Effect of Particle-Wall Interaction in Disperse Flows

Direct numerical simulations of fully-developed gas-particle flows in a rectangular channel have been done using a point force method to calculate the forces exerted by particles on the gas. Particle transport in two flow configurations, i.e., (1) gas-solid flow in which particles bounce on the walls and (2) gas-liquid disperse flow of an annular pattern in which particles are injected from wall sources and are removed when they hit a wall are examined. The particles are represented by solid spheres with a density ratio of 1000. The effect of gravity is ignored. A volume fraction, alpha = 1 x 10(-4), which is sufficiently small to ignore inter-particle collisions, is assumed. A significant effect of particle-wall interaction is observed in the near-wall region of the concentration field and of the particle velocity field. Large concentrations in the near-wall region of the gas-solid flow are due to the particles that lose their momentum by slipping against the gas flow after bouncing on the wall. Damping of gas turbulence, which is caused by decreases in the gas Reynolds shear stresses to accommodate the particle stresses, shows to be larger in the gas-liquid flow than is observed in the gas-solid flow. The injection and deposition mechanisms that decrease concentrations in the near-wall region show to be effective in drag reduction.