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The Capillary Flow Experiments (CFEs) are a suite of fluid physics flight
experiments designed to investigate large length scale capillary
flows and phenomena in low gravity. The CFE data to be obtained
will be crucial to the Space Exploration Initiative, particularly
as it pertains to fluids management systems such as fuels and
cryogen storage systems, water collection and recycling, thermal
control systems, and materials processing in the liquid state. NASA’s
current plans for exploration missions assume the use of larger
liquid propellant masses than have ever flown on interplanetary
missions. Under low-gravity conditions, capillary forces can be
exploited to control fluid orientation so that such large mission-critical
systems perform more reliably.
CFE is a simple fundamental
scientific study that can yield quantitative results from safe,
low-cost, short time-to-flight, handheld fluids experiments. The
experiments aim to provide results of critical interest to the capillary
flow community that cannot be achieved in ground-based tests such
as tests to probe dynamic effects associated with a movingcontact
boundary condition, capillary-driven flows in interior corner networks,
and critical wetting phenomena in complex geometries. Specific applications
of the results center on particular fluids challenges concerning
propellant tanks. The knowledge gained will help spacecraft fluid
systems designers increase system reliability, decrease system mass,
and reduce overall system complexity.
CFE encompasses three experiments with two unique experimental units
per experiment. There are multiple tests per experiment. Each of the
experiments employs parametric ranges and test cell dimensions that
cannot be achieved in groundbased experiments. All units use similar
fluid injection hardware, have simple and similarly sized test chambers,
and rely solely on video for highly quantitative data. Silicone oil
will be used for these tests. Differences between units are primarily
fluid properties, wetting conditions, and test cell geometry. The
experiment procedures are simple and intuitive.
CFE Interior
Corner Flow (CFE–ICF)
Spontaneous capillary flows in containers of increasing complexity
have been designed to determine important transients for low-g propellant
management. Significant progress has been made for complex containers
that are cylindrical, but many practical systems involve containers
with geometries that are tapered.
The taper of the irregular polygonal cross section of the test cells
provides particular design advantages in preferentially locating the
liquid where desired. Passive capillary flow in such containers is
called imbibition and cannot be tested on the ground for large three-dimensional
geometries with “underdamped fluids”—a most common
characteristic of low-g fluids systems. The equations governing the
process are known but have not been solved analytically to date because
of a lack of experimental data identifying the appropriate boundary
conditions for the flow problem. Experimental results will guide the
analysis by providing the necessary boundary condition(s) as a function
of container cross section and fill fraction. The benchmarked theory
can then be used to design and analyze capillary devices such as three-dimensional
vane networks and tapered screen galleries for bubble-free collection
and positioning of fuels for satellites, an important and outstanding
problem for propellant management aboard spacecraft.
CFE Vane
Gap (CFE–VG)
A complicated critical wetting condition arises between interior corners
that do not actually contact; such as in the gap formed by a vane
and tank wall of a large propellant storage tank (a commonality in
practice), or near the intersection of vanes in a tank with complex
vane network. Two CFE units will be employed to investigate this phenomenon
using a right cylinder with elliptic cross section with a single central
vane that does not contact the container walls.
The vane can be pivoted varying the angle between the vane and the
wall and also varying the size of the vane-wall gap. The vane is slightly
asymmetric so that two gaps can be tested for each container. All
static interface shapes recorded by video will be compared quantitatively
with shapes computed using a computer algorithm. A major goal of this
experiment is to carefully observe all interface configurations during
the rotation of the vane and to test the repeatability and reversibility
of the critical wetting phenomena.
CFE Contact
Line (CFE–CL)
Two CFE units will be used to study a fundamental and practical concern
for low-g fluid phenomena—the impact of the dynamic contact
line. The contact line controls the interface shape, stability, and
dynamics of capillary systems in low g. The CFE–CL experiments
will provide a direct measure of the extremes in behavior expected
from an assumption of either the free or pinned contact line condition.
The two units are identical except for their respective wetting characteristics.
Status
The CL–1, the ICF, and VG units are complete and awaiting launch.
CFE–CL unit 2 (CL–2) was launched to the International
Space Station (ISS) on Progress 13 in January 2004. ISS Science Officer
Michael Fincke, operated the CL–2 unit on August 28 and again
on September 18, 2004. CL–2 operations were also performed on
December 20, 2005 and on April 18, 2006 by ISS Science Officers, William
McArthur and Jeff Williams, respectively. Video data was taken and
is currently being analyzed. Tests include a variety of fluid disturbances,
such as tap, slide, multiple slide, push, swirl, and axial perturbations.
Preliminary results were recently reported1 where it has been observed
that the correct contact line boundary condition is pivotal to accurate
modeling of large length scale capillary surface dynamics. In addition,
it was observed how large amplitude multiple slide disturbances act
to form ‘hourglass’ configurations in the smooth cylinder
while the surface remains pinned in the pinning cylinder. Axial mode
drop ejection tests were performed and obvious differences in settling
time and natural frequency as a function of contact line condition
and disturbance type were observed. Publication of the quantified
data awaits further analysis of the data. |
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CFE–ICF
flight units: tapered isosceles vessel (top) and tapered rectangular
vessel (bottom). |
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CFE–VG
flight unit. |
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A
CFE–CL flight unit with smooth cylinder on left and
pinning
cylinder on right. |
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