Research Containing: hydrodynamic
To understand the boiling crisis mechanism, one can take advantage of the slowing down of boiling at high pressures, in the close vicinity of the liquid-vapor critical point of the given fluid. To preserve conventional bub- ble geometry, such experiments need to be carried out in low gravity. We report here two kinds of saturated boiling experiments. First we discuss the spatial experiments with SF6 at 46 ◦ C. Next we address two ground-based experi- ments under magnetic gravity compensation with H2 at 33 K. We compare both kinds of experiments and show their complementarity. The dry spots under vapor bubbles are visualized by using transparent heaters made with metal oxide films. We evidence two regimes of the dry spots growth: the regime of circular dry spots and the regime of chain coalescence of dry spots that immediately pre- cedes the heater dryout. A recent H2 experiment is shown to bridge the gap between the near-critical and low pressure boiling experiments.
Capillary Channel Flow (CCF) EU2–02 on the International Space Station (ISS): An Experimental Investigation of Passive Bubble Separations in an Open Capillary Channel
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.
For over two decades nighttime satellite imagery from the Operational Linescan System (OLS) has been used to detect impervious surfaces. However, OLS-based maps suffer from the sensor’s coarse resolution (2.7 km/pixel), overglow, and saturation in urban areas, resulting in inaccurate estimates of the extent and degree of impervious surfaces. In order to provide more reliable estimates of impervious surface extent, we used high resolution (~10 m/pixel) nighttime photography from the International Space Station (ISS). Focusing on the city of Berlin in Germany, we produced a map of the extent of impervious surfaces. Our classification was 85% accurate for both user and producer measures. Impervious surfaces omitted by ISS photography were mainly transit roads and airport runways, while green areas and water bodies within the city were falsely identified. An analysis based on ISS imagery classified 55.7% of the study area as impervious, which is only 3.9% less than ground truth (while the OLS-based estimate was 40% higher than ground truth). ISS imagery failed to provide reliable information about the degree of imperviousness for individual pixels (±20% errors); nevertheless it accurately estimated the spatially-averaged degree of imperviousness for the whole study area (30.2% vs. the reference value of 30.1%). These results show that ISS photography is an important source of nighttime imagery for mapping the extent of impervious surfaces, and represents a considerable improvement over OLS capabilities.
We study the singular event which took place when conducting an experiment with a liquid bridge aboard the International Space Station. The liquid bridge vibrated unexpectedly for several tens of seconds with an oscillation amplitude larger than 15% of its radius. At first glance, the analysis of the mass force measured by the accelerometer during the oscillation did not show any significant perturbation. However, our study reveals the existence of two small-amplitude vibrations of the experimental setup with practically the resonance frequency of the first lateral mode. These vibrations occurred a few tens of seconds before the liquid bridge oscillation reached its maximum amplitude, produced a mass force with a magnitude of the order of 10−5g. The numerical integration of the non-linear Navier–Stokes equations reproduces remarkably well the free surface oscillations measured in the experiments. It allows us to reconstruct the three-dimensional liquid bridge motion which took place in the experiment. The present study illustrates the sensitivity of liquid bridges in a microgravity environment, where tiny perturbations may produce significant vibrations which survive over long periods of time.