Experimental study of turbulent flow of spherical particles in Newtonian and drag reducing viscoelastic fluid.*

Particle image velocimetry in suspensions of large inertial spherical particles in Newtonian as well as drag reducing viscoelastic fluid is performed using refractive index-matched particles. Two geometries are investigated: a horizontal square duct and a round pipe. For the square duct, turbulent flow statistics, pressure drop, and particle concentration distribution are measured at a bulk Reynolds number Re of 11000. The particles are nearly neutrally-buoyant and the duct height to particle diameter ratio is 10. The volume fraction Φ is varied between 0-20%. Addition of particles causes a monotonic increase in the pressure drop with concentration. However, the rate of increase is faster in viscoelastic fluid. The Reynolds shear stress decreases with increasing Φ, more for viscoelastic fluid. Similar to Newtonian fluid, particles migrate towards the core as well as towards the wall.

For the round pipe, only pressure drop is measured for a range of Re from 5000 to 35000 for three particle sizes: D/d_p = 43, 21 and 10. For all cases, increasing Φ and size leads to increased pressure drop. At high Re and high Φ, the largest particles cause an abrupt increase in drag, a feature similar to Newtonian flow.