A circumferentially microgrooved capillary evaporator is here proposed as a reliable alternative for ground and spacecraft thermal control system applications. In this paper, experimental results concerning the start-up and thermal behavior of a capillary evaporator at steady state operation are presented. A capillary pumped loop was developed and tested at ground and microgravity conditions, using deionized water as the working fluid. The capillary evaporator has internally machined circumferential grooves with an average opening of 33 μm opening at 215 μm step into a 19.05 mm (3/4 in) diameter aluminum tube. The corresponding capillary pumping pressure is about 1.5 kPa. In both tests, power inputs up to 10 W (4.55 kW/m2) were successfully applied to the external surface of the evaporator, showing good performance under ground and microgravity conditions. The capillary evaporator as proposed proved to be a reliable alternative for industrial and space applications.
Research Containing: Capillary
The Vane Gap Capillary Flow Experiments are part of a suite of low-g experiments ﬂown onboard the International Space Station to observe critical wetting phenomena in ‘large length scale’ capillary systems. The Vane Gap geometry consists of a right cylinder with elliptic cross-section and a single central vane that does not contact the container walls. The vane is slightly asymmetric so that two gaps between the vane and container wall are not of the same size. In this study, we identify the critical wetting conditions of this geometry using the Concus-Finn method for both perfectly and partially wetting ﬂuids as a function of container asymmetry. In a cylindrical container in zero-g, single-valued ﬁnite height equilibrium capillary surfaces fail to exist if a critical wetting condition is satisﬁed. This nonexistence results in signiﬁcant redistribution of the ﬂuids in the container. It will be shown that there could be three critical geometric wetting conditions that include one in each gap region and one for a global shift of bulk ﬂuid which, among the three, is the most signiﬁcant.
This paper provides a current overview of the in-flight operations and experimental results of the capillary flow experiment (CFE) performed aboard the International Space Station (ISS) beginning August 2004 to present, with at least 16 operations to date by five astronauts. CFE consists of six approximately 1–2 kg experiment units designed to probe certain capillary phenomena of fundamental and applied importance, such as capillary flow in complex containers, critical wetting in discontinuous structures, and large length scale contact line dynamics. Highly quantitative video images from the simply performed experiments provide direct confirmation of the usefulness of current analytical design tools as well as provide guidance to the development of new ones. A description of the experiments, crew procedures, performances and status of the data collection and reduction is provided for the project. The specific experimental objectives are briefly introduced by way of the crew procedures and a sample of the verified theoretical predictions of the fluid behavior is provided. The potential impact of the flight experiments on the design of spacecraft fluid systems is discussed in passing.
Preliminary Results from the Capillary Flow Experiment Aboard ISS: The Moving Contact Line Boundary Condition
The Capillary Flow Experiment (CFE) consists of six approximately 2kg test vessels constructed by NASA to probe certain capillary phenomena of fundamental and applied importance. The light weight, low-volume hardware can be shipped to orbit on short notice as cargo space permits and the experiment performed in stand-alone mode by a single crewmember on, for example, the Maintenance Work Area (workbench) of the International Space Station. Video images from the simply performed crew procedures provide highly quantitative data for the confirmation of current analytical design tools as well as directions for further theoretical development. This paper presents a narrative of preliminary results from the first Capillary Flow Experiment (CFE) conducted aboard ISS in August-September 2004. The tests are performed as per of NASA’s Saturday Morning Science Program on ISS and completed in good order by Astronaut Michael Fincke who collected approximately 100 data sets that compare large length scale capillary surface oscillations and damping for two otherwise identical cylindrical tanks differing only in respect to a critical yet uncertain boundary condition at the contact line. Linear, nonlinear, and destabilizing slosh, swirl, axial, and other disturbances are studied. The large data set is being reduced for comparisons to the blind predication of a group of numerical analysis assembled to gauge the accuracy of present methods to predict large length scale capillary dynamics critical to fluids management in spacecraft (i.e. fuels, cryogens, water). The success of the experiment reported herein serves as a testimony to astronaut ingenuity and the perhaps surprisingly flexible fluids laboratory of the ISS for safe and simple fluids experimentation.
A beverage cup comprised by an open top and at least one channel defined by a corner with an acute angle so placed that the channel runs along the cup side from the cup bottom to the cup rim. In the absence of significant gravitational force as found in microgravity, weightless or weightlessness of spacecraft or the International Space Station, capillary forces between the beverage and the cup wall allow the beverage to creep along the channel and be in near proximity to the open cup rim. Lips placed at or near the channel at the rim can readily sip, drink, and consume the beverage without the need for a straw and without undue spillage for normal drinking motions including toasting. The channel conducts the beverage via capillary forces from the bottom of the cup to the rim until the beverage has been consumed.
This paper provides a summary of the experimental, analytical, and numerical results of the Capillary Flow Experiment (CFE) performed onboard the International Space Station (ISS) from Increment 9 (beginning August, 2004) through Increment 16 (ending December, 2007), with 19 operations by 7 astronauts; M. Fincke, W. McGarthur, J. Williams, S. Williams, M. Lopez-Alegria, C. Anderson, and P. Whitson. CFE consists of 6 approximately 1 to 2kg experiment units designed to probe certain capillary phenomena of fundamental and applied importance, such as capillary flow in complex containers, critical wetting in discontinuous structures, and large length scale contact line dynamics. Highly quantitative video images from the simply performed flight experiments provide immediate confirmation of the usefulness of current analytical design tools as well as provide guidance to the development of new ones. A brief review of the experiments and procedures is provided before reporting the status of the data collection, reduction, and comparisons with both analytic and numerical predictions. The products of the work include design tools for modeling capillary interface dynamics relevant to spacecraft engineering systems. The CFE experimental program was initiated in February 2003 as part of a fast-paced unscheduled payloads/experiments program. All six of the units were performed on standby or at times as part of NASA Saturday Science and all units have been returned to Earth for post flight analysis. The experiments were conducted in stand-alone mode by a single crewmember on the Maintenance Work Area of the ISS.
urrent experiments aboard the International Space Station (ISS) illustrate an extent to which liquid behavior aboard spacecraft can be controlled by wetting and container geometry. The experiments are referred to as the 'Vane-Gap' experiments and are part of a more general set of simple handheld Capillary Flow Experiments1) (CFE) designed and developed at NASA's Glenn Research Center for conduct on ISS. The CFE-Vane Gap experiments highlight the sensitivity of a capillary fluid surface to container shape and how small changes to said shape may result in dramatic global shifts of the liquid within the container. Understanding such behaviors is central to the passive management of liquids aboard spacecraft and in certain cases permits us the ability to move (pump) large quantities (potentially tons) of liquid by a simple choice of container shape. In particular, the Vane-Gap experiments identify the critical geometric wetting conditions of a vane structure that does not quite meet the container wall-a construct arising in various fluid systems aboard spacecraft such as liquid fuel and cryogen storage tanks, thermal fluids management, and water processing equipment. In this paper experimental results are compared with preliminary theoretical and numerical analyses.
Thermophysical characteristics of a wickless heat pipe in microgravity – Constrained vapor bubble experiment
Wickless heat pipes are being studied for use in cooling critical components of spacecraft. The wickless design is thought to produce a simpler and lighter heat transfer system than heat pipes containing wicks or mechanically driven systems. The constrained vapor bubble experiment (CVB) is one such system tested on the International Space Station where the Bond Number (ratio of gravitational force to surface force) is small maximizing the affects of capillarity. The CVB is essentially a square, fused silica spectrophotometer cuvette evacuated and then partially filled with pentane as the working fluid. Along with temperature and pressure measurements, the two-dimensional thickness profile of the menisci formed at the corners of the quartz cuvette was determined using an interferometry based system contained with the station’s Light Microscopy Module (LMM). The CVB can be viewed as a hollow fin and its behavior analyzed using a simple, one-dimensional heat transfer model. That model, coupled with the visual observation of the vapor–liquid distribution inside the fin, provides an enhanced understanding of what the measured temperature and pressure profiles represent and the heat transfer mechanisms controlling the operation of the device. The internal heat transfer processes were found to be very complicated, multi-dimensional, and greatly dependent on internal and external radiative heat transfer. Internal radiative exchange was found to be more significant than originally anticipated as was the effect Marangoni forces on internal convective heat transfer. An analysis of the temperature profiles in conjunction with vapor–liquid interface mapping showed that the system could be separated into a number of discrete operation zones depending on the dominant mode of heat transfer.
A counterintuitive, thermocapillary-induced limit to heat- pipe performance was observed that is not predicted by current thermal-fluid models. Heat pipes operate under a number of physical constraints including the capillary, boiling, sonic, and entrainment limits that fundamentally affect their performance. Temperature gradients near the heated end may be high enough to generate significant Marangoni forces that oppose the return flow of liquid from the cold end. These forces are believed to exacerbate dry out conditions and force the capillary limit to be reached prematurely. Using a combination of image and thermal data from experiments conducted on the International Space Station with a transparent heat pipe, we show that in the presence of significant Marangoni forces, dry out is not the initial mechanism limiting performance, but that the physical cause is exactly the opposite behavior: flooding of the hot end with liquid. The observed effect is a consequence of the competition between capillary and Marangoni-induced forces. The temperature signature of flooding is virtually identical to dry out, making diagnosis difficult without direct visual observation of the vapor-liquid interface.
Structures in directionally solidified Al–7 wt.% Si alloys: Benchmark experiments under microgravity
Microgravity offers a unique opportunity to achieve solidification in the limit of diffusive transport. Directional solidification experiments in Al–7 wt.% Si alloys were carried out under such conditions on board the International Space Station in orbit around the Earth. Microstructural characterizations include the elongation factor and equivalent diameter of the dendritic grains, together with the dendrite arm spacing and the percentage of eutectic. The experimental investigations reveal that coarse randomly oriented dendritic grains promote non-uniform distribution of eutectic and enhance intergranular segregation. The columnar-to-equiaxed transition (CET) observed in the dendritic grain structure of the refined alloys, sharp or progressive, is defined and characterized based on the profile of the averaged elongation factor. Progressive CET is revealed by an intermediate zone where elongated and equiaxed dendritic grains coexist, sandwiched between the columnar and the equiaxed zones. Fragmentation is also observed in non-refined alloy experiments by electron backscattered diffraction analyses. Capillary-driven detachment during coarsening is suggested to explain this finding, while dendrite fragments cannot cause CET under microgravity because of the absence of convection. This unique set of well-characterized experiments serve as benchmark data for direct numerical simulation of structures and segregations.