Fast liquid mixing by cross-flow impingement in millimeter channels

A fast liquid mixing process was implemented by the cross-flow impingement of thin liquid sheets in the confined mixing channels with the width of millimeter(s). The species transport between the two liquids was studied by visualizing the 2-D concentration field of Rhodamine dye with the planar laser induced fluorescence (PLIF) technique, on which the intensity of segregation (IOS) and the 95% mixing time (τ₉₅) were calculated to evaluate the mixing quality. Due to the reduced spatial scale of liquid mixing and the high energy dissipation rate of ∼1000 to produced by the strong impingement between the liquid sheets, fast mixing of liquids was achieved at a time scale of milliseconds. The effects of operating conditions and the mixer geometry on the mixing behavior were investigated comprehensively by both experiments and computational fluid dynamics (CFD) simulations. Good agreement of the CFD predictions with the experimental data was obtained by the k-? model with species transport, where dependence of the CFD predictions on the turbulent Schmidt number (i.e. Sc_t) was discussed in detail. The results show that for this turbulence-induced mixing procedure the momentum ratio and the cross-flow angle between the two liquids play significant roles in the mixing efficiency. The absolute liquid velocity has little effect on the species transport in space, i.e. the mixing distance to reach IOS of 5%. Nevertheless, the mixing time is shortened at higher velocity conditions. The fluctuation of the transient concentration signals shows stronger interaction at the interface between the two liquid sheets. And the local concentration fluctuations can be well described by the β-PDF (probability density function) model.