Experimental study of particle-laden turbulent horizontal pipe flows up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si19.svg" display="inline" id="d1e4122"> <mml:mrow> <mml:mi>R</mml:mi> <mml:msub> <mml:mrow> <mml:mi>e</mml:mi

To understand the effects of particle settling and Reynolds number, experiments are conducted in a smooth-wall horizontal pipe with a diameter D = 10 cm , and using particles of diameter d p = 250 μ m at viscous-scale Stokes numbers of S t + = 0 . 96 –5.93, a volume fraction of ϕ v = 0 . 022 % , and a particle-to-fluid density ratio of ρ p / ρ f = 1 . 05 . Measurements of the streamwise and radial velocities of both the fluid and particle phases are obtained using planar imaging techniques (PIV and PTV). Two friction Reynolds numbers, R e τ ≈ 850 and 2 050 , corresponding to bulk Reynolds numbers of R e b = 32 000 and 86 000 , respectively, are studied, along with an additional experimental dataset at R e τ ≈ 190 with the same ρ p / ρ f . We observe that turbulence modulation depends on R e τ both at the pipe centreline and off-centreline. In general, the streamwise intensity of the fluid phase increases relative to the unladen case, the radial intensity decreases, and the Reynolds stress is reduced. The velocity statistics of the particle phase generally match those of the fluid phase, except for the radial intensity, which is higher for the particles than for the fluid, highlighting the importance of particle settling. To quantify settling due to gravity, we introduce a new non-dimensional number and use it to classify different experiments in channels and pipes. We also perform a drag decomposition, finding that at the highest R e τ , particle stress can contribute more than 10% of the total drag. Since we do not observe any overall change in drag in particle-laden flows, the decomposition shows that the turbulence contribution, via Reynolds shear stress, decreases and is replaced by particle stress, maintaining approximately the same overall drag. A detailed analysis of the Reynolds shear stress through quadrant analysis reveals that the introduction of particles reduces sweep and ejection contributions and tends to randomise the quadrant contributions, effectively reducing the overall Reynolds shear stress. • Increasing R e τ slightly enhances streamwise turbulence intensity but significantly suppresses radial intensity and Reynolds shear stress. • The particles at a low density ratio settle toward the lower pipe wall with a maximum concentration at about 1 / 4 D from the lower wall. • A parameter G (ratio of fluid acceleration to effective gravitational acceleration) is introduced and combined with P a S t to classify channel and pipe experiments. • Although the total drag remains nearly unchanged, particle stresses can contribute more than 10% of the total drag at high R e τ .