Melting Phase Change Research Articles

A general non-Fourier Stefan problem formulation that accounts for memory effects

The Stefan problem is the classical model of a melting phase change. In heterogeneous systems, such phase changes can exhibit non-Fourier (anomalous) behaviors, where the advance of the melt interface does not follow the expected time scaling. These situations can be modeled by replacing the derivatives, in the governing partial differential equations, with fractional order derivatives. In particular, replacing the time derivatives leads to non-Fourier models that account for memory effects in the system. In this work, by using appropriate time convolution integrals, a general thermodynamic balance statement for melting phase problems, explicitly accounting for memory effects, is developed. From this balance, a general model formulation applicable to problems involving melting over a temperature range (i.e., a mushy region) is derived. A key component in this model is the representation of memory effects through the use of fractional derivative based constitutive models of the enthalpy and heat flux. On shrinking the mushy region to a single isotherm, a general sharp interface melting model is obtained. Here, in contrast to the classic Stefan problem, the fractional derivatives induce a natural regularization, such that the constitutive models for enthalpy and heat flux are continuous at the melt interface; a result confirmed through numerical simulation. To further support the theoretical findings, a physical example of a non-Fourier Stefan problem is presented. Overall the development and results in this paper underscore the importance of explicitly relating the development of fractional calculus models to the appropriate thermodynamic balance statements.

Numerical studies on a fin-foam composite structure towards improving melting phase change

The low thermal conductivity of phase change materials limits the large-scale application of latent heat storage technology. It is meaningful to improve the thermal conductivity of phase change materials by corresponding means. In order to improve the performance of latent heat storage device, this paper takes square latent heat storage device as the research object, and uses fin and foam metal to enhance its melting performance. A 3-D numerical model is established and verified by visualization experiment. The quantitative comparison of melting properties of four different structures (pure paraffin, fin, metal foam, fin-metal foam) is presented. The results show that compared with pure paraffin structure, the complete melting time of phase change materials corresponding to fin, metal foam, and fin-metal foam structure is reduced by 47.48%, 79.53%, and 83.68%, respectively. The temperature uniformity increased by 28.97%, 79.37%, and 91.12%, and the total heat storage decreased by 6.0%, 4.6%, and 11.64%, respectively. It shows that the addition of fin and foam metal is beneficial to improve the melting performance and the overall temperature uniformity of the device, but it has a negative effect on total heat storage. Dynamic temperature studies were conducted to further explore the effect of fins and metal foam on the internal melting process compared with pure paraffin structure.

Numerical Study on Flow-Melt Characteristics of Ice Slurry in Horizontal Straight Pipe with a Local Large Heat Flux Segment

Ice slurry is a high energy density coolant with excellent flow, phase change, and thermophysical properties. In order to investigate the recovery of ice slurry flowing through a local large heat flux segment, a 3D Eulerian-Eulerian model based on the granular kinetic theory considering the flow and melting phase change of ice slurry is developed. Sensitivity analysis of interphase forces is carried out. A comparison of the pressure drop, solid phase velocity, and heat transfer coefficient with empirical data is carried out, respectively. The calculated results are in good agreement with the experimental results, indicating that the numerical model could accurately describe the flow and melting characteristics. Thermophysical field distributions, the axial variation of ice volume fraction (IVF), recovery curve, the average heat transfer coefficient, as well as the re-uniformization length are obtained. After passing through local large heat flux segment, due to shear stress action, the IVF and the particle uniformity of the cross section have recovery characteristics. The gradient of the recovery curve decreases with increasing inlet IVF as well as with increasing Reynolds number. After the local large heat flux increases to a certain value, its effect on the recovery curve of the ice slurry is small. The re-uniformization length increases as the local large heat flux increases. The average heat transfer coefficient of local large heat flux segment increases due to damage of the boundary layer. These results can provide a theoretical basis for the design of ice slurry systems in practical application.

Experimental and numerical studies on melting/solidification of PCM in a horizontal tank filled with graded metal foam

Although solar energy is a clean and abundant resource, it has an unstable nature. It is demonstrated that latent thermal energy storage (LTES) systems have been an excellent way to utilize solar energy fully and widely. However, LTES has the problem of insufficient thermal conductivity. For this reason, it is inevitable to consider effective methods to intensify the thermal conductivity of LTES system. In the current study, experiment and numerical simulation are used to study the influence of non-uniform metal foams on heat transfer during phase transition. In this study, a horizontal shell-and-tube LTES test system is established. Moreover, the phase change melting rate of radially filled metal foams with different porosity gradients is compared. According to the numerical simulation results of phase interface, velocity field and temperature field, natural convection can accelerate the melting of PCM. However, there is no distinct effect on the solidification process. When the equivalent porosity is 0.94, the optimal combination (melting process is 0.84-0.92-0.99 and solidification process is 0.87-0.94-0.97), compared with the uniform structure, can shorten the total consumption time by 9.7% and 6.2%, respectively. • The radial positive gradient structure accelerates the melting, but it has little effect on the solidification. • In the current study, the optimal melting/solidification combinations are 0.84-0.92-0.99 and 0.87–0.94-0.97, respectively. • The optimized design of metal foam gradient can reduce the cost of mobile thermal storage system.

Template-free facile preparation of mesoporous silica from fly ash for shaped composite phase change materials

Traditional sol-gel template preparation method for mesoporous silica has limited its large-scale production and applications due to low production quantity, complex preparation process and high cost. In this paper, we proposed a novel template-free process to prepare mesoporous silica-based materials by acid etching of activated fly ash to improve the support performance for shaped composite phase change materials. The mechanisms of mineral phase transformation and pore formation during preparation of mesoporous silica from fly ash were systematically investigated, and the structure, morphology and thermal properties of as-prepared shaped composite phase change materials (SCPCMs) were characterized and analyzed. The results show that stable mullite and quartz could be transformed into potassium aluminosilicate minerals after calcined with K2CO3, and mesoporous silica-based materials with different pore shapes and specific surface area ranging from 330 to 490 m2/g could be prepared from as-calcined fly ash. The PEG/MS-I (ink bottle-shaped material) achieved the largest loading efficiency of 64.88%, and the highest melting phase change enthalpy of 88.38 J/g, which is 30.7% and 57.7% higher than that of PEG/MS-S (silt-shaped material) and PEG/MS-W (wedge-shaped material), respectively. After 20 thermal cycles, the loss of melting enthalpy for PEG/MS-I was only 3%, and the thermal energy storage and release time were 23.1% and 17.0% shorter than that of pure PEG, respectively. In all, the PEG/MS-I composite exhibited reliable thermal storage performance and temperature regulation ability, which provides the feasibility of application of fly ash-derived composite phase change material in energy storage field.

Enhancement of phase change material melting using nanoparticles and magnetic field in the thermal energy storage system with strip fins

In this work, the numerical analysis of heat transfer and the enhancement of the melting of phase change material (PCM) in a three-dimensional cylindrical thermal energy storage system has been investigated. The RT82 is as PCM and the finite volume method is as the numerical solution method. Porosity-enthalpy method has been used to simulate the phase change melting process. The effects of strip fins, Fe3O4 nanoparticles, and uniform and non-uniform magnetic fields on the PCM melting are studied in details. Different modes have been considered for the non-uniform magnetic fields, both constant and variable with location. The predicted results display that the PCM melting time of the energy storage system using the new arrangement of strip fins is reduced by about 51 % compared to the that without fins. The addition of nanoparticles in PCM with volume fractions of 2.5 % and 5 % has improved the melting time by about 15 % and 22 % compared to those without nanoparticle. The use of a uniform magnetic field does not have much effect on the melting process without fins. But in the finned cases, in the initial melting (<1000 s), it has a noticeable effect on the amount of releasing energy in the desired system compared to the that without magnetic field, the mount of releasing energy is almost doubled. Using a variable magnetic field has a positive effect on the melting process of thermal energy storage and has improved the phase change process by about 39 % compared to the case without a field. It has also been concluded that in the case where the changes of the origin of the variable magnetic field (electric voltage) in the z-direction are parabolic, >60 % reduction in the melting time is noted.

Flexible and Biocompatible Silk Fiber-Based Composite Phase Change Material for Personal Thermal Management

Phase change materials (PCMs) are regarded as an effective passive personal thermal management strategy. However, the preparation of flexible and biocompatible PCMs remains a great challenge. In this study, a silk fiber (SF)-based composite PCM for wearable personal thermal management was prepared using renewable natural silkworm cocoons and biocompatible capric acid (CA). The SF/CA composite PCM is flexible, biocompatible, and dyeable. The results show that the SF and CA are physically bonded, and the crystal structure of CA is not influenced by SF. The melting phase change enthalpy and temperature of the SF/CA composite PCM are 123.4 J/g and 30.6 °C, respectively. It has excellent shape stability, thermal stability, and cycling stability. Particularly, the SF/CA composite PCM has excellent performance for wearable personal thermal management under three scenarios including variable temperature mode, isothermal mode, and light irradiation mode. It can also reduce the temperature fluctuation of the human body in a hot or cold environment. Therefore, the SF/CA composite PCM has bright application prospects for wearable personal thermal management in hot and cold environments.

Tailored calcium chloride hexahydrate as a composite phase change material for cold storage

In this study, a water-salt system composite phase change material with calcium chloride hexahydrate (CCH, CaCl2·6H2O) as the main cold storage functional body was successfully tailored. The phase change temperature of CaCl2·6H2O was reduced by adding urea and ammonium chloride (NH4Cl), and its supercooling and phase separation were suppressed by adding strontium chloride hexahydrate (SrCl2·6H2O) and sodium carboxymethyl cellulose (CMC). The preferred composite phase change material (CCH-UNSC4) consisted of 76 wt% CaCl2·6H2O, 11 wt% urea, 6 wt% NH4Cl, 4 wt% SrCl2·6H2O, and 3 wt% CMC, which had a melting phase change temperature of 9.51 °C, a latent heat of melting of 99.08 J g−1, and a supercooling degree of only 0.39 °C. The thermal conductivity of CCH-UNSC4 reached 0.330 W m−1 K−1. Its phase change properties were basically unchanged after 50 thermal cycles, showing good cycling stability. Cold storage and release experiments and fruit storage experiments showed that CCH-UNSC4 can play its advantages in cold chain transportation.

Heat transfer characteristics of ceramic foam/molten salt composite phase change material (CPCM) for medium-temperature thermal energy storage

Molten salts have been widely used as energy storage materials in medium- and high-temperature thermal energy storage. However, pure salt commonly suffers from low thermal conductivity and many conventional methods of heat transfer enhancement do not apply due to the serious corrosion and the extremely high temperature. In the current study, the open-cell SiC ceramic foam was integrated with solar salt (60wt% NaNO3 + 40wt% KNO3) to enhance the heat transfer of salt and avoid severe corrosion issues. A visualised experiment was for the first time conducted to investigate the melting phase change heat transfer in the ceramic foam/molten salt composite phase change material (CPCM). A representative elementary volume (REV)-scale simulation was simultaneously performed and the computational results were compared with experimental data. It is found that the conduction-friendly ceramic skeleton remarkably enhances the heat transfer in molten salt, especially in the region far away from the heat source. The spatial temperature difference across the composite is decreased in both horizontal and vertical directions and the local superheating is mitigated. The enhancement of heat conduction is greater than the suppression of natural convection; as a result, the melting rate of the CPCM is increased by 41.3%. This study provides crucial benchmark data of phase change heat transfer for medium-temperature thermal energy storage and paves the way for system design and optimization.

Properties of eco-friendly foam concrete containing PCM impregnated rice husk ash for thermal management of buildings

Thermal energy storage (TES) through the use of construction materials incorporating phase change materials (PCMs) can prevent temperature fluctuations and allow energy saving in buildings. With this background, aim of this work is to develop new kind eco-friendly foam concrete (FC) containing lauryl alcohol (LA)-impregnated rice husk ash (RHA) composite PCM. RHA, an agricultural waste product, was used as a carrier material to eliminate the leakage problem of LA. Thus, leakage-free composite PCM (LFCPCM) was first prepared, and then such an RHA-based LFCPCM was integrated with cementitious FC for the first time in this study. The fabricated novel FCs were subjected to detailed examinations in terms of morphological, mechanical, physical, and TES properties. The DSC outcomes indicated that LFCPCM showed melting phase change at 19.97 °C and had a latent heat TES capacity of 99.60 J/g, while the PCM into FC-LFCPCM50 melted at 20.01 °C and had a latent heat TES capacity of 16.55 J/g. Solar thermoregulation performance test results revealed that compared to the reference FC (RFC), the FC-LFCPCM50 wallboard provided about 1.29 °C warmer indoor temperature during the cold weather hours, whereas the room center temperature was about 2.8 °C lower during the daytime in hot weather conditions. An energy-saving of 14.28 kW h per day is obtained by FC-LFCPCM50 wallboard. The carbon emission equivalences of this energy-saving amount account for 38 kg-CO2, 37.7 kg-CO2, and 6.19 kg-CO2 for coal, natural gas, and electricity, respectively. These results suggest that the fabricated novel FC-LFCPCM50 can be effectively evaluated as green building materials for thermo-regulation and energy saving of buildings.

Compression effect of metal foam on melting phase change in a shell-and-tube unit

• Compressed foam greatly reduces complete melting time by 13.9%, under the same mass of metals. • A proper compressive ratio leads to a synchronized in melting for the top and bottom region. • Compressed metal foam significantly improves thermal conductivity and bottom conduction. • Over compressing metal foam causes an increment more than 129.4% in full melting time. • Excessive compression leads to serious non-uniform features of different regions. Metal foams have high thermal conductivity and can be used to markedly improve the low effective conductivity in phase change materials (PCMs) during the process of melting and solidification. Furthermore, natural convection as a main way to heat transfer should be achieved attention in thermal enhancement. To further accelerate the energy storage rate, porous metal foam was compressed and saturated with PCMs. The latent heat thermal energy storage tubes packed with compressed metal foams under various compression ratios were designed and analyzed compared with uncompressed tubes. Good agreement between experimental results and numerical simulations assessed the applicability of the established numerical model. The observations on the melting process including melting fraction, temperature response distribution and uniformity, velocity field, heat flux and energy storage were further discussed. Results showed that the compressed metal foam had a better performance on improving phase change, achieving a reduction of 13.9% for complete melting time. The enhancement of thermal conductivity and the strengthening of natural convection were synergized. However, over compressing metal foam will not help reduce the melting time, leading a prolonged melting time by 129.4% and serious non-uniformity in the melting process of different regions, as well.

EFFECT OF SUPERGRAVITY ON MELTING PHASE CHANGE IN METAL FOAM

EFFECT OF SUPERGRAVITY ON MELTING PHASE CHANGE IN METAL FOAM

Study on Heat Transfer of Waxy Crude Oil Melting Phase Change Based on a Novel Model

Study on Heat Transfer of Waxy Crude Oil Melting Phase Change Based on a Novel Model

Preparation and characterization of polyethylene glycol/polyurea composites for shape stabilized phase change materials

AbstractA linear polyurea (LPU) and a crosslinked polyurea (CPU) were respectively synthesized by condensation between 4,4′‐diaminodiphenyl methane and multivariate isocyanates, and then were used as supporting material to load polyethylene glycol (PEG) for shape‐stabilized phase change materials (ssPCMs). LPU has an incompact flocculent morphology with higher surface area and pore volume than CPU, and CPU is composed of contiguous bead particles with many interspaces. The shape stability of PEG/CPU ssPCMs is better than that of PEG/LPU ssPCMs with the same PEG mass fraction due to the different molecular interactions between PEG and the polymer matrix. The maximum PEG mass fraction in PEG/LPU and PEG/CPU ssPCMs LPU are 65% and 75%, respectively, and the corresponding melting phase change enthalpies are 82.66 J/g and 98.13 J/g, respectively. The heat storage efficiency of PEG/CPU ssPCMs is smaller than that of PEG/LPU ssPCMs with the same PEG content, caused by the greater movement restriction of PEG chains from the crosslinked network of CPU. The prepared ssPCMs have favorable reusability after 100 heating–cooling thermal cycles and have good thermal stability below 250°C, which is helpful to widen extent of application for latent heat storage.

Large-scale fabrication of flexible EPDM/MXene/PW phase change composites with excellent light-to-thermal conversion efficiency via water-assisted melt blending

Thermal therapy based on phase change materials (PCMs) has broad application prospects in the field of personal thermal care. Herein, a serious of EPDM/MXene/PW (EMP) based shape-stable phase change composites (SSPCCs) with excellent flexibility was prepared in large scale via water-assisted melt blending. The MXene nanosheets were dispersed well in the EPDM matrix during the melt blending processes with the assistance of water. With the introduction of a small amount of MXene nanosheets (only 0.8 wt%), the light-to-thermal conversion efficiency of SSPCCs were improved obviously. The melting phase change enthalpy and freezing phase change enthalpy of EMP-0.8 are about 120 J/g and 115 J/g, respectively. And after 200 times thermal cycles and 25 times light-thermal cycles, the phase change enthalpy of obtained EMP-0.8 SSPCC hardly changes. All the results show that the obtained EMP based SSPCCs can be fully utilized in the field of personal thermal therapy.

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.

Experimental study on the variation of unfrozen water content and pore water in unsaturated soil during thawing process

Based on nuclear magnetic resonance (NMR) technique, the variation characteristics of unfrozen water content and the occurrence law of unfrozen water in pore structure of silty clay, red clay and remolded mudstone were analyzed by thawing experiments under different temperature gradients. The results show that the thawing process can be divided into negative temperature without phase change temperature rise stage, violent phase change melting stage and positive temperature without phase change temperature rise stage, and there is no superheating phenomenon corresponding to the supercooling phenomenon. The ice in the small pores continued to thaw during the whole test process. The ice stored in the macropores began to thaw in 30mins, and basically thawed in 40mins. The temporal order of pore ice melting is caused by the difference of hydrothermal kinetic potential energy. And the temperature gradient has no significant effect on the sequence of pore ice melting.

Enhanced cold storage performance of Na2SO4·10H2O/expanded graphite composite phase change materials

Emerging phase change cold storage material named SBCKN tailored by sodium sulfate decahydrate (Na2SO4·10H2O, SSD), borax (B), carboxymethyl cellulose (CMC), potassium chloride (KCl), and ammonium chloride (NH4Cl), has successfully suppressed supercooling and phase separation, and its melting phase change temperature is 8.01 °C. Expanded graphite (EG) was added to improve the shape stability and thermal conduction properties of the SBCKN. The results indicated that the melting phase change temperature of SBCKN/EG5 added with 5% EG was 5.92 °C, whose melting and cooling latent heat of melt values were 99.35 J g−1 and 66.39 J g−1. Compared to SSD, the supercooling degree of SBCKN/EG5 was reduced by 18.93 °C. The thermal conductivity of SBCKN/EG5 was 0.43 W m−1 k−1, which was 1.75 times that of the SBCKN, as demonstrated in both the thermal storage and release experiment and transient temperature experiment. The crystal structure and chemical bonds of the tailored composites were further investigated, and the composites had good chemical stability, which possessed potential application in cold chain transportation. Moreover, the mechanism of the phase transition process of the SBCKN/EG composites was revealed.

Charging optimization of multi-tube latent heat storage comprising composite aluminum foam/nano-enhanced coconut oil

The fast charging of thermal energy storage (TES) systems is a requirement for the practical application of these systems. The thermal energy should be stored in a unit within a reasonable time. The present research aims to minimize the thermal charging time of a latent heat TES unit by using aluminium foams, nanoparticles (copper oxide), and geometrical optimization of the unit. The melting phase change was simulated in the TES unit by using FEM. The Taguchi optimization method was invoked to maximize the melting rate during two hours of thermal charging. The results showed that the geometrical design of the unit and the porosity of foam are the most influential design parameters on the thermal energy storage and melting rate. The variation of the design parameters could improve the melting rate by 41%. The presence of nanoparticles could only improve the melting rate by less than 2%. The optimal design of the TES unit can be fully charged in two hours. Such fast-charging time could be advantageous in solar systems and transient heat recovery.

Natural convection improvement of PCM melting in partition latent heat energy storage: Numerical study with experimental validation

This study examines an innovative design to enhance the natural convection in a rectangular latent heat energy storage unit (LHSU). The new design divides the rectangular plane into different small partitions (1, 2, 4, 8, 16 and 32) to trap the melted phase change material (PCM) and create several melting fronts which accelerate the melting process than with a normal 1-partition LHSU. The number of these partitions was varied by changing the PCM cavity aspect ratio with values 2, 1, 0.5, 0.25, 0.125 and 0.0625 taken. To validate the numerical model developed herein, LHSUs with one and two partitions were tested experimentally. The numerical model using the ANSYS FLUENT, extended the analysis to other partition numbers. Results showed that the natural convection was significantly strengthened by partitioning the LHSU, which shortens the time required for PCM melting. However, an optimal number of partitions were obtained after which improvement ratio was declined. An LHSU with 8 partitions (0.25 aspect ratio partitions) was found as an optimum. In addition, the comparison with a normal 1-partition LHSU found that the PCM melting process was enhanced by 31.2%, 53.6%, 65% and 68% for the LHSUs with 2, 4, 8, and 16 partitions, respectively.