PARTICLE-TURBULENCE INTERACTIONS IN THE PRESENCE OF A ROUGH WALL

Abstract

Experiments were conducted over smooth and rough walls in a low Reynolds number horizontal turbulent channel flow laden with small (64 μm) glass particles. A particle image velocimetry technique was used to measure velocities of both the carrier fluid and particles. Various turbulent characteristics were examined to investigate the impact of wall roughness on the particle-turbulence interactions. The results show that particles increased the turbulent intensities near the wall, and reduced them in the outer layer, but these effects were dampened for the rough wall. On the contrary, particles increased the peak value of the Reynolds shear stress in the presence of the rough wall when compared to the unladen flow. Particle velocity fluctuation intensities matched those of the unladen fluid for the smooth wall, but the peak velocity fluctuation intensities were enhanced in the presence of wall roughness due to particle-wall collisions. The effect is larger for the streamwise velocity fluctuation intensity than the wall-normal velocity fluctuation intensity. The present results indicate that the particle motion is more responsive to the presence of the rough wall than the particle-laden fluid. INTRODUCTION Turbulent flows laden with particles are common in many engineering applications. Examples include fluidized beds, pneumatic conveying and pollution control systems. Understanding these flows as well as developing their model representations demands knowledge of the interaction between particles and fluid turbulence. It has been suggested that depending on the size, density ratio and particulate phase loading, the interaction may lead to modification of the fluid turbulence level. The type of interaction between the particles and fluid is described by the particle volume fraction, Φv, defined as the volume occupied by the particles per unit volume of the particlefluid mixture. For dilute loadings (Φv < 10), particles act as passive tracers, and the particle-fluid interaction is described as one-way coupling. For intermediate loadings (10 < Φv < 10), particles do not only respond to the fluid motion but also modulate the fluid turbulence level. This type of interaction called two-way coupling is the subject of numerous previous experimental and numerical investigations aimed at quantifying the exact impact of particles on turbulence. Tests conducted in channels (Maeda et al. 1980; Tsuji et al. 1984; Liljegren and Vlachos 1990; Kulick et al. 1994; Kiga and Pan 2002; Rani et al. 2004) and boundary layers (Rashidi et al. 1990; Best et al. 1997; Kaftori et al. 1998; Righetti and Romano 2004) by far have produced conflicting results regarding the effects of particles on the fluid mean and turbulent characteristics. Some studies indicate that the carrier fluid mean velocity is enhanced compared to the unladen flow (Maeda et al. 1980; Hagiwara et al. 2002; Righetti and Romano 2004), while others reported the mean velocity to be reduced (Best et al. 1997; Kaftori et al. 1998; Kiga and Pan 2002). Similar discrepancies exist for the carrier fluid turbulence as well. For instance, turbulence has been reported to be attenuated by small particles and augmented by large particles, with the degree of modification increasing with volume fraction (Maeda et al. 1980; Rashidi et al. 1990; Kulick et al. 1994; Pan and Banerjee 1996), whereas others (Liljegren and