Plasmas are considered the fourth state of matter, making up 99.99% of the observable matter in the universe. Their applications are very diverse, and plasmas are found and utilized in everything from commercial products (neon lights, fluorescent lamps, TVs) power generation (e.g. fusion reactors), plasma gasification technology (waste-to-energy), plasma thrusters/electric propulsion for spacecraft, electronic switching devices, materials processing for manufacturing integrated circuits/semiconductors and devices in the electronics (e.g. MEMS), chemical, automotive and aerospace industries.When voltage is applied between two electrodes situated in close proximity to each other (10-100 µm), a weakly ionized, low temperature plasma discharge can be generated.&nbsp; This in turn creates a plasma sheath, an electrically ionized boundary layer (typically of the order of 10’s to 100’s of microns), where space charge effects dominate.&nbsp; The sheath acts like a virtual capacitor, with the plasma behaving as an inductor.&nbsp; Changes in pressure (and shear if unconfined) influence the plasma morphology (shape, thickness), thus making the plasma the transduction mechanism.&nbsp; The change in plasma morphology in turn translates to a change in one of the following plasma properties:&nbsp; sheath potential, φpl(t), electron number density, ne, and floating potential φf(t).&nbsp; By measuring one or more of these plasma properties simultaneously, it is then possible to discern changes in the local pressure dynamic field.&nbsp; Plasma properties (ne, φpl, φf) are not a strong function of the sensor’s surface area, therefore making the transducer active area small is not detrimental to sensitivity.&nbsp; Miniaturization then becomes a pragmatic exercise, given the materials available and the fabrication/assembly process that can be leveraged.&nbsp; A cast method, whereby electrodes are placed in a mold that is filled with a ceramic, is a viable approach for large scale production that is straight forward and low cost.&nbsp;&nbsp; A porous cap to access the flow field can be integrated at the tip to confine the plasma region, to shield the emitted electro-magnetic radiation (if conductive) and to increase reliability for long duration use.&nbsp; Furthermore, tailoring the porosity pattern of the cap can yield sensor tuning to one or more physical parameters induced by flow interaction.&nbsp; High temperature ceramics along with tungsten electrodes comprise the body of the probe.&nbsp;&nbsp; High purity alumina is usable in both oxidizing and reducing atmospheres to 1925°C, while at 3420 °C, tungsten has the highest melting point of all metals.&nbsp;The attraction to the use of plasma discharge as a transduction method for fluid flow property measurement stem from the fact that it lends itself to a probe implementation that is simple in design, can be miniaturized, and at the same time offers unmatched capability for handling ultra-high temperature environments.&nbsp; Sensing plasma discharge characteristics and their variation due to flow interaction can be done electrically, but also optically to yield time-varying intensity and spectral information from fluid-plasma interaction.The current paper will focus on the deployment of a dual micro-plasma sensor system as a new novel multi-parameter sensing approach for surface flow measurement. Results on pressure dynamics, shear flow, and other possible engineering parameters will be discussed in the context of results from several bench-level experiments.&nbsp;

Boundary Layer Multi-Property Flow Measurements Using a Micro-Plasma Sensor

Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2021/volume/85284

ASME 2021 Fluids Engineering Division Summer Meeting (FEDSM2021)

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.