Transient Melting Research Articles

Effect of Fin Orientation on Melting Process in Horizontal Double Pipe Thermal Energy Storage Systems

Abstract Immersion of fins in latent heat thermal energy storage (LHTES) systems has been used as an influential approach to remedy the poor thermal conductivity of phase-change materials (PCMs). This paper numerically investigates heat transfer and phase-change improvement by means of longitudinal fins in a double pipe thermal energy storage unit. The main aim of this study is to investigate the effect of fin orientation on the performance of the thermal storage unit. Six configurations of different fin numbers (2, 4, and 8 fins) and orientations (π/2, π/4, and π/8) are tested. For simulations, a two-dimensional mathematical model incorporating the enthalpy-porosity method and finite volume techniques are established and solved by ansys-fluent. The numerical predictions are successfully validated by comparison with experimental and numerical data from the literature. Heat transfer characteristics and melting process are analyzed through streamlines, isotherms, mean temperature, heat flux (HF), and heat transfer coefficient (HTC) as well as transient melting front position and liquid fractions. Results show that orientation of fins has a significant impact on the charging time for two cases (2 and 4 fins) whereas no significant reduction in charging time was obtained for the case of 8 fins. In case of utilizing 2 fins, a fin orientation of 0 deg (vertical fins) shortens the charging time by up to 2.5 folds compared with the horizontal fins (90 deg). These results could help designing efficient latent thermal energy storage units.

Effect of the inclination angle on the transient melting dynamics and heat transfer of a phase change material

We report two-dimensional simulations and analytic results on the effect of the inclination on the transient heat transfer, flow, and melting dynamics of a phase change material within a square domain heated from one side. The liquid phase has Prandtl number Pr = 60.8, Stefan number Ste = 0.49, and Rayleigh numbers extend over eight orders of magnitude 0≤Ra≤6.6·108 for the largest geometry studied. The tilt determines the stability threshold of the base state. Above a critical inclination, there exists only a laminar flow at the melted phase, irrespective of the Rayleigh number. Below that inclination, the base state destabilizes following two paths according to the inclination: either leading to a turbulent state for angles near the critical inclination or passing through a regime of plume coarsening before reaching the turbulent state for smaller angles. We find that the Nusselt and Reynolds numbers follow a power law as Nu∼Raα, Re∼Raβ in the turbulent regime. Small inclinations reduce very slightly α and strongly β. The inclination leads to subduction of the kinematic boundary layer into the thermal boundary layer. The scaling laws of the Nusselt and Reynolds numbers and boundary layers are in agreement with different results at high Rayleigh convection. However, some striking differences appear as the stabilization of turbulent states with further increasing of the Rayleigh number. We find as well that the turbulent regime exhibits a higher dispersion in quantities related to heat transfer and flow dynamics on smaller domains.

Melting performance enhancement of a phase change material using branched fins and nanoparticles for energy storage applications

This study presents the application of branched fins and nanoparticles to enhance the melting of phase change material (PCM) placed in a horizontally configured latent heat thermal energy storage (LHTES) unit. The mathematical model governing the transient melting of PCM is numerically solved for 2D planar geometries. The numerical solution procedure is validated against literature as well as our experimental results. First, the effects of different fin designs on melting performance are analyzed using pure PCM. All-straight, single-branched and double-branched fin designs are seen to enhance melting through efficient heat transfer resulting in time savings of 22.9%, 35.4% and 45.9%, respectively, as compared to a Y-oriented triple-fin Base Case. Additionally, the energy storage capacity is observed to increase in the same order with the double-branched design storing the highest at ∼3600 kJ. Next, the combined effects of the most efficient double-branched fin design and Al2O3 nanoparticles on melting performance are assessed. An addition of 1%, 5% and 10% nanoparticles by volume in the PCM results in an additional time savings of 11.5%, 19.2% and 26.8%, respectively, taking pure PCM case as the reference. The corresponding energy storage, however, is seen to decrease with an increase in the nanoparticles concentration.

Influence of ultrasound irradiation on transient solidification characteristics in DC casting process: Numerical simulation and experimental verification

It is well known that solidification of molten metals occurs under the influence of transient unsteady phenomena, which are difficult to control in actual casting operation. Examples include buoyancy flows, natural heat convection, repeated remelting and solidification in mushy zone and solid shell at the mold interface. This paper presents the results suggesting that ultrasound waves, irradiated into the hot-top mold offers an attractive way to control the transient phenomena in DC casting of aluminum alloy billets. A novel mathematical model was developed to simulate the DC casting with considering transient melt flow, heat transfer, ultrasound propagation, acoustic streaming and solidification. It was found that ultrasound irradiation alters the solidification behavior of melt and sump evolution especially in the earlier stage of casting, however as the billet length increases, the sump profile becomes almost the same for both the ultrasonic and conventional castings. In this condition, the casting speed becomes the key parameter influencing the sump evolution and mushy zone volume. The results reveal that ultrasound-driven acoustic streaming and turbulent oscillations result in a suppression of buoyancy flow in the sump and lead to an increase of frequency of the melt temperature oscillations at the solidification interface. One of the useful results of these phenomena can be a more frequent repetition of remelting/solidification cycle and improvement of the billet quality, particularly the billet surface morphology. The effect of ultrasound irradiation on the billet morphology was verified experimentally using a pilot DC caster.

THERMAL ENERGY CHARGING IMPROVEMENT OF A LATENT THERMAL ENERGY STORAGE SYSTEM VIA FRACTAL-BRANCHED FINS

The current study achieves a melting improvement of the latent thermal energy storage (LTES) system using fractal-branched fins (i.e., Y-type and T-type fins). A transient melting model with free convection in LTES systems is developed and numerically simulated with the enthalpy-porosity method. The effects of branched fins on the melting performance are discussed and compared to the traditional plate fin with emphasis on the roles of fin parameters. Besides, the optimized configuration is obtained by the response surface methodology (RSM) in terms of the total charging time. The results indicate that the branched fin expedites the melting process, especially for the T-type branched fin. Due to a more desirable fin layout, the LTES system using T-type branched fins presents a more uniform temperature field and faster melting rate, manifesting as the reduced total melting time by 5.6% compared to the Y-type branches. The melting process includes the pre-heat conduction dominated, free convection dominated, and post-heat conduction dominated stages. A T-type branched fin with moderate branch level ([Formula: see text], larger length index ([Formula: see text], and lower width fractal dimension ([Formula: see text] is favorable for the coordinated enhancement of heat convection and conduction. The RSM optimization implies the length index and branch level play more remarkable roles than the width fractal dimension in melting enhancement. To maximize the charging efficiency of LTES systems, the optimal parameters of T-type fin are [Formula: see text], [Formula: see text], and [Formula: see text], which shortens the total charging time by 52.9% compared to the traditional plate fin.

Numerical simulation and experimental verification of constrained melting of phase change material in cylindrical enclosure subjected to a constant heat flux

The transient melting process of the phase change material (PCM) is controlled by heat conduction and natural convection. Several experimental investigations are essential to understand the heat transfer mechanisms and performance for different designs of PCM thermal storage systems. Therefore, the present study experimentally investigated paraffin wax's melting process in a vertical cylindrical enclosure. The PCM, initially at 30 °C, was heated using an electric heater located at the center of the enclosure. The heat flux density varied to 1300, 1000, and 700 W/m2. Local temperatures of the PCM were measured, and the solid-liquid interface was tracked. A CFD (computational fluid dynamic) model was developed to numerically investigate the melting process with and without considering the convection effect. The CFD model was validated with the current experiments and with data from the literature. A very good agreement was obtained. The results showed that; the melting was initially dominated by conduction at an early stage, and this period increased with a decrease in heat flux. Natural convection was promoted, at a later time, leading to a curved shape of the solid-liquid interface. Numerical results indicated a robust thermal stratification of the molten liquid in the upper half of the storage unit. It was observed that increasing the input power from 700 W/m2 to 1000 W/m2 and 1300 W/m2 decreased the total melting time by 24.82% and 43.58%, respectively. It is recommended for the future modeling of the PCM melting process to consider the convection effect.

The MEMOS-U code description of macroscopic melt dynamics in fusion devices

The MEMOS-U physics model, addressing macroscopic melt motion in large deformation and long displacement regimes, and its numerical schemes are presented. Discussion is centred on the shallow water application to the metallic melts induced by hot magnetized plasmas, where phase transitions and electromagnetic responses are pivotal. The physics of boundary conditions with their underlying assumptions are analysed and the sensitivity to experimental input uncertainties is emphasized. The JET transient tungsten melting experiment (Coenen et al 2015 Nucl. Fusion 55 023010) is simulated to illustrate the MEMOS-U predictive power and to highlight key aspects of tokamak melt dynamics.

Ion tracks in silicon formed by much lower energy deposition than the track formation threshold

Damaged regions of cylindrical shapes called ion tracks, typically in nano-meters wide and tens micro-meters long, are formed along the ion trajectories in many insulators, when high energy ions in the electronic stopping regime are injected. In most cases, the ion tracks were assumed as consequences of dense electronic energy deposition from the high energy ions, except some cases where the synergy effect with the nuclear energy deposition plays an important role. In crystalline Si (c-Si), no tracks have been observed with any monomer ions up to GeV. Tracks are formed in c-Si under 40 MeV fullerene (C60) cluster ion irradiation, which provides much higher energy deposition than monomer ions. The track diameter decreases with decreasing the ion energy until they disappear at an extrapolated value of ~ 17 MeV. However, here we report the track formation of 10 nm in diameter under C60 ion irradiation of 6 MeV, i.e., much lower than the extrapolated threshold. The diameters of 10 nm were comparable to those under 40 MeV C60 irradiation. Furthermore, the tracks formed by 6 MeV C60 irradiation consisted of damaged crystalline, while those formed by 40 MeV C60 irradiation were amorphous. The track formation was observed down to 1 MeV and probably lower with decreasing the track diameters. The track lengths were much shorter than those expected from the drop of Se below the threshold. These track formations at such low energies cannot be explained by the conventional purely electronic energy deposition mechanism, indicating another origin, e.g., the synergy effect between the electronic and nuclear energy depositions, or dual transitions of transient melting and boiling.

Universal process diagrams for laser sintering of polymers

The laser irradiation step and the thermo-viscous flow during selective laser sintering process applied to polymeric materials are analyzed both theoretically and by transient numerical simulations using the smoothed particle hydrodynamics method to spatially resolve individual powder particles. Taking into account the strongly different time scales of laser motion, thermal diffusion and viscous flow, normalized master curves for the transient melt pool temperature and the surface temperature are presented. Infrared camera measurements of the surface temperature confirm the validity of the theoretical framework. Based on these master curves a novel ratio of laser energy input and energy required to melt the polymer – the attenuation melt ratio (AMR) – is introduced, which puts material properties and process parameters into relation. Based on the AMR, normalized process diagrams, which relate depth and temperature of the melt pool to the laser energy input, are constructed. The process diagrams include process windows for optimal part properties. The predictive quality of the process windows is confirmed by the assessment of mechanical properties of laser sintered parts made of different polymers.

Performance evaluation of cascaded storage system with multiple phase change materials

The charging enhancement of latent heat thermal energy storage system is of great significance for the efficient transfer of non-uniform thermal sources. In order to solve the problem of long charging time caused by the low thermal conductivity of phase change materials (PCMs), multiple PCMs - uneven fins combination as a compound enhancement approach was proposed to improve thermal storage rate. The transient heat transfer and melting process of PCMs was numerically studied by the finite volume method and enthalpy-porosity method. Firstly, the filling ratio of high melting point PCM to high melting point PCM was optimized as 60%: 40%. Under the optimal ratio, the charging time was reduced by 29% and the thermal storage rate was increased by 9%. Then, the effect of natural convection on multiple PCMs melting characteristics was analyzed. Finally, the thermal storage performance of multiple PCMs - uneven fins combination was investigated. The results indicated that the reasonable fin arrangement (upper dense - lower sparse fins) strengthens the local convection and improves the melting uniformity, which reduces the melting time by 16% and increases the thermal storage rate by 7%. Therefore, multiple PCMs - uneven fins combination can obviously reduce the charging time and increase the thermal storage capacity, which contributes to promote efficient utilization of fluctuating thermal energy.

Evidence for the presence of chondrule‐ and CAI‐derived material in an isotopically anomalous Antarctic micrometeorite

AbstractWe report the discovery of a unique, refractory phase‐bearing micrometeorite (WF1202A‐001) from the Sør Rondane Mountains, East Antarctica. A silicate‐rich cosmic spherule (~400 µm) displays a microporphyritic texture containing Ca‐Al‐rich inclusion (CAI)‐derived material (~5–10 area%), including high‐Mg forsterite (Fo98‐99) and enstatite (En98‐99, Wo0‐1). The micrometeorite also hosts a spherical inclusion (~209 µm), reminiscent of chondrules, displaying a barred olivine texture. Oxygen isotopic compositions of the micrometeorite groundmass (δ17O = –3.46‰, δ18O = 10.43‰, ∆17O = –1.96‰) are consistent with a carbonaceous chondrite precursor body. Yet, a relict forsterite grain is characterized by δ17O = –45.8‰, δ18O = –43.7‰, ∆17O = –23.1‰, compatible with CAIs. In contrast, a relict low‐Ca pyroxene grain (δ17O = –4.96‰, δ18O = –4.32‰, ∆17O = –2.71‰) presumably represents a first‐generation silicate grain that accreted 18O‐rich gas or dust in a transient melting scenario. The spherical inclusion displays anomalous oxygen isotope ratios (δ17O = –0.98‰, δ18O = –2.16‰, ∆17O = 0.15‰), comparable to anhydrous interplanetary dust particles (IDPs) and fragments from Comet 81P/Wild2. Based on its major element geochemistry, the chondrule size, and oxygen isotope systematics, micrometeorite WF1202A‐001 likely sampled a carbonaceous chondrite parent body similar to, but distinct from CM, CO, or CV chondrites. This observation may suggest that some carbonaceous chondrite bodies can be linked to comets. The reconstructed atmospheric entry parameters of micrometeorite WF1202A‐001 suggest that the precursor particle originated from a low‐inclination, low‐eccentricity source region, most likely either the main belt asteroids or Jupiter family comets (JFCs).

Recent changes in pan-Antarctic region surface snowmelt detected by AMSR-E and AMSR2

Abstract. Surface snowmelt in the pan-Antarctic region, including the Antarctic ice sheet (AIS) and sea ice, is crucial to the mass and energy balance in polar regions and can serve as an indicator of climate change. In this study, we investigate the spatial and temporal variations in surface snowmelt over the entire pan-Antarctic region from 2002 to 2017 by using passive microwave remote sensing data. The stable orbits and appropriate acquisition times of the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) and the Advanced Microwave Scanning Radiometer 2 (AMSR2) enable us to take full advantage of daily brightness temperature (Tb) variations to detect surface snowmelt. The difference between AMSR-E/2 ascending and descending 36.5 GHz Tb values in vertical polarization (DAV36) was utilized to map the pan-Antarctic region snowmelt, as this method is unaffected by snow metamorphism. We evaluated the DAV36 algorithm against ground-based measurements and further improved the method over the marginal sea ice zone by excluding the effect of open water. Snowmelt detected by AMSR-E/2 data was more extensive and persistent than that detected by the Special Sensor Microwave/Imager (SSM/I) data. Continuous melt onset (CMO) ranged from August in the marginal sea ice zone to January in the Antarctic inland, and the early transient melt events occurred several days to more than 2 months earlier. The pan-Antarctic region CMO was significantly correlated (R=0.54, p<0.05) with the summer Southern Annular Mode (SAM). The decreased AIS melt extent was very likely linked (R=-0.82, p<0.01) with the enhanced summer SAM.

Numerical benchmark of transient pressure-driven metallic melt flows

Fluid dynamics simulations of melting and crater formation at the surface of a copper cathode exposed to high plasma heat fluxes and pressure gradients are presented. The predicted deformations of the free surface and the temperature evolution inside the metal are benchmarked against previously published simulations. Despite the physical model being entirely hydrodynamic and ignoring a variety of plasma-surface interaction processes, the results are also shown to be remarkably consistent with the predictions of more advanced models, as well as experimental data. This provides a sound basis for future applications of similar models to studies of transient surface melting and droplet ejection from metallic plasma-facing components after disruptions.

Thermal performance of nanoparticle enhanced phase change material in a concentric cylindrical enclosure

This paper presents the flow and heat transfer characteristics of melting behavior of phase change material (PCM) in the presence of nanoparticles. The melting process occurs in a concentric cylindrical enclosure filled with PCM (ice) and aluminum oxide (Al2O3) nanoparticles. The inner and outer walls of the concentric cylinder are considered as hot and cold walls and maintained at a constant wall temperature. The problem is modeled as unsteady, two-dimensional turbulent multiphase flow using the Eulerian multiphase model and SST k-ω turbulence model. The numerical simulation is performed using Ansys Fluent 19.2. The primary and secondary phases are specified with thermo-physical properties of ice and aluminum oxide. The transient melting behavior of PCM is investigated for different nanoparticle volume concentrations by varying the temperature difference between the hot and cold walls of concentric enclosure. The results are analyzed by plotting the streamline, velocity and temperature contours. The results indicate that the dispersion of aluminum oxide nanoparticles into phase change material increases the heat transfer rate and decreases the melting temperature of PCM. The results also indicate that the melting rate and thermal performance are significantly enhanced with increase in volume concentrations and temperature difference.

Melting heat transfer enhancement of a horizontal latent heat storage unit by fern-fractal fins

The latent heat storage (LHS) technique is of crucial importance in chemical energy engineering. Inspired by multi-bifurcated fern leaves, a mimic fern-fractal fin is designed to improve the thermal energy charging efficiency. This paper develops a transient melting model of a rectangular LHS unit using fern-fractal fins, and their melting behaviors are compared with the conventional fins. Besides, a parametric optimization of fern-fractal fins is conducted for maximizing the thermal efficiency based on the response surface method (RSM). The results indicate that the temperature uniformity is more superior and the melting duration is shorter for the fern-fractal LHS unit when compared with the conventional one. Interestingly, the fern-fractal LHS device presents a slower heat storage rate during the initial conduction-dominated and early convection-dominated melting stages, while a prominent melting enhancement is achieved during the later melting stage. The shortest melting time is obtained based on the RSM technique when a fern-fractal fin with length ratio α = 0.94 and branch angle θ = 54.7° is utilized. Compared with a conventional fin, the averaged heat storage rate increases by 88.3%, and the total melting time is declined by 40.3% for an optimized fern-fractal fin.

Resolidification-controlled melt dynamics under fast transient tokamak plasma loads

Studies of macroscopic melt motion induced by fast transient power loads and the ensuing damage to plasma-facing components are critical for ITER. Based on state-of-the-art experiments, heat transfer is argued to be strongly entangled with fluid dynamics. The physics model of the MEMOS-U code is introduced. Simulations are reported of recent tokamak experiments concerning deliberate transient melting of beryllium main chamber tiles (JET) and tungsten divertor components (ASDEX Upgrade, JET). MEMOS-U is demonstrated to capture the main physics responsible for melt dynamics and to reproduce the observed surface deformation. The intricate role of resolidification is elucidated.

Computer Simulation of Thermal Processing of Steel

The review of the numerical modelling of casting and welding of steel workpieces was studied. The contemporary industry needs tools to optimize and control parameters of processes by considering the favourite chemical composition of steel. In recent time, very efficient tool for optimizing and controlling of the process parameters is numerical modelling of engineering processes. At casting and welding numerical modelling is focused on optimization of distribution of microstructure contents, mechanical and physical properties distribution and tolerated distortions and residual stresses. Mechanical properties and prevention of distortion and cracking can be accomplished by prediction and control of residual stresses and microstructure distribution. The numerical modelling of thermal processing of metals consists of the research of multi-scale processes and multi-physics processes, such as phase transformations, heat transfer and mechanics of materials, and research of numerical methods for computer interpretation of the problem. By means of the numerical modelling and computer simulation of thermal processing should calculate the transient temperature field, melting and solidification processes, as well as, to calculate distribution of mechanical and physical properties and microstructure composition. In this work, the numerical models of studied thermal processes are based on the finite volume method. Values of surface heat transfer coefficient, heat capacity and heat conductivity coefficient were obtained by the inversion method. The microstructures composition has been predicted by CCT diagrams and additionally improved by the kinetic equations. The mathematical model has accomplished to solve thermal processing of 3-D axially symmetric specimens.

Experimental study of the effect of inclination angle on the paraffin melting process in a square cavity

Abstract The transient melting process of a phase change material (PCM) is strongly affected by its inclination angle caused by the behavior of buoyancy-driven natural convection. An experimental study is carried out to investigate the effect of this inclination on the thermal behavior during PCM melting contained in a square cavity. The range of inclination angle from 0° (vertical), 45° (inclined) and 90° (bottom) is considered. Paraffin with a melting range of 49-54°C is used as PCM. The volumetric shrinkage of PCM during solidification preceding the melting experiments leads to the development of a large void in the free surface PCM-air. The effects of inclination and shrinkage phenomenon are analyzed by visualizing the solid-liquid interface and measuring temperatures by thermocouples and infrared camera. Results show that the inclination angle has a great influence on the natural convection behavior, and affects the melting front progression and the heat transfer rate. The total melting time for the bottom and inclined cavities cases were, on average, 56% and 48% less than the vertical one, respectively. The volumetric shrinkage disrupts significantly the melting process in the vertical and inclined cases. The natural convection is more intense in the bottom case and the global average Nusselt number increases during the strong convection by about 10% and 21% compared to the inclined and vertical cases, respectively.

A Quantitative Optical Microscopy Method for Investigating the Laser-Induced Transient Melting Behavior of a Nanoparticle-Laden Polymer System in a Microenvironment

The photothermally induced transient heating and melting of a nanoparticle-filled system in microenvironments underlie many of the emerging applications from photothermal cancer therapies to energy...

Criteria for the onset of convection in the phase-change Rayleigh–Bénard system with moving melting-boundary

Here, for the first time, we report the criterion for the onset of convection in a low Prandtl number phase-change Rayleigh–Bénard (RB) system with an upward moving melt interface in a two-dimensional square box for a wide range of Rayleigh number Ra and Stefan number Ste (defined as the ratio between the sensible heat to the latent heat). High fidelity simulations were performed to study the phenomenon of the onset of convection. Unlike the classical RB system in the phase-change RB system, it was found that the onset of convection depended on Ste and Fourier number τ, in addition to Ra. The phase-change RB system with upward moving melt interface can be classified into two groups: slow expanding phase-change RB system (Ra ≤ 104) and moderate/fast melting phase-change RB system (Ra > 104). The slow melting phase-change RB system becomes unstable when the effective Rayleigh number based on the melt height is ≈1295.78, consistent with the finding by Vasil and Proctor [“Dynamic bifurcations and pattern formation in melting-boundary convection,” J. Fluid Mech. 686, 77 (2011)]; however, moderate and fast melting phase-change RB systems become unstable when the product of the local Rayleigh number Ra based on the melt-layer height hyt and the Fourier number based on the melt-layer height reaches a threshold value. Interestingly, it is seen that the criteria for the onset of convection for moderate and fast melting phase-change RB systems show a power law kind of form such that Racrτcr = aSteb + c. In addition, during the initial conduction regime before the onset of convection, it is seen that the Nusselt number at the hot wall is Nuh ∼ τ0.5, and during the onset of convection, i.e., during the formation of the initial convection rolls, the Nusselt number at the hot wall is Nuh ∼ τd, where the value of the exponent d is 2 for low Rayleigh numbers and 4 for higher Rayleigh numbers. This study reports some general characteristics of the onset of convection and some organized behavior in the transient melting phase-change RB system, which are not yet explored and reported in the open literature. This work may lead to significant understanding of different applications of fluid-dynamical melting phase-change RB systems in both natural and engineering systems.