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首页> 外文期刊>Journal of Heat Transfer >Numerical Simulation of the Effect of the Size of Suspensions on the Solidification Process of Nanoparticle-Enhanced Phase Change Materials
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Numerical Simulation of the Effect of the Size of Suspensions on the Solidification Process of Nanoparticle-Enhanced Phase Change Materials

机译：悬浮液尺寸对纳米相增强相变材料凝固过程影响的数值模拟

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Nanostructure-enhanced phase change materials (NePCM) have been widely studied in recent years due to their enhanced thermal conductivity and improved charge/discharge in thermal energy storage applications. In this study, the effect of the size of the nanopar-ticles on the morphology of the solid-liquid interface and the evolving concentration field during solidification is reported. Combining a one-fluid-mixture approach with the single-domain enthalpy-porosity model for phase change and assuming a linear dependence of the liquidus and solidus temperatures of the mushy zone on the local concentration of the nanoparticles subject to a constant value of the segregation coefficient, thermal-solutal convection as well as the Brownian and thermophoretic effects are taken into account. A square cavity containing a suspension of copper nanoparticles (diameter of 5 and 2 nm) in water was the model NePCM considered. Subject to a 5℃ temperature difference between the hot (top) and cold (bottom) sides and with an initial loading of the nanoparticles equal to 10 wt. % (1.22 vol. %), the colloid was solidified from the bottom. The solid-liquid interface for the case of NePCM with 5 nm particle size was almost planar throughout the solidification process. However, for the case of the NePCM with particle size of 2 nm, the solid-liquid interface evolved from a stable planar shape to an unstable dendritic structure. This transition was attributed to the constitutional supercooling effect, whereby the rejected particles that are pushed away from the interface into the liquid zone form regions of high concentration thus leading to a lower solidus temperature. Moreover, for the smaller particle size of 2 nm, the ensuing solutal convection at the liquid-solid interface due to the concentration gradient is affected by the increased Brownian diffusivity. Due to size-dependent rejection of nanoparticles, the frozen layer that resulted from a dendritic growth contains regions of depleted concentration. Despite the higher thermal conductivity of the colloids, the amount of frozen phase during a fixed time period diminished as the particle size decreased.
机译：近年来，由于纳米结构增强相变材料（NePCM）的导热性增强和在热能存储应用中的充放电得到改善，因此已被广泛研究。在这项研究中，报道了纳米粒子的尺寸对凝固过程中固液界面形态和浓度场变化的影响。将单流体混合方法与用于相变的单域焓-孔隙率模型相结合，并假设糊状区域的液相线和固相线温度对纳米粒子局部浓度的线性依赖性，而纳米粒子的局部浓度服从恒定的偏析值系数，热对流以及布朗和热泳效应都考虑在内。 NePCM模型是一个方形空腔，其中包含铜纳米颗粒（直径为5和2 nm）在水中的悬浮液。高温（顶部）和低温（底部）之间的温度差为5℃，且纳米颗粒的初始负载等于10 wt。 ％（1.22vol。％），胶体从底部固化。在整个固化过程中，对于具有5 nm粒径的NePCM而言，固液界面几乎是平面的。但是，对于具有2 nm粒径的NePCM，固液界面从稳定的平面形状演变为不稳定的树枝状结构。该转变归因于结构上的过冷效应，由此，从界面推入液体区域的废颗粒形成高浓度区域，从而导致较低的固相线温度。此外，对于2 nm的较小粒径，由于浓度梯度而导致的液固界面上的对流随之受到布朗扩散系数的增加的影响。由于纳米颗粒的尺寸依赖性排斥，由树枝状生长导致的冷冻层包含耗尽浓度的区域。尽管胶体的导热系数较高，但随着粒径的减小，固定时间段内的冷冻相数量会减少。

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