Effect of feedback and inter-particle collisions in an idealized gas–liquid annular flow

Abstract

An idealized gas–liquid annular flow in a vertical rectangular channel was considered. Point sources of solid spheres were located on the walls to represent an atomizing wall film. The decrease in the deposition coefficient with increasing volume fraction, observed in laboratory studies, was examined by using a direct numerical simulation, which does not fully resolve scales of the size of particles, to calculate the fluid turbulence. The influence of particles on the fluid turbulence was modeled simply by introducing point forces at particle loci. Inter-particle collisions were also considered. Significant attenuation of fluid turbulence at very low concentrations could be observed as resulting from the feedback effect of particles. In the fluid momentum balance the fluid Reynolds shear stresses decrease with increasing the volume fraction in order to accommodate the particle forces. This leads to a decrease in the production of fluid turbulence and, therefore, to a decrease in particle turbulence. Particle turbulence is augmented by elastic inter-particle collisions and can be attenuated by inelastic inter-particle collisions. The decrease in the deposition coefficient, observed in gas–liquid annular flows, can be explained by the feedback effect if droplet collisions are highly inelastic. When feedback and elastic inter-particle collisions are considered, changes in the particle turbulence are mainly associated with elastic inter-particle collisions at large enough volume fractions. The observed influence of point forces on fluid turbulence has a kinship to findings on polymer drag reduction in that polymer molecules (or aggregates) create local stresses in the fluid and the Reynolds shear stresses decrease to accommodate these polymer stresses.