Mass transfer and mixing by pulsatile three-dimensional chaotic flow in alternating curved pipes

We present an experimental and numerical study of three-dimensional pulsatile flow in a twisted pipe in order to show the effects of chaotic advection on mixing in this flow configuration. The numerical study is done by CFD code with a pulsatile velocity field imposed as an inlet condition. The experimental setup is composed principally of a “Scotch-yoke” pulsatile flow generator and a twisted duct. The twisted duct consists of six 90° bends of circular cross-section; the plane of curvature of each bend is at 90° to that of its neighbors. The secondary flow, generated by centrifugal force, the pulsating velocity field and also due to the change in curvature plane, leads to irregular fluid particle trajectories. Velocity measurements were made for a range of stationary Reynolds numbers (300 ⩽ Rest ⩽ 1200) and frequency parameters (1 ⩽ α < 20) and for two velocity-amplitude ratios (β); agreement between the numerical and experimental results is satisfactory. In the first bend, for certain control parameter values, the secondary flow becomes more complex due to the pulsation, and in some cases Lyne instability and a siphon phenomenon appear. However, in the other bends, one passes from four cells (Lyne instability) in the first bend with two cells in the other bends. The numerical and experimental study revealed modifications in the trajectories’ evolution due to pulsation. The superposition of an oscillating flow on a stationary curved-pipe flow, in some cases, causes the destruction of the trapping zones (KAM structures). The number of regular zones that disappear with an increase in the number of bends decrease with pulsation frequency and velocity-amplitude ratio. Both these phenomena contribute to the mixing and mass transfer enhancement in the flow.