Phase Change Process Research Articles

A consistent phase-field model for three-phase flows with cylindrical/spherical interfaces

A phase-field model for three-phase flows with cylindrical/spherical interfaces is established by combining the Navier-Stokes (NS), the continuity, and the energy equations, with an explicit form of curvature-dependent modified Allen-Cahn (AC) and Cahn-Hilliard (CH) equations. These modified AC and CH equations are proposed to solve the inconsistency of the phase-field method between flat and curved interfaces, which can result in “phase-vanishing” problems and the break of mass conservation during the phase-changing process. It is proved that the proposed model satisfies the energy dissipation law (energy stability). Then the icing process with three phases, i.e., air, water, and ice, is simulated on the surface of a cylinder and a sphere, respectively. It is demonstrated that the modification of the AC and CH equations remedies the inconsistency between flat and curved interfaces and the corresponding “phase-vanishing” problem. The evolution of the curved water-air and the water-ice interfaces are captured simultaneously, and the volume expansion during the solidification owing to the density difference between water and ice agrees with the theoretical results. A two-dimensional icing case with bubbles rising is simulated. The movement and deformation of bubbles, as well as the evolution of the interfaces, effectively illustrate the complex interactions between different phases in the icing process with phase changes.

Simultaneous close-contact melting on two asymmetric surfaces: Demonstration, modeling and application to thermal storage

The present study deals with melting in a geometry suitable for Latent-Heat Thermal Energy Storage (LHTES) systems, which are of importance for future industrial installations utilizing solar energy or waste heat. The intrinsically high thermal resistance of phase-change materials (PCM) can be remedied by taking advantage of close contact melting (CCM), where the solid phase is separated from a hot surface by only a thin liquid layer. The basic CCM takes place on a horizontal flat surface, but during the last decade its application in various finned LHTES units has been demonstrated experimentally and analyzed numerically.Specifically, the configuration studied in the present work is relevant to a horizontal double-pipe concentric storage unit with a longitudinally finned inner tube. While this rather simple geometry has been extensively studied in the past, the present work is completely novel in its demonstration and analysis of simultaneous close-contact melting on two generally asymmetric surfaces created by the longitudinal fins. A unique experimental apparatus is introduced, based on the so-called 'Mercedes' configuration of the fins. It is designed transparent, allowing for real-time observation of melting sequences. Close-contact melting is achieved by supplying heat to the outer shell of the unit: the solid phase is detached from the shell and moves, by translation and rotation, in the liquid phase. The results from these experiments demonstrate the feasibility of CCM on two asymmetric walls within this system, hinting at the potential for optimizing the melting rate by utilizing a larger portion of the extended surface for CCM.A key component of this study is the development of a reliable numerical approach, which goes beyond the limitations of widely-applied enthalpy-porosity method and combines the general enthalpy formulation, convective heat transfer and general rigid body motion that includes rotation. Therefore, a new numerical model has been devised, using an in-house code realized in MATLAB. A full set of the governing conservation equations is solved using a finite-difference framework, integrated with advanced numerical methods for the fluid-solid interaction and an enthalpy formulation for the phase change process. The model is validated carefully, and then numerical studies are conducted to elucidate the melting process.The present paper presents a further proof of the special role that close-contact melting (CCM) can play in properly designed thermal energy storage units and finned systems in general. It is demonstrated that the fins, when properly designed and oriented, can induce CCM in their vicinity, contributing to a very significant increase in the melting rate, which reflects charging of the unit.

Transient thermal management of laser systems using Plate-Fin phase change heat Exchangers: Experimental and computational study

To mitigate transient thermal shocks in lasers and reduce thermal stresses caused by temperature fluctuations, the use of phase change materials (PCMs) in thermal management systems is a viable solution. This study proposes an innovative two-dimensional transient heat transfer model specifically designed for plate-fin phase change heat exchangers (PFPCHEs). The model meticulously simulates the complex heat transfer phenomena within the heat exchanger, including fluid convection, solid thermal conduction, and the phase change processes of PCM. The convective heat transfer coefficient between fluid and plate is calculated using the Wieting correlation. An advanced pulse-mode experimental test platform was constructed to validate the model, dynamically monitoring and recording outlet temperatures and heat exchange performance for stringent experimental comparison. The research explores the impact of key operating parameters such as initial temperature, flow rate, and inlet temperature of the cooling cycle on the performance of the heat exchanger, providing valuable design and control strategies for transient thermal management of laser systems. The experimental results confirm that the model accurately predicts the dynamic response characteristics and temperature distribution of PFPCHEs under multi-cycle pulse loads, with 96% of the predictions within a 10% error margin. A significant finding is that the initial temperature has negligible influence on the heat transfer characteristics when it is below the solid phase temperature of the PCM. Moreover, by meticulously adjusting the flow rate and inlet temperature of cooling cycle, it is possible to effectively maintain the stability of the outlet temperature throughout the entire pulse cycle. This is crucial for minimizing temperature fluctuations within the thermal management system and extending the service life of electronic components.

Melting and heat management of phase change material using smart ternary-hybrid coolant

This research investigates the effectiveness of using a smart ternary-hybrid nanofluid to enhance the melting rate and convective behavior of electrically conducting tin (Sn) in a rectangular enclosure under the influence of a uniform magnetic field. The enclosure has adiabatic vertical walls with hot and cold temperatures on the bottom and top walls. The finite element method (FEM) is used to solve the governing equations with appropriate boundary conditions using Galerkin's weighted residual approach. The study focuses on applying tin as the phase change material (PCM), with the highest temperature of 508 K, the lowest temperature of 503 K, and the melting interface temperature of 505 K. To enhance the heat transfer performance, tin-based ternary (graphene (G), silicon carbide (SiC), and nickel (Ni)) hybrid smart coolant is applied into the system. To investigate the mechanism of the melting and convective thermal transfer process, the results of the present study are reported with time for various values of the magnetic field (Ha) and solid concentration of ternary hybrid nanoparticle (ϕ). This study represents the streamlines, isothermal lines, melting interface, melting fraction, and heat transfer for the above-mentioned parameters. The results show that increasing the magnetic field reduces the rate of thermal transport by 38.96 % at t = 4000s. However, at a particular time of 2500s, increasing the solid volume fraction of nanoparticles enhances the melting fraction by approximately 8.34 %. Two regression equations are derived for the Nusselt number and melting fraction, with multiple response variables. This article improves understanding of natural convective heat transport during phase change processes for various coolants in different engineering applications.

Simulation and Experimental Investigation of the Effect of Pore Shape on Heat Transfer Behavior of Phase Change Materials in Porous Metal Structures.

With the gradual increase in energy demand in global industrialization, the energy crisis has become an urgent problem. Due to high heat storage density, small volume change, and nearly constant transition temperature, phase change materials (PCMs) provide a promising method to store thermal energy. In this work, we designed and fabricated three kinds of porous metal structures with hexagonal, rectangular, and circular pores and explored the phase change process of PCMs within them. A two-dimensional numerical model was established to investigate the heat transfer process of PCMs within different shapes of porous metal structures and analyze the influence of heat source location on the thermal performance of the thermal storage units. Visualization experiments were also carried out to reveal the melting process of PCMs within different porous metal structures by a digital camera. The results show that paraffin in a porous metal structure with hexagonal pores has the fastest melting rate, while that in a porous metal structure with circular pores has the slowest melting rate. Under the bottom heating mode, the melting time of the paraffin in porous metal structures with hexagonal pores is shortened by 18.6% compared to that in porous metal structures with circular pores. Under the left heating mode, the corresponding melting time is shortened by 16.7%. These findings in this work will offer an effective method to design and optimize the structure of porous metal and improve the thermal properties of PCMs.

On the numerical simulation of a phase change material melting inside periodic structures: from validation to optimization

Nowadays, numerical models are considered powerful design tools, which are largely used in several engineering applications. When applied to heat transfer applications, they have already been shown to be reliable, especially in single phase systems. When considering two-phase heat transfer, and in particular solid-liquid phase change processes, the numerical tools must be carefully handled. Thus, this paper follows the critical literature on the application of numerical methods to simulate and optimize latent thermal energy storages to show how, in most cases, the common geometrical and numerical approximations should not be applied because they might lead to misleading solutions. Starting from reliable experimental results collected during melting process of a paraffin wax inside 3D periodic structures, different simulation approaches are proposed, and their advantages and disadvantages are discussed. The results show that, when dealing with certain complex 3D structures, 2D approximations cannot reliably capture the phase-change phenomenon. The validated tool also allowed to identify the optimum pore size to maximize the solid-liquid phase change. This work can be considered as a reference for future numerical studies on the solid-liquid process inside 3D structures.

Comparative experimental study of the effect of loading rate on the typical mechanical properties of bubble and clear ice cubes

Icing is a common liquid–solid phase change process that usually has negative effects. Bubbles will form in ice since air is significantly less soluble in ice than it is in water and these bubbles will affect the mechanical properties of ice cubes. To quantitatively investigate the effect of air bubbles on the mechanical strength of ice cubes, an experimental setup is built to explore the mechanical strength and modulus of clear and bubble ice cubes at different loading rates. The results show that even a low volume fraction of air bubbles in the ice cubes as low as 1.98% can have a significant effect on their mechanical properties. The weakening of tensile, compressive and bending strengths by air bubbles is almost less than 20%, while the weakening of shear strength by air bubbles reaches almost 60%. The effect of air bubbles on the four typical mechanical moduli does not exceed 20%. This research helps to optimize the de-icing technique and also provides methods and ideas for preparing ice.

Novel phase‐change PDA‐Ni@GNS/CNF‐C/SA/PEG composites with ultrahigh shape stability and latent heat as thermal management material for microelectronic devices

AbstractPhase‐change materials (PCMs) have become a hot spot in the development of modern thermal management materials because of their absorbing and releasing heat while keeping the temperature constant during phase change process. However, their relatively low thermal conductivity (TC) considerably restricts their further application. In this work, graphene nanosheets (GNS) were initially nickel‐plated to increase the contact area between adjacent GNS (in the form of Ni@GNS) by constructing a “point‐surface” structure. Then, the prepared Ni@GNS was coated with polydopamine (PDA) to prepare PDA‐Ni@GNS, which helped to effectively reduce the interface thermal resistance (ITR) between the filler and the matrix, and also made the prepared filler more easily dispersed in the polymer matrix. By constructing well‐defined three‐dimensional (3D) thermally conductive pathways in the matrix, the PCMs exhibited excellent TC and effectively prevented the leakage of polyethylene glycol (PEG) even at low filler loading. The results of thermogravimetry (TG), differential scanning calorimeter (DSC), and cyclic DSC tests jointly showed that the PDA‐Ni@GNS/CNF‐C/SA/PEG PCMs displayed excellent thermal storage properties, high latent heat as well as good cyclic thermal stability. The present study provides a facile way of preparing PEG‐based PCMs, which are applicable in thermal management area.

Preparation and Properties of Stearic Acid/Nanocellulose Composite Phase Change Energy Storage Materials

To solve the problems of liquid phase leakage and poor thermal conductivity of organic phase change energy storage materials, a novel composite phase change energy storage material (SA/CNF) based on stearic acid (SA) and nanocellulose (CNF) was prepared in this study and added graphene to enhance its thermal conductivity. For the prepared composite phase change materials (SFs) with different ratios, shape stability, and testing experiments showed that the maximum adsorption rate of CNF on SA reached 80 %. The properties of SA/CNF were then characterized by various instruments. The results show that SA and CNF are compounded together by physical interaction. More than 93 % of SA in the composite phase change energy storage material SA/CNF is able to store and release heat through the phase change process, and the latent heat of phase change of pure SA is 206.3 J/g, while the latent heat of phase change of the composite phase change material SA/CNF with 20 % of CNF is 156.2 J/g, and they both have good cycling stability. The addition of graphene enhances the thermal conductivity of SA/CNF to a large extent, and the thermal conductivity at a graphene ratio of 5 % is 261 % higher than that of SA/CNF without graphene. SA/CNF is an environmentally friendly energy storage material with very high application prospects.

The multi-cycle dynamics of the cavitation bubble near the solid wall with an air-entrapping hole or a hemispherical air bubble: A numerical study

Investigating the interaction between the near-wall cavitation bubble and the air bubble has great significance for understanding the mechanism of air entrainment to alleviate cavitation in actual hydraulic engineering. To quantify the effect of the air bubble on the multi-cycle dynamics of the near-wall cavitation bubble, a more comprehensive compressible three-phase model considering the phase-change process was developed based on OpenFOAM, and corresponding validation was performed by comparing the simulated bubble shape with the published experimental values. The key features of the multi-cyclical evolution of the cavitation bubble are nicely reproduced based on the current numerical model. For the cavitation bubble near the solid wall containing a hemispherical air bubble, the simulated results reveal that the air bubble can reflect the shock wave and thus prevent it from impacting directly on the solid wall, which will help to uncover the microscopic mechanism of aeration avoiding cavitation damage. The dynamical features of the cavitation bubble at different dimensionless distances (γ1) and dimensionless sizes (ε) are investigated and analyzed. For the near-wall cavitation bubble with an air-entrapping hole, the air hole plays a crucial role in the multi-cycle dynamics of the cavitation bubble, leading to the bubble that is always far away from both the air hole and the solid wall. Thus, the current results may provide a potential application for preventing the wall damage caused by the impact of the liquid jet.

Study on the influence factors of rock breaking by supercritical CO2 thermal fracturing

At present, there is a growing demand for safe and low-pollution rock-breaking technology. The rock breaking technology of supercritical CO2 thermal fracturing has many advantages, such as no dust noise, no explosion, high efficiency, controllable shock wave and so on. Fully considering the combustion rate of energetic materials, heat and mass transfer, CO2 phase change and transient nonlinear flow process, a multi-field coupled numerical model of rock breaking by supercritical CO2 thermal fracturing was established based on the existing experiments. The influence factors of CO2 thermal fracturing process were studied to provide theoretical guidance for site construction parameters optimization. The numerical simulation results were in good agreement with the experimental observation results. The results showed that the maximum temperature of CO2 and the growth rate of CO2 pressure during the fracturing process would decrease accordingly with the increase of CO2 initial pressure. But the change in CO2 peak pressure wasn't significant. Appropriately increasing the heat source power could improve the heating and pressurization rate of CO2 and accelerate the damage rate of rock. The relevant results were of great importance for promoting the application of rock breaking by supercritical CO2 thermal fracturing technology.

Numerical Voltammetry of Phase Separating Materials Using Phase Field Modeling

Higher capacity materials, such as Si and Sn are known to have phase separating behavior during the (de)lithiation. While initial models for lithiation in graphite electrode were based on single phase diffusion, with the introduction of Si and Sn, disposition of the models has shifted to the two-phase diffusion. It is important to understand the interaction of various phenomenon in materials which show phase change during (dis)charging cycles. In this work, we present a phase field model to simulate two-phase lithiation. This model is used to study the electrochemical response of the system by conducting numerical voltammetry. The main goal of this effort is to highlight the difference in electrochemical response occurring during single-phase diffusion and two-phase diffusion and explain the ensuing physics. Furthermore, effect of elasticity which governs the phase-change process and also alters corresponding voltammograms is also studied in detail. The voltammograms show clear shift in current peaks’ size and position for the changing diffusion behavior. Also as elasticity affects the two-phase diffusion, change in nucleation timing and diffusion rate are visible in voltammograms. Additionally, it is also observed how elasticity can cease the phase separation behavior and voltammogram for two-phase diffusion can become identical to single-phase diffusion.

Theoretical investigation of VO2 smart window with large-scale dynamic infrared emittance adjustment for adaptive thermal management

Thermochromic windows based on vanadium dioxide (VO2) are widely used in architectural windows due to its reversible phase change process. However, traditional VO2 windows only manage solar radiative transmittance and their emittance variation trend in the mid-infrared contradicts realistic requirements, which seriously hinders the further development of thermochromic windows. To address this issue, a VO2 full-spectrum smart window based on theoretical calculations is proposed for adaptive adjustment of solar spectral transmittance and thermal emittance. We cleverly utilize a thin Ag layer to construct a Fabry-Perot (FP) resonant cavity with the VO2 layer to achieve forward modulation of the emittance in the atmospheric window. Notably, the smart window has 83.8 % emittance modulated ability, which enables effective control of radiative cooling and has greatly exceeded the reported performance of the smart windows while maintaining 72.8 % high visible transparency. Energy consumption analysis indicates that the smart window has high energy-saving potential worldwide, achieving over 60 MJ/m2 energy-saving effect. This simple and easy-to-manufacture smart window expands the research scope of windows and broadens application prospects in thermal management, infrared camouflage, and building energy conservation.

Single‐step preparation of intrinsic solid–solid phase change materials with excellent anti‐leakage and form‐stable performance

AbstractPhase change materials (PCM) often encounter severe problems of leakage and shape collapse concerns. Chemical modification methods, utilizing chemical bonds to bind phase change molecular chains, have the potential to address the above‐mentioned problems radically. However, this approach often entails the use of expensive raw materials and complex processes. Therefore, a simple one‐pot preparation method had been reported using earth‐abundant and inexpensive octadecyl methacrylate (OMA) and 1,6‐hexanediol diacrylate (HD) as raw materials to prepare PCM with a molecular structure of cross‐linked polymer main chains and phase change side chains through free radical polymerization. Owing to the three‐dimensional polymer main chain structure, it exhibited excellent leakage‐proof performance, maintaining its form‐stable nature at 70°C with no obvious leakage observed during the phase change process, confirming it to be an excellent intrinsic solid–solid PCM (SSPCM). Additionally, the introduction of the cross linking segment significantly improves its thermal stability, and the 50% degradation temperature increased significantly from 228.8 to 355.9°C. Moreover, the latent heat increased with the decrease of the crosslinking density until the molar ratio of HD/OMA was reduced to 1/40 and then tended to reach constant. At the optimal ratio the latent heat of SSPCM can reach 88.23 J g−1. Excessive crosslinking density will affect the stacking tightness of molecular chains during the crystallization process, thereby reducing the latent heat.

Editor's Note for article entitled “Optimization of phase change process in a sinusoidal-wavy conductive walled cylinder with encapsulated-phase change material during magnetohydrodynamic nanofluid convection”

Editor's Note for article entitled “Optimization of phase change process in a sinusoidal-wavy conductive walled cylinder with encapsulated-phase change material during magnetohydrodynamic nanofluid convection”

Scaling Laws for the Influence of Gravity and Its Gradient on Dropwise Condensation: A Simulation Study.

Gravity is essential for the shedding of condensed droplets on hydrophobic surfaces, whose influences on condensation parameters under unconventional gravity conditions remain unclear and are hard to probe through experiments. A simulation framework is designed here to investigate such phase-change processes. We find clear scaling laws between heat flux Q, residual volume V, gravitational acceleration g, and nucleation density N0 with Q ∼ g1/6N01/3 and V ∼ g-1/2N00. We also identify a critical gravitational acceleration determined by nucleation density, above which a counterintuitive trend emerges: the heat flux decreases with increasing gravitational acceleration. This deviation is attributed to the sharp decrease in heat flux contributed by droplets larger than the effective radius. In addition, for zero-gravity scenarios, a centrifugal strategy is proposed to simulate Earth's gravity by introducing artificial gravity with a spatial gradient. We reveal that the gradients have a significant influence on the residual volume but a minor one on the heat flux. The conclusions are informative for the estimation and design of condensation heat transfer systems for future space applications.

Efficient utilization of cold energy enabled by phase change cold storage brine gels with superior thermophysical properties towards biochemical reagent cold chain

Phase change cold storage, as an emerging cold chain method of maintaining a low-temperature environment and effectively ensuring the quality of biochemical reagents, is extensively utilized because of its benefits of high energy density, low cost, energy conservation and environmentally friendly mode. However, the low thermal conductivity of working medium affects its cold charging/discharging rate and operating efficiency. Meanwhile, the high leakage characteristic in the phase change process leads to a decrease of cold storage capacity and contamination of items. This work proposes the efficient utilization of cold energy enabled by leakage-free phase change cold storage brine gels with extraordinary high thermal conductivity towards biochemical reagent cold chain. Phase change thermal storage gel is prepared by confining brine in the sodium polyacrylate‑calcium alginate network and porous adsorption of expanded graphite. The prepared materials present leak free characteristics and almost 100% mass retention rate. Expanded graphite addition enables the thermal conductivity increase from 0.542 W m−1 K−1 to an extremely high value of 2.766 W m−1 K−1 (an increase of 510%). The enthalpy value of the cold storage brine gel is as high as 144 Jg−1, stably releasing cold at around −24 °C. By regulating the distribution of cold storage working medium, the minimum temperature difference inside the apparatus can be reduced from 6.7 °C to about 1 °C. Finally, performance-enhancing phase change cold storage materials and apparatus in the cold chain of biological reagents is fruitful, effectively providing a long-lasting and uniform low-temperature environment.

Saturated phase change threshold of plume flow gas species under low temperature and rarefied environment

Gas species in high altitude plume flow are all exposed to low temperature, thus making possible phase change processes on those other than H2O vapor. In order to obtain saturated threshold of gas species, which is normally abundant in plume flow, this study establishes Gibbs phase calculation method for CO2, CO, H2, and N2. The non-continuum effect caused by rarefied gas environment is quantitatively calculated by ratio between molecular free path scale and characteristic length of investigated object. Validation on Gibbs method is guaranteed by comparison between calculated results and data from American National Standards Institute (ANSI), while non-continuum effect method is verified by revealing essential reason for gas-solid phase change processes resembling droplet condensation phenomenon observed in high altitude environment. According to the results, non-continuum effect only influences gas-solid boundary. Phase change for rarefied gas species can only occur under low temperature. Saturated threshold deflects toward solid phase region below specific molecules number density, thus decreasing gas to solid phase change trend. Both increasing on characteristic length and existence of other gas components contamination decreases non-continuum effect. Variation on non-continuum effect shows biggest influence on CO2 saturated phase change threshold, while shows smallest influence on that of N2.

Corrosion, permeation and mass transfer mechanisms of alkali metals in corundum refractories

During the operation of the waste liquid incinerator, the alkali metal slag might adhere to the surface of the refractory material to form an inhomogeneous solid slag layer, causing the furnace lining to be simultaneously corroded by three phases of alkali metals: vapor, molten salt, and slag. This phenomenon intensifies the damage to the refractory material. Therefore, this paper investigates the mass transfer and permeation process, phase change process and corrosion rate of Na2CO3 inside corundum refractory materials in three phases: Na vapor, molten Na2CO3 and Na-slag. The results showed that alkali vapor penetrated the interior through the pores, and the NaAlO2 and β-Al2O3 generated by the reaction led to the volume expansion and microcracks. Na vapor continued to penetrate the interior along the cracks and eroded the sample, and the higher the temperature the greater Na vapor penetration. In vapor phase corrosion, the effect of corrosion time on the erosion resistance of corundum refractories is less than that of temperature, and the increase in corrosion time does not lead to the formation of additional new phases. In the molten salt corrosion experiments, it was found that the molten salt corrosion was accompanied by vapor phase corrosion at 1100 °C, and the amount of Na2CO3 has a greater effect on the corroded mass than the temperature. Comparing the corrosion of refractory materials by the three phases of Na2CO3, the molten salt corrosion rate was the highest, followed by the vapor phase corrosion, and finally the slag corrosion. It is concluded that the slag layer can effectively prevent the corrosion of the refractory material by alkali metal molten salts and vapors, thus prolonging the service life of refractories.

Experimental and numerical assessment of thermal characteristics of PCM in a U-shaped heat exchanger using porous metal foam and NanoPowder

The utilization of Latent Heat Thermal Energy Storage (LHTES) has gained significant attention to address the disparity between energy supply and demand. One of the key advantages lies in the use of phase change materials (PCM). The purpose of this research is to overcome this obstacle by focusing on enhancing the thermal efficiency of an advanced thermal energy storage system specifically designed for solar domestic and industrial application. to overcome this obstacle by focusing on enhancing the thermal efficiency of an advanced thermal energy storage system specifically designed for solar domestic and industrial application. Through computational and experimental studies, a novel and small LHTES system with parallel U-shaped heat exchanger (USHX) has been created and investigated. To improve performance, two approaches are employed: optimizing thermal efficiency by dispersing nano-sized graphite powders into the paraffin material, and/or incorporating metal foams. The PCM is RT35HC, and the hot/cold heat transfer fluid is H2O, which travels via the U-shaped tube. The model incorporates the enthalpy-porosity technique to account for phase change phenomena. After comparing the numerical outcomes with the experiments herein run, data are shown in terms of liquid fraction, temperature evolution, stored energy, and a dimensionless parameter that characterizes the phase change process. The findings suggest that the proposed methods for enhancing heat transfer can enhance the thermal efficiency of systems. The outcomes illustrate that by addition of all methods, reduces the melting time by 13.39 %, 60.77 %, and 71.93 %, when compared to system with pure PCM.