Experimental and numerical characterisation of mixing in a steady spatially chaotic flow by means of residence time distribution measurements

This work describes an experimental study and a numerical simulation of res idence time distributions (RTD) in a spatially chaotic three-dimensional fl ow. The experimental system is made up of a succession of bends in which ce ntrifugal force generates a pair of streamwise Dean roll-cells. Fluid parti cle trajectories become chaotic through geometrical perturbation obtained b y rotating the curvature plane of each bend +/-90 degrees with respect to t he neighbouring ones. Different numbers of bends, ranging from 3 to 33, wer e tested. RTD is experimentally obtained by using a two-measurement-point c onductimetric method, the concentration of the injected tracer being determ ined both at the inlet and at the outlet of the chaotic mixer. The experime ntal RTD is modelled by a plug flow with axial dispersion volume exchanging mass with a stagnant zone. RTD experiments were conducted for Reynolds num bers between 30 and 13,000. Peclet number based on the diameter of the pipe Pe(D) = (W) over bar D/D-ax) increases with Reynolds number, whatever the number of bends in the system. This reduction in axial dispersion is due to the secondary Dean flow and the chaotic trajectories. Globally, the flowin g fraction increases with Reynolds number, whatever the number of bends, to reach a maximum value between 90 and 100%. For Reynolds numbers between 50 and 200, the flowing fraction increases with the number of bends. The stag nant zone models fluid particles located close to the tube wall. The pathli nes become progressively chaotic in small zones in the cross section and th en spread across the flow as the number of bends is increased, allowing mor e trapped particles to move towards the tube centre. In order to characteri se more completely the efficiency of the device, a criterion is proposed th at takes into account both the mixing characteristics and the pressure drop . The RTD for low Reynolds numbers has also been obtained numerically using a flow model based on Dean's asymptotic perturbation solutions of the mean flow in a curved pipe. At the end of each bend, the velocity field is rota ted by +/-90 degrees before entering the next bend. The RTD is calculated b y following the trajectories of 250,000 'numerical' particles along the dev ice. Numerical results are in good agreement with experiments in the same R eynolds number range. (C) 2000 Elsevier Science Ltd. All rights reserved.