Dilute bidispersed tube flow: Role of interclass collisions at increased loadings

This paper presents measurements and analysis of a bidispersed two-phase flow system at low (11%) and moderate (110%) mass loadings in a fully developed pipe flow. Our goal is to provide data and analysis showing the role of interclass collisions. The mass distribution of the particles mixture is therefore chosen in order to promote collisions between large 90μm and 60μm glass beads. Velocity statistics up to and including the third-order moments at both mass loadings are presented. The radial turbulent transport of longitudinal and radial fluctuating kinetic stresses is much higher for the particles than for the fluid phase. Dominant effects in the radial balance of the kinetic stresses are discussed. The signature of interparticle collisions is observed by comparing measurements at high- and low-mass loadings. In particular, at higher-mass loading, we measure a decrease of the streamwise kinetic stress of the large particles over the entire section of the tube. On the other hand, the streamwise kinetic stress of the 60μm particles decreases in the high shear region only. The most striking result is the strong increase of the radial fluctuating velocities of the glass beads and of the radial transport of the radial fluctuating kinetic energy. The expected phenomenology is an important contribution of interparticle collisions to the radial variance νp′ associated with a damping of the longitudinal kinetic stresses. This damping is more important for the 90μm glass beads than for the 60μm glass beads because the 60μm particles receive kinetic energy from 90μm particles. The redistribution mechanisms via collisions are expected to be largest in the near wall region where the anisotropy of the fluctuating motion is high. An analysis of the collision frequency in different regions of the flow shows that, even at Mj=110%, the mean time scale between collisions is of the order or larger than the particles aerodynamic time constant. Therefore, collisions cannot significantly decrease the mean free path of the particle fluctuating motion, and transport effects of both longitudinal and radial fluctuating kinetic stresses toward the core region are still high. For a stronger increase in mass loading, leading to dominant effects of collisions, one would expect these transport terms to be of minor importance.