Near-Wall Velocimetry in the Impingement-Zone of a Micro-Droplet and a Round Jet Stream

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.

A versatile set-up to study plasma/microwave sources for liquid fuel ignition

Motivated by the high demand for an alternative, more reliable, high energy ignition source to facilitate the re-ignition of lean-burn combustion chambers which are necessary to reduce pollutant emissions, a new set-up has been designed to study plasma/microwave sources. The use of a waveguide-based resonant cavity leads to very low power plasma ignition. An example in this paper shows that a plasma at atmospheric pressure can be maintained with less than 2 W input power. Such a performance is possible using the large variety of possible adjustments (resonance frequency, different kind of initiators, etc.) that this versatile set-up offers. To illustrate the wide range of possible studies, another example is given and discussed : minimum ignition energy for an ethanol droplet stream with aluminum and stainless steel initiators. The results show that the initiator material and its surface quality have an influence on the minimum ignition energy, especially for large gaps. Depending on the gap size we can get down to under 10 W entering the cavity to ignite the droplet stream.