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A two-phase lattice Boltzmann method (LBM) is used to study the natural convection heat exchange characteristics of Al2O3–H2O nanofluids and (Al2O3/Cu)–H2O composite nanofluids in a rectangular cavity in this paper. It can be found that the two-phase LBM is suitable for the study on natural convection heat exchange characteristics of nanofluids, and its correctness is verified. Firstly, the effects of temperature differences (∆T = 1 K and ∆T = 11 K), nanofluids volume fractions (φ = 0.01, 0.03 and 0.05) and kinds of fluid (water, Al2O3–H2O nanofluids and Al2O3/Cu–H2O nanofluids) on the free convection are researched. Then, temperature and streamline distributions of Al2O3/Cu–H2O nanofluids are investigated, respectively. Lastly, local Nusselt number and average Nusselt number distributions of different nanofluids along with Y direction are investigated. It can be found that Al2O3/Cu–H2O nanofluids show a larger enhancement than Al2O3–H2O nanofluids and water. Al2O3/Cu–H2O and Al2O3–H2O nanofluids can strengthen the heat transfer performance by 9.9% and 5% at the extreme compared with deionized water, respectively. Besides, the maximum enhancement ratio of Al2O3/Cu–H2O and Al2O3–H2O nanofluids is 9.8% and 4.5% at small temperature difference (ΔT = 1 K), respectively, which is greater than that (1.6% and 0.36%) at big temperature difference (ΔT = 11 K). [ABSTRACT FROM AUTHOR]

The working fluids with higher solar thermal conversion performance within broadband spectrum ranges are of great concern for direct absorption solar collectors (DASCs). Both metal nanoparticles with localized surface plasmon resonance (LSPR) effects and carbon nanomaterials have unique spectral absorption behaviors and have shown better photothermal performance in DASCs. In this paper, we attempted to prepare composite nanofluids including plasmonic bimetallic alloy and carbon nanomaterials to realize enhanced solar absorption and photothermal conversion performance. By taking ZIF-8-derived nitrogen-doped graphitic polyhedrons (ZNGs) as carrier, plasmonic bimetallic Ag-Au alloy nanoparticles were loaded on them by an impregnation-reduction method successfully. Ag-Au/ZNGs ethylene glycol nanofluids showed significant broadband absorption in the visible and near-infrared spectrum range at a lower concentration. Comparing to ethylene glycol, the photothermal conversion effeiency of all ZNGs nanofluids increased remarkablely. Plasmonic bimetallic Ag-Au alloy nanoparticles further improved the photothermal conversion efficiency, which was up to 74.35% for Ag-Au ZNGs nanofluids compared with 72.41%, 70.35% for Au/ZNGs, Ag/ZNGs respectively. This work presents a new way to enhance solar energy absorption and improve solar thermal conversion efficiency of nanofluids for DASCs. • Dual plasmonic Ag-Au/nitrogen-doped graphitic polyhedrons (ZNGs) nanofluids were designed in solar thermal application. • Ag-Au/ZNGs nanofluids show significant broadband absorption in the visible and near-infrared spectrum. • Photo-thermal conversion efficiency of the Ag-Au/ZNGs nanofluids increased remarkably due to synergistic effect. [ABSTRACT FROM AUTHOR]