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CCF is a versatile experiment for studying a critical variety of inertial-capillary
dominated flows key to spacecraft systems that cannot be studied
on the ground. Applications of the results of CCF are direct to
the portion of the aerospace community challenged by the containment,
storage, and handling of large liquid inventories (fuels, cryogens,
water) aboard spacecraft. The results are immediately useful for
the design, testing, and instrumentation for verification and
validation of liquid management systems of current orbiting, design
stage, and advanced spacecraft envisioned for future lunar and
Mars missions. The results will also be used to improve life support
system design, phase separation, and enhance current system reliability
by designing into the system passive, in this case capillary redundancies.
Technologies for liquid management in space use capillary forces to
position and transport liquids, since the hydrostatic pressure is absent
which gives the liquid a defined surface and enables easy withdrawal
from the tank bottom. But the effect of capillary forces is limited
on earth to a few millimeters. In space these forces affect free
surfaces that extend over meters. For the application of open channels
in propellant tanks of spacecrafts, design knowledge of these limitations
are a requirement, predicated with a bubble free liquid restriction
prior to entering the thrusters.
Currently, spacecraft fuel tanks rely on an additional reservoir
to prevent the ingestion of gas into the engines during firing. Research
is required to update these current models, which do not adequately
predict the maximum flow rate achievable through the capillary vanes.
These results will lead to improved reliability design for liquid and
propellant management in space, equating to more efficient storage utilization.
Capillary channels (or vanes) in connection with the dominating surface
tension forces are used to pump and / or position the liquid as well
as achieve a connection between different liquid bodies, i.e., between
the bulk in the tank and the reservoir. Critical flow rate
knowledge is required to avoid rupture of the connection. Induced ruptures
study ways to avoid the withdrawal of liquid from reservoirs. Capillary
channels geometrically formed to be tested are parallel plates, grooves,
wedges and a liquid bridge. These results will be used to validate and
test the theoretical models.
CCF will test the theoretical predictions for the free surface shapes
and the critical flow velocities for open capillary channel (vane) flows
in microgravity. CCF is designed to validate the assumptions used to
develop the governing equations. Open capillary channels are used in
surface tension tanks to transport liquid / propellant to reservoirs
or directly to the thruster. This experiment will provide the verifications
for the flow rate limits and corresponding critical flow velocities.
From a fluid mechanical point of view, a characteristic velocity must
exist which cannot be exceeded by any means. To find this velocity and
the point of collapse of the free surface, the surface profile must
be measured with great accuracy. Furthermore, the local flow velocity
at dedicated points of the channel must be known.
The CCF experiment is configured to emphasize applications over fundamentals.
The following highlights CCF’s capabilities:
- Provide performance limits for capillary dominated
systems such as passive fluids management (i.e., capillary collection,
pumping, and containment) and processes such as passive phase
separation and transport. This is a current and pressing requirement
for a wide range of spacecraft fluid systems.
- CCF experiments will be performed to quantify
the impact of non-ideal wetting conditions brought about by high
surface energy fluids such as water, fluid contamination in the
form of particulates and in/soluble surfactants, and non-ideally
wetted substrates (i.e., rough surfaces, chemically inhomogeneous
surfaces, etc.). The processes studied are from both a surface
roughness and surface wet-ability point of view. Direct
applications to failing and flaking biocide coatings on condensing
surfaces, contaminant boundaries resulting from repeat dryout
and rewetting events, and fluid borne contaminants and particulates
are investigated.
- Water is employed as a test liquid in one of
the two experiments test units since water is common to many life
support systems with a purity content that does not require maintenance:
the water will be allowed to contaminate over time in a similar
manner to certain water recycling systems (i.e., condensing heat
exchanger). The impact of contamination is monitored by in-situ
measurements of surface tension, contact angle, and contact angle
hysteresis.
- CCF will use test cell geometries and variable
parameter ranges to investigate the ability of capillary systems
to passively change multiphase flow regimes. It will also provide
(100%) phase separation and permit access to single phase gas
or liquid to mechanically pumped lines modeling up/downstream
unit operations in the water processing cycles. CCF will study
capillary dominated multiphase flow that may be exploited to assist
other active or passive systems.
- CCF will also provide critical data for the
uniquely low-g inertial-capillary flow regime important to liquid
fuels and cryogen storage and management. A test liquid similar
to coolants and propellants will be implemented.
Of the myriad of geometries envisioned for the capillary
control of fluids in low-g environments, the CCF exploits a variable
plate geometry capable of creating parallel plate, gapped plate, and
interior corner capillary conduits. This universal geometry represents
a class of practical capillary geometries successfully implemented
in the fuels and tank community of the aerospace industry. Spacecraft
water processing equipment is replete with such constructs. Validation
of current models developed for such geometries is expected to lend
confidence to the application of theory to other geometries pertinent
to advanced systems development.
Methods
Forced liquid flows through open capillary channels
with free liquid surfaces will be investigated in the Microgravity
Science Glovebox (MSG) onboard ISS. If a certain critical flow
is exceeded, the flow does not remain steady, the surfaces collapse,
and gas ingestion occurs at the outlet.
To achieve a high degree of flexibility, the experiment
was designed as a modular system consisting of the Fluid Management
System (FMS), the Board Computer (BC), and the Test Units (TU). The
FMS is equipped with the required components to establish the flow (pumps,
plunger, valves), while the TU contains the test channel, a phase separation
chamber, (PSC), a compensation tube (CT), cameras for the video observation
as well as the required illumination. The experiment control, the sampling
of the housekeeping data as well as the communication with the MSG interfaces
and the ground station (PI site) is performed by the BC. For the investigation
of the selected channel geometries (parallel channel, groove channel,
wedge-shaped channel, and a liquid bridge) and the different channel
dimensions, the TU is exchangeable. This enables the use of the set-up
for other projects with similar technology driven research objectives. |
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