Mixing enhancement by pulsating chaotic advection

The purpose of this study is to investigate transverse mixing enhancement by superposition of periodic time dependence, in the form of pulsation, on a twisted pipe flow in which the fluid particles trajectories are spatially chaotic. The pulsation makes the secondary flow structure more complex, resulting in stronger velocity gradients that enhance stretching and folding, the main mechanisms of chaotic mixing. Here, the chaotic configuration is six alternating 90° curved pipes. The imposed pulsating conditions range as follows: steady Reynolds numbers 420 ≤ Rest ≤ 1000, velocity component ratios 1 ≤ (β = Umax,osc/Um,st) ≤ 4 and frequency parameters 8.37 < (α = r0(ω/ν)0.5) < 24.5. The secondary velocity fields are measured by particle image velocimetry. The axial vorticity and transverse strain rate at the outlet of each curved pipe in pulsatile flow are compared with those of the steady flows. Analysis of these criteria for mixing assessment shows that β ≥ 2 and α ≤ 15 are favourable pulsating conditions for transverse mixing enhancement. Moreover, in some pulsation conditions, the cell centres visit a zone in the flow cross-section that is much larger than in the steady case, implying that pulsation also contributes to mixing homogenization.