Slip-Flow and Heat Transfer of a Non-Newtonian Nanofluid in a Microtube

The slip-flow and heat transfer of a non-Newtonian nanofluid in a microtube is theoretically studied. The power-law rheology is adopted to describe the non-Newtonian characteristics of the flow, in which the fluid consistency coefficient and the flow behavior index depend on the nanoparticle volume fraction. The velocity profile, volumetric flow rate and local Nusselt number are calculated for different values of nanoparticle volume fraction and slip length. The results show that the influence of nanoparticle volume fraction on the flow of the nanofluid depends on the pressure gradient, which is quite different from that of the Newtonian nanofluid. Increase of the nanoparticle volume fraction has the effect to impede the flow at a small pressure gradient, but it changes to facilitate the flow when the pressure gradient is large enough. This remarkable phenomenon is observed when the tube radius shrinks to micrometer scale. On the other hand, we find that increase of the slip length always results in larger flow rate of the nanofluid. Furthermore, the heat transfer rate of the nanofluid in the microtube can be enhanced due to the non-Newtonian rheology and slip boundary effects. The thermally fully developed heat transfer rate under constant wall temperature and constant heat flux boundary conditions is also compared

Multiband selective absorbers made of 1D periodic Ag/SiO_2/Ag core/shell coaxial cylinders horizontally lying on a planar substrate

In this paper, we present a one-dimensional periodic microstructure for multiband selective absorbers of thermal radiation. The microstructure is made of Ag/SiO2/Ag core/shell coaxial cylinders horizontally lying on top of a SiO2 dielectric spacer and an opaque silver substrate. The spectral-directional absorptivity of the proposed structure was numerically investigated with the finite element based Comsol Multiphysics software. Multiband selective absorption in the wavenumber range from 2500 to 20000 cm(-1) for TM-wave incidence was obtained. Physical mechanisms responsible for the multiband selective absorption were elucidated due to the resonance of magnetic polaritons in the SiO2 spacer shell, excitation of surface plasmon polaritons at the SiO2/Ag interface, and the effect of Wood's anomaly. Furthermore, the effects of a silver core radius, spacer shell thickness, a confocal elliptical core/shell cylinder on the property of multiband absorption, and the absorptivity of the structure with one core/four shells coaxial cylinders were explored. (C) 2017 Optical Society of AmericaNational Natural Science Foundation of China (NSFC) [51576004]SCI(E)ARTICLE8A208-A2222

Near-Field Radiative Heat Transfer Between Materials With Dielectric Coatings

Radiative heat transfer between materials with dielectric coatings is numerically studied based on the fluctuational electrodynamics and the fluctuation-dissipation theorem. The results show that whereas a dielectric coating (SiC) enhances the far-field radiative heat transfer between two bulk metals, it will suppress the radiative heat transfer in the near-field and the suppression is only for the s-wave contribution. The total radiative heat flux continuously decreases as the coating thickness increases up to I gm. A further increase in the coating thickness will cause the total radiative heat flux to increase slightly before it saturates. In addition, a much smaller coating thickness than the coating's skin depth is enough to significantly change the total radiative heat flux in the near-field region. On the contrary, a thin dielectric coating that supports surface polaritons can greatly enhance the radiative heat transfer between a metal and a dielectric in the case that the coating is on the metal. The large enhancement is due to surface polaritons excited on the two surfaces of the air gap boundaries.Engineering, Electrical & ElectronicEngineering, MechanicalNanoscience & NanotechnologyEICPCI-S(ISTP)