Natural Convection Effect on Solidification Enhancement in a Multi-Tube Latent Heat Storage System: Effect of Tubes’ Arrangement
TL;DR: In this article, the effect of the arrangement of the tubes in a multi-tube heat exchanger was examined during the solidification process with the focus on the natural convection effects inside the phase change material (PCM).
Abstract: The solidification process in a multi-tube latent heat energy system is affected by the natural convection and the arrangement of heat exchanger tubes, which changes the buoyancy effect as well. In the current work, the effect of the arrangement of the tubes in a multi-tube heat exchanger was examined during the solidification process with the focus on the natural convection effects inside the phase change material (PCM). The behavior of the system was numerically analyzed using liquid fraction and energy released, as well as temperature, velocity and streamline profiles for different studied cases. The arrangement of the tubes, considering seven pipes in the symmetrical condition, are assumed at different positions in the system, including uniform distribution of the tubes as well as non-uniform distribution, i.e., tubes concentrated at the bottom, middle and the top of the PCM shell. The model was first validated compared with previous experimental work from the literature. The results show that the heat rate removal from the PCM after 16 h was 52.89 W (max) and 14.85 W (min) for the cases of uniform tube distribution and tubes concentrated at the bottom, respectively, for the proposed dimensions of the heat exchanger. The heat rate removal of the system with uniform tube distribution increases when the distance between the tubes and top of the shell reduces, and increased equal to 68.75 W due to natural convection effect. The heat release rate also reduces by increasing the temperature the tubes. The heat removal rate increases by 7.5%, and 23.7% when the temperature increases from 10 °C to 15 °C and 20 °C, respectively. This paper reveals that specific consideration to the arrangement of the tubes should be made to enhance the heat recovery process attending natural convection effects in phase change heat storage systems.
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TL;DR: In this paper , the authors evaluated the effect of arc-shaped fins on the melting capability of a triplex-tube confinement system filled with phase-change materials (PCMs).
Abstract: This study numerically intends to evaluate the effects of arc-shaped fins on the melting capability of a triplex-tube confinement system filled with phase-change materials (PCMs). In contrast to situations with no fins, where PCM exhibits relatively poor heat response, in this study, the thermal performance is modified using novel arc-shaped fins with various circular angles and orientations compared with traditional rectangular fins. Several inline and staggered layouts are also assessed to maximize the fin’s efficacy. The effect of the nearby natural convection is further investigated by adding a fin to the bottom of the heat-storage domain. Additionally, the Reynolds number and temperature of the heat-transfer fluid (HTF) are evaluated. The outcomes showed that the arc-shaped fins could greatly enhance the PCMs’ melting rate and the associated heat-storage properties. The melting rate is 17% and 93.1% greater for the case fitted with an inline distribution of the fins with a circular angle of 90° and an upward direction, respectively, than the cases with uniform rectangular fins and no fins, which corresponded to the shorter melting time of 14.5% and 50.4%. For the case with arc-shaped fins with a 90° circular angle, the melting rate increases by 9% using a staggered distribution. Compared to the staggered fin distribution, adding an extra fin to the bottom of the domain indicates adverse effects. The charging time reduces by 5.8% and 9.2% when the Reynolds number (Re) rises from 500 to 1000 and 1500, respectively, while the heat-storage rate increases by 6.3% and 10.3%. When the fluid inlet temperature is 55°C or 50°C, compared with 45°C, the overall charging time increases by 98% and 47%, respectively.
7 citations
TL;DR: In this paper , the effects of various gap widths in the modified geometries are investigated and the advantages of using frustum tubes in increasing the discharging rate and reducing the solidification time compared with that of the straight tube unit due to the higher natural convection effect.
Abstract: This study aims to study the discharging process to verify the influence of geometry modifications and heat transfer flow (HTF) patterns on the performance of a vertical triplex-tube latent heat container. The phase change material (PCM) is included in the middle tube, where the geometry is modified using single or multi-internal frustum tubes instead of straight tubes to enhance the discharging rate. The effects of the HTF flow direction, which is considered by the gravity and opposite-gravity directions, are also examined in four different cases. For the optimal geometry, three scenarios are proposed, i.e., employing a frustum tube for the middle tube, for the inner tube, and at last for both the inner and middle tubes. The effects of various gap widths in the modified geometries are investigated. The results show the advantages of using frustum tubes in increasing the discharging rate and reducing the solidification time compared with that of the straight tube unit due to the higher natural convection effect by proper utilization of frustum tubes. The study of the HTF pattern shows that where the HTF direction in both the inner and outer tubes are in the gravity direction, the maximum discharging rate can be achieved. For the best configuration, the discharge time is reduced negligibly compared with that for the system with straight tubes which depends on the dimensions of the PCM domain.
3 citations
TL;DR: In this article , the existence and uniqueness of the stationary double-diffusive natural convection model were proved based on the fixed point theorem, and three finite element iterative methods for the considered problem were designed.
Abstract: In this paper, we consider the stationary double-diffusive natural convection model, which can model heat and mass transfer phenomena. Based on the fixed point theorem, the existence and uniqueness of the considered model are proved. Moreover, we design three finite element iterative methods for the considered problem. Under the uniqueness condition of a weak solution, iterative method I is stable. Compared with iterative method I, iterative method II is stable with a stronger condition. Moreover, iterative method III is stable with the strongest condition. From the perspective of viscosity, iterative method I displays well in the case of a low viscosity number, iterative method II runs well with slightly low viscosity, and iterative method III can deal with high viscosity. Finally, some numerical experiments are presented for testing the correctness of the theoretic analysis.
2 citations
TL;DR: In this article , the orientation of oval tubes (horizontal, vertical, and oblique) in phase change material (PCM, RT35)-based shell-tube heat exchanger was analyzed with respect to the metal foam type (Graphite, Copper, and Nickel) in comparison to the case of pure PCM.
Abstract: One of the likely methods for enhancing heat transfer in a latent thermal energy storage system is the conception of a thermal unit. In this work, the orientation of oval tubes (horizontal, vertical, and oblique) in phase change material (PCM, RT35)-based shell-tube heat exchanger was analyzed with respect to the metal foam type (MF: Graphite, Copper, and Nickel) in comparison to the case of pure PCM. For this purpose, a two-dimensional mathematical model was developed to investigate the thermal efficiency of the PCM-metal foam based composite energy storage unit. It was concluded that the orientation of the oval tubes (oblique, horizontal, and vertical) has a negligible impact on the performance of the thermal unit during the melting/solidification processes. Based on the liquid/solid fraction, total enthalpy and the average temperature in the annular space, the performance of a heat exchanger during fusion/solidification periods is in the order: Copper-MF > Graphite-MF > Nickel-MF > Pure PCM. Whatever the adopted metal foam or the geometry of tubes, the melting process is expedited compared to the solidification mechanism.
1 citations
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TL;DR: In this paper, the phase change problem has been formulated using pure conduction approach but the problem has moved to a different level of complexity with added convection in the melt being accounted for, which makes it difficult for comparison to be made to assess the suitability of PCMs to particular applications.
Abstract: This paper reviews the development of latent heat thermal energy storage systems studied detailing various phase change materials (PCMs) investigated over the last three decades, the heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy and the formulation of the phase change problem. It also examines the geometry and configurations of PCM containers and a series of numerical and experimental tests undertaken to assess the effects of parameters such as the inlet temperature and the mass flow rate of the heat transfer fluid (HTF). It is concluded that most of the phase change problems have been carried out at temperature ranges between 0 °C and 60 °C suitable for domestic heating applications. In terms of problem formulation, the common approach has been the use of enthalpy formulation. Heat transfer in the phase change problem was previously formulated using pure conduction approach but the problem has moved to a different level of complexity with added convection in the melt being accounted for. There is no standard method (such as British Standards or EU standards) developed to test for PCMs, making it difficult for comparison to be made to assess the suitability of PCMs to particular applications. A unified platform such as British Standards, EU standards needs to be developed to ensure same or similar procedure and analysis (performance curves) to allow comparison and knowledge gained from one test to be applied to another.
1,630 citations
TL;DR: A review of the latest publications on the use of phase change materials (PCM) in buildings is presented in this article, where the authors present a classification of materials, materials available and problems and possible solutions on the application of such materials in buildings.
Abstract: In recent years the use of thermal energy storage with phase change materials has become a topic with a lot of interest within the research community, but also within architects and engineers. Many publications have appeared, and several books, but the information is disseminated and not very much organised. This paper shows a review of the latest publications on the use of phase change materials (PCM) in buildings. The paper compiles information about the requirements of the use of this technology, classification of materials, materials available and problems and possible solutions on the application of such materials in buildings.
1,389 citations
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TL;DR: A comprehensive review of phase change materials (PCMs) with phase transition temperatures between 0 and 250°C is presented in this paper, with the focus on this temperature range is due to potential CO 2 emissions reduction able to be achieved replacing conventional heating and cooling applications in the domestic, commercial and public administration sectors, which represented around a quarter of the UK's final energy consumption in 2015.
755 citations
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484 citations