Given the well-recognized challenges related to nanoparticle stability, especially at higher volume fractions (e.g., 0.15 vol%), how do the authors justify the use of a single-phase homogeneous model to represent nanofluid behavior, particularly for Cu and CuO nanofluids, which are prone to agglomeration and sedimentation under shear forces and thermal gradients? In twisted geometries, where secondary flows and recirculation zones are prominent, such stability issues may significantly influence local viscosity distribution and overall heat transfer performance.
In my view, it would be more appropriate to consider the use of a two-phase approach (e.g., Eulerian–Eulerian or discrete phase model) to more accurately capture nanoparticle migration, concentration non-uniformity, and potential near-wall accumulation effects.