Effect of particle size on characteristics of transitional particle-laden pipe flow*

Neutrally buoyant particles in laminar pipe flow exhibit a phenomenon called the tubular pinch effect, where they accumulate at a certain radius near the wall. Particles also promote or hinder the transition to turbulence. The transitional regime is marked by intermittent turbulent structures called puffs. These phenomena are characterized by the particle-to-pipe diameter ratio(D/d), the particle volume fraction (ϕ), and Reynolds Number (Re). In the current study, we examine the interaction between polystyrene beads in a 20% glycerol-water solution (ρ=1046 kg-m^-3) and puffs using PTV. Even at small loadings (ϕ=0.2%), larger particles (D/d=43) lead to transition at lower Re than single-phase flow while smaller particles (D/d=86) delay the transition to higher Re. In the laminar regime, the particle concentration was found to peak near 0.88R. Puffs disrupt this accumulation, with the particles migrating radially inwards. The radial velocities of the particles are significant over a distance of 10D near the trailing edge of the puff, and the disruption persists for more than 100D. We examine the radial trends of particle velocity, concentration, and recovery with respect to particle size and volume fraction.