Experimental characterization of puff–particle interaction in transitional pipe flow

In laminar pipe flow, neutrally buoyant particles accumulate at a certain radius near the wall depending on the pipe-to-particle diameter ratio ( D / d ) and pipe Reynolds number ( R e ). Transitional pipe flow containing intermittent puff structures and low particle volume fractions ( ϕ ) was investigated to understand the interactive effects. For D / d = 44 , puff spacing and transitional Reynolds number dropped substantially even at low ϕ (0.25%). By contrast, D / d = 84 particles caused very little change in the transitional Reynolds number. Planar particle tracking velocimetry was performed to evaluate particle distribution and motion within puffs. Both particle sizes were found to accumulate at 0 . 93 R in laminar flow. Puffs disrupted this particle accumulation by transporting many of these particles inward to locations between r / R = 0 . 7 and 0.9, somewhat flattening the radial distribution. Particle distributions took more than 90 D to recover to their initial values. Streamwise particle velocities matched closely with expected fluid velocities in the laminar part of the flow. Within the turbulent part of puffs, radial RMS fluid and particle velocities greatly exceeded values in fully developed turbulent flow. Longer particle trajectories evaluated in the turbulent part of the puff were unidirectional over time scales that corresponded closely with coherent vortical structures identified in single-phase flow. The disruption of particle accumulation near the wall was associated with wall-normal fluid ejections in single-phase transitional flow near the puff trailing edge.