Near-wall fluid velocimetry in the impingement zone of a micro-droplet stream on a flat surface is reported utilizing micro-Particle Tracking Velocimetry . The results are then compared with the near-wall fluid velocimetry in the impingement region of a steady micro-jet stream. The presence of tracer particles in the fluid results in a small movement of the droplets away from the orifice axis, causing a change in the location of the droplet impingement center. A new method to find the center of impingement is described, and an algorithm is developed to obtain the radial velocities in the impingement zone at three out-of-plane heights of 2µm, 7µm, and 10±2µm from the wall. Single-frame double-exposed images of fluorescent tracer particles at low loading are used for the experiments. As the impingement frequency of the droplet stream is much higher than the image-capturing rate of the camera, each double-exposed image corresponds to a different random instance within the impingement period of the droplets. The presented results show the occurrence of a higher normalized root mean square along with positive skewness of the measured radial velocity values for the droplet stream. These indicate higher velocity fluctuations or fluid mixing characteristics induced by the droplet-crown propagation for the droplet stream when compared to that of a jet stream. The near-wall velocity measurements support previously reported observations of the enhanced convection heat transfer characteristics for a droplet stream case.

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