Complete Melting Research Articles

Simulation study on charging performance of the latent energy storage heat exchanger with a novel conical inner tube

The application of a latent heat thermal energy storage (LHTES) system can effectively solve the problem of the mismatch between the energy supply and demand. However, most studies focus on the traditional cylindrical configuration with a low heat storage rate, which limits the wide application of LHTES systems. The configuration optimization of a vertical shell-and-tube LHTES unit is carried out in this paper to enhance heat storage rate, and the effects of cone angle values (0–10°) in the conical shell and conical tube configurations on the heat storage performance are explored by numerical simulations. The results show that these two kinds of conical configurations both significantly improve the melting rate of the phase change material (PCM) compared with the cylindrical unit. However, when the cone angle is the same, the heat transfer enhancement effect of the latter is found to be superior to that of the former. Moreover, the complete melting time is shortened by 46 % as the cone angle of the inner tube increases from 0° to 10°. More importantly, a novel conical tube configuration design is proposed to eliminate the hard-to-melt zone at the bottom of the LHTES unit with the conical tube, then the effects of different tilting heights (60–170 mm) and bottom annulus distances (0–10 mm) are investigated. The results demonstrate that the melting rate increases with the increase in the tilting height, but the increased amplitude of that gradually reduces. Besides, the average heat storage rate shows a trend of first increasing and then decreasing with the increase in the distance of the bottom annulus. When the bottom annulus distance is 2 mm, the average heat storage rate reaches the maximum value, which is 94.84 W. The research results can provide a reference for the engineering design and configuration optimization of the vertical shell-and-tube phase change heat exchanger.

Study of thermal behavior during the directed energy deposition (DED) of Ti6Al4V alloy using finite element modeling (FEM)

Directed energy deposition (DED) offers the ability for a high deposition rate as compared to other additive manufacturing (AM) processes generally due to the high layer thickness compared to other AM processes. Finite element modeling (FEM) of a DED process can predict the melt-pool and temperature profile without extensive experimentation, thus saving considerable time, material and money. In the current study, FEM of a high layer thickness DED process is developed by considering the Gaussian heat source model to investigate the melt-pool and temperature profiles under different process conditions. The current model is in agreement with the experimental data. Results showed insufficient melting at the very start of the process, but complete melting is achieved as the process continues. From the 2nd layer onwards, there is partial remelting of previous layers. The model predicted a rise in temperature and melt-pool dimensions when the power increases and the opposite effect when the scan speed is increased. It showed the accurate implementation of the Gaussian distributed heat source model for a high-layer thickness DED model.

Effects of laser printing parameters on molten pool formation, microstructure evolution and mechanical properties of laser directed energy deposition of difficult-to-process tungsten heavy-alloy

Laser directed energy deposition (LDED) additive manufacturing is promising in the fabrication of high-performance W-based heavy-alloys which are normally difficult-to-process due to their unique nature such as the extremely high melting point and high ductile-brittle transition temperature. In this work, a novel 95 W heavy alloy modified by nanosized Ni and Fe particles was manufactured by LDED process. The effects of laser power on the formation mechanisms of molten pool were investigated. It was found that a relatively high laser power could promote the complete melting of W powder and formation of regular molten pool morphology. The microstructures of laser-processed W heavy alloy showed columnar grains, cellular grains and equiaxed grains from bottom to top within the molten pool along the building direction. The three-dimensional finite element simulation revealed the temperature gradient ( G ), cooling rate ( C ) and solidification velocity ( S ) within the molten pool were different, which was responsible for the microstructural development. The bulk-form 95 W heavy-alloy part free of defects was produced at the laser power of 1400 W. The optimally processed 95 W heavy-alloy by LDED showed a high average nanohardness of 8.6 GPa and a sound wear performance with a low coefficient of friction (COF) of 0.5. The tensile tests also revealed the achievement of a combination of an excellent tensile strength of 541 MPa and an elongation of 0.99%. This work demonstrated that LDED was able to manufacture high-performance tungsten heavy-alloys in bulk form with outstanding mechanical properties.

Effect of charging/discharging temperatures upon melting and solidification of PCM-metal foam composite in a heat storage tube

Solar energy has addressed the significant issue of energy conservation and emission reduction through providing an alternative solution to the building energy utilization. To solve the inherent intermittency problem, latent heat phase change module is designed to even the fluctuation of energy supply. This paper designs a phase change heat storage/release unit and builds a visual test system for phase change heat storage/release. To study the influence of different heating and cooling temperatures upon thermal cycle of melting and solidification, a series of experiments are carried out. Melting front evolution, temperature field and response, and uniformity analysis are assessed. Experimental results demonstrate that the phase interface changes from top to bottom in the melting process. However, solidified covers appear evenly up and down during solidification process. Reasonably increasing the heating temperature effectively shortens the complete melting time. 56.0% reduction in full melting time is achieved if increasing the heating temperature from 65 °C to 85 °C. However, this is done at the expense of decreasing the temperature uniformity of the PCM. The results proved guidance for the design strategy on passive building energy utilization.

Numerical analysis of the effect of the iso-surface fin redistribution on the performance enhancement of a shell-and-tube latent heat thermal energy storage unit for low-temperature applications

In the present study, the melting and solidification processes of a Phase Change Material (PCM) are numerically carried out in eight geometrical configurations of horizontal Latent Heat Thermal Energy Storage (LHTES) units for low-temperature applications. The PCM considered in this study is water undergoing solidification below its density-temperature inversion point (T < 4 °C) at atmospheric pressure, and the Heat Transfer Fluid (HTF) considered is a water/glycol mixture solution. Among the eight geometrical configurations studied, four are made of longitudinal finned tubes and a shell and the other four are made of radial finned tubes and a shell. All the geometrical configurations studied have the same PCM volume, compactness and total heat transfer surface. The objective of the study is to compare their thermal performances in order to highlight the most efficient configurations during melting and solidification processes. Effects of fin redistribution at a constant total heat transfer surface on the local melting and solidification mechanisms are investigated. Variations in the solid volume fraction during melting and solidification and the associated kinetics are compared and subjected to quantitative analysis. Natural convection and its effect on the melting and solidification mechanisms are studied and analysed using the commercial CFD code Star CCM + V12.02. This study shows that increasing the number of fins, at a constant PCM volume, compactness and total heat transfer surface, reduces the thermal performance of the heat storage unit when complete melting or solidification is desired. The opposite behaviour is observed at low melting/solidification rates. Among the configurations studied, the best thermal performances at the end of the melting/solidification phases are obtained in heat storage units with a smaller number of fins. A solidification rate higher than 82 % was reached with the 4 longitudinal fins configuration, compared with 75.7 % with the 8 longitudinal fins configuration, 63.3 % with the 16 longitudinal fins configuration and 55 % with the 32 longitudinal fin configuration after 4 h of solidification process. A similar succession of solidification rates was observed with radial fin configurations. During the melting process, the same trends that had been observed during the solidification process were noted: better performance from configurations with 4 longitudinal or radial fins than from the other configurations with larger numbers of fins after 4 h of the melting process. Conversely, at the very beginning of the melting/solidification process, better performances were obtained in the configurations with the largest number of fins. Therefore, for given values of heat transfer surface augmentation and PCM volume, in shell-and-tube storage units used for low-temperature applications, it is recommended to prioritize finned-tube configurations with small numbers of tall fins rather than those with large numbers of short fins when complete melting or solidification is desired.

Thermal hysteresis analysis of finned-heat-pipe-assisted latent heat thermal energy storage application for solar water heater system

This work numerically investigates the performance of an innovative compact-design solar water heater (SWH) using evacuated tube heat pipe solar collectors (ETHPSCs) coupled with a latent heat storage tank (LHS tank). This system is thought to be an unwritten fourth generation of solar water heaters known as heat-pipe-assisted thermal batteries. The basic and explicit hypothesis is to store energy than a bulk of hot water, which is the key distinction in compared to the prior classical sorts. Using three phase change materials (PCMs) for high and low solar intensity categories in the tropical region of South East Asia, a computational effort is conducted to model the system and the given temperature headed for the supply output hot water. This research is validated by two interrelated experimental and analytical investigations. According to the results of 12-h charging (melting) process, the maximum mean temperature of the PCMs belongs to PCM C, Rubitherm (RT70HC), which indicates 101 °C during the defined typical high daily solar radiation. This value implies 2.02% error in compared to the reference study. Referring to the solid-liquid interface parameter, the maximum liquefied thickness is also for PCM C with 76 mm thickness. However, reaching the complete melting is required by further studies.

Samarium: from a distorted-fcc phase to melting under dynamic compression using in-situ x-ray diffraction

Lattice and electronic structure interactions for f-electrons are fundamental challenges for lanthanide equation of state development. Difficulties in first-principles calculations, such as density functional theory (DFT), emphasize the need for well-characterized experimental data. Here, we measure in-situ x-ray diffraction of shocked samarium (Sm) and temperature along the Hugoniot for the first time, providing direct evidence for phase transitions. We report direct evidence of a distorted fcc (dfcc) phase at 23 GPa. Shocked samarium melts from the dfcc phase starting at 33 GPa (1333 K), with complete melt at 40 GPa (1468 K). Previous work indicated shock melt at 27 GPa (1200 K), underscoring the significance of x-ray measurements for detecting phase transitions. Interestingly, our observed melting is in sharp contrast with the melting reported by a diamond anvil cell study. These experimental data can tightly constrain first principles calculations and serve as key touchstones for equation of state modeling.

No fins attached? Numerical analysis of internal–external fins coupled PCM melting for solar applications

Thermal energy storage/sink is imperative for any type of solar energy harnessing system/technology. However, the inevitable challenge remains in developing and optimizing a thermal energy storage (TES) system suitable for long cyclic operation. This work numerically analyses the melting characteristics of phase change material (PCM; RT42)-based latent thermal energy storage (LTES) as a two-dimensional rectangular enclosure. The consequence of utilizing internal–external extended surfaces (fins) with realistic convective boundary condition is studied for photovoltaic thermal (PVT) applications. The application of internal–external fins is intended for enhancing the charging (melting) duration according to the long cyclic solar operation and better dissipation of heat after complete melting. Three kind of fins-rectangular, triangular, and Y-type- are investigated for two orientations of storage/sink enclosures-upright ( θ = 90°) and inclined ( θ = 35°). Moreover, three fin configurations are analyzed-equal, decreasing and increasing-stepped-based on the arrangement of fins along the enclosure height. Two-dimensional, transient numerical simulations are conducted for governing PCM melting, contemplating the consequences of natural convection. The performance assessment of each storage unit (orientation/fin-type/fin-arrangement) is made based on parameters such as time enhancement ratio, suppression ratio, liquid fraction, temperature distribution, melt velocity magnitude, storage capacity/rate, and enhancement in Nusselt number. The melting is delayed for triangular and Y-fins enclosures as compared to that for rectangular fins. Equal and increasing-stepped Y-fin arrangements yielded the largest time enhancement ratios of 42.38% and 29.86% for inclined and upright enclosures, respectively. Moreover, Y-fins-based enclosure produced a larger average suppression ratio as compared to other types of fins. A 2.56, 2.67, and 2.64 times enhancement in Nusselt number is observed for increasing-stepped fin arrangement of rectangular, triangular, and Y-finned upright enclosures, respectively. However, for the inclined enclosure, a 2.32 times enhancement in Nusselt number is reported for increasing-stepped Y-fin. • Internal–external fin-coupled thermal storage for cyclic solar PVT applications. • Utilization of rectangular, triangular, and Y-fins in augmenting PCM melting. • Assessment of 3 configurations/fin-immersions for intensifying convection currents. • Effect of cavity inclination on melting characteristics for long duty operation. • Maximum 2.67 times Nusselt number enhancement for increasing triangular fin.

Thermal energy storage enhancement of a forced circulation solar water heater’s vertical tank unit using phase change material

This work aims to investigate the thermodynamic effect of phase change material integration within vertical storage tanks that are connected to forced circulation solar water heaters, on their thermal energy storage capability. The phase change material is encapsulated in cylindrical and elliptical capsules, which are integrated at the bottom, middle and top sections of the tank. Hence, three parametric studies were carried out depending on the geometry of the encapsulation, besides to their integration position inside the tank, through computational fluid dynamic calculations. The numerical model was established under “OpenFOAM” and the enthalpy-porosity method for the phase change phenomena modeling was validated against the experimental data of the literature. For this purpose, the following thermal performance indicators were used: temperature, liquid fraction contours and the tank’s energy storage efficiency. It was found that the time required for a complete melting of the phase change material inside the cylindrical capsules is higher than that of the elliptical ones (beyond 17.50 s), for a fixed integrating location. Otherwise, 21, 23 and 17.50 s were necessary for the melting of the phase change material filling the vertical ellipse (top configuration), horizontal ellipse (bottom configuration) and cylindrical cavity (middle configuration) respectively. In addition, results showed that the shapes of phase change materials affect the thermal storage efficiency. For example, 25, 28 and more than 40 s were sufficient to reach 100% of the vertical tank’s storage efficiency for the following top configurations: vertical ellipse, horizontal ellipse and cylindrical cavity, respectively. Finally yet importantly, integrating the phase change materials at the middle section of the tank can be considered as the optimal configuration, because it allows a gain of 2 °C compared to the bottom and top zones.

The Use of Ultrasound in Shaping the Properties of Ice Cream with Oleogel Based on Oil Extracted from Tomato Seeds

In this study, the possibility of using ultrasound technology as an alternative to traditional pasteurization and homogenization in ice cream production was presented. Three types of ice cream with different proportions of oleogel (5, 6, and 7%) prepared using tomato seed oil were studied. The fatty acid contents of the oil were analyzed. Using chemical analysis, dry matter, fat, protein, dietary fiber, ash, and pH of the ice cream samples were determined. The physical analysis included analysis of the ice cream samples using a differential scanning calorimeter (DSC) and determination of their first drop time, complete melting time, overrun, viscosity, hardness, and adhesiveness. The structure of the samples was evaluated using scanning electron microscopy. Fourier transform infrared spectroscopy spectra were measured using a dedicated QATR-S Single-Reflection ATR ACCESSORY with a diamond prism. With the increase in the proportion of oleogels, the fat and carbohydrate contents, the amount of freezable water, and the overrun of the samples were increased, whereas their viscosity and hardness were decreased. Oleogels were found to be a promising alternative to fat in ice cream rich in unsaturated fatty acids, and the ice cream samples prepared using ultrasound pasteurization showed lower overrun and viscosity and higher hardness.

Weakening of Indian Summer Monsoon Synoptic Activity in Response to Polar Sea Ice Melt Induced by Albedo Reduction in a Climate Model

AbstractThe effect of polar sea ice melt on low latitude climate is little known. To understand the response of the Indian summer monsoon (ISM) synoptic activity to the sea ice melt, we have run a suite of coupled and uncoupled climate model simulations. In one set of simulations, the albedo of sea ice is reduced so that it would melt due to increased absorption of solar radiation. The coupled model simulation with a reduced sea ice albedo resulted in an almost complete melting of the sea ice in summer in both hemispheres. A high‐resolution (50 km) atmospheric general circulation model (AGCM) is forced with the climatological annual cycles of sea surface temperature (SST) and sea ice concentrations (SIC) from the coupled model outputs to better resolve synoptic scale variability. In the high‐resolution AGCM simulations forced with SST and SIC from the sea ice melt experiments, the ISM circulation weakened substantially, and the monsoon low‐pressure systems (LPS) activity experienced an overall decline of 23%, with a widespread weakening in the south and a moderate strengthening over the north, in response to a decline of 78% (24%) in SIC over the Arctic (Antarctic) in the June–September season. The changes in the LPS activity in response to polar sea ice melt are found to be mostly driven by the changes in low‐level absolute vorticity and vertical shear over the Bay of Bengal.

Melting dynamics analysis of a multi-tube latent heat thermal energy storage system: Numerical study

This numerical study investigates the melting dynamics of paraffin wax as a phase change material (PCM) in a multi-tube latent heat thermal energy storage system (MT-LHTESS). The performance of an arrangement with four tubes is compared with a concentric single tube arrangement keeping the same cross-section area of heat transfer fluid tubes and the same PCM quantity. Four longitudinal fins of two types are also incorporated to further improve the melting. Thirteen cases with different arrangements of tubes and fins are compared in this study. The study attempts to select a suitable arrangement of the tubes. A two-dimensional enthalpy-porosity model in ANSYS-Fluent is used to solve the governing equations. The melting results are depicted as the contours of liquid fraction, temperature, and flow streamlines. The transient performance is assessed based on progress plots of liquid fraction and average domain velocity. It is observed that splitting into multiple tubes and using fins enhance the melting performance. The generation and coalescence of small vortices in the molten PCM accelerate the melting performance. The staggered array, larger eccentricity, and (+ type) finned tubes outperform their counterparts. The lowest melting time is observed in Case 2(+), having four staggered tubes, at 30 mm eccentricity, and with (+ type) fins. This case takes 75 min for complete melting, and 1411.1 kJ/m heat at a rate of 322 W/m is stored. The melting time gets reduced by 53.3–71.1% using four tubes and by 65.78–83.33% using finned four tubes as compared to the single tube case.

Analysis of Methods and Technologies for Composition Assessing of Corium Formed as a Result of the Fukushima Daiichi NPP Accident

This paper analyzes the methods and technologies for assessing the method of formation, composition, characteristics and features of corium, which is a mixture of nuclear and structural materials of the nuclear reactor core, formed as a result of an accident accompanied by partial or complete core melting. The study is based on data from the study of corium formed as a result of the accident at the Fukushima Daiichi nuclear power plant, which are in the public domain and are the result of the work of many scientific organizations around the world. Corium research is one of the main issues in the framework of improving nuclear safety in the future and is one of the objectives of the successful procedure for eliminating the consequences of the accident at the Fukushima Daiichi nuclear power plant. Without a detailed analysis of the neutronic, materials science, gravimetric and other characteristics of the corium, as well as the creation of a complex model of the corium that combines these data, it is impossible to organize an efficient and safe process for removing nuclear materials from the damaged units of the Fukushima Daiichi nuclear power plant. The objective of this work is to combine the existing research results into a data set that allows modeling of the corium using neutronic calculation codes and includes such data as the size, density and morphology of corium samples and their approximate nuclide composition. Such modeling allows not only to perform tasks related to increasing the level of safety in the implementation of the procedure for eliminating the consequences of the accident at the Fukushima Daiichi nuclear power plant, but also to serve as an international benchmark for modeling a mixture containing nuclear materials.

A numerical investigation on the finned storage rotation effect on the phase change material melting process of latent heat thermal energy storage system

Latent Heat Thermal Energy Storage (LHTES) is one of the most significant ways to mitigate solar energy intermittency using Phase Change Material (PCM). In the present study, a novel simultaneous utilization of longitudinal fins and storage rotation techniques to reduce the charging time and increase the amount of energy stored is considered in a double-pipe heat exchanger by developing a two-dimensional numerical model. Seven main cases have been defined, and finally, an optimal case with the least complete melting time has been introduced. The results indicated that embedding the fin on the lower half of the fixed storage accelerates the melting process. Rotating the finned storage can further reduce the melting time by up to 18.5 %, and embedded fins in rotating storage trap the melted PCM and reduce the complete melting time by 70 %. Finally, it is observed that simultaneous use of storage rotation and embedded longitudinal full-scale fins can achieve a 72 % reduction in complete melting time and a 115 % increase in total cumulative stored energy, which is related to the optimal case. The optimal case has four fins with an angle difference of 90° and two vertical stop steps with an angle of 180°.

Thermal conductivity enhancement and thermal saturation elimination designs of battery thermal management system for phase change materials based on triply periodic minimal surface

Phase change material (PCM) exhibits great application prospects for battery thermal management system (BTMS) due to their high enthalpy of phase change. However, the low thermal conductivity of PCM has limited its application in fast thermal response. Furthermore, the risk of battery thermal runaway will be exacerbated by the thermal saturation of PCM after complete melting. In this study, triply periodic minimal surface (TPMS) sheet structure is proposed for thermal conductivity enhancement and thermal saturation elimination of PCM. The thermal performance of the BTMS is studied by simulations and experiments. The results show that the TPMS sheet structure improves the heat transfer performance and decreases the battery temperature at 1C discharge rate. When the PCM is combined with the liquid cooling, the average battery temperature of the PCM/liquid/TPMS-based BTMS is 40% lower than that of the PCM/TPMS-based BTMS at 2C discharge rate, eliminating the thermal saturation of PCM. At 1C discharge rate, the phase transition temperature of PCM can affect the cooling performance of the BTMS without liquid cooling. For the PCM/TPMS-based BTMS, at 1C discharge rate, the average battery temperature with phase transition temperature of PCM at 35 °C is 8.5% lower than that at 40 °C.

High pressure-temperature phase equilibrium studies on Martian basalts: Implications for the failure of plate tectonics on Mars

Mars lacks ongoing tectonic activities such as volcanism and mountain-building processes. Modern plate tectonic movements on the Earth's surface are driven primarily by the descent of subducted slabs into the mantle. Slab crust made of dense eclogite metamorphosed from the Mid-Ocean Ridge Basalt provides an important driving force for slab subduction. Thus, mantle convection inside Mars can be hindered if the density contrast between Martian slab crust and the ambient Martian mantle is sufficiently smaller than that of Earth. To evaluate this hypothesis, we carried out high pressure-temperature phase equilibrium experiments on three different Martian basalts: Yamato 980459, NWA 8159, and GUSEV basalt (Humphrey). The GUSEV basalt and NWA 8159 undergo partial or complete melting along the Martian areotherm due to their high Fe content, suggesting that both compositions are geochemically evolved. Yamato 980459, the nearly primitive Martian basalt, on the other hand, would transform to a low-density eclogite at a depth of ∼250 km. The density contrast between a Martian crustal slab made of Yamato 980459, and the ambient Martian mantle is much smaller than that for Earth's mantle. Calculated slab sinking torques and velocities further suggest that sustained buoyancy-driven subduction of thin slabs was unlikely early in Martian history. Additional experiments exploring wider composition and pressure-temperature ranges are needed to fully understand the consequences of Martian mantle compositions and cooling history for the tectonic history of Mars.

Numerical Investigation of Heat Transfer Performance and Structural Optimization of Fan-Shaped Finned Tube Heat Exchanger

Latent heat storage technology is widely used in solar power generation. Aiming to enhance the energy utilization rate to a greater extent, an innovative fan-shaped structure has been proposed to construct the metal fins of the shell-and-tube thermal storage device. The enthalpy method is used to simulate the heat storage process and focuses on the influence of inlet conditions on heat transfer. The influence of the fin structure on the melting properties of phase change material has been studied. The results show that increasing inlet temperature and inlet flow rate is a convenient and effective way to improve energy efficiency. As the inlet temperature is increased from 343 K to 358 K, the total heat storage and energy efficiency are improved by 13.4% and 10.2%, respectively, and the melting time is reduced by 36.2%. As the flow rate is increased from 3 L/min to 9 L/min, the complete melting time is reduced by 33.4%. Energy efficiency peaks at a flow rate of 5 L/min. Reasonable optimization of the fin structure can enhance the natural convection circulation during the melting process and further improve the energy efficiency. The research results can guide the design and structural optimization of the finned tube heat storage device.

Design and assessments on a hybrid pin fin-metal foam structure towards enhancing melting heat transfer: An experimental study

Solar energy, as a kind of renewable energy, offers a large reserve to be harvested at a reasonably low cost for engineering applications. To decouple the temporal and spatial relevance of the continuous energy supply of solar energy, latent heat thermal energy storage can deal with this problem at different temperatures. Aiming to improve energy efficiency, a novel hybrid metal foam-pin fin structure is designed and assessed. Upon conducting measurements on a well-designed experimental bench, the phase change processes of paraffin that is filled in fins, metal foam, and a combination of both (hybrid structure) are evaluated. During the experiments, the transient melting interface is snapshotted and temperature development is documented under five different heat source temperatures of 61 °C, 63 °C, 65 °C, 68 °C, and 70 °C. In the foreground of the novel hybrid structure, each segment of the hybrid is also justified and discussed. Results indicate that the hybrid structure augments marked heat transfer. Compared to pure PCM, complete melting time decreases by 63.4% and simultaneously the temperature response rate increases by 143.9% as implementing the hybrid. Attempts to design hybrid structure find a solution to assess and operate thermal storage applications for solar engineering.

Insight into the Distribution of High-pressure Shock Metamorphism in Rubble-pile Asteroids

Shock metamorphism in ordinary chondrites allows for reconstructing impact events between asteroids in the main asteroid belt. Shock-darkening of ordinary chondrites occurs at the onset of complete shock melting of the rock (>70 GPa) or injection of sulfide and metal melt into the cracks within solid silicates (∼50 GPa). Darkening of ordinary chondrites masks diagnostic silicate features observed in the reflectance spectrum of S-complex asteroids so they appear similar to C/X-complex asteroids. In this work, we investigate the shock pressure and associated metamorphism pattern in rubble-pile asteroids at impact velocities of 4–10 km s−1. We use the iSALE shock physics code and implement two-dimensional models with simplified properties in order to quantify the influence of the following parameters on shock-darkening efficiency: impact velocity, porosity within the asteroid, impactor size, and ejection efficiency. We observe that, in rubble-pile asteroids, the velocity and size of the impactor are the constraining parameters in recording high-grade shock metamorphism. Yet, the recorded fraction of higher shock stages remains low (<0.2). Varying the porosity of the boulders from 10% to 30% does not significantly affect the distribution of pressure and fraction of shock-darkened material. The pressure distribution in rubble-pile asteroids is very similar to that of monolithic asteroids with the same porosity. Thus, producing significant volumes of high-degree shocked ordinary chondrites requires strong collision events (impact velocities above 8 km s−1 and/or large sizes of impactors). A large amount of asteroid material escapes during an impact event (up to 90%); however, only a small portion of the escaping material is shock-darkened (6%).

Intermolecular forces at ice and water interfaces: Premelting, surface freezing, and regelation.

Using Lifshitz theory, we assess the role of van der Waals forces at interfaces of ice and water. The results are combined with measured structural forces from computer simulations to develop a quantitative model of the surface free energy of premelting films. This input is employed within the framework of wetting theory and allows us to predict qualitatively the behavior of quasi-liquid layer thickness as a function of ambient conditions. Our results emphasize the significance of vapor pressure. The ice-vapor interface is shown to exhibit only incomplete premelting, but the situation can shift to a state of complete surface melting above water saturation. The results obtained serve also to assess the role of subsurface freezing at the water-vapor interface, and we show that intermolecular forces favor subsurface ice nucleation only in conditions of water undersaturation. We show that ice regelation at ambient pressure may be explained as a process of capillary freezing, without the need to invoke the action of bulk pressure melting. Our results for van der Waals forces are exploited in order to gauge dispersion interactions in empirical point charge models of water.