The influence of viscosity on micromixing in turbulent flows

Abstract

Viscosity has a negligible effect, if any, on macromixing parameters (e.g. velocity distribution and blending time when turbulent flow is fully developed). It does, however, influence micromixing parameters (e.g. Batchelor concentration microscale (νD2/ϵ)14 and time constant for decay of fine-scale concentration fluctuations). The product distribution of two rapid, competitive, consecutive reactions (diazo coupling between 1-naphthol and diazotized sulphanilic acid) is sensitive to reagent concentration gradients on the molecular scale. It is shown that if all the independent micromixing parameters are kept constant, except the viscosity, the product distribution changes. The viscosity was varied by dissolving less than 0.5wt.% carboxymethyl cellulose (Hercules, type 7MF) in the aqueous reagent solutions. The viscosity then depends upon many factors (CMC concentration, temperature, shear rate, pH, chemical composition of the solution and mixing sequence during make-up of the solution), and CMC is not an ideal additive. Nothing better seems to be available. At least the spectrophotometric analytical method and the rate constants are unaffected by low CMC concentrations (⪯0.5 wt.%). Three reactors (rotorstator high intensity mixer, flow in a pipe, stirred tank) were operated in the turbulent flow regime. Increasing the viscosity caused more secondary product to be formed. This effect was described quantitatively by our earlier micromixing model.