Inertial focusing phenomena of neutrally buoyant particles suspended dilutely in square duct flows*

Neutrally buoyant spherical particles suspended in laminar rectangular duct flows are known to cross streamlines, focusing on certain points at downstream cross sections. For square duct flows, recent experimental and numerical studies showed that there are several focusing patterns of particles over the duct cross section, depending on the Reynolds number (Re) and the particle-to-duct-size ratio. In the case of the size ratio around 0.1, four focusing points, called channel face equilibrium positions (CFE), appear near the center of duct walls at low Re. This pattern is named “CFE pattern”, tentatively. At higher Re, new focusing points, called channel corner equilibrium positions (CCE), appear on the diagonal near duct corners, together with the CFE (CFE-CCE pattern). For larger size ratios (>0.3) at low Re, on the other hand, the appearance of CCE instead of CFE was predicted by a recent numerical study (CCE pattern). In the present study, we have aimed to elucidate the focusing patterns of large particles (size ratio 1/3) in square duct flows over a wide range of Re, by experimental and numerical studies. We have confirmed the appearance of the CCE pattern at low Re. An increase in Re was found to yield the CFE-CCE pattern, followed by the CFE pattern.