Convective mass transfer boundary conditions

The prescription for convective mass transfer boundary conditions is derived in a linearized form as a function of the mass fraction, with the mixture convection flux and the transferred substance-state as the coefficient and value, respectively. It is shown that all the basic boundary conditions of computational fluid dynamics; inlet, outlet, wall, etc. may be essentially seen as specific instances derived from this simple prototype. Details of how to calculate boundary values and gradients from the driving force are provided. In addition to being applicable to mass transfer problems, where the dependent variable is mass fraction or concentration; the theory is also relevant to heat and momentum transfer for fluid flow problems with injection/suction along the boundaries. The key concepts of the convective mass transfer boundary condition are derived for a basic finite-volume cell, with a critical discussion of the magnitude of the errors that are introduced when simplified formulations are employed at the boundaries. This study bridges and extends traditional mass transfer theory and best practices in computational fluid dynamics. • Recommends boundary conditions be prescribed with transferred substance state value. • Generalizes/combines several common boundary conditions (inlets, outlets, walls). • Common numerical schemes are compared with exact solutions. • Wall type mass transfer boundary conditions add unnecessary complication.