) with possible multi exposures. The camera is triggered by the PDA and/or the pyrometer. It is then possible to calculate for each molten particle its Sommerfeld parameter characterizing its impact mode (rebounding, deposition or splashing) when no solidification occurs during flattening. The substrate are made of stainless steel 304L rapidly covered by alumina splats resulting in a Ra~5-6µm. They are kept at 300°C, temperature at which splats are disk shaped on smooth substrate (Ra<0.05µm). The very preliminary results obtained show that unmolten or partially molten particles rebound in all directions but not elastically : the rebounding particle velocity is 3 to 5 times lower than that of the impacting one. For fully molten particles, splashing occurs in all cases even for low Sommerfeld numbers. It thus seems that the substrate roughness plays a key role in splashing.

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