Drop Media Interactions and Demisting in Microgravity

When liquid droplets pass through a fibrous media, the liquid phase may be captured by the fibers while the gaseous phase passes through it. The ability of a fibrous media to capture the liquid from such a ‘mist’ is commonly used in processes such as fog harvesting. Such ‘demisting’ processes are significantly impacted by the presence of gravity which limits droplet size ranges, impact phenomena, sedimentation and adhesion within the media, and drives the ever-present ‘downward’ capillary wicking flows at increased saturation levels. An increased understanding of demisting in the unique microgravity environment could make a significant contribution to the Space Technology Roadmap TABS element 6.1.2: Water Recovery and Management. Free liquid droplets are a product of many processes associated with space travel. A small sample of such processes includes vigorous boiling, sweep gas droplet entrainment, pipetting, droplet spray watering systems for plants, lid removal, and condensing heat exchangers. As briefly mentioned above, demisting in microgravity is fundamentally different from the terrestrial case. For example, in terrestrial environments the diameter of air-bound droplets are usually limited to less than 100 micrometers due to sedimentation, but in microgravity environments suspended droplets can be 100,000 times larger in size leading to droplet-laden flows of enormous size distribution. Further, for demisters on Earth, gravity tends to limit media saturation to approximately centimetric lengths before gravity-driven draining occurs. But no natural drainage mechanism exists in most microgravity systems; droplets may accumulate saturating the filter material, possibly even extending beyond the extents of the filter as large unruly bodies of liquid, drastically altering the performance of such media regarding total media hold-up, blow through limits, and pressure losses as a function of low-g unique saturation distribution. Unfortunately, as a result of limited access to low-gravity environments, the lack of effective engineering design tools for microgravity demisting has led to systems of reduced reliability due to increased complexity, weight, volume, noise, and power draw (i.e., centrifuge separation, ‘stop-gap’ filters, etc.). This research aims to describe fundamental aspects of demisting such as: droplet impact phenomena including capture, rebound, and breakup; filter saturation phenomena including stability and reentrainment, equilibrium geometry, accumulation dynamics, and migration/wicking of captured liquid; and system performance including pressure drop and filtration efficiency. We will investigate a wide range of fibrous media compositions, including purchased and purpose-built media (i.e., 3-D printed), and fluid thermophysical properties with tests performed in the Dryden Drop Tower at Portland State University and concurrent numerical analyses providing insight into microgravity demisting.

This research aims to describe fundamental aspects of demisting such as: droplet impact phenomena including capture, rebound, and breakup; filter saturation phenomena including stability and reentrainment, equilibrium geometry, accumulation dynamics, and migration/wicking of captured liquid; and system performance including pressure drop and filtration efficiency. We will investigate a wide range of fibrous media compositions, including purchased and purpose-built media (i.e., 3-D printed), and fluid thermophysical properties with tests performed in the Dryden Drop Tower at Portland State University and concurrent numerical analyses providing insight into microgravity demisting.

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