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The present experimental study addresses the flow of a Yield Stress Fluidwith some elasticity (Carbopol gel) in a square duct. The behaviour of twofluids with lower and higher yield stress is investigated at multiple Reynoldsnumbers $Re^* \in$ (1, 200) and Bingham numbers $Bi \in$ (0.01, 0.35). Asecondary flow consisting of eight vortices is observed to recirculate thefluid from the corners to the core. Their extent and intensity grows withincreasing $Re^*$. The second objective of this study is to explore the changein flow in the presence of particles. Almost neutrally-buoyant finite-sizespherical particles with duct height, $2H$, to particle diameter, $d_p$, ratioof 12 are used at two volume fractions $\phi$ = 5 and 10\%. Particle TrackingVelocimetry (PTV) is used to measure the velocity of theserefractive-index-matched spheres, and PIV to extract the fluid velocity. Simpleshadowgraphy is also used for qualitatively visualising the development of theparticle distribution along the streamwise direction. The particle distributionpattern changes from being concentrated at the four corners, at low flow rates,to being focussed along a diffused ring between the center and the corners, athigh flow rates. The presence of particles induces streamwise and wall-normalvelocity fluctuations in the fluid phase; however, the primary Reynolds shearstress is still very small compared to turbulent flows. The size of the plug inthe particle-laden cases appears to be smaller than the corresponding singlephase cases. Similar to Newtonian fluids, the friction factor increases due tothe presence of particles, almost independently of the suspending fluid matrix.Interestingly, predictions based on an increased effective suspension viscosityagrees quite well with the experimental friction factor for the concentrationsused in this study.

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