Capillary Channel Flow (CCF) - 07.29.15
Capillary flow is the natural wicking of fluid between narrow channels in the opposite direction of gravity. Tree roots are one example of a capillary system, drawing water up from the soil. By increasing understanding of capillary flow in the absence of gravity, the Capillary Channel Flow (CCF) experiment helps scientists find new ways to move liquids in space. Capillary systems do not require pumps or moving parts, which reduces their cost, weight and complexity.
Science Results for Everyone
Burst those bubbles! Generally, moving liquids around in microgravity requires mechanically-driven electrically-powered pumps and complex equipment. If capillary forces can be used to drive fluid flows then they can replace finicky complex equipment and increase system reliability—especially life support systems. The capillary channel flow (CCF) experiment observes that, in microgravity, capillary flow rates reach a maximum "critical point" above which the flow collapses ingesting bubbles the can cause further problems downstream because they don’t rise and can reside anywhere blocking flow passages and deactivating equipment. Experimenting with wedge-shaped channels, scientists find that bubbles flowing in wedge-shaped channels can be effectively and passively removed. With such knowledge, simple conduits can be designed that perform the bubble separation process without moving parts. Experiment Details
Michael E. Dreyer, Ph.D., University of Bremen, Bremen, Germany
Jörg Klatte, University of Bremen, Bremen, Germany
Peter Canfield, University of Bremen, Bremen, Germany
Mark Milton Weislogel, Ph.D., IRPI LLC, Wilsonville, OR, United States
Yongkang Chen, Ph.D., Portland State University, Portland, OR, United States
Aleksander Grah, University of Bremen, Bremen, Germany
Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
Human Exploration and Operations Mission Directorate (HEOMD)
ISS Expedition Duration
September 2010 - September 2014
Previous ISS Missions
ISS Expedition 25/26 was the first operation of CCF in microgravity.
- 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 current models, which do not adequately predict the maximum flow rate achievable through the capillary vanes.
- CCF tests 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. The experiments provide the verifications for the flow rate limits and corresponding critical flow velocities.
Capillary Channel Flow (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. The results of CCF will help innovate existing and inspire new applications in the portion of the aerospace community that is challenged by the containment, storage, and handling of large liquid inventories (fuels, cryogens, and water) aboard spacecraft. The results will be 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.
Since hydrostatic pressure is absent in microgravity, technologies for liquid management in space use capillary forces to position and transport liquids. On Earth, the effect of capillary forces is limited to a few millimeters. In space, these forces still affect free surfaces that extend over meters. For the application of open channels in propellant tanks of spacecraft, design knowledge of the limitations of open capillary channel flow is a requirement.
Current spacecraft fuel tanks rely on an additional reservoir to prevent gases from entering the engines during firing. Capillary systems larger than a few millimeters in length are impossible to study on the ground, but the weightlessness on the International Space Station makes research on a variety of capillary channel sizes possible. The results are useful for the design and testing of liquid management systems in current use and for future missions.
Rocket fuel tank designs and construction models from space research help to advance new technology and manufacturing capabilities on Earth.
Forced liquid flows through open capillary channels with free liquid surfaces will be investigated in the Microgravity Science Glovebox (MSG) onboard ISS. In open capillary channels, if a certain critical flow rate is exceeded, the flow becomes unsteady, the surfaces collapse, and gas ingestion occurs at the outlet. From a fluid mechanical point of view, a characteristic critical velocity must exist at which the steady subcritical flow turns into an unsteady supercritical flow, which involves the collapse of the free surfaces. To find this velocity and the location of collapse of the free surface, the surface profile must be measured with great accuracy. Furthermore, the local flow velocity must be known at dedicated points of the channel.
The crew involvement is limited to installation of the CCF hardware in the MSG work volume and exchange of experiment units between sessions.
The CCF Experiment investigates forced liquid flow through partially open capillary channels aboard the International Space Station (ISS). The flow channel is made up of either two parallel plates or an open wedge conduit. Results collected show favorable agreement with predictions of critical flow rates and bubble separation rates. The results also indicate the nature of desabilizations and the myriad outcomes of gas liquid flows in the microgravity environment. Regarding critical flow rate limitations, in general, steady uninterrupted flow is possible as long as it is below the bubble-ingestion speed. However right at this “critical” point, the flow speed of the fluid is counteracted equally by resisting waves going the opposite direction and quickly gives rise to the instability, called choking, which causes ingestion of bubbles into the moving fluid. At this point, the maximum flow rate is achieved briefly before the free fluid surface collapses and ingests air. The results suggest that these limitations are primarily governed by flow velocity, intrinsic properties of the liquid (i.e., surface tension, density, viscosity, wetting condition), and the steady balance of directional forces exerted on the moving fluid. For passive bubble separations in wedge sectioned conduits the studies have suggest devices that can be applied directly to perform such task beyond such fundamental investigations. Researchers are continuing to experiment with different capillary channel designs, fluids, and flow velocities to study and improve computational models for predicting capillary channel flow behavior in space which can translate into fabricating efficient fluid transport systems for fuel, life support, and energy systems for space exploration.^ back to top
Jenson RM, Wollman AP, Weislogel MM, Sharp L, Green RD, Canfield P, Klatte J, Dreyer ME. Passive phase separation of microgravity bubbly flows using conduit geometry. International Journal of Multiphase Flow. 2014 June; epub. DOI: 10.1016/j.ijmultiphaseflow.2014.05.011.
Canfield P, Bronowicki PM, Chen Y, Kiewidt L, Grah A, Klatte J, Jenson RM, Blackmore W, Weislogel MM, Dreyer ME. The capillary channel flow experiments on the International Space Station: experiment set-up and first results. Experiments in Fluids. 2013 May 8; 54(5): 1519. DOI: 10.1007/s00348-013-1519-1.
Conrath M, Canfield P, Bronowicki PM, Dreyer ME, Weislogel MM, Grah A. Capillary channel flow experiments aboard the International Space Station. Physical Review E. 2013; 88(6): 063009. DOI: 10.1103/PhysRevE.88.063009.
Bronowicki PM, Canfield P, Grah A, Dreyer ME. Free surfaces in open capillary channels—Parallel plates. Physics of Fluids. 2015 January; 27(1): 012106. DOI: 10.1063/1.4906154.
Grah A, Canfield P, Bronowicki PM, Dreyer ME, Chen Y, Weislogel MM. Transient capillary channel flow stability: Experiments on the International Space Station. Microgravity Science and Technology. 2014 December; 26(6): 385-396. DOI: 10.1007/s12217-014-9403-z.
Ground Based Results Publications
ISS Research Project - CCF
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