Flattening and cooling of millimetre and micrometre sized alumina drops

摘要

An experimental set-up has been developed, at the SPCTS Laboratory, to produce fully melted, millimetre-sized, ceramic or metallic drops with impact velocities up to 10 m/s. Such impact velocities allow reaching impact Weber numbers, close to those of the plasma spray process (We = 2300). A fast camera (4000 image/s) combined to a fast pyrometer (4000 Hz), allows following the drop flattening. For studding the flattening at the micrometre scale, a DC plasma torch is used to melt micrometer sized alumina particles (around 45 μm). The experimental set-up is composed of a fast (50 ns) two-color pyrometer and two fast CCD cameras (one orthogonal and other tangential to the substrate). The flattening of millimetre and micrometre sized particles is compared. First are studied impacts of alumina drops (millimetre sized) with impact velocities up to 10 m/s. Then are considered micrometre sized alumina particles (about 45 μm in diameter) sprayed with a DC plasma torch. A correlation has been found between both flattening scales and, in spite of the lower impact velocity at the millimetre scale, ejections are also found at the micrometre scales. This work shows that to compare phenomena at the two different scales it is mandatory to have Weber numbers as close as possible in both cases.