Mass transport analysis of non-Newtonian fluids under combined electroosmotically and pressure driven flow in rectangular microreactors

Abstract

Hydrodynamically fully developed flow of power-law fluids under combined action of electroosmotic and pressure gradient forces in rectangular microreactors is analyzed considering heterogeneous catalytic reactions. The Poisson-Boltzmann, Cauchy momentum, and concentration equations are considered in two dimensions and after being dimensionless are numerically solved applying a finite difference algorithm. Variation of axial concentration gradient, and axial and horizontal mass diffusions are taken into account as well. To accomplish a more general analysis, the velocity distribution is obtained by solving continuity and Cauchy momentum equations and is not considered as an average axial velocity for solving the concentration equation. Profiles of mean concentration of reactants are analyzed as functions of operating variables. Results reveal that mean concentration increases with increasing Péclet number and dimensionless Debye-Hückel parameter. It is also a decreasing function of Damkohler number. Presence of pressure gradient will affect the decreasing trend of mean concentration through the channel. Shear-thinning fluids, due to their nature, are most strongly influenced by pressure gradient and Newtonian and shear-thickening fluids are less affected, respectively. Favorable pressure gradient will decelerate mean concentration rate of decrease and adverse pressure gradient acts the opposite. Moreover, a larger aspect ratio leads to slower descending rate of mean concentration in purely electroosmotic and pressure-assisted flows while in a pressure-opposed flow, the behavior might change depending on the strength of pressure gradient and the value of channel aspect ratio.