TRANSIENT AND STATIONARY AXIAL-DISPERSION IN VORTEX ARRAY FLOWS .1. AXIAL SCAN MEASUREMENTS AND MODELING OF TRANSIENT DISPERSION EFFECTS

Whereas conventional RTD experiments yield incomplete and potentially misleading information, the axial scan method is shown to be a powerfu l technique to analyze the transient dispersion effects in vortex arra y flows (VAFs). Applying different initial tracer distributions to a g iven vortex, and making axial scans of the spreading tracer distributi on, allowed, for the first time, to quantify and classify the complete set of strongly different transient dispersion modes. As a model syst em, the laminar Couette-Taylor flow has been selected. By working unde r high viscosity conditions, the time scale of the different acting ph enomena has been enlarged to such an extent that even the fastest disp ersion events could be extensively studied and quantified. It is shown that in laminar VAFs effective axial dispersion coefficients can be o btained which vary over orders of magnitude, just by applying differen t initial tracer distributions to a given vortex. A first principles t wo-dimensional model (valid when the mixing along the streamlines occu rs fast) with which all observed transient dispersion effects can be a ccurately represented is proposed. The insights obtained in the presen t study are especially useful for the development of VAF reactors for the treatment of strongly viscous fluids. (C) 1997 Elsevier Science Lt d. All rights reserved.