  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.
