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A wide range of applications involving mixed-convection studies can be found in various engineering processes such as thermal discharge of water bodies, float glass production, heat exchangers, nuclear reactors, and crystallization process. The present study focuses on understanding the thermal mixing scenarios for mixed-convection lid-driven flow in a square cavity using heatlines. Thermal mixing is analyzed for four different thermal boundary conditions, and heat flow patterns in mixed convection are analyzed using Bejan's heatlines concept for wide ranges of parameters (Pr = 0.015À7.2, Re = 1À100, and Gr = 10 3 À10 5 , where Pr, Re, and Gr denote the Prandtl, Reynolds, and Grashof numbers, respectively). The results indicate that, at low Pr values (Pr = 0.015), the transport is conduction-dominant irrespective of the values of Gr and Re. The trends of heatlines and streamlines are identical near the core for high-Re cases. A single circulation cell was observed in the streamlines for any Pr g 0.7 at high Re and low Gr values for uniform heating of the bottom surface with cold side walls. It was observed that thermal mixing increased significantly with subsequent rises in Gr for high-Pr fluids. Patterns of heatlines and multiple circulation cells of heatlines were found to lead to enhanced thermal mixing, with the thermal boundary layer much compressed toward the walls for linearly heated side walls. The heat-transfer rates along the walls are illustrated by the local Nusselt number distribution based on gradients of heatfunctions for the first time in this work. Nusselt numbers with infinitely large magnitudes were observed at hotÀcold junctions, illustrating high heattransfer rates. An oscillatory distribution in the local heatfunction rate was observed as a result of sinusoidal heating of the bottom surface for high-Pr fluids. Negative heat-transfer rates or local Nusselt numbers were observed along the side walls when side wall(s) was/were linearly heated, as explained based on negative heatfunction gradients. Also, the effect of Gr on the local and average Nusselt numbers in different cases can be adequately explained based on heatlines. Dense heatlines signifying higher overall heattransfer rates along the bottom surface and side walls were observed for uniform bottom surface heating, whereas lower heat-transfer rates were observed for sinusoidal heating. Nonmonotonic distributions in overall heat-transfer rates along the bottom surface and left wall were observed when both walls were linearly heated, whereas a smooth and exponential increase was observed when the right wall was isothermally cooled.

Mixed transition metal oxide films are emerging as efficient and inexpensive potential alternatives to multilayer cermet spectrally selective coatings. However, to replace the current standards involving a complex metal–dielectric structure, oxides must be optimized in terms of their electronic structure and mainly their film morphology. In the present work, a simple ultrasonic nebulized spray pyrolysis technique is used to deposit CuCo2O4 films for solar absorber coatings. Their photothermal efficiencies are studied for solar thermal energy harvesting for different film thicknesses obtained by varying the deposition time. The film surface attributes are studied using atomic force microscopy and scanning electron microscopy. The films deposited for 5 and 10 min show relatively high visible absorptance (∼0.79) and relatively low thermal emittance (∼0.1) and thus are promising candidates for spectrally selective coatings. Meanwhile, increasing the deposition time (>10 min) increases the thickness, thereby increasing the solar absorptance. However, this results in an uncontrolled increase in the surface roughness, which affects the spectral selectivity adversely, leading to the films having higher thermal emittance of between 0.1 and ∼0.25. Analysis of the specular reflection contribution shows that this deterioration is governed predominantly by interference effects due to surface attributes. This study is important for the technological applications of spectrally selective coatings and makes a significant quantitative contribution to emphasize the importance of surface morphology in optics.

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