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Research Containing: Bubbles

Capillary Channel Flow (CCF) EU2–02 on the International Space Station (ISS): An Experimental Investigation of Passive Bubble Separations in an Open Capillary Channel

by cfynanon 22 August 2016in Physical Sciences No comment

It would be signi cantly easier to design uid systems for spacecraft if the uid phases behaved similarly to those on earth. In this research an open 15:8 wedge- sectioned channel is employed to separate bubbles from a two-phase ow in a micro- gravity environment. The bubbles appear to rise in the channel and coalesce with the free surface in much the same way as would bubbles in a terrestrial environ- ment, only the combined e ects of surface tension, wetting, and conduit geometry replace the role of buoyancy. The host liquid is drawn along the channel by a pump and noncondensible gas bubbles are injected into it near the channel vertex at the channel inlet. Control parameters include bubble volume, bubble frequency, liq- uid volumetric ow rate, and channel length. The asymmetrically con ned bubbles are driven in the cross- ow direction by capillary forces until they at least become inscribed within the section or until they come in contact with the free surface, whereupon they usually coalesce and leave the ow. The merging of bubbles en- hances, but does not guarantee, the latter. The experiments are performed aboard the International Space Station as a subset of the Capillary Channel Flow experi- ments. The ight hardware is commanded remotely and continuously from ground stations during the tests and an extensive array of experiments is conducted identi- fying numerous bubble ow regimes and regime transitions depending on the ratio and magnitude of the gas and liquid volumetric ow rates. The breadth of the pub- licly available experiments is conveyed herein primarily by narrative and by regime maps, where transitions are approximated by simple expressions immediately useful for the purposes of design and deeper analysis.

Related URLs:
http://ntrs.nasa.gov/search.jsp?R=20160001341

Bubble Formation and Transport During Directional Solidification in Microgravity: Model Experiments on the Space Station

by cfynanon 9 June 2015in Physical Sciences No comment

Flow Visualization experiments on the controlled melting and solidification of succinonitrile were conducted in the glovebox facility of the International Space Station (ISS). The experimental samples were prepared on ground by filling glass tubes, 1 cm ID and approximately 30 cm in length, with pure succinonitrile (SCN) under 450 millibar of nitrogen. Porosity in the samples arose from natural shrinkage, and in some cases by direct insertion of nitrogen bubbles, during solidification of the liquid SCN. The samples were processed in the Pore Formation and Mobility Investigation (PFMI) apparatus that is placed in the glovebox facility (GBX) aboard the ISS. Experimental processing parameters of temperature gradient and translation speed, as well as camera settings, were remotely monitored and manipulated from the ground Telescience Center (TSC) at the Marshall Space Flight Center. During the experiments, the sample is first subjected to a unidirectional melt back, generally at 10 microns per second, with a constant temperature gradient ahead of the melting interface. The temperatures in the sample are monitored by six in situ thermocouples. Real time visualization of the controlled directional melt back shows bubbles of different sizes initiating at the melt interface and, upon dislodging from the melting solid, migrating at different speeds into the temperature field ahead of them, before coming to rest. The thermocapillary flow field set up in the melt, ahead of the interface, is dramatic in the context of the large bubbles, and plays a major role in dislodging the bubble. A preliminary analysis of the observed bubble formation and mobility during melt back and its implication to future microgravity experiments is presented and discussed.

Related URLs:
http://dx.doi.org/10.2514/6.2004-627

Observation of an Aligned Gas – Solid Eutectic during Controlled Directional Solidification aboard the International Space Station – Comparison with Ground-based Studies

by cfynanon 9 June 2015in Physical Sciences No comment

Direct observation of the controlled melting and solidification of succinonitrile was conducted in the glovebox facility of the International Space Station (ISS). The experimental samples were prepared on ground by filling glass tubes, 1 cm ID and approximately 30 cm in length, with pure succinonitrile (SCN) in an atmosphere of nitrogen at 450 millibar pressure for eventual processing in the Pore Formation and Mobility Investigation (PFMI) apparatus in the glovebox facility (GBX) on board the ISS. Real time visualization during controlled directional melt back of the sample showed nitrogen bubbles emerging from the interface and moving through the liquid up the imposed temperature gradient. Over a period of time these bubbles disappear by dissolving into the melt. Translation is stopped after melting back of about 9 cm of the sample, with an equilibrium solid-liquid interface established. During controlled re-solidification, aligned tubes of gas were seen growing perpendicular to the planar solid/liquid interface, inferring that the nitrogen previously dissolved into the liquid SCN was now coming out at the solid/liquid interface and forming the little studied liquid = solid + gas eutectic-type reaction. The observed structure is evaluated in terms of spacing dimensions, interface undercooling, and mechanisms for spacing adjustments. Finally, the significance of processing in a microgravity environment is ascertained in view of ground-based results.

Related URLs:

Transient Capillary Channel Flow Stability

by cfynanon 9 June 2015in Physical Sciences No comment

Capillary channel techniques with free liquid surfaces provide very reliable means for liquid management in space. However, capillary channel flow is subject to limitation due to liquid surface instabilities when a critical flow rate is reached. Steady flow rate limitation is a consequence of the choking effect and well understood. Critical steady flow rate computation with a one-dimensional model is related to a numerical singularity which occurs at critical flow. For transient flow the singularity does not occur. Therefore, a new transient stability model is defined. It is based on the steady model, a simplified transient momentum balance, the consideration of the capillary pressure of typical observed surface shapes, and on a simplified dynamic inside the channel. The balance and dynamic are defined by liquid and geometrical properties only and therefore significantly easier to compute than a transient differential equation system. In 2011, experiments were performed in cooperation with NASA on the International Space Station (ISS) to confirm the model for steady flow and validate the new transient model. A new phenomenon is discussed, the flexibility effect, which provides significant additional transient stability for channels of sufficient length. An undesired feedback effect, provoked by the reuse of the liquid in a circular loop of the experimental setup, and which influenced the measurements, is compensated by a semi-empirical model for a feedback ratio.

Related URLs:
http://dx.doi.org/10.1007/s12217-014-9403-z

Capillary channel flow experiments aboard the International Space Station

by cfynanon 9 June 2015in Physical Sciences No comment

In the near-weightless environment of orbiting spacecraft capillary forces dominate interfacial flow phenomena over unearthly large length scales. In current experiments aboard the International Space Station, partially open channels are being investigated to determine critical flow rate-limiting conditions above which the free surface collapses ingesting bubbles. Without the natural passive phase separating qualities of buoyancy, such ingested bubbles can in turn wreak havoc on the fluid transport systems of spacecraft. The flow channels under investigation represent geometric families of conduits with applications to liquid propellant acquisition, thermal fluids circulation, and water processing for life support. Present and near future experiments focus on transient phenomena and conduit asymmetries allowing capillary forces to replace the role of gravity to perform passive phase separations. Terrestrial applications are noted where enhanced transport via direct liquid-gas contact is desired.

Related URLs:
http://link.aps.org/doi/10.1103/PhysRevE.88.063009

Does water foam exist in microgravity?

by cfynanon 9 June 2015in Physical Sciences No comment

Liquid foams are omnipresent in everyday life, but little is understood about their properties. On Earth, the liquid rapidly drains out of the foam because of gravity, leading to rupture of the thin liquid films between bubbles. Several questions arise: are liquid foams more stable in microgravity environments? Can pure liquids, such as water, form stable foams in microgravity whereas they do not on Earth? In order to answer these questions, we performed experiments both in parabolic flights and in the International Space Station.

Related URLs:
http://dx.doi.org/10.1051/epn/2014303

How foams unstable on Earth behave in microgravity?

by cfynanon 9 June 2015in Physical Sciences No comment

Foams made of gas bubbles dispersed in a liquid have limited stability and disappear rapidly unless surface active species are used. Foams can be a nuisance or very much sought after, however the control over the foaming and stability is still hampered because of the limited understanding of foam properties. On Earth, liquid rapidly drains out of the foam because of gravity, the liquid films formed between bubbles thin and break. In microgravity conditions, gravity drainage is suppressed and stability is expected to be greatly enhanced. We describe investigations of foams that are very unstable on Earth, including foams made with liquids containing antifoaming agents. Experiments performed in the International Space Station (ISS) show that foam generation can still be limited, however once created these foams are very stable.

Related URLs:
http://www.sciencedirect.com/science/article/pii/S0927775714005238

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