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

TL;DR

This study experimentally investigates how spherical particles of different sizes affect turbulence and pressure drop in a square duct, revealing size-dependent effects on flow resistance and turbulence attenuation.

Contribution

It provides new experimental data on turbulent suspensions of spherical particles in a square duct, highlighting size and volume fraction effects on flow behavior and pressure drop.

Findings

Pressure drop decreases with particle size at low volume fractions.

At high volume fractions, pressure drop shows non-monotonic behavior with particle size.

Turbulence attenuation correlates with reduced drag at low particle concentrations.

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 neutrally-buoyant with respect to the suspending fluid. Refractive Index Matched - Particle Image Velocimetry is used for fluid velocity measurement even at the highest particle volume fraction (20%) and Particle Tracking Velocimetry 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 $Re_{2H}\approx$ 10000 whereas, at the highest $Re_{2H}\approx$ 27000, particles are in almost full suspension. The friction factor of the suspensions is found to be significantly larger than that of single-phase 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. The pressure drop is found to decrease with the particle diameter for volume fractions lower than $\phi$ = 10% for nearly all $Re_{2H}$. However, at the highest volume fraction $\phi$ = 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 $\phi$ = 20%, however, occurs despite this large reduction of the turbulent stresses, and it is therefore due to significant particle-induced stresses.