Papers by Keyword: Phase Change

Abstract: To understand the nano phase formation, cooling experiments of a hypereutectic Zn-Al alloy containing 6 wt% of Al are carried out under two different cooling rates of 0.04 and 10.00 °C/s. The applied cooling rates significantly influence the phase change behavior of the investigated alloy. The liquidus temperature (T_N) for the nucleation of the primary phase decreases from 390.3 to 382.9 °C, and the undercooling increases from 0.7 to 8.1 °C, as the cooling rate rises from 0.04 to 10 °C/s. The eutectic and eutectoid temperatures decrease from 381.5, 277.7 to 375.6 and 267.6 °C, respectively, when the cooling rate increases from 0.04 to 10.00 °C/s. The SEM and EDS analyses reveal that the solidified alloy contains the primary γ-ZnAl phase, the eutectic β-Zn phase, and the eutectoid α-Al and eutectoid β-Zn phases. The fast phase change and transformation caused by rapid cooling results in the formation of nano eutectoid phases and fine microstructure.

Abstract: Despite many desirable properties, most phase change materials (PCMs) undergo timing issues during the phase change process due to a low thermal conductivity, which limits their application in heat storages. Thus, many techniques have been pointed out to overcome these disadvantages and improve heat transfer, such as coupling PCMs with metal inserts, like high porosity open-cell metal foams. Indeed, the presence of a metal foam increases the effective thermal conductivity of the composite medium and speeds up the charging and discharging phases. In the present paper, a numerical model developed in COMSOL ambient has been calibrated by comparison with experimental results on the melting of pure and metal-foam loaded PCMs, placed in a small case and heated from the top by an electric resistance. The numerical model considers the metal foam as a static solid, filled with a phase changing fluid and employs a literature correlation to evaluate the effective thermal conductivity of the composite medium. The performance of two different paraffinic PCMs (RT35 and RT35HC by Rubitherm GmbH, D), loaded either with a copper foam (20 PPI, 95% porosity, by Porometal, China) or with an aluminum one (10 PPI, 96% porosity, by Recemat, NL) has been investigated in terms of speed rate of the phase change front, time required to complete the melting process, temperature distribution and effect of foam porosity. The obtained results clearly evidence the significant heat transfer improvement yielded by metal foams, whose presence increases the effective thermal conductivity of the composite medium (from 0.2 to 7.03 W/mK for copper foam and to 3.52 W/mK for aluminum one), leading to a significant decrease of the charging time and to a lower temperature gradient within the PCM (from about 16 to 3 K).

Abstract: The thermal management processes for PhotoVoltaic (PV) cooling applications, increase PV systems’ overall efficiency and yield to a maximized power generation. Accordingly, this paper investigates recent PV thermal management methods, which involve the use of Phase Change Material (PCM) under the back of PV modules. Compared to other cooling methods (such as air and water based methods) PCM based techniques show less need for maintenance, are environment-friendly, and have a longer life cycle. Since PCM are diverse in nature, and many methods exist to guide their selection procedure, this paper begins by revealing different types of PCM, which are found to be as Organic, Inorganic, Eutectic and Commercial PCM, with the characteristics of each. After acknowledging different PCM natures, a selection process is established based on either the melting temperature, latent heat, or thermal conductivity of PCM. Results have shown that Commercial PCM are the best option followed by Organic PCM, due to their improved chemical aspects when compared with Inorganic and Eutectic PCM. Concerning PCM selection criteria, the easiest yet sufficient process is based on the melting temperature method, due to the simplified calculations when compared to other thermic quantities. At the end, future work recommendations are declared, related to PCM lifecycle assessment and cooling/heating cycles effects on PV entropy.

Abstract: Phase change buoyancy control systems (PBCS) driven by phase change materials have unique advantages over conventional hydraulically actuated buoyancy control systems, especially in their high adaptability for deep-sea exploration and seamless interaction with humans. Studying the heat transfer process and working mechanism of PBCS is helpful to research the precise control mode of the phase change buoyancy regulation system. In this study, we chose paraffin wax as the phase change material for PBCS. A buoyancy generator for phase change buoyancy regulation is designed and manufactured. By studying the relation between its expansion volume and heating time and heat input, the mathematical model of the volume change of PBCS is established. We obtain the calculation results of the PBCS working process by using the computer-aided software MATLAB. Experimental results show that the theoretical volume expansion velocity and volume expansion rate of paraffin are almost consistent with the experimental results. The mathematical calculation model is available and has a significant reference value for further research on the theoretical basis of the diving depth control of PBCS.

Abstract: The influence of applying an external distributed pressure along the upper surface of the molten metal (Aluminum ) during the solidification process on the temperature reduction profile was studied including the time of solidification of the cast and the phase change moving boundary location for two mould wall thicknesses (10mm and 15mm). A 3D model was built up by Solidworks and simulated by ANSYS FLUENT; each mould wall thickness was discussed for two press cases (1bar and 3bar) sequentially, comparing with no press cases. The discussion includes the ambient temperature effect, which is taken (300K then 310K), the overall cases that studied was 7 cases. The study shows a remarkable effect of press on the temperature reduction profile especially when mix with the mould thickness effect as well as the ambient temperature which has a great order in guiding the results. The results showed that the heat reduction increases by increasing the mould thickness as well as the applied pressure. Moreover, this effect will reduce the solidification time and the moving of the boundary of phase change become faster in appearance.

Abstract: In the present numerical study, the convection diffusion phenomena associated with solid-liquid phase transition processes during phase change material (PCM) melting within a rectangular cavity is studied. The cavity is heated from left wall with a sinusoidal temperature distribution. Initially the enclosure was filled by solid gallium at melting temperature 29.78°C. The enthalpy-based lattice Boltzmann method (LBM) with D2Q9 particle velocity model is used to solve density, velocity and temperature fields. Influence of Rayleigh number ranging from 10³ to 4×10⁵ on streamlines, isotherms and liquid fraction is analyzed. The results indicate that natural convection of liquid phase change material (PCM) plays a significant role in the melting heat transfer of PCM. It is found that the rate of the melting increases with the increase in the values of the Rayleigh number.

Abstract: The study on heat flow in welding is essential as the quality of the weld depends on mainly heat flow through the welded plate. The heat input from welding source flows in a limited zone, and it subsequently flows into the workpiece by conduction. In this study, an attempt is taken to predict the transient temperature distribution and solidification pattern through a numerical model and the associated mathematical technique considering the solidification and heat transfer, of molten weld pool when it is covered with flux and without flux in arc welding process. The numerical model developed in this study solves fluid flow and heat transfer considering solidification and melting phase change the along with natural convection in the meltpool. It was found that the flux is functioning as insulation on the welded pool, hence it restricts rapid solidification and increases the mushy zone width.

Abstract: Gallium is a liquid metal at near room temperature and in recent decades has become a key element in both electronic and optoelectronic applications. We demonstrate for the first time highly ordered spherical Ga nanoparticle (NP) arrays were fabricated by glancing angle deposition (GLAD) technique. GLAD is a simple method based on self-assembly that can produce highly ordered one-dimensional plasmonic NP chains. The real time (in-situ) monitoring of optical properties of Ga NPs plays an important role in measurement of optical behaviour in phase change. Also this provides information on the growth mechanism and allows production of structures with the desired optical characteristics. Reflectance anisotropy spectroscopy (RAS) has been used for monitoring of in-situ optical properties of the liquid-solid transition in Ga NP arrays. The results show stability and phase transition sensitivity of plasmonic resonance of Ga NP arrays. In order to confirm experimental results an analytical model was adapted by using Transfer Matrix Formalism and experimental parameters of Ga NP arrays were used for simulation.

Abstract: With the increase of the speed of vehicle, the thermal protection system of its powerplant requires higher insulation materials. Phase change materials can absorb large amounts of heat in short time. So the introduction of phase change materials in thermal insulation materials can achieve efficient insulation in a limited space for a short time. In this paper, a new phase change thermal insulation material was prepared by pressure molding with microporous calcium silicate as matrix and Li₂CO₃ as phase change material. The morphology stability, exudation and heat insulation of the materials were tested. The results show that the porous structure of microporous calcium silicate has a good encapsulation when the phase transition of Li₂CO₃ is changed into liquid. And the material has no leakage during use. The thermal performance test also shows that the insulation performance of the material has obvious advantages in the short term application.

Abstract: This paper examines a model for coupled heat and mass transfer for freezing in a porous matrix with Dirichlet and convective boundary conditions. Variables include porosity, heat transfer coefficients, thermal and mass diffusivity, density, latent heat and boundary temperatures. It is shown that heat and mass transfer balance at the interface can affect stability. The effect of boundary conditions on the velocity of freezing is computed for some cases, and applications to physical problems highlighted