Effect of particle concentration on puff characteristics in transitional particle-laden pipe flow

In single phase pipe flow, laminar to turbulent transition occurs around Re=2040. In transitional flow, turbulent-like structures known as puffs are observed. Previous studies have shown that adding particles to the flow changes the critical Reynolds Number. The impact of the particles on the flow depends on the pipe to particle diameter ratio (D/d) and the volume fraction of particles(ϕ). In the current study, we examine the behavior of neutrally buoyant polystyrene beads in a 20% glycerol-water solution (ρ=1046 kg-m^-3) as they interact with isolated puffs. Experiments are performed with D/d =43 and ϕ=5×10^-3 , 1×10^-2 and 1.5×10^-2. Using a DSLR camera and backlit LED panel, planar imaging is performed. The particle positions are determined, and particle velocities are computed by PTV. We also evaluate puff passage frequency by varying Re and the particle concentration. We find that the critical Reynolds number decreases initially for ϕ=5×10^-3 and 1×10^-2, but then increases for ϕ=1.5×10^-2 and beyond. The change in puff frequency at higher concentrations will be discussed. We will also discuss particle velocity and concentration variation as puffs pass through the system.