Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

surprising findings of these studies are that superfluid droplets are ”sticky” in the sense that they exhibit an anomalously large
contact angle hysteresis, and that superfluid drops require a substantial force to initiate flow. Despite the very low viscosity, the
drops have a mechanical Q factor similar to water. We are in the process of developing a next generation 3-He based optical cryostat
that will allow us to extend our studies to lower temperatures. We plan to study superfluid spreading on a dry wettable surface,
which requires a cesium dam to form a dry spot. Temperatures below 0.3K are required to eliminate vapor phase mass transport.
We will also study the mechanical Q of drops in the limit of zero vapor pressure and unity superfluid fraction. The dynamics of
dewetting and film rupture can be studied by forming metastable films of superfluid on a substrate whose thermodynamically
stable state is dry.
Author (revised)
Liquid-Solid Interfaces; Superfluidity; Liquid Helium; Wetting; Wettability
20010024985 National Center for Microgravity Research on Fluids and Combusiton, Cleveland, OH USA
Splashing Droplets
VanderWal, R. L., National Center for Microgravity Research on Fluids and Combusiton, USA; Tryggvason, Gretar, Michigan
Univ., USA; Kizito, John P., National Center for Microgravity Research on Fluids and Combusiton, USA; Alexander, Iwan, Case
Western Reserve Univ., USA; Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference; December
2000, pp. 1395-1396; In English; See also 20010024890; No Copyright; Abstract Only; Available from CASI only as part
of the entire parent document
In practical applications involving sprays for cooling, atomization, coating and fuel injection, the small spatial scales and
short temporal scales prohibit detailed study of the internal fluid dynamics and surface and internal temperature profile during
the droplet-surface interaction. Such information is critical to defining ranges of behavior, testing existing models of droplet-wall
interactions and providing benchmark data for development of theoretical models. In a low-gravity environment, enhanced spatial
and temporal scales may be achieved while maintaining comparable ranges of surface tension, viscous and inertial effects, typical
of practical applications with well-defined initial conditions uninfluenced by gravity. The relative magnitude of surface tension
and viscosity effects will depend upon the kinematics of the impaction process, liquid properties and ambient conditions. Experiments
in both the film evaporation and film boiling regimes will be conducted to illuminate surface tension gradient and viscosity
effects upon droplet breakup. Parameters that will be varied include the initial bulk liquid surface tension, viscosity, degree of
liquid subcooling, impact Weber number, ambient pressure and impact angle. Comparison between results will be used to delineate
the role and quantify the relative magnitude of surface tension effects compared with inertial, viscous and wetting contributions
to droplet breakup or ”splashing”. A second thrust of the work proposed here will be to test equivalent dimensionless groups:
for example, for droplets of different size, adjustment of velocity, surface tension and viscosity will be used to equalize Weber
and Reynolds numbers to test for hidden property dependencies. These tests will be used to evaluate the applicability of droplet
breakup studies using widely different conditions. Experiments in the film evaporation region will provide tests of theory apart
from phase change while including wetting behavior and contact angle effects. Experiments in the film boiling regime will include
phase change effects while eliminating surface wetting and contact angle effects. Experiments in the nucleate and transition boiling
regimes will determine whether splashing can occur prior to droplet disruption by boiling and serve to identify alternative
droplet breakup behavior. The experimental data is expected to serve as input to practical spray models while detailed numerical
modelling will be developed by comparison to experimental results. The experimental studies will be complemented by detailed
simulation of the drop collision with the walls. A so-called on-fluid formulation will be used where a single set of equations is
written for all the phases involved. In this approach the phase boundary is treated as an imbedded interface by adding the appropriate
source terms to the conservation laws. These source terms are in the form of delta functions localized at the interface and are
selected in such a way to satisfy the correct matching conditions at the phase boundary. More specifically, the head-on impact of
a drop on a hot surface will be simulated using an axisymmetric numerical model. The model will include local grid refinement
to capture the draining of the film between the drop and the surface, the energy equation to compute the temperature distribution,
temperature dependent surface tension, and mass transfer from the surface due to evaporation. For the case of drops colliding with
solid walls where no slip boundary conditions must be enforced, quantitative predictions will require very fine resolution. The
simulations will be set up to mimic the experimental setup as closely as possible and detailed comparisons will be made. by turning
various physical properties on and off and examining the sensitivity of the results to changes in the modeling of the relation
between the surface tension and the temperature, for example, it will be possible to determine exactly what forces are important
during the collision of drop with a hot surface and the evolution of splashed products.
Author (revised)
Drops (Liquids); Interfacial Tension; Computerized Simulation; Dimensionless Numbers; Splashing
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