The importance of interstitial fluid effects on particle fluctuating motion in gas-particle or liquid-particle flows with significant particle-particle collisions can be characterized by the ratio of the coefficient of restitution for inelastic particle-particle collisions in a fluid ef to the coefficient of restitution for collisions in a vacuum es. In previous research, the ratio ef/es has been found to depend on the impact Stokes number St, which relates the particle inertia to the viscous force. The value of the impact Stokes number is known to be a function of the particle velocity at impact, the size and density of the particle, and the fluid viscosity. In the present work, the effect of the interstitial fluid on particle fluctuating motion is investigated experimentally using LDV/PDPA in a vertical pipe flow configuration. The influence of the variation in the particle loading and Reynolds number, which affects the particle impact velocity and, hence, ef, is probed using particles of two different densities with comparable sizes. Specifically, the downward flow of 70 micron glass beads (density = 2500 kg/m3) and 60 micron hollow ceramic microspheres (density = 700 kg/m3) is investigated for solids loadings (ratio of solids mass flowrate to gas mass flowrate) from 0.4 to 7 and Reynolds numbers which vary from 6000 to 13,000. For this range of conditions, the values for ef range between 0.65 and 0.94 (0.94 is the value for es for glass beads colliding in a vacuum). Trends in the mean and fluctuating velocities for both the gas and particle phases, as well as the gas turbulence modulation in the presence of particles, are discussed in relation to the degree of influence of the interstitial fluid on the details of the particle motion.

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