Experimental study of inertial particles in horizontal turbulent pipe flows

Particle-laden horizontal turbulent pipe flow is studied experimentally at a friction Reynolds number of 190 in the two-way coupling regime with a focus on particle-to-fluid density ratio ρ_p/ρ_f =1 and 1.05. Particle volume fraction φ_v up to 1% and viscous Stokes numbers ranging from St⁺ ≈1.2 to St⁺ ≈ 3.8 are investigated using time-resolved two-dimensional particle image and tracking velocimetry. Substantial differences are observed between the statistics of neutrally buoyant (i.e. ρ_p/ρ_f =1) and denser (i.e. ρ_p/ρ_f =1.05) settling particles (with settling velocities 0.12 - 0.32 times the friction velocity), which, at most instances, show opposing trends compared to unladen pipe flow statistics. Neutrally buoyant particles show a slightly increased overall drag and suppressed turbulent stresses, but elevated particle-fluid interaction drag and resulting in elongated turbulent structures compared to the unladen flow, whereas ρ_p/ρ_f =1.05 particles exhibit a slight overall drag reduction, even with increased radial turbulent stresses, and shorter streamwise structures compared to the unladen flow. These differences are enhanced with increasing St⁺ and φ_v, and can be attributed to the small but non-negligible settling velocity of the denser particles, which also leads to differing statistics in the upper and lower pipe halves. Towards the end, we will also present some preliminary results at higher Reynolds numbers of 850 and 2050 at ρ_p/ρ_f =1.05.