| The heat generated during operation of electronics has been highly increased due to the increasingly improved extent of integration of such devices. The research and development of efficient thermal management systems has been realized as one of the bottlenecks of the development of modern electronics. Thermal management based on thermal energy storage has shown to be an efficient passive cooling method. The heat generated by the target electronics can be effectively absorbed and dissipated by means of the latent heat of fusion during melting of phase change materials. The temperature of phase change materials will almost remain at their melting points during melting so as to provide an additional protection of the target devices from overheating. Obviously, the performance of thermal energy storage-based heat sinks strongly depends on the thermophysical properties of the phase change materials utilized. Despite the relatively high specific heat and latent heat associated with the common candidates of phase change materials, e.g., paraffin wax, their ability of thermal conductance is undesirably low, which significantly suppresses the efficacy of the thermal management systems.Enhancement of the thermal conductivity of phase change materials has received great attention. A promising solution is to disperse highly-conductive fillers, e.g., metal fins, foam metals, and carbon fibers, etc, into phase change materials to form composite materials that possess greater effective thermal conductivity. Following the development of nanoscale materials technologies, it has turned to utilization of ultrafine fillers, e.g., carbon nanotubes and metal nanoparticles, to attain greater thermal conductivity enhancement and better dispersity. This novel family of composite phase change materials is coined as nano-enhanced phase change materials. It has been shown by abundant experimental data that the effective thermal conductivity of phase change materials can be increased considerably due to the introduction of nanoscale fillers, whereas the energy storage capacity, i.e., specific heat and latent heat, is decreased to some extent. Considering both the cost of fillers and stability of the composites, the loading of nanoscale fillers poses an optimization problem for real-world applications of nano-enhanced phase change materaisls.In an effort to assess the applicability of nano-enhanced phase change materials in thermal management systems for electronics and their potential under various operation conditions, a parametric study of the loading of nanoscale fillers was conducted numerically. In the numerical study, nano-enhanced phase change materials were treated as "customized" materials with homogeneous effective thermophysical properties. In other words, the presence and loading of nanoscale fillers was only represented by the variations of the effective thermophysical properties of the composite phase change materials. The effective thermophysical properties, including density, thermal conductivity, viscosity in liquid phase, specific heat, and latent heat, of nano-enhanced phase change materials were predicted using the effective media theory and the existing empirical equations/models. All the computations were performed in the environment of a control volume-based commercial computational fluid dynamics code, where the built-in enthalpy-porosity model was adopted to simulate solid-liquid phase change.The structure and configuration of a finned heat sink for cooling of electronics were considered in the present work. Melting of nano-enhanced phase change materials in a rectangular cavity heated from below was investigated as it is a representative model of the operation of the heat sink. Eicosane and carbon nanotubes were chosen as the phase change material and filler, respectively. The effects of three different Grashof numbers were studied. It was shown that the introduction of nanoscale fillers leads to two competing effects. On the one hand, melting is expedited due to the enhanced thermal conductivity of the phase change material. On the other hand, a negative effect exists that natural convection during melting is suppressed owing to the increase of viscosity in liquid phase. In relatively big cavities, the extent of expediting of melting due to the enhanced thermal conductivity will be deteriorated by the suppressed natural convection. This negative effect, however, may be disregarded as natural convection plays a negligible role in relatively small cavities. The apparent effect is determined by the relative weights associated with the effective transport properties, i.e., thermal conductivity and viscosity.Considering the relatively small size of the heat sink simulated, natural convection was neglected during melting and only the heat conduction equations were solved. The performance of the heat sink was first investigated based on a simplified two-dimensional model, where paraffin wax and copper nanoparticles were selected as the phase change material and nanoscale fillers, respectively. It was shown that the performance of the heat sink can be improved by introduction of highly-conductive nanoparticles. At the end of heating the maximum temperature rise was significantly mitigated with increasing loading of nanoparticles. The maximum temperature rise was decreased by9%when the copper nanoparticles were introduced at a volume fraction of0.1. Within the range of loading of nanoparticles investigated, the improvement of the performance of the heat sink was shown to be linearly related to the loading. Despite a decrease of the storage capacity due to the presence of nanoparticles, the melting process was extended because heat was able to transfer faster by the phase change materials with enhanced thermal conductivity, thus leading to a longer time period of protection of the target electronics from overheating. Carbon-based nanoscale fillers, e.g., carbon nanotubes, are better candidates than metal nanoparticles since they possess extremely high thermal conductivity as well as lower density. A three-dimensional study was then performed on the same heat sink because the simplification for the two-dimensional model may have led to significant deviations between the computational and physical models. According to the above-mentioned conerns, eicosane and carbon nanotubes were considered as the phase change material and nanoscale fillers, respectively. A comparative analysis was carried out for the performance of the heat sink as a function of the loading of fillers under both periodic and pulsed heat loads. It was shown that the extent of performance enhancement was related logrithmically to the loading of nanoscale fillers. Nano-enhanced phase change materials were shown to be suitable for the thermal management of electronics under pulsed heat load. Under periodic heat load, the performance enhancement was less than1%even at the greatest loading of nanoscale fillers. The introduction of10%fillers, however, resulted in a mitigation of the temperature on the heated surface of the heat sink and a reduction of the thermal resistance by8℃and14%, respectively.It is noted that further increasing the loading of nanoscale fillers may not be helpful, although the performance of the heat sink was shown to be gradually improved with increasing loading within the range investigated. Evidently, a high loading of nanoscale fillers will not only lead to undesirably high cost and instability of the composites, but also result in insufficient availability of storage capacity that will restrict the performance of the heat sink. The primary purpose of the future plan will be to amend the prediction models of the effective thermophysical properties, especially thermal conductivity and viscosity in liquid phase, of nano-enhanced phase change materials, and to verify the computational results obtained in the present work using experimental data. |
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