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Physical properties of NaNO3 and stainless steel
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Heat transfer simulations and predictions of the thermal energy storage capability using encapsulated phase change materials (EPCM) at high temperatures are conducted. NaNO3 is considered as a phase change material (PCM). The PCM is encapsulated by a stainless steel shell. Two dimensional simulations of a cylindrical capsule are considered. The eff...
Context in source publication
Context 1
... results for a 2-dimensional simulation of a cylindrical capsule are presented for a capsule diameter of 50.8 mm & 76.2mm with a shell thickness of 1.5875 mm. The properties of the PCM (NaNO 3 ) and the shell (stainless steel) are given in Table 1. The melting temperature of NaNO 3 is 308°C (581 K). ...
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Citations
... It was concluded from the study that the presence of voids affects thermal performance and phase change behaviour. Solomon et al. [14] investigated the heat transfer and energy storage capabilities of encapsulated phase change materials (EPCM) at high temperatures in the presence of the void. The authors concluded that the void location within the EPCM capsule profoundly affects the shape of the solid/liquid interface and the rate at which the PCM melts. ...
... However, most of the studied materials had a low solid thermal conductivity which vastly reduces the heat transfer rate during solidification. Research into various means of increasing the heat transfer rate is a wide topic that includes embedding a highly conductive material into the PCM [9-14], encapsulation [4,5,[15][16][17][18][19][20][21][22], embedding fins into the PCM [23][24][25], and using heat pipes [26][27][28][29][30][31][32][33][34][35]. ...
The suitability of stainless steel 316L and Inconel 625 for use in a latent heat thermal energy storage (TES) system was investigated. A NaCl–NaF eutectic mixture with a melting temperature of 680 °C was used as the phase change material (PCM). Containers were filled with the PCM prior to heating to 750 °C, then examined after 100 and 2500 h of high-temperature exposure by analyzing the material surface and cross-section areas. A small amount of corrosion was present in both samples after 100 h. Neither sample suffered significant damage after 2500 h. The undesirable inter-granular grain boundary attack found in SS316L samples was in the order of 1–2 µm in depth. On Inconel 625 sample surface, an oxide complex formed, resisting material dissolution into the PCM. The surface morphology of tested samples remained largely unchanged after 2500 h, but the corrosion pattern changed from an initially localized corrosion penetration to a more uniform type. After 2500 h, the corrosion depth of Inconel 625 remained at roughly 1–2 µm, indicating that the corrosion rate decelerated. Both materials demonstrated good compatibility with the chosen NaF–NaCl eutectic salt, but the low corrosion activity in Inconel 625 samples shows a performance advantage for long term operation.
... Most prominently is the low thermal conductivity of the studied materials, particularly in the solid phase, which has a limiting effect on the heat transfer during the discharging process. Numerous methods of improving the heat transfer rate have been studied, including adding a highly conductive material to the PCM [8][9][10][11][12][13], employing multiple PCMs with varying melting temperature [14][15][16], macro-and micro-encapsulation [5,6,[17][18][19][20][21][22][23][24][25], and embedding fins into the PCM [26][27][28][29]. Another option is to use heat pipes. ...
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The understanding of melting characteristics and mechanism of high temperature encapsulated phase change material (EPCM) is important for its application in heat storage systems.
In this study, a two-dimensional axisymmetric numerical model is established to simulate the melting of spherical EPCMs with air void on the top. Besides buoyancy driven natural convection, Marangoni convection due to the thermocapillary force at the air-molten PCM interface is considered for the first time in this topic. To estimate thermocapillary force more accurately, the air-PCM interface is tracked by the coupled level-set and VOF method. The numerical simulations are conducted for EPCMs with Al2O3 shell and NaNO3 core with the different EPCM diameters and under the different gravity levels. It is found that Marangoni convection mainly affects the early and middle stages of PCM melting process, and its importance relative to buoyancy driven natural convection increases as the diameter of EPCM and gravity level decreases. For the EPCMs with diameter greater than 24 mm under the earth gravity, Marangoni convection has negligible effect, while for 12 mm diameter EPCM under the moon gravity, the complete melting time could be 19% shorter under the additional role of Marangoni convection. The solid PCM profile, fluid flow structure, melting ratio and heat transfer rate during the melting of EPCM under the effects of natural convection and Marangoni convection are also analyzed in detail.
A visualization experiment is carried out to investigate the void-affected unconstrained melting process in a spherical container. Factors including the diameter of spherical container (20.05-70.30 mm), filling ratio of PCM (0.8-0.98), heating temperature (32-47 °C) and initial temperature (7-22 °C) are studied. Melting behaviors concerning the evolution of solid-liquid interface and the motion of solid PCM are revealed, from which a new unconstrained melting mode (i.e., floating melting which occurs before the close-contact melting) is observed due to the existence of void in solid PCM. In addition, the quantitative results of floating melting time, total melting time and liquid fraction are obtained. The results show that as the melting mode changes from floating melting to close-contact melting, the melting rate is slightly enhanced. With the increase of spherical container diameter, the floating melting time increases first and then decreases, while the total melting time has a continually increasing trend. The higher heating temperature and higher initial temperature will decrease the floating melting time and total melting time. As the filling ratio of PCM increases, the floating melting time and the total melting time increase at first and then decrease. Finally, the correlations of dimensionless total melting time and liquid fraction are proposed.
