Near-Wall Velocimetry in the Impingement-Zones of a Microdroplet and a Round Jet Stream

Near-wall fluid velocimetry in the impingement zone of a microdroplet stream on a flat surface is reported utilizing microparticle tracking velocimetry (μPTV). The results are then compared with the near-wall fluid velocimetry in the impingement region of a steady microjet stream. The presence of tracer particles in the fluid results in a small random 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 2, 7, and 10 ± 2 μm from the wall. Single-frame double-exposed images of low loading fluorescent tracer particles 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 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.