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.

Contacts at NASA Glenn Research Center
Project Manager: Donna Y. Bohman
Donna.Y.Bohman@nasa.gov
216-433-8860
Project Scientist: Dr. Robert Green, NASA GRC
Robert.D.Green@nasa.gov
216-433-5402
Principal Investigator: Prof. Mark Weislogel, Portland State University
mmw@cecs.pdx.edu