Unsteady mass transfer from a core-shell cylinder in crossflow

Abstract

Mass transfer from a composite cylinder - made of an inner core and an outer enveloping semipermeable shell - under channel crossflow is studied numerically using two-dimensional lattice-Boltzmann simulations. The core is initially loaded with a solute that diffuses passively through the shell towards the fluid. The cylinder internal structure and the initial condition considered in this study differ and thus complement the classical studies dealing with homogeneous uncoated cylinders whose surfaces are sustained at either constant concentration or constant mass flux. Here, the cylinder acts as a reservoir endowed with a shell that controls the leakage rate of the encapsulated solute. The transition from steady to unsteady laminar flow regime, around the cylinder, alters the released solute spatial distribution and the mass transfer efficiency, which is characterized by the Sherwood number (the dimensionless mass transfer coefficient). Moreover, the reservoir involves unsteady and continuous boundary conditions, which lead to unsteady and nonuniform distribution of both the concentration and the mass flux at the cylinder surface. The effect of adding a coating shell is highlighted, for a given ratio of the cylinder diameter to the channel width, by extracting a correlation from the computed data set. This new correlation shows explicit dependency of the Sherwood number upon the shell solute permeability (the shell mass transfer coefficient).