Evolution Of Melting Front Research Articles

A comprehensive approach combining gradient porous metal foam and the magnetic field to regulate latent heat storage performance

This study proposes a comprehensive regulation method of gradient porous metal structure combined with the magnetic field to improve the controllability of heat utilization of latent heat thermal energy storage (LHTES) unit. Melting and solidification are considered. The enthalpy porosity method and magnetic force model describe the phase change of phase change material (PCM) and the movement of Fe3O4 nanoparticles under the magnetic field and assume the local thermal non-equilibrium model of metal foam. The numerical model is consistent with the visual experimental results of melting front evolution. The effects of magnetic nanoparticle concentration and gradient porosity on heat transfer and thermal energy change during melting and solidification are numerically studied. The results show that the phase change rate increases with the concentration of nanoparticles. The positive magnetic field further accelerates the phase change and heat energy change. The addition of copper foam considerably shortened the phase change time. The positive magnetic field accelerated the melting and heat energy change rate of PCM in copper foam by 18.2% and 23.1%, but the improvement of solidification was weak. Based on magnetic field promotion, the setting of porosity spacing with different gradients can regulate the phase interface uniformity, total heat energy, and heat energy change rate. Finally, according to different heat utilization scenarios of the LHTES system, this study gives recommended cases to optimize the performance of melting, solidification, and the phase change cycle.

Experimental study on the melting performance of magnetic NEPCMs embedded in metal foam subjected to a non-uniform magnetic field

This experimental study aims to evaluate the melting performance of non-magnetic and magnetic nano-enhanced phase change materials (NEPCMs) composited with metal foam inside a cavity heated from the left in a non-uniform magnetic field via thermochromic liquid crystal (TLC) visualization. The evolutions of solid-liquid interface and mushy zone during melting are captured by TLC technique and temperature distributions are recorded by ten thermocouples. The effects of particles concentrations of NEPCM (0 wt%, 1 wt%, 3 wt%, 5 wt%), filling arrangements of metal foam (filling height, porosity gradient direction) and directions of magnetic field (left, right) are studied with emphasis on the heat transfer and energy storage characteristics including liquid fraction, energy stored and Nusselt number. The results show that comparing to pure PCM, the 3 wt% Al 2 O 3 NEPCM maximizes the enhancement of melting performance for decreasing complete melting time as 29.32% and increasing energy storage as 0.26%. For partial porous filling, as the filling height increases, the melting rate firstly increases then decreases, but energy stored shows the opposite trend. The filling height of 3/4h is considered as the optimal filling height with 43.51% decreasing of complete melting time and only 6.81% decreasing of energy stored. For gradient porosity filling, positive gradient has faster melting rate (62.16% amplification), but negative gradient has less energy loss (only 6.30% reduction). Under a non-uniform magnetic field, the convection of magnetic NEPCM is greatly suppressed by the Kelvin force, with 6.76% and 19.59% increasing of complete melting time and 0.07% reduction and 2.45% growth of energy stored for left and right magnet cases, respectively. • Conduct experiments to explore the effect of magnetic field on NEPCM melting performance. • Use TLC technique to capture evolution of melting front and mushy zone. • Gradient porosities of full filling can improve the melting performance but decrease energy storage. • Kelvin force suppresses the convection of magnetic NEPCM and reduces the width of mushy zone.

Numerical Analysis of Heat Transfer Behaviours of Melting Process for Ice Thermal Storage Based on Various Heat Source Configurations

Ice thermal storage (ITS) performance for cooling systems is greatly influenced by the poor thermal conductivity of phase change material (PCM). The effect of natural convection on the melting process is significant for heat transfer enhancement. Thus, the melting performance of PCM in a shell-and-tube latent heat storage (STLHS) unit is numerically studied by considering natural convection in terms of various heat source positions and configurations, i.e., central position, eccentric position, and flat-tube type. Temperature distribution, melting time, and the overall heat transfer coefficient during the process are investigated. The results show that the circulation vortex formed by natural convection is a dominant factor that affects melting front evolution and the overall heat transfer coefficient. When input heat flux is relatively weak, PCM below the heat source is liquefied first. In contrast, PCM in the upper part melts earlier when the heat flux is excellent. The overall heat transfer coefficient decreases sharply with the increase in melting time in the early stage. Then, the heat transfer coefficient tends to be constant. PCM in an STLHS unit with a heat source in a lower position and a configuration of vertical flat-tube type has a desirable performance when compared with other cases, which could provide good support for ITS application.

Simulation of Phase Change Material Absorbers for Passive Cooling of Solar Systems

One of the main challenges that face the reliable use of photovoltaic solar systems in hot arid regions is the prevailing high temperatures during the day. To overcome this issue, Phase Change Materials (PCM) are used for passive cooling providing different options to attain sufficient thermal management solutions for different applications. Passive cooling can be achieved by adjusting a heat sink to the solar PV module. This can be realized by attaching a PCM layer or sensible heat storage to the backside of PV panels. Few studies have reported on simplified modeling and numerical procedures using the apparent heat capacity formulation and volume averaging technique as a robust approach to solving such sophisticated problems with minimal computational efforts, high accuracy, and in a short period of time. However, there is still a need to bridge the large-scale gap between the macroscale within the PCM layer, with a moving melting front, and the length scale of PV modules. Hence, this work focuses on modeling and simulating PCM-Matrix Absorbers (PCM-MA) that consist of fibrous aluminum cellular structure filled with PCM for passive thermal management of PV panels using apparent heat capacity formulation and homogenization based on volume averaging technique. COMSOL Multiphysics FEM software was used for the numerical simulation of the phase change problem by using a Smoothed Heaviside step function to overcome the singularity of PCM challenge that arises with sharp melting temperatures. To validate the proposed model, it has been compared with a benchmark analytical solution for an ice melting problem, i.e., the Stefan problem, in a semi-finite slab, i.e., Neumann’s solution under the same assumptions and boundary conditions. The specific characteristics of phase change and evolution of melting front with time, heat capacity change with the temperature at different times, and with locations along the slab height are presented. As the phase change is modeled to take place over a mushy region, i.e., a narrow temperature interval rather than a sharp melting point, the results show a good coincidence of the heat capacity profile and its peak at different times and locations. The validated model can be used for the optimization of PCM-MA for any specific geographical location and other applications such as the passive cooling of buildings with PCM integrated with the outer envelope. To this end, the results of the simulation in this work are shown to be in agreement with those obtained from the analytical solution.

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.

Experimental study on heat transfer characteristics of metal foam/paraffin composite PCMs in large cavities: Effects of material types and heating configurations

This study explores the effects of material types and structural parameters of metal foam, and heating configurations on the evolution of melting front, heat transfer characteristics, and temperature distribution of metal foam/paraffin composite phase change materials (PCMs) in large cavities. Copper and iron foams were used as the heat transfer enhancement medium, while paraffin with a melting point of 45 °C was chosen as the PCM to store heat. This study identified the heat transfer regimes in large cavities during the melting process. The experimental results on different heating configurations indicated that different from small cavities, both conduction and convection regimes played a fundamental role in the melting process. In addition, the heating configuration controlled the evolution of the melting front of the pure PCM and composite PCM. On the basis of these results, a non-dimensional correlation validated against a large dataset of experiments was fitted to model the melting process of the composite PCMs.

Melting of hybrid nano-enhanced phase change material in an inclined finned rectangular cavity for cold energy storage

The aim of this paper is to study the potential of two heat transfer enhancement techniques, namely adding fins and hybrid nanoparticle (Cu and Al2O3), for improvement of the melting process of PCM (water) in an inclined rectangular enclosure as model of cold energy storage systems. Phase change and heat transfer of PCM are modelled using an in-house built code based on Lattice Boltzmann Method, which is validated with experimental and numerical results from literature. Enhancement effect is investigated through the main parameters including number of fins, fin length ratio (W/H) and nanoparticle volume fraction (ϕ). Heat transfer and fluid flow, liquid fraction, transient evolution of melting front, required time for full melting, and thermal energy storage are analysed in detail. The results show that the melting rate is about two times higher for the fin length ratio of W/H = 0.75, compared to W/H = 0.25. Besides, nanoparticle loading further enhances the melting rate. The highest melting rate is attained in the case of 3-fins and nanoparticle concentration of ϕ = 6 vol% with a 33.5% reduction in the complete melting time. Incrementing the fin length ratio increases the thermal energy stored, and the melting time can be shortened by up to 64%.

Melting assessment on the angled fin design for a novel latent heat thermal energy storage tube

Latent heat thermal energy storage was widely used to solve the problem of the intermittency in time and space for solar energy systems. To improve the energy storage efficiency, metal fins were typically further added to the shell side to increase thermal conductivity of phase change materials (PCMs). However, the positive effect of metal fins on enhancing heat transfer has not been fully utilized. In this paper, a novel design on angled fins was proposed to improve the thermal transport for PCMs in a shell-and-tube thermal storage unit. A numerical model was built and verified by comparing with experimental observations on the melting front evolution and temperature history. Bending angles for fins ranging from 0° to 75° were designed upwardly and downwardly and quantified on the melting front shape and location, melting time, and temperature field. Results demonstrated that melting rates were improved for the angled-fin cases other than cases with large bending angles. The case with a 10° downward angle exhibited as much as 55.41% reduction in full melting time. Simultaneously, the temperature uniformity can be improved by 20.00%. The novel design strategy on fin angles engendered a promising solution to enhancing thermal storage for emerging engineering applications.

Mean-field model of melting in superheated crystals based on a single experimentally measurable order parameter

Melting is one of the most studied phase transitions important for atomic, molecular, colloidal, and protein systems. However, there is currently no microscopic experimentally accessible criteria that can be used to reliably track a system evolution across the transition, while providing insights into melting nucleation and melting front evolution. To address this, we developed a theoretical mean-field framework with the normalised mean-square displacement between particles in neighbouring Voronoi cells serving as the local order parameter, measurable experimentally. We tested the framework in a number of colloidal and in silico particle-resolved experiments against systems with significantly different (Brownian and Newtonian) dynamic regimes and found that it provides excellent description of system evolution across melting point. This new approach suggests a broad scope for application in diverse areas of science from materials through to biology and beyond. Consequently, the results of this work provide a new guidance for nucleation theory of melting and are of broad interest in condensed matter, chemical physics, physical chemistry, materials science, and soft matter.

New Equivalent Thermal Conductivity Model for Size-Dependent Convection-Driven Melting of Spherically Encapsulated Phase Change Material.

Spherically encapsulated phase change materials (PCMs) are extensively incorporated into matrix material to form composite latent heat storage system for the purposes of saving energy, reducing PCM cost and decreasing space occupation. Although the melting of PCM sphere has been studied comprehensively by experimental and numerical methods, it is still challenging to quantitatively depict the contribution of complex natural convection (NC) to the melting process in a practically simple and acceptable way. To tackle this, a new effective thermal conductivity model is proposed in this work by focusing on the total melting time (TMT) of PCM, instead of tracking the complex evolution of solid–liquid interface. Firstly, the experiment and finite element simulation of the constrained and unconstrained meltings of paraffin sphere are conducted to provide a deep understanding of the NC-driven melting mechanism and exhibit the difference of melting process. Then the dependence of NC on the particle size and heating temperature is numerically investigated for the unconstrained melting which is closer to the real-life physics than the constrained melting. Subsequently, the contribution of NC to the TMT is approximately represented by a simple effective thermal conductivity correlation, through which the melting process of PCM is simplified to involve heat conduction only. The effectiveness of the equivalent thermal conductivity model is demonstrated by rigorous numerical analysis involving NC-driven melting. By addressing the TMT, the present correlation thoroughly avoids tracking the complex evolution of melting front and would bring great convenience to engineering applications.

Thermal energy storage in a confined cylindrical heat source filled with phase change materials

Thermal energy storage in the phase change materials (PCMs) around a confined heated cylindrical heat source has been investigated numerically. This work aims to explore three major objectives namely; evolution of melting front around the cylindrical heat source, quantification of overall rate of heat transfer and identifying the factors to maximize the energy storage. The enthalpy-porosity formulation is utilized to model melting of PCMs. The flow and energy equations with relevant boundary conditions have been solved using finite element method for range of parameters as Rayleigh number (102≤Ra≤106), Prandtl number (102≤Pr≤103), Stefan number (0.01≤Ste≤0.5) and aspect ratio (0.1≤AR≤0.5). The evolution of melting front and melt fraction, streamlines, isotherms, Nusselt number and thermal energy storage are reported as a function of Rayleigh number (Ra), Prandtl number (Pr), Stefan number (Ste), and aspect ratio (AR). Stefan number and the aspect ratio have shown to be a positive influence on the amount of thermal energy absorbed during the melting of PCMs. At low Rayleigh and Stefan number, melting kinetics is shown to be sluggish. All else being equal, the energy storage exhibits three distinct regimes depending upon the Fourier number namely, the lag phase, the exponential phase and stationary phase. For process design calculations, the gross engineering parameters like Nusselt number and steady-state melt fraction are often required, and therefore a predictive correlation for the value of the average Nusselt number is proposed to facilitate the estimation of average Nusselt number at intermediate values of dimensionless numbers for a given practical application.

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.

Effect of fin-metal foam structure on thermal energy storage: An experimental study

Thermal energy storage (TES) has been playing a crucial role in addressing the issue of solving the intermittent and random problems in solar energy utilization. The objective of this study is to squarely address and clarify the contribution of metal fin and foam to enhancing the phase change heat transfer by experiments. A novel hybrid fin-foam tube is proposed and its thermal performance is evaluated by experimentally comparing with other three competing structures including bare, fin, and metal foam tubes. A well-designed test rig is built and the energy storage features for the designed four TES tubes are analyzed by the complete melting time, melting front evolution, temperature variation and uniformity, and the temperature response rate. Results show that the fin-foam hybrid structure outperforms the other competing ones, demonstrating a reduction of 83.35% in complete melting time (compared with the bare tube). The transient temperature response is maximized by 529.1%. As for the single structure, both fins and metal foam can improve conductivity of phase change materials. The metal foam does a good favor to improve the uniformity of the temperature field inside the TES tube, but the fins weakened the uniformity. If the design target is temperature uniformity, adding metal foams other than fins can fulfil the task.

Metal foam/PCM melting evolution analysis: Orientation and morphology effects

Phase Change Materials (PCM) are promising materials for thermal energy storage systems. Since they present a relatively low thermal conductivity, they are often embedded in an open cell a metallic foam to enhance the overall thermal conductivity. In this paper, both experimental and numerical results on PCMs coupled with aluminum foams under different heat fluxes, porosities, number of Pores Per Inch (PPIs) and orientation are presented. The test cell is equipped with a Zincum Selenide window that allows to capture the whole temperature distribution by means of a IR camera. The melting front position in time is tracked by means of a MATLAB® algorithm based on IR camera images that are useful for a more robust tracking of melting front. Numerical simulations are performed with references to the porous media volume-averaged approach, under the assumption of local thermal non-equilibrium between the two phases. The most updated correlations for the porous media closing coefficients are taken from the literature. All the experiments are compared with numerical simulations, showing a very good agreement. After showing the effects of the different input parameters on melting front evolution, an analysis in terms of different convective heat losses to the environment and melting temperature range is presented to appreciate how these two variables affect the melting front position. Finally, total melting front evolution has been compared between experiments and simulations, showing a good agreement. This has been evaluated for different conditions, showing that a decrease in the porosity drastically reduces the melting time, while PPI has no relevant effect and small effects can be observed from orientation.

Design of non-uniformly distributed annular fins for a shell-and-tube thermal energy storage unit

The shell and tube latent heat thermal energy storage systems are widely recognized as one of the most effective ways to store and utilize solar energy due to their high energy density, constant storage/releasing temperature, structural feasibility and rational price. The application of fins is an effective method to enhance heat exchange through extend the heat transfer surface area. However, there existed strong uneven melting behavior including melting fraction, temperature uniformity in a vertical thermal energy storage unit. To improve its uniformity of melting fraction and temperature, this study designed various conditions for fin pitch and positions in a non-uniform pattern to reduce the inhomogeneity for the melting process. Numerical models were established and validated by the experimental measurement conducted in this study. The effects of fin pitch and position on the thermal performance of the melting process were quantified via analyzing the melting front evolution, temperature and velocity distribution, melting rate and temperature uniformity. Results demonstrated that a 62.8% and 34.4% reduction in full melting time and average temperature difference in the phase change material region was separately obtained for the thermal energy storage unit with non-uniformly distributed annular fins, compared to the original case with uniform fins. Besides, the melting rate uniformity was also improved by 84.7%. The non-uniform design on fin position and pitch give a perspective to enriching the design methods for practical application in engineering.

Simulations of paraffine melting inside metal foams at different gravity levels with preliminary experimental validation

In this work, the results of a numerical code based on the porous media Local Thermal Non-Equilibrium (LTNE) and the apparent heat capacity methods, are compared with experiments aiming at a preliminary validation. The test cell consists in a 50 mm aluminum foam cube filled with a paraffin wax, heated and cooled on the same face. The heat flux is measured by two miniaturized sensors, while the temperature is measured in three different locations along the cube edge. Finally, one side is equipped with a Zinc Selenide window which is transparent to the long wave InfraRed. This system allows to track the paraffin melting front evolution together with the temporal trend of the whole temperature distribution simplifying the comparison with the numerical outputs at different time steps.The numerical model is then set with the same boundary conditions (heat flux) to predict the experimental temperature fields, considering both conduction in the solid domain and natural convection in the liquid domain. The preliminary validation shows that the numerical results match the experimental data with good agreement. Results are also presented for different gravity levels. This study can be a starting point for all those applications where gravity has a major role.

Lattice Boltzmann simulation of melting heat transfer in a composite phase change material

Thermal performance enhancement of the phase change material (PCM) is of particular significance to the energy storage applications. The composite PCM with metal particles is introduced here for maximizing the thermal performance. Based on the description of a composite PCM by the QSGS method, a lattice Boltzmann model of the melting composite PCM was developed and numerically solved. The melting front evolution and transient temperature response in the composite PCM are analyzed and compared to the pure PCM. Furthermore, the effects of the dispersity, volume fraction, and diffusion direction of the metal particles on the melting performance of composite PCMs are comprehensively analyzed. The results indicate that the presence of metal particles benefits the thermal performance enhancement of PCMs. The melting evolution and the heat transfer in the composite PCM are much quicker due to the enhanced thermal conduction. Besides, the melting front morphology maintains an approximate line during the whole melting process arising from the natural convection suppression. For maximizing the melting heat transport, the metal particles in the composite PCM are recommended to present a larger dispersity, and their diffusion direction is suggested to be in the same direction as the heat flow. Interestingly, even though the volume fraction of the metal particles is identical, the melting performance of composite PCM is also relevant to the dispersity and diffusion direction of the metal particles.

Gradient design of pore parameters on the melting process in a thermal energy storage unit filled with open-cell metal foam

In this paper, a novel latent heat thermal energy storage unit filled by metal foams with gradient in pore parameters was proposed to enhance thermal energy storage performance. Two kinds of gradients, namely positive and negative gradients, in porosity and pore density for metal foams were designed. Physical and numerical models were established for this problem to predict the melting front evolution, temperature and velocity field during the melting process. An experimental test rig was built and measurements on temperature variation in paraffin/metal foam composite were conducted to validate the numerical model, achieving good agreement. The temperature variations and evolution of melting front were explored and recorded. The numerical results demonstrated that the positive gradient in porosity can provide the better heat transfer enhancement than the uniform and negative gradient cases. A 17.9% reduction in full melting time can be achieved with the positive gradient design of porosity and simultaneously a better temperature uniformity was also obtained in comparison with the other arrangements. Improving the arrangement of pore density contributed little to the melting process but the temperature uniformity for the case with positive gradient in pores density increased by 9.1% compared to the uniform arrangement. An optimization recommended on a gradient in porosity of 0.89–0.95–0.98 to further reduce the full melting time by 21.1%.

Experimental study on melting performance of phase change material-based finned heat sinks by a comprehensive evaluation

Efficient thermal management has become an important challenge for the integration and miniaturization of electronic devices with high power density. Phase change material (PCM)-based finned heat sink is an efficient passive cooling technology for an intermittent-use electronic device with high power density. The present work reports an experimental study on the transient thermal performance of PCM-based finned heat sinks for thermal management of electronic devices, with particular emphasis on the roles of heat load and PCM volumetric fraction. The temperature response and melting front evolution in heat sinks are analyzed and compared with the corresponding cavity. Moreover, the effects of heat load, fin geometry and PCM volumetric fraction on the melting performance of heat sinks are discussed. The results indicate the presence of fins effectively improves the thermal performance of PCM-based heat sinks, and a more fin number leads to a lower operating temperature and hence the duration of electronic devices is longer in an accepted temperature. Moreover, a higher input heat flux results in a higher operating temperature and a shorter duration of low operating temperature. The increase in PCM volumetric fraction enhances the thermal storage capacity of finned heat sinks and reduces the operating temperature. Therefore, the finned heat sinks filled with PCM ensure sufficiently low operating temperature for perfect reliability and duration of high-power-density electronic devices.

Melting performance enhancement of a latent heat storage unit using gradient fins

Latent heat storage (LHS) is critical for the large-scale utilization of sustainable energy. The present paper designs the gradient fins (i.e., fins with gradient thickness and gradient central-angle) to enhance the melting performance of a LHS unit. A two-dimensional model of the melting heat transfer process for phase change material with Paraffin wax RT58 in a finned LHS unit is developed and numerically solved to investigate the melting behaviors. The melting front evolution and the dynamic temperature distribution are analyzed, and the effects of natural convection, fin layout, and Stefan number are investigated as well. Moreover, the fin configuration is optimized by the response surface method (RSM) with the objective function of the complete melting time. The results indicate that the natural convection seriously affects the melting behaviors in LHS units, and the melting rate is improved by 180% as compared with the case without natural convection. The LHS unit with gradient fins has a faster melting rate and a more uniform temperature distribution due to the enhancement in heat transfer synergy between natural convection and thermal conduction. Moreover, the increase of Stefan number leads to a faster melting rate and a shorter complete melting time. The RSM optimization indicates that the optimal gradients of fin thickness and central angle in this paper are 2.846 mm and 14.96°, respectively, and the complete melting time is reduced by 30.5% for the optimized LHS unit as compared with the corresponding uniform fin. Therefore, the gradient fins are recommended to be configured in a LHS unit in the engineering applications, where the fins are distributed with gradient angle, and they are thinner at the upper part and thicker at the lower part.