Observation of concentration dispersion in unsteady deflected flows

Abstract

The aim of this paper is to investigate the dependence on fluid dynamic condition of the concentration dispersion caused by an oscillatory flow through a channel with ladder-like flow deflectors and in which vortex waves are generated by an oscillatory flow. By varying the amplitude and the frequency of the oscillation the vortex wave may be increased in strength and extended longitudinally so as to control both transverse mixing and axial transport. By directly measuring the distribution of the cross-sectional averaged concentration along the axial direction, concentration dispersion can be modelled as a diffusive process with a constant apparent diffusivity (the dispersion coefficient). This provides a global parameter to indicate the efficiency of mixing and transport. Two physical parameters, the oscillation amplitude and frequency, lead to two non-dimensional parameters, the Reynolds number, Re, and Strouhal number, St. We have presented the experimental results for different Re and St numbers and for different deflector spacing in order to show the effect of eddy motions on dispersion. We show that large dispersion enhancement is obtained when strong vortex waves are generated. Numerical experiments have been used in a parallel study and these provide a quantitative comparison for the dispersion coefficient. The results show that in addition to the controllability of dispersion, measurements for low Reynolds number flows give good agreement between numerical simulation and experimental measurement but poorer agreement for high Reynolds number flows and in channels with long ladder spacing.