Unsteady mass transfer from a core-shell cylinder in crossflow

Mass transfer from a composite cylinder - made of an inner core and an outer\nenveloping semipermeable shell - under channel crossflow is studied numerically\nusing two-dimensional lattice-Boltzmann simulations. The core is initially\nloaded with a solute that diffuses passively through the shell towards the\nfluid. The cylinder internal structure and the initial condition considered in\nthis study differ and thus complement the classical studies dealing with\nhomogeneous uncoated cylinders whose surfaces are sustained at either constant\nconcentration or constant mass flux. Here, the cylinder acts as a reservoir\nendowed with a shell that controls the leakage rate of the encapsulated solute.\nThe transition from steady to unsteady laminar flow regime, around the\ncylinder, alters the released solute spatial distribution and the mass transfer\nefficiency, which is characterized by the Sherwood number (the dimensionless\nmass transfer coefficient). Moreover, the reservoir involves unsteady and\ncontinuous boundary conditions, which lead to unsteady and nonuniform\ndistribution of both the concentration and the mass flux at the cylinder\nsurface. The effect of adding a coating shell is highlighted, for a given ratio\nof the cylinder diameter to the channel width, by extracting a correlation from\nthe computed data set. This new correlation shows explicit dependency of the\nSherwood number upon the shell solute permeability (the shell mass transfer\ncoefficient).\n