Experimental investigation of turbulence modification by inertial particles in a horizontal turbulent pipe flow

Particle-laden pipe flow is widely encountered in various industries where efficient and reliable prediction on the effect of particles on turbulence is required. Recently, numerical simulations have outpaced experimental investigations with little cross-validation, and most experiments are in vertical pipes to minimise the gravity effect. As such, relatively little is known about the changes to the dynamics of turbulence owing to inertial particles under the influence of gravity. In this study, the effects of volume fraction, Stokes number and solid-fluid density ratio on turbulent pipe flow are experimentally investigated. The experiments were conducted in a smooth-wall horizontal pipe with diameter D=20.5mm, using two inertial particles with diameters Dp=250μm and 437μm in the range of friction Stokes number St=1.09-3.87 at volume fractions (0-1%), Reτ≈195, and at particle-fluid density ratios of 1 (not gravity effect) and 1.05 (influenced by gravity). Particle image velocimetry technique is employed to record the two-phase behaviours and investigate the intricate turbulent modification. Results reveal that the mean streamwise velocity, fluctuating intensities and Reynolds shear stress decrease more at higher Stokes number and volume fraction than in a neutrally-buoyant scenario. The presence of inertial particles induces an apparent turbulent attenuation, which is also verified by the pre-multiplied energy spectra of streamwise fluctuating velocity and instantaneous vertical fluctuating velocity field. Due to the deposition of solid phase at density ratio 1.05, the bottom-half of pipe displays a more significant turbulent attenuation than the top-half.