Multiscale investigation of particle suspension in solar nanofluids: Impacts on radiative characteristics and optical-thermal performance

• A multiscale Eulerian-Lagrangian model is developed to address particle migration in solar based nanofluids. • Particle migration and slip affect both thermal and radiative properties of nanofluids. • At Re ≥ 1, lift forces cause particles to migrate from walls to the channel center. • At Re < 1, Brownian motion dominates and single-phase models remain accurate. • Migration leads to strong concentration gradients and non-uniform optical properties. • Wall regions show lower extinction; core shows enhanced absorption in λ < 1.4 μm range. Conventional single-phase Eulerian models often fail to accurately predict the performance of nanofluid-based volumetric absorption collectors (VACs) due to their inherent assumption of uniform particle distribution. The current investigation proposes a multiscale Eulerian-Lagrangian (EL) framework to investigate the crucial role of particle dynamics in rectangular channel flow. The introduced model is validated against several experimental and numerical benchmarks well-established in the literature. We demonstrate that hydrodynamic lift forces induce significant particle migration, particularly at Reynolds numbers relevant to solar applications (Re ≥ 1). Particles migrate from high-shear near-wall regions to the low-shear channel core, creating remarkable non-uniformities in concentration. This migration directly impacts the nanofluid’s optical behavior, resulting in spatially varying extinction coefficients: lower near the walls due to particle depletion and significantly higher in the central region due to accumulation. Consequently, radiative penetration increases near boundaries while absorption becomes concentrated centrally, especially in the visible/near-infrared spectrum (λ < 1.4 μm). These findings, supported by spectral radiative flux analysis, highlight substantial deviations from single-phase predictions, which overestimate near-wall extinction and underestimate core extinction at Re ≥ 1. Conversely, at low Re (<1), Brownian motion dominates, migration is negligible, and single-phase models suffice. The results underscore the necessity of employing multi-phase models that capture particle dynamics for accurate prediction and optimization of nanofluid solar systems.