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• Heat transfer mechanisms of alumina aerogel-fiber-opacifier composites were studied. • An analytical thermal conductivity model of composites was developed. • Solid-gas coupling heat transfer in aerogels was found to be a dominant mechanism. • Contribution of gaseous conduction was comparable to solid conduction and radiation. • The optimal diameters and volume fractions of fibers and opacifier were obtained. As compared with silica aerogels, alumina aerogels have excellent high-temperature stability, and hence, are regarded as a promising candidate for high-temperature thermal insulation materials. However, heat transfer mechanisms for such materials remain insufficiently explored. In this study, based on real material microstructures, an analytical thermal conductivity model for alumina aerogel-fiber-opacifier composites is developed via a modified parallel-series equivalent electrical circuit approach. The model takes into account solid, gaseous, and solid-gas coupling heat conduction in aerogels, solid heat conduction in both fibers and opacifier particles, and radiation in aerogels, fibers, and opacifier particles. The model is validated by comparing its predictions with measured thermal conductivity data. The model predictions reveal that there are two kinds of solid-gas coupling heat transfer mechanisms, which occur in the corner regions between interconnected secondary particles and in the slit regions between adjacent chains, respectively. Because of this coupling, the contribution of gaseous heat conduction is comparable to that of solid heat conduction and radiation of aerogels, fibers, and opacifier particles, which is well supported by experimentally measured data of thermal conductivity. The model is also used to analyze effects of various parameters on the thermal insulation performance of the composites, such as diameters and volume fractious of both fibers and opacifier particles. [Display omitted] [ABSTRACT FROM AUTHOR]

A new design of double-layered microchannel heat sinks, which combines wavy microchannels with porous vertical ribs, is proposed in this work. The flow and heat transfer behaviors of the new design are investigated via a three-dimensional solid–fluid conjugate heat sink model. The superiority of the new design is highlighted by comparing the new design with three existing double-layered designs under constant pumping power constraints. The three existing designs include a design with straight microchannels and solid-ribs, a design with straight microchannels and porous-ribs, and a design with wavy microchannels and solid-ribs. The results show that porous-ribs reduce the pressure drop across heat sinks, whereas wavy microchannels induce Dean vortices and enhance coolant mixing. In a low pumping power region of 0.005 to 0.01 W, the coolant inlet velocity is found to be the most dominant factor to determine the cooling capacity of heat sinks, whereas in a high pumping power region of 0.1 to 0.2 W, the coolant mixing becomes the dominant one. The new design has both wavy microchannels and porous-ribs, thus providing the best cooling capacity among the four double-layered heat sinks in the whole pumping power range of 0.005 to 0.2 W. The new design is further optimized by single-parameter analyses. It is found that with a fixed pumping power of 0.05 W as constraint, there is an optimal wavelength or an optimal amplitude of wavy units to achieve the lowest thermal resistance; however, the thermal resistance monotonously reduces with the increase in porous permeability or the decrease in quadratic drag factor. [ABSTRACT FROM AUTHOR]

In this study, microchannel heat sinks with symmetric and parallel wavy microchannels are studied in a wide range of Reynolds numbers from 50 to 700 via a three-dimensional fluid-solid conjugate model. The results show that the symmetric configuration yields higher Nusselt numbers than the parallel one, and it is especially true at higher Reynolds numbers and larger amplitude-to-wavelength ratios. The heat transfer enhancement can be attributed to the fact that the symmetric configuration induces four Dean vortices in the cross sections perpendicular to the flow path, whereas there are only two Dean vortices in the parallel configuration, leading to stronger coolant mixing and hence the higher Nusselt numbers in the symmetric configuration. However, because of the presence of channel throats, the symmetric configuration also achieves a significantly high pressure penalty. As a result, the overall performance of the symmetric configuration is slightly lower than that of the parallel configuration. To enhance the performance of wavy microchannel heat sinks, modified symmetric and parallel wavy configurations are proposed, in which several transverse gaps are added into ribs to connect adjacent microchannels. The results demonstrate that such a secondary branch design significantly enhances Nusselt numbers for both the parallel wavy configurations owing to the enhanced fluid mixing between adjacent microchannels. Moreover, it is found that, as compared with its original wavy configurations, secondary branches slightly increase the pressure drop across the modified parallel configuration. Thus, the overall performances are markedly enhanced for the modified parallel configurations. As expected, secondary branches greatly reduce the pressure drop across the modified symmetric configuration. Beyond expectation, the suction effect of channel throats due to secondary branches can weaken the Dean vortices in the valley vs valley regions and worsen the heat transfer performance, especially for the heat sink with larger Re and amplitude-to-wavelength ratios. As a result, the modified symmetric wavy configurations are suggested to employ with a wider gap and small amplitude-to-wavelength ratio. Generally speaking, the modified parallel wavy configuration is more dominant than the symmetric one. • Symmetric and parallel wavy microchannel heat sinks are studied numerically. • Parallel configuration shows higher Nu but larger pressure drop penalty. • Modified wavy configurations are proposed with secondary branch design. • Modified parallel configurations yield better overall performance. • Modified symmetric configuration achieves better overall performance partly. [ABSTRACT FROM AUTHOR]