A Model for the Anomalous Velocity-Undercooling Behaviour of Levitated Al-Ni Alloys On-board the International Space Station

Al-Ni alloys (for Ni < 45 at.%) show a unique property in that, over at least part of the accessible undercooling range, the recalescence velocity measured in electromagnetically levitated samples is observed to decrease as the undercooling increases. This result has been subject to careful validation, including microgravity experiments utilising the TEMPUS levitation facility on-board the International Space Station (ISS). In these experiments, anomalous growth is observed to coincide with a recalescence morphology comprising multiple circular growth fronts [Herlach et al. Phys. Rev. Mat. 3, 073,402 (2019)], termed “scales”. In this paper we present an analysis of high speed video data from the ISS experiments in which we show that such scale-like growth is consistent with a recalescence front that is initially confined to a thin layer on the surface of the sample. This then nucleates a slower, radial inward growth, which is consistent with microstructures observed in Al-Ni droplets. We show that such surface recalescence would be favoured for samples which were surface enriched in Ni, wherein the recalescence velocity (at fixed nucleation temperature) increases rapidly with Ni-concentration. Moreover, it is shown that the anomalous velocity behaviour can be matched in all compositions studied if the surface enhancement in Ni is a linear function of the nucleation temperature with a gradient of 0.03 at.% K−1. Analysis of historical results from the literature indicates that such surface Ni-enhancement may have been present, but overlooked, in other experiments on Al-rich Al-Ni droplets.

Effect of Wettability on Vaporization of Hydrocarbon Solvents in Capillary Media

Summary Tight rock reservoirs have gained popularity and become a subject of great interest because of their huge recovery potential. A substantial portion of the potential hydrocarbon could be removed from the reservoir by injecting solvent gases [hydrocarbon or carbon dioxide (CO2)] as an enhanced-oil-recovery (EOR) application. Achieving precise modeling of these processes and an accurate description of hydrocarbon dynamics requires a clear understanding of phase-change behavior in a confined (capillary) medium. It was previously shown that early vaporization of liquids could occur in channels that were larger than 1000 nm. The surface wettability plays a critical role in influencing the vaporization and condensation nature in confined systems. This paper studies the influence of the medium wettability on phase-transition temperatures of liquid hydrocarbons in macrochannels (greater than 1000 nm) and nanochannels (less than 500 nm) by using different types of rock samples. The boiling temperature of hydrocarbon solvents was measured in two extreme wetting conditions: (1) strongly water-wet and (2) strongly oil-wet. Boiling temperatures of heptane and octane in sandstone, limestone, and tight sandstone were observed to be lower than their bulk boiling points by 13% (4% in Kelvin units), on average. Altering rock wettability characteristically changes the average hydrocarbon nucleation temperatures, being as critical as the pore size. Changing sandstone’s wettability to strongly oil-wet shifted the average nucleation temperature of heptane and octane by 6% (1.3% in Kelvin units) and 15% (0.8% in Kelvin units), compared with cases before wettability alteration. The experimental outcomes also showed that reducing the solvent adsorption on clays in Berea sandstone lowers the nucleation temperature of heptane and octane from their normal phase-change temperatures by 20% (4.3% in Kelvin units) and 30% (6.5% in Kelvin units). In comparison with the medium wettability alteration, reducing the solvent adsorption had a greater influence on nucleation temperatures. Such a phenomenon shows that molecule-solid interactions have more control of altering the phase behavior of solvents than of medium wettability.

Introducing Ice Nucleating Particles functionality into the Unified Model and its impact on the Southern Ocean short-wave radiation biases

Insufficient reflection of short-wave radiation especially over the Southern Ocean region is still a leading issue in many present-day global climate models. One of the potential reasons for this observed bias is an inadequate representation of clouds. In a previous study, we modified the cloud micro-physics scheme in the Unified Model and showed that choosing a more realistic value for the capacitance or shape parameter of atmospheric ice-crystals, in better agreement with theory and observations, benefits the simulation of short-wave radiation over the Southern Ocean by brightening the clouds. However, attempts to modify the cloud phase by directly adjusting the micro-physics process rates like capacitance tend to affect both the hemispheres symmetrically whereas we seek to brighten only the high-latitude Southern Hemisphere clouds. In this study we implement a simple prognostic parametrisation whereby the heterogeneous ice nucleation temperature is made to vary three-dimensionally as a function of the mineral dust distribution in the model. As a result, those regions with less dust number density would have lower nucleation temperature compared to the default global value of −10 °C. By using mineral dust as an indicator for ice nucleating particles in the model, this parametrisation thus captures the impact of ice nucleating particles on the cloud distribution due to its general paucity over the Southern Ocean region. This approach thus improves the physics of the model with minimal complexity.

The Effect of Static Electric Field and Nucleator Agent on the Solidification of CaCl2·6H2O and Ca(NO3)2·4H2O

The addition of energy from the electric field is one way in the active method to overcome the nucleation barriers of inorganic phase change materials (PCM) e.g. salt hydrate. The effort is to aim at improving the performance of PCM as a thermal energy storage system. Moreover, the passive method commonly uses a chemical substance called nucleator agent to induce the nucleation and to reduce the phase separation that typically occurs during the freezing-thawing cycle of salt hydrate PCM. In this paper, we report an experimental study to conduct the effect of the static electric field (DC voltage) and nucleator agent as a combination of passive and active methods on the nucleation of salt hydrates consisting of CaCl2·6H2O and Ca(NO3)2·4H2O. In general, the nucleation temperature of CaCl2·6H2O and CaCl2·6H2O+BaSO4 (0.1 wt%) become higher with the increase of the intensity of the electric field, leading to the decreases of supercooling degree. Besides that, the electric field also induces the increase in the nucleation rate, as measured by the shorter induction time. Meanwhile, the case for Ca(NO3)2·4H2O and Ca(NO3)2·4H2O+Ba(OH)2·8H2O (1 wt%) show that the nucleation temperature tends to become smaller with increase the intensity of the electric field, leading to increases the supercooling degree. However, the addition of the nucleator agent, Ba(OH)2·8H2O (1 wt%) to Ca(NO3)2·4H2O has not provided a significant result in terms of nucleation probability.

Effect of Copper Electrode Geometry on Electrofreezing of the Phase-Change Material CaCl2·6H2O

The development of effective active thermal energy storage systems requires an understanding of how electrode geometry affects the electrofreezing process. This study aimed to observe the nucleation behavior of an inorganic phase-change material, CaCl2·6H2O, using a DC electric field and various copper electrode geometries. The effects of both the electrode diameter (d=0.5 and 0.7 mm) and the tip shape (flat and sharp end surfaces) were investigated. Data analysis was performed to reveal the nucleation temperature, freezing temperature, supercooling degree, supercooling time, and crystallization time period. The copper electrode with the larger diameter was found to result in a higher nucleation temperature, a smaller supercooling degree, faster nucleation, and a shorter crystallization time period. Moreover, changing from a flat tip to a sharp tip decreased the nucleation temperature and increased the supercooling degree. This study showed that the electrode geometry plays an important role in the phase-change behavior of CaCl2·6H2O.

Effects of modifying the hypoeutectic AlMg5Si2Mn alloy via addition of Al10Sr and/or Al5TiB

This work demonstrates that the combined addition of Al10Sr and Al5TiB master alloys to the AlMg5Si2Mn effectively refines the grain microstructure and partially modifies the eutectic Mg2Si phase. Thorough spectroscopic characterization reveals that the grain refinement effect is due to Al3Ti particles acting as nucleation sites for α-Al grains, and the increased nucleation temperature of α-Al is due to Al10Sr addition. It is also determined that TiB2 particles can act as nucleation substrates for the primary Mg2Si phase. The prepared alloy sample with the finest microstructure (treated with both Al10Sr and Al5TiB) exhibits the greatest corrosion resistance among all tested samples.