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Nanofluids as thermal absorption media or optical filters have showed great potential for application in direct absorption solar collector or hybrid photovoltaic/thermal (PV/T) collectors. Due to their nanoscale size and high surface energy, particles in nanofluid have a great tendency to aggregate into large secondary agglomerate, even in the presence of the anti-agglomeration agent, which could in turn lead to significant change in the thermal and optical properties of the nanofluid. However, rare work has been carried out to investigate the relation between the aggregation and optical properties of the nanofluid, which in our view, is of great importance for the enhanced performance of the nanofluid in solar collectors. In this study, taken alumina nanofluids with controlled particle aggregation properties as example, we were able to investigate the underlying correlation between the size distribution of the aggregate in nanofluid and their resulting absorption coefficients. It was found that inhibited aggregation and resultant smaller aggregated particle size could lead to larger absorption coefficient. Both Rayleigh scattering and Mie scattering theories have been attempted to predict the optical properties of the nanofluid after particle aggregation. The results indicate that values obtained by Rayleigh scattering theory are several orders higher than experimental values but that predicted by Mie scattering theory can reasonably match with the experimental results. Specially, it was demonstrated that by introducing the projected areas of the nanoparticle aggregate corresponding to smallest sphere enclosing diameter, more accurate absorption coefficient prediction based on Mie theory can be obtained. Our study is expected to provide a valuable guidance for the regulation of the optical properties of nanofluids and to improve their performance in solar collectors. [ABSTRACT FROM AUTHOR]

In this study, evolution and final state of particle size distribution (PSD) of TiO 2 /water suspension were described by a modified population balance equation considering both natural convection and Brownian motion. The calculation was verified by experimental measurement of scanning electron microscope and Laser particle analyzer. Based on the predicted PSD, Rayleigh scattering theory (for d < 4 λ ) and Fraunhofer diffraction theory (for d > 4 λ ) were then employed to determine the scattering and absorption coefficients of TiO 2 /water suspensions with various particle concentrations and at various light wavelengths. UV–Vis investigation showed that the predicted optical properties of TiO 2 /water suspension considering PSD are in much better agreement with the experimental results than that without considering PSD, at all particle concentrations investigated. It is traditionally accepted that for colloid suspension the scattering coefficient can be neglected compared to the absorption coefficient and therefore the extinction, which is the combination of the absorption and scattering coefficients, can be approximately replaced by the absorption coefficient. However, our finding indicates that, when particle aggregation is considered, the scattering coefficient of TiO 2 /water suspension could be much higher than the absorption coefficient. Consequently, using the absorption coefficient alone to replace the extinction coefficient even at long wavelengths could result in significant calculation errors and is not recommended. [ABSTRACT FROM AUTHOR]