The Constrained Vapor Bubble (CVB) experiment concerns a transparent, simple, "wickless" heat pipe operated in the microgravity environment of the International Space Station (ISS). In a microgravity environment, the relative effect of Marangoni flow is amplified because of highly reduced buoyancy driven flows as demonstrated herein. In this work, experimental results obtained using a transparent 30 mm long CVB module, 3 mm x 3 mm in square cross-section, with power inputs of up to 3.125 W are presented and discussed. Due to the extremely low Bond number and the dielectric materials of construction, the CVB system was ideally suited to determining if dry-out as a result of Marangoni forces might contribute to limiting heat pipe performance and exactly how that limitation occurs. Using a combination of visual observations and thermal measurements, we find a more complicated phenomenon in which opposing Marangoni and capillary forces lead to flooding of the device. A simple one-dimensional, thermal-fluid flow model describes the essence of the relative importance of the two stresses. Moreover, even though the heater end of the device is flooded and the liquid is highly superheated, boiling does not occur due to high evaporation rates.
Research Containing: Fluid physics
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.
Boiling phenomena in the two-phase region of SF6 close to its critical point have been observed using the high-quality thermal and optical environment of the CNES dedicated facility ALI-DECLIC on board the International Space Station (ISS). The weightlessness environment of the fluid, which cancels buoyancy forces and favorites the three-dimensional spherical shape of the gas bubble, is proven to be an irreplaceable powerful tool for boiling studies. To identify each key mechanism of the boiling phenomena, the ALI-DECLIC experiments have benefited from (i) the well-adapted design of the test cells, (ii) the high-fidelity of the ALI insert teleoperation when long-duration experiment in stable thermal and microgravity environment are required and (iii) the high repeatability of the controlled thermal disturbances. These key mechanisms were observed by light transmission and interferometry technique independently with two sample cells filled with pure SF6 at a near-critical density. The fluid samples are driven away from thermal equilibrium by using a heater directly implemented in the fluid, or a surface heater on a sapphire optical window. In the interferometry cell, the bulk massive heater distinguishes two symmetrical two-phase domains. The modification of the gas bubble shape is observed during heating. In the direct observation cell, the gas bubble is separated by a liquid film from the thin layered transparent heater deposited on the sapphire window. The liquid film drying and the triple contact line motion during heating are observed using light transmission. The experiments have been performed in a temperature range of 10 K below the critical temperature Tc, with special attention to the range 0.1 mK ≤ T c − T ≤ 3 mK very close to the critical temperature. The unique advantage of this investigation is to provide opportunities to observe the boiling phenomena at very low heat fluxes, thanks to the fine adjustment of the liquid–vapor properties, (e.g. surface tension), by precise control of the distance to the critical point. We present the new observations of the gas bubble spreading over the heating surface which characterizes the regime where vapor bubbles nucleate separately and grow, as well as liquid drying, vapor film formation, triple contact line motion, which are the key mechanisms at the origin of the boiling crisis when the formed vapor film reduces the heat transfer drastically at the heater wall.
Geoflow: First Results from Geophysical Motivated Experiments inside the Fluid Science Laboratory of Columbus
Objective of GeoFlow experiment is to study thermally-driven rotating fluids, in order to investigate the stability, pattern formation, and transition to turbulence of viscous incom-pressible fluids contained between concentric, co-axially rotating spheres. These physical mechanisms are important for a large number of astrophysical and geophysical problems showing flows in spherical geometry driven by rotation and convection: for example, to explain the mantle convection of the Earth, or the flow in a planet's interior. The European microgravity experiment GeoFlow, which is executed in the Fluid Science Laboratory (FSL) of Columbus module on the International Space Station (ISS), is an experiment investigating pattern formation and stability of thermal convection in rotating spherical shells under the influence of an artificial central symmetric buoyancy field and eliminated gravity. In this paper we present numerical preliminary studies of this spherical Rayleigh-Bnard problem under a central dielectrophoretic force in microgravity environment and first experimental results from ISS. Numerical simulations are done for a range of parameter values for Rayleigh and Taylor number. For the experiment flow visualization is realized using the Wollaston-shearing method.
This paper reports some important results obtained from a series of microgravity experiments on the Marangoni convection that takes place in liquid bridges. This project, called Marangoni Experiment in Space (MEIS), started from August 22, 2008 as the first science experiment on the Japanese Experimental Module “KIBO” at the ISS. Two series of experiments, MEIS-1 and 2, were conducted in 2008 and 2009, respectively. The experimental methods used are explained in some detail. The maximum size of the liquid bridge that could be realized during these experiments was 30 mm in diameter and 60 mm in length, giving an aspect ratio of 2.0. The results are obtained for a wide range of aspect ratios of the liquid bridges, including the values that cannot be reached in 1 g experiments, and therefore, they provide indispensable amount of data for the study of instability mechanisms of the Marangoni convection.
First identification of sub- and supercritical convection patterns from ‘GeoFlow’, the geophysical flow simulation experiment integrated in Fluid Science Laboratory
Physical mechanisms of thermally driven rotating fluids are important for a large number of geophysical problems, e.g. to explain the convection of the Earth's liquid outer core. Objective of the ‘GeoFlow’ experiment is to study stability, pattern formation, and transition to chaos of thermal convection in fluid-filled concentric, co-axially rotating spheres. This experiment is integrated in the Fluid Science Laboratory of the European COLUMBUS module on International Space Station. Fluid dynamics of the experiment was predicted with numerical simulations by means of a spectral code. In the non-rotating case the onset of convection bifurcated into steady fluid flow. Here patterns of convection showed co-existing states with axisymmetric, cubic and pentagonal modes. Transition to chaos was in the form of sudden onset. For the thermal convection in rotating spheres the onset of first instability showed an increase of modes for higher parameter regime. Transition was from steady via periodic to chaotic behaviour. Convection patterns of the experiment are observed with the Wollaston shearing interferometry. Images are in terms of interferograms with fringe patterns corresponding to special convective flows. A first glance at the images showed the classification of sub- and supercritical flow regimes. Aligned with numerical data a shift between experiment and numerical simulation was identified. Identification of convection patterns in interferograms was demonstrated for the example of a supercritical flow.
Experimental and numerical analysis of mass transfer in a binary mixture with Soret effect in the presence of weak convection
One of the targets of the experiment IVIDIL (Influence Vibrations on Diffusion in Liquids) conducted on-board ISS was to study the response of binary mixtures to vibrational forcing when the density gradient results from thermal and compositional variations. Compositional variations were created by the Soret effect and can strengthen or weaken the overall density gradient and, consequently, the response to vibrational forcing. We present the results of two experimental runs conducted on-board ISS in the frame of the experiment IVIDIL for low and strong vibrational forcing. The experimental observations revealed that a significant mean flow is set within 2 minutes after imposing vibrations and later in time it varies weakly and slowly due to the Soret effect. A mathematical model has been developed to compute the thermal and concentration fields in the experiment IVIDIL and verify the accuracy of picture processing based on the classical approach used in non-convective systems with the Soret effect. The effect of temperature and concentrations perturbations by joint action of vibrational convection and Soret effect on long time scale are carefully examined. The model demonstrates that image processing used for non-convective systems is suitable for the systems with vibration-affected thermodiffusion experiment.
Disruption of an Aligned Dendritic Network by Bubbles During Re-melting in a Microgravity Environment
The Pore Formation and Mobility Investigation (PFMI) utilized quartz tubes containing succinonitrile and 0.24 wt% water “alloys” for directional solidification (DS) experiments which were conducted in the microgravity environment aboard the International Space Station (ISS; 2002–2006). The sample mixture was initially melted back under controlled conditions in order to establish an equilibrium solid-liquid interface. During this procedure thermocapillary convection initiated when the directional melting exposed a previously trapped bubble. The induced fluid flow was capable of detaching and redistributing large arrays of aligned dendrite branches. In other cases, rapidly translating bubbles originating in the mushy zone dislodged dendrite fragments from the interface. The detrimental consequence of randomly oriented dendrite arms at the equilibrium interface upon reinitiating controlled solidification is discussed.
Effect of ambient fluid flow upon onset of oscillatory thermocapillary convection in half-zone liquid bridge
Onset of the oscillatory thermocapillary convection in a half-zone liquid bridge is known to be sensitive to heat transfer at free surface of the liquid bridge and the ambient air motion. The effect of the heat transfer through the free surface upon the onset of the transition mechanism and three-dimensional nature of the oscillatory flow, however, is not yet fully understood. In the present study, the thermal fluid behavior of the ambient gas and its effect upon the criticality were investigated experimentally and numerically. In the experiment, flows in the liquid bridge and the ambient air were visualized by suspending tracer particles in both fluids. Volume of the ambient air region was adjusted by placing two partition disks perpendicular to the liquid bridge. The onset of oscillation depended on the distance between the partition disks; the critical Marangoni number increased with the decreasing distance. Three-dimensional simulation of the liquid bridge and the ambient air has been performed by the finite difference method in order to determine the onset of the oscillation. The present calculations demonstrate significant influence of the heat transfer at the free surface on the onset of oscillation. The results of calculations yield a good agreement with the experimental critical values.
Modeling of the Fluid Merging Viscosity Measurement (FMVM) International Space Station experiment with COMSOL MultiPhysics
The purpose of FMVM was to measure the rate of coalescence of two highly viscous liquid drops and correlate the results with the liquid viscosity and surface tension. The experiment takes advantage of the low gravitational force free floating conditions in space allowing the unconstrained coalescence of two nearly spherical drops. The merging of the drops is accomplished by deploying them from a syringe and suspension on Nomex threads. An astronaut’s slow manipulation of one of the drops toward a stationary droplet till there is contact initiates the droplet coalescence. Coalescence and merging occurs due to shape relaxation and reduction of surface energy, being resisted by the viscous drag within the liquid. Experiments were conducted onboard the International Space Station in July of 2004 and subsequently in May of 2005. The coalescence was recorded on video and down-linked near real-time. When the coefficient of surface tension for the liquid is known, the increase in contact radius can be used to determine the coefficient of viscosity for that liquid. The viscosity is determined by fitting the time to achieve contact neck diameter equal to half of the initial droplet diameter. This time is compared with a relaxation time scaling coefficient to arrive at the liquid viscosity. Recent fluid dynamical numerical simulations with COMSOL MultiPhysics of the coalescence process will be presented. The results are important for a better understanding of the coalescence process. The experiment is also relevant to liquid phase sintering, free form in-situ fabrication, and as a potential new method for measuring the viscosity of viscous glass formers at low shear rates.