Experimental investigation of turbulent suspensions of spherical particles in a square duct

Abstract

We report experimental observations of turbulent flow with spherical particles in a square duct. Three particle sizes namely: 2H/d p = 40, 16 and 9 (2H being the duct full height and d p being the particle diameter) are investigated. The particles are nearly neutrallybuoyant with a density ratio of 1.0035 and 1.01 with respect to the suspending fluid. Refractive Index Matched -Particle Image Velocimetry (RIM-PIV) is used for fluid velocity measurement even at the highest particle volume fraction (20%) and Particle Tracking Velocimetry (PTV) for the particle velocity statistics for the flows seeded with particles of the two largest sizes, whereas only pressure measurements are reported for the smallest particles. Settling effects are seen at the lowest bulk Reynolds number Re 2H ≈ 10000 whereas, at the highest Re 2H ≈ 27000, particles are in almost full suspension. The friction factor of the suspensions is found to be significantly larger than that of singlephase duct flow at the lower Re 2H investigated; however, the difference decreases when increasing the flow rate and the total drag approaches the values of the single phase flow at the higher Reynolds number considered, Re 2H = 27000. The pressure drop is found to decrease with the particle diameter for volume fractions lower than φ = 10% for nearly all Re 2H investigated. However, at the highest volume fraction φ = 20%, we report a peculiar non-monotonic behavior: the pressure drop first decreases and then increases with increasing particle size. The decrease of the turbulent drag with particle size at the lowest volume fractions is related to an attenuation of the turbulence. The drag increase for the two largest particle sizes at φ = 20%, however, occurs despite this large reduction of the turbulent stresses, and it is therefore due to significant particle-induced stresses. At the lowest Reynolds number, the particles reside mostly in the bottom half of the duct where the mean velocity significantly decreases; the flow is similar to that in a moving porous bed near the bottom wall and to turbulent duct flow with low particle concentration near the top wall. † Email address for correspondence: luca@mech.kth.se arXiv:1809.06607v1 [physics.flu-dyn]

Turbulence modulation by finite-size spherical particles in Newtonian and viscoelastic fluids

“…It can be seen that the drag for φ > 10% in VEF is higher than the single phase NF. It is worth noting that in our previous study [54], in NF with slightly larger particles (2H/d p = 9) at a slightly lower Re 2H,η S = 27195 (cf table 1), we observed a negligible change in drag at φ = 5 and 10% compared to single phase NF. For φ = 20%, the drag increase was around 8%, which is still less than the 25% increase that we observe in VEF for the present case.…”

“…Optical techniques fail with opaque particles where it becomes impossible to see in the core of the flow. Refractive index matched particles have been used with quite some success to overcome this hurdle [52,8,28,54] and have been used in this study.…”

Near-wall dynamics of a neutrally buoyant spherical particle in an axisymmetric stagnation point flow

“…To understand flows in arbitrary geometries, we must focus attention beyond parallel shear flows, e.g. Poiseuille or Couette flows, the study of which (Segré & Silberberg 1962;Ho & Leal 1976;Vasseur & Cox 1976;Matas, Morris & Guazzelli 2004;Loisel et al 2015) has led to a well-developed characterization of inertial migration of particles, and to a better understanding of the modulation of flow turbulence by suspended particles (Matas, Morris & Guazzelli 2003;Shao, Wu & Yu 2012;Wang, Abbas & Climent 2017;Zade et al 2018). In other geometries, such as the flow around an obstacle or in a pipe bend, there are regions where the flow is incident onto a surface.…”

Finite-size spherical particles in a square duct flow of an elastoviscoplastic fluid: an experimental study

“…The experimental set-up used in this study has been previously used in Zade et al (2018) for investigating particle laden turbulent flows of Newtonian and non-Newtonian flows up to φ = 20%, and is shown in figure 1. Only the most important features are described here and the interested reader is referred to the previous publications for additional details.…”