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The thermal performance of a heat sink is analyzed according to the position of the partially heated surface. Numerical models for simulating forced convection were used to analyze the heat transfer between the heat sink and ambient air. The optimal partial heating position was discussed in terms of the effects of total heat transfer rate, air velocity, the ratio of total heat sink length to partially heated surface width, the thermal conductivity of the heat sink, and the thickness of the heat sink base. Finally, a correlation was suggested to determine the partial heating position that maximizes thermal performance by using the experimental design method. It was thus possible to reduce the thermal resistance of the heat sink by up to approximately 30% by finding the optimal partial heating position. [ABSTRACT FROM AUTHOR]

• Heat and mass transfer analogy was proposed under cryogenic conditions. • A dimensionless correlation was developed for the frost density. • The correlation for thermal conductivity was derived via regression analysis. An experimental study on the phase change occurring on a cryogenic cooling surface was conducted under forced convection conditions. The mass and surface temperature of frost and the heat flux at the cooling surface were measured in the experiment. The heat and mass transfer analogy was investigated under cryogenic conditions. The mass estimated under various experimental conditions using the modified analogy differed by up to 8% from the experimentally measured mass. The density and thermal conductivity of frost at cryogenic temperatures were lower than those of general-low temperature frost; the dimensionless correlations of density and thermal conductivity of frost were derived via regression analysis. The calculated frost density and thermal conductivities were differed from the measured value by up to 9% and 11%, respectively. Therefore, a model was built to predict the heat and mass transfer and improve the thermal performance on the cooling surfaces of various cryogenic devices using the presented frost density and thermal conductivity correlations. [ABSTRACT FROM AUTHOR]

We report the fabrication of a three-dimensional (3D) metal-graphene network-based phase change composite with a tunable size of individual phase change materials under the same amount of metal contents. Mixing the granules of phase change material with metal paste and subsequent hot pressing effectively forms 3D metal networks among phase change materials with a minimal inclusion of metal. Specifically, the formation of a 3D silver network with 6 volume percentages among pure erythritol increases the thermal conductivities of pure erythritol by 2.7-fold, while achieving a heat capacity that is comparable to that of pure erythritol. Decreasing the size of the individual erythritol part with the same metal content significantly suppresses the subcooling phenomena of erythritol by 24 °C due to the effectively increased interfacial surface areas for active heterogeneous nucleation. The addition of graphene sheets between the erythritol granules and 3D metal network further enhances the thermal conductivities of phase change composites by 4.7-fold compared to those of pure erythritol. Finally, the stable operation of the 3D metal or metal-graphene network-based phase change composite during repeated melting and solidification cycling revealed the good structural integrity of the fabricated phase change composite. [ABSTRACT FROM AUTHOR]

Metallic micro-cell UO 2 pellet with high thermal conductivity has received attention as a promising accident-tolerant fuel. Although experimental demonstrations have been successful, studies on the potency of current metallic micro-cell UO 2 fuels for further enhancement of thermal performance are lacking. Here, we numerically investigated the thermal conductivities of micro-cell UO 2 Mo pellets in terms of the amount of Mo content, the unit cell size, and the aspect ratio of the micro-cells. The results showed good agreement with experimental measurements, and more importantly, indicated the importance of optimizing the unit cell geometries of the micro-cell pellets for greater increases in thermal conductivity. Consequently, the micro-cell UO 2 Mo pellets (5 vol% Mo) with modified geometries increased the thermal conductivity of the current UO 2 pellets by about 2.5 times, and lowered the temperature gradient within the pellets by 62.9% under a linear heat generation rate of 200 W/cm. [ABSTRACT FROM AUTHOR]

• Phase change materials (PCMs) are embedded to the base plate of fin-type heat sinks. • Thermal performance of heat sinks for high power electronics is investigated. • PCM-integrated fin-type heat sinks have similar thermal resistances with conventional ones. • PCM-integrated ones show excellent thermal capacitive effects in reduced cooling conditions. We report the enhanced cooling performance of the phase change material (PCM)-integrated fin-type heat sink compared to conventional fin-type heat sink in high power electronics with two localized hot spots. The PCM-integrated fin-type heat sink is fabricated by embedding the phase change composite to the base plate of the heat sink. As an effort to effectively utilize thermal capacitive effects of PCM, the phase change composites with paraffin infiltrated to copper foams are deployed within circular hole arrays in the base plate, which is subsequently covered by a graphite sheet, to achieve excellent heat spreading characteristics. Considering the cooling environments of commercial high power electronics (insulated-gate bipolar transistor (IGBT)), thermal performance of the PCM-integrated and the conventional fin-type heat sinks is experimentally and numerically investigated upon the heating powers of 400∼800 W. While the PCM-integrated fin-type heat sinks have similar heat sink thermal resistance with the conventional fin-type heat sinks, the PCM-integrated fin-type heat sinks exhibit an effective time delay up to ∼27.3% of the hot-spot temperature rise until 80 ℃ of the heat sinks in reduced cooling conditions, showing the potential as an effective thermal managing platform of the PCM-integrated heat sinks in convection-limited cooling environments. [ABSTRACT FROM AUTHOR]

Deploying high thermal conductive materials as additives to UO 2 has been highlighted due to their enhanced thermal performance as a potential candidate of Accident-Tolerant Fuels (ATFs). Herein, we investigate thermomechanical behaviors of UO 2 – 5 vol% Mo fuel pellets with a sandwiched configuration where Mo layers are positioned on the top and bottom of a UO 2 part. The successfully fabricated sandwich-type UO 2 fuel pellets with an inclusion of 5 vol% Mo by using a conventional sintering method exhibit up to 3.0 and 2.5 times higher effective thermal conductivities at 800 °C than standard UO 2 and dispersion-type UO 2 – 5 vol% Mo fuel pellets, due to the effective heat transfer characteristics in the Light Water Reactor (LWR) fuel. Accordingly, the sandwich-type fuel pellets not only effectively reduce the thermal stress gradients across the pellets, but also significantly decrease the maximum tensile hoop stresses of fuel pellets. Finally, the radial deformation behaviors of the sandwich-type fuel pellets are also investigated under a simulated irradiation condition, showing that an hourglass-shaped deformation of UO 2 pellets are considerably mitigated. • Thermomechanical behaviors of UO 2 – 5 vol% Mo fuel pellet with a sandwiched configuration are investigated. • Sandwich-type pellets exhibit up to 3.0 times higher effective thermal conductivities at 800 °C than standard UO 2. • Sandwich-type pellets significantly reduce the maximum hoop stress and strain of standard UO 2. • Radial deformation behaviors of sandwich-type pellets in a simulated irradiation are investigated. [ABSTRACT FROM AUTHOR]

• Layer-by-layer assembled phase change composite with 90 vol% paraffin is fabricated. • Phase change composite shows up to 285 times higher thermal conductivities. • Phase change heat spreaders lower hot spot temperature by ~7 °C, compared to aluminum. This study reports fabrication of layer-by-layer assembled phase change composites by locating phase change material (paraffin) into the network of aluminum meshes after which they are sandwiched by ultra-thin graphite sheets. The fabricated phase change composites with 90 vol% phase change materials exhibit up to 285 times higher thermal conductivities than bare paraffin with directional characteristics of heat flow due to the incorporation of graphite sheets which have anisotropic thermal conductivities. We further examine thermal performance of bare paraffin, aluminum, and phase change composite heat spreaders by monitoring maximum hot-spot temperatures under different cooling conditions. As a result, the phase change composite heat spreaders perform superior cooling capacities to bare paraffin and aluminum heat spreaders in high power intensity conditions with high heating and low cooling rates by lowering their hot-spot temperatures by up to 121 °C and 7 °C, respectively. It is enabled by the effective utilization of thermal capacitive effects of phase change materials with significantly enhanced thermal conductivities. This study represents an effort to develop novel heat spreaders with enhanced cooling capacities for the potential applications of high thermal budgets with limited cooling resources. [ABSTRACT FROM AUTHOR]