Melting Interface Research Articles

In situ visualization of traveling solvent growth of thin Si0.7Ge0.3 crystals

A thin compositionally uniform silicon germanium crystal was grown by the traveling solvent method (TSM). In situ observation of the melting and crystal growth behavior with different temperature profiles was carried out to study the growth mechanism for the first time. It was found that the temperature gradient and the stability of melting interface was crucial for planar growth. Otherwise, the constitutional supercooling would induce faceted, and further dendritic growth. A temperature gradient of 1 K/mm was found suitable for planar growth. Above the critical temperature gradient of 1.4 K/mm, the growth interface became faceted. The stable growth velocity was also consistent with the theory indicating that the growth was driven by diffusion. From the electron backscatter diffraction (EBSD) analysis, it was confirmed that a single crystal was successfully grown from the seed. It was clarified that the strong segregation effect and positive concentration gradient from the seed side to the feed side caused solvent trapping in the faceted tips.

Theoretical study on melting of phase change material by natural convection

Natural convection plays a crucial role in latent heat storage system. Determining how natural convection affects the melting of phase change material (PCM) will provide a better understanding of the melting process and give helpful advice on strengthening techniques. The research content of this paper is a mathematical model is established to explain the melting process of PCM affected by natural convection. The mathematical model of phase change melting process is given by introducing the natural convection and the temperature gradient. A coordination factor (Co) based on the mathematical model, which is an instantaneous quantity and proportional to the heat exchange rate and the intensity of natural convection, is introduced to explain the melting process. A square-shaped and shell-tube (concentric, eccentric) PCM heat storage unit were numerically analyzed in more detail. It is found that the Co factor is closely related to the melting rate. It will not promote the melting rate when Co less than 0 while it will promote the melting rate when Co higher than 0, and the higher the Co value, the faster the melting rate. This model gives a clear and quantitative explanation for the problem of melting interface movement affected by natural convection, as well as that increasing the heating temperature and changing only the shape of the heat storage unit can greatly enhance the heat storage rate in this study. The model also provides theoretical guidance for the study of the phase change heat storage enhancement.

Natural convection in phase change material: Experimental study

Natural convection in a melting material (hexadecane) is studied experimentally using Magnetic Resonance Imaging. This imaging technique allows to discriminate solid and liquid phases and to measure velocity vectors in the volume sample, even through/in opaque phases. Within our experimental conditions, the convection occurs above Ra≈1430. The thermo-convective flow affects the melting interface leading to higher liquid height in uprising flow and lower liquid height in downward flow regions. Our experiments show that (i) the transient evolution of the mean liquid height h¯ is enhanced with convection since h¯∝t0.8 (h¯∝t12−3β with β=1/4), while h¯∝t0.5 in the conductive regime similarly to the Stefan’s problem; (ii) the steady averaged liquid height h¯ increases 4 times larger in the convective regime than in the conductive regime. In our experiments the range of A, ratio between the solid and liquid heights, is such as A>1.5. Within this range of values we obtain convective pattern under the form of hexagons / polygons. As the melting boundary grows, we observe a decrease in the polygon number. The polygonal pattern is characterized by a constant dimensionless wavelength, i.e. λ/h¯≈2, and thus a constant wavenumber around 3.1 similarly to the primary bifurcation in the classical Rayleigh–Bénard Convection (RBC). Heat transfer is evaluated via the Nusselt number Nu. Close to the onset of convection and above, our results highlight a scaling law Nu∝Raβ with β=1/4 similarly to the classical RBC. The kinetic energy Ec is also evaluated via velocity measurements leading to Ec/E0∝Nu4 and therefore to Re∝Ra2β∝Ra1/2. Within the frame of our experiments, all scaling laws are converging to β=1/4. They reflect clearly an increase in the convective intensity from both thermal and dynamical points of view.

A new lattice Boltzmann method for melting processes of high Prandtl number phase change materials

The lattice Boltzmann method has shown its advantages to model melting processes in thermal energy storage systems. However, the organic phase change materials which are popularly used in modern thermal energy storage systems and featured by a large Prandtl number, have raised a big challenge for the simulation by the lattice Boltzmann method, as a large Prandtl number will cause a large discrepancy between the relaxation times for flow and temperature fields in the lattice Boltzmann method. And such discrepancy will cause serious numerical instability. In this work, a new lattice Boltzmann model is proposed to address this challenge. Three benchmark test cases are adopted to validate the capability of the present lattice Boltzmann model for simulating melting processes of organic phase change materials with large Prandtl numbers (Pr up to 56.2). The present numerical results are in good agreement with the existing data obtained numerically and experimentally. Compared with the available lattice Boltzmann approaches, the present model can capture the melting interface accurately with much less grid density.

Lattice Boltzmann simulation on thermal performance of composite phase change material based on Voronoi models* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51876184, 51725602, and 51806147) and the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20180102).

Phase change materials (PCMs) are important for sustaining energy development. For the thermal performance enhancement, the composite PCM with metal foam reconstructed by the Voronoi method is investigated in this work. The lattice Boltzmann method (LBM) is used to analyze the melting process on a pore scale. The melting interface evolution and temperature contour of the composite PCM are explored and compared with those of pure PCM. Moreover, structure parameters including the pore density, porosity and irregularity are investigated comprehensively, indicating that the additive of metal foam strengthens the melting performance of PCM obviously. Compared with pure PCM, the composite PCM has quick rates of the melting front evolution and heat transfer. The heat conduction plays a great role in the whole melting process since the convection is weakened for the composite PCM. To improve the melting efficiency, a larger pore density and smaller irregularity are recommended in general. More significantly, a suitable porosity is determined based on the requirement for the balance between the melting rate and heat storage capacity in practical engineering.

Melting phase change heat transfer in a quasi-petal tube thermal energy storage unit.

In the present study, the thermal energy storage of a hot petal tube inside a shell-tube type Thermal Energy Storage (TES) unit was addressed. The shell is filled with the capric acid Phase Change Material (PCM) and absorbs the heat from a hot U-tube petal. The governing equations for the natural convection flow of molten PCM and phase change heat transfer were introduced by using the enthalpy-porosity approach. An automatic adaptive mesh scheme was used to track the melting interface. The accuracy and convergence of numerical computations were also controlled by a free step Backward Differentiation Formula. The modeling results were compared with previous experimental data. It was found that the present adaptive mesh approach can adequately the melting heat transfer, and an excellent agreement was found with available literature. The effect of geometrical designs of the petal tube was investigated on the melting response of the thermal energy storage unit. The phase change behavior was analyzed by using temperature distribution contours. The results showed that petal tubes could notably increase the melting rate in the TES unit compared to a typical circular tube. Besides, the more the petal numbers, the better the heat transfer. Using a petal tube could increase the charging power by 44% compared to a circular tube. The placement angle of the tubes is another important design factor which should be selected carefully. For instance, vertical placement of tubes could improve the charging power by 300% compared to a case with the tubes’ horizontal placement.

Effect of inclination angle on melting process of phase change materials in a square cavity under mechanical vibration

The effect of inclination angle on melting processes of solid–liquid phase change materials has been investigated numerically and experimentally. However, the available studies did not consider mechanical vibration which exists in most thermal systems. To bridge the knowledge gap on this important topic, in this work the effect of inclination angle on paraffin melting process under mechanical vibration condition is investigated for the first time. The results show that, under low vibration frequency condition, the inclination angle has a significant influence on the melting rate, melting heat transfer and melting interface. However, the effect of inclination angle on melting characteristics will be weakened by moderate/high frequency mechanical vibration. Mechanical vibration can accelerate the melting processes for all inclination angles, but the enhancement effect relies closely on the inclination angle. The present numerical results for the stationary condition are consistent to the published experimental exploration that did not consider mechanical vibration.

Thermal transport in phase change materials embedded in metal foam: evaluation on inclination configuration

Melting features of phase change materials (PCM) in metal foams with distinctive pore parameters were numerically studied. Metal foams infiltrated by PCM were filled in an inclined rectangle enclosure. The effect of enclosure inclination upon melting behaviors was intensively analyzed by means of melting time for different melting fractions, temperature field and uniformity. Numerical models with tilted configuration were built and verified by experimental results regarding both melting interface evolution and temperature data. Satisfactory agreement was found for both two parameters between the two methods. The analysis on the plenty of results demonstrated that the time required for full melting increased significantly with porosity for metal foam regardless of inclination angles. However, the complete melting time was less affected by the variation in pore density. Maximal difference in the complete melting time under the four inclination angles was within 6.27% (porosity: 0.98; pore density: 10 PPI). This revealed that the inclination had limited influence on the complete melting time for PCM embedded in metal foams with various kinds of pore parameters.

Effects of backwall on inner thermal structure in opposed-flow horizontal flame spread of thick PMMA panel

Flame spread on solid fuel is an important aspect of fire safety engineering. In this work, an unsteady numerical model was developed to investigate the opposed-flow horizontal flame spread mechanism of thick polymethyl methacrylate (PMMA) on backwall with different thermal properties. The temperature fields in both the condensed fuel and backwall material, the fuel gas distribution and the pyrolysis profiles along the fuel bed were analysed in detail. The results showed that the thermal properties of the backwall play an important role in the structure of the melting interface of the PMMA flame front. The molten layer will not penetrate the sample obviously for the case of gypsum backwall. However, the molten liquid penetrates the PMMA sample with more molten liquid generated on the aluminium backwall, thus resulting the higher possibility of molten droplets. It is also indicated that the heat accumulation at the preheated region was enhanced by the heat transfer through the aluminium backwall from the flame and pyrolysis region. On the other hand, the aluminium wall behind the flame tip normally remain at relatively high temperatures, which in turn improved the pyrolysis rate, and lead to higher gas temperature. Therefore, the flame spread of polymer would be accelerated obviously for the case of metal backwall. This work provides important information for the acceleration mechanism of metal backwall on the opposed-flow horizontal flame spread of polymer and the design of polymer components, and also provides an important basis for engineering application to evaluate the fire risk of materials by considering the thermal performance of the backwall.

Numerical simulation of melting behavior of nano-enhanced phase change material in differentially heated cavity

The melting and solidification process in enclosures filled with phase change materials has wide applications in metal casting, thermal energy storage, building insulation and battery thermal management. In the current study, the melting and heat transfer characteristics of Nano Enhanced Phase Change Material (NEPCM) are numerically investigated in a differentially heated square cavity. The NEPCM consists of ice as the phase change material (PCM) into which Single Walled Carbon Nano-tubes (SWCNT) particles of uniform size is dispersed. The left and right walls of the enclosed cavity are maintained at a constant wall temperature and considered as hot and cold walls and the remaining walls are treated as adiabatic and the simulation is performed using Ansys Fluent 19.2. The temperature difference between the hot and cold walls are varied and further investigations are performed by considering the effects of nanoparticle volume fraction on the melting behavior of NEPCM. The results are analyzed by plotting the stream function, melting interface temperature and velocity contours. The results indicate that the addition of SWCNT nanoparticles significantly influences the heat transfer characteristics and melting rate of phase change material. It is also found that the increase in nanoparticle volume fraction enhances the average melting and heat transfer rate of phase change material. The present results are validated and are in excellent agreement with the benchmark results available in literature.

Investigation on charging enhancement of a latent thermal energy storage device with uneven tree-like fins

The charging intensification of latent thermal energy storage (LTES) devices has an important meaning for solar energy applications. For a more uniform temperature and faster melting rate of LTES devices, uneven tree-like fins are applied and optimized here. Numerical research of melting behaviors in tree-like finned LTES devices is performed with a particular focus on fin layouts. The melting interface transformation and transient temperature distribution in four LTES devices are examined. Moreover, the roles of fill angle and central-angle gradient in melting performance are studied for the optimization of uneven tree-like fins. The results indicate that uneven tree-like fins considerably improve energy charging performance since the heat conduction enhancement exceeds the free convection suppression. Compared with uniform tree-like fins, the uneven tree-like fins lead to a more uniform temperature distribution and consistently faster melting rate by the synergetic improvement of free convection and heat conduction in LTES devices. More significantly, the optimal fill angle and the central-angle gradient are suggested to be 300° and 8° for maximizing the melting performance. Correspondingly, the complete melting time of the optimal LTES devices with heterogeneous and gradient tree-like fins respectively decrease by 49% and 46% compared to LTES devices with uniform tree-like fins.

Numerical Simulation of the Melting Behavior of Steel Scrap in Hot Metal

The current study focuses on the melting behavior of a scrap bar with low carbon content in hot metal which contains high carbon concentration by applying experiments and mathematical modelings. The experiments suggest that higher temperature is favorable for the melting of the bar and the melting rate of the bar is initially high while decreased to a relative stable level after 90 s in the current conditions. It can be found from the mathematical results that the bar temperature is increased near to bath temperature in about 20 s after it was immersed into the bath, and the temperature in the axis of the bar is not distributed evenly during the temperature increase stage. Moreover, the mathematical results shows that a bath circulation flow would be formed in the bath under the effects of temperature and carbon distribution during the melting process. The bath flow near the melting interface would influence the carbon concentration of the molten phase, in turn, affects the melting rate of the bar in the vertical direction. Both the experimental and mathematical results show that the melting rate in the upper part, which is in the upstream of the bath flow, is higher than that of the middle part, followed by the down part of the bar in the downstream of the flow, in which the carbon concentration is much lower than that of the bath. At this period, the main factor that dominate the bar melting is not the temperature but the carbon distribution at the melting interface after the bar temperature is increased to the bath temperature.

Lattice Boltzmann method simulation of ice melting process in the gas diffusion layer of fuel cell

A 2D lattice Boltzmann model combining the enthalpy-based method (EBM) is established to investigate the ice melting process in the gas diffusion layer (GDL). The effects of GDL melting position, porosity, Rayleigh number and Stefan number on the melting rate, ending time of melting, temperature distribution and solid-liquid interface distribution are investigated. It is found that the GDL melting positions have a significant effect on the melting process, the melting rate in the In-plane position increases fastest. When the porosity is large, the effect of the carbon fiber layer number on the melting process is more significant than that of the carbon fiber length. In the In-plane position, the increase of natural convection intensity has a promoting effect on the ice melting process. The increasing of Stefan number can promote the melting process significantly when the Stefan number is not high. And reducing the GDL porosity can effectively promote the melting process.

Hot Water Deicing Method for Insulators Part 2: Analysis of Ice Melting Process, Deicing Efficiency and Safety Distance

In this paper, the heat balance integral method was employed to solve the internal temperature of ice layer and the position of melting interface by stages, due to its main advantage of converting the governing partial differential equation to an ordinary differential one, which can effectively reduce numerical efforts. Then the deicing time of hot water for insulator was calculated and analyzed. Taking LXY-120 insulator as the verification object, after natural icing at Xuefeng mountain natural icing test base, the validation experiments of deicing time were carried out in the natural environment. The initial water temperature and outlet pressure were optimized for deicing efficiency. Finally, the leakage current of water jet was measured. The research results indicate that the deicing rate increases and the energy consumption gradually decreases as the initial temperature of hot water increases. As the outlet pressure increases, the deicing rate increases. For the deicing condition of 1.2 mm nozzle, ambient temperature of -1 °C, wind speed of 3 m/s, ice weight of 2 kg/piece and deicing distance of 2 m, when the initial temperature of hot water and outlet pressure are 85.21 °C and 3.35 MPa respectively, the comprehensive deicing efficiency is the highest. The leakage current through water jet decreases with the increasing distance. The leakage current increases with the increase of water conductivity and outlet pressure. Hot water deicing for insulator in 110 kV system can be carried out according to the safety distance of the existing regulation.

A three-dimensional pore-scale lattice Boltzmann model for investigating the supergravity effects on charging process

Latent heat thermal energy storage with metal foams has been considered as a promising candidate for thermal management in aerospace systems. Thus, there is good cause to deeply explore the heat transfer mechanisms of phase change material (PCM) melting with metal foams. In order to get close to the real situation, a three-dimensional, pore-scale lattice Boltzmann model is explored based on a three-dimensional reconstructed porous structure morphology taken from experimentally observed in this paper, to characterize the distribution of flow and temperature fields during charging of porous PCM. The gravity effects on heat transfer performance are documented by comparison of charging at different accelerates. During the charging process, nonuniform temperature distributions and inclined melting interfaces are presented at the latter stages, caused by the interplay of primary natural convection in the melting direction and secondary convection in the transverse direction. More inclined melting interfaces are observed as gravitational acceleration increases, yielding faster PCM melting in the upper region while melting in the bottom region almost terminated. This implies that natural convection gradually dominates heat transfer and leads to the temperature nonuniformity. Similar shape characteristics of melting interface are observed in two-dimensional model, while there is an apparent difference of the melting fraction, showing it gradually growing from 4.1% to 8.6% with increasing gravitational acceleration. These results indicate that secondary convection effect neglected in the two-dimensional model, leading to a significant error in the prediction of heat transfer performance.

Experimental study on thermal structure inside flame front with a melting layer for downward flame spread of XPS foam.

Thermal structure inside flame front during downward flame spread was experimentally measured for XPS foam with thicknesses of 1.6, 2.4, 3.4, and 4.4 cm. The temperature distribution and temperature gradient in the condensed phase, as well as the shape of the molten liquid, were obtained by 2-D heat transfer equations and experimental measurement. The results show that both the temperature and its temperature gradient decrease from the sample surface to the inside of the condensed phase, which results in the inclination of the melting interface. The molten layer is the thinnest near the sample surface and thicker inside the condensed phase. The adhering of the molten liquid to the wall greatly increases the thickness of the molten layer near the back wall, and the higher the thickness, the more molten material adheres to the wall. Finally, there is a relatively flat solid-liquid interface near the sample surface and a large inclined solid-liquid interface near the back of the sample, especially for the thicker samples of 3.4cm and 4.4cm. It is indicated that the distribution of the molten layer in the direction perpendicular to the plane of the XPS sheet is a significant factor for dripping and collapsing.

Experimental and numerical investigation of spatiotemporal characteristics of thermal energy storage system in a rectangular enclosure

Abstract In this study, experimental and numerical investigation for melting of paraffin wax is carried out for a portable thermal energy accumulation system in a rectangular enclosure of size 200 mm × 160 mm × 20 mm. The studied paraffin wax is typically used for a wide variety of applications including thermal management of buildings and solar energy storage. The melting behaviour was also investigated numerically with the help of FVM (Finite Volume Method) approach based commercial code Ansys Fluent. The melting behavior is observed for 1500, 1930, 2500 and 3100 W/m2 of heat flux values that suits for thermal accumulation and thermal regulation of the buildings. The images of the PCM domain have been captured at regular intervals of 30 min and the temperature readings at fifteen different locations in the PCM domain have been recorded simultaneously. The differences between experimental and numerical results were observed to be in acceptable range. The investigation reveals the facts about the transient behaviour of melting interface, melted proportion and the stored energy for melting cycle of 5–5.5 .

Capillary viscous flow and melting dynamics: Coupled simulations for additive manufacturing applications

The rate of melting of a solid and the rate of deformation of the resulting melt due to capillary forces are comparable in additive manufacturing applications. This dynamic structural change of a melting solid is extremely challenging to study experimentally. Using meshless numerical simulations we show the influence of the flow of the melt on the heat transfer and resulting phase change.We introduce an accurate and robust Incompressible Smoothed Particle Hydrodynamics (ISPH) method to simulate melting of solids and the ensuing fluid-solid interaction. We present validations for the heat transfer across the free surface and the melting interface evolution, separately. We then present two applications for this coupled multiphysics simulation method — the study of rounding of an arbitrarily shaped particle during melting and the non-linear structural evolution of three spheres undergoing agglomeration. In both the studies we use realistic transport and thermal properties for the materials so as to demonstrate readiness of the method for solving engineering problems in additive manufacturing.

Image Segmentation Based on Dynamic Particle Swarm Optimization for Crystal Growth.

In order to realize the intelligent production of sapphire crystal, it is important to obtain the growth status from the furnace by charge coupled device (CCD). However, a significant challenge is that traditional approaches are often not valid to separate the images of the melting interface well due to the low contrast and uneven brightness from the heater. In this paper, an improved Otsu algorithm based on dynamic particle swarm optimization (DPSO) is proposed to find the exact threshold band as contour of the crystal. In this method, the Otsu method is constructed firstly, then DPSO is used to find the optimal threshold band. Experimental results show that the proposed algorithm can separate the texture of crystal growth images well and has high robustness.

Melting of a Nano-enhanced Phase Change Material (NePCM) in partially-filled horizontal elliptical capsules with different aspect ratios

Melting of a Nano-enhanced Phase Change Material (NePCM), i.e., n-octadecane paraffin dispersed with Cu nanoparticles, in partially-filled horizontal elliptical capsules for a Rayleigh number of 1.744 × 106 is investigated numerically using the collocated finite volume method. To accommodate the increase in the NePCM volume during melting, a 15% air void within the capsule is considered. The simulations are carried out using the enthalpy-porosity technique and the Volume of Fluid (VOF) model. In this work, three volumetric concentrations (Φ = 0, 1 and 3 vol%) of nanoparticles and various aspect ratios (AR = 2.0, 1.0 and 0.5) of the capsule are adopted. The accuracy of the numerical procedure is validated through the comparison of two test cases with those available in the literature. The results are demonstrated in terms of the isotherms, streamlines, melting interface and the air-NePCM interface. It is concluded that for a given AR, the presence of nanoparticles enhances the melting rate and decreases the volume change of NePCM as compared to the pure PCM case. In addition, the highest and the lowest melting rates are associated with the AR = 2.0 and AR = 1.0, respectively.