Effect of stratification on the mixing and reaction yield in a T-shaped micro-mixer

Abstract

The effect of a small density difference, i.e. lower than 12%, between the two miscible liquid streams fed to a T-junction, is investigated experimentally and through numerical simulations.Micro-PIV experiments allowed detailed support to the numerical analysis of how stratification influences flow features in different flow regimes. From dimensional analysis, we find that gravitational and inertial fluxes balance each other at a distance L = d/ √ Ri from the confluence along the mixing channel, where d is the hydraulic diameter and Ri is the Richardson number. In general, at distances |y| L, the influence of gravity can be neglected, while at |y| L the two fluids are fully segregated; in particular, at the confluence, the flow field is the same as the one that we obtain assuming that the two inlet fluids are identical. Thus, in the segregated regime, the contact region separating the two fluids of the inlet streams remains vertical at distances |y| L along the mixing channel while it becomes progressively horizontal at |y| ≈ L. In the vortex regime as well, near the confluence the flow field presents a mirror symmetry, with a very small resulting degree of mixing; however, as we move down the mixing channel, when |y| > L, gravity becomes relevant, leading to a symmetry breaking that promotes convection and enhances mixing. Further increasing the Reynolds number, in the engulfment regime, the degree of mixing becomes much larger due to the mixing induced by the flow instability at the confluence and thus the successive stratification appears to have a small effect on the flow topology, with a degree of mixing that continues to grow very slowly in the mixing channel, similarly to what happens in the case of identical inlet fluids. As expected, the onsets of the vortex and engulfment regimes occur at values of the Reynolds numberRe that hardly depend on the density difference between the two inlet fluids, provided that Re is defined in terms of the fluid properties of a homogeneous fluid mixture. Finally, the reaction yield along the mixing channel is computed both from numerical and experimental data. In agreement with theoretical predictions, we found that the reaction yield depends on the Damköhler number and the kinetic constant, while it is independent of the density ratio, at least within the range of the investigated conditions.

Effect of Chamber Depth Modifications on Flow Regimes and Mixing Performance in Cross-Shaped Mixers

“…The intensity of segregation (IOS) is used to evaluate the mixing in cross-shaped mixers, which is defined as, ,,, IOS = C ′ 2 ̅ / false( C̅ × ( 1 − C̅ ) false) C = C̅ + C ′ where C is normalized dye concentration, C̅ is normalized average dye concentration, C ′ is normalized dye concentration deviation, and C ′ 2 is normalized dye concentration variance. At least 200 consecutive visualization images on plane z / h = 5 are time-averaged for each IOS in current study.…”

“…Discretized partial differential equations were solved by a finite volume CFD solver FLUENT. For steady flows, the pressure–velocity coupling was treated with the SIMPLE algorithm and a second-order upwind scheme was used for spatial discretization of momentum and pressure equations. ,, The convergence was ensured by low values of normalized residuals for all equations, i.e., less than 10 –12 . , For unsteady flows, the pressure-implicit with the splitting of operators (PISO) was adopted for pressure–velocity coupling. Second-order upwind and pressure staggering options (PRESTO) were chosen for the spatial discretization of momentum and pressure equations, respectively .…”

“…For steady flows, the pressure−velocity coupling was treated with the SIMPLE algorithm and a second-order upwind scheme was used for spatial discretization of momentum and pressure equations. 11,28,34 The convergence was ensured by low values of normalized residuals for all equations, i.e., less than 10 −12 . 8,21 For unsteady flows, the pressure-implicit with the splitting of operators (PISO) was adopted for pressure−velocity coupling.…”

“…As an efficient mixing method, impinging jets have received great attraction in the last two decades. Numerous impinging jets mixers were designed and reported, such as T-, Y-, arrow-, and cross-shaped mixers. − Of these, T-shaped mixers with two opposed inlets and one outlet have been extensively studied. Engler et al, Wong et al, Bothe et al, Soleymani et al, Andreussi et al, and Sultan et al carried out visualization experiments on flow regimes in T-shaped mixers, and three laminar flow regimes, segregated flows, vortex flows, and engulfment flows were detected in the mixing chamber.…”

A Study on the Effect of Flow Unsteadiness on the Yield of a Chemical Reaction in a T Micro-Reactor

“…In our recent works [32,33], we attempted to fill this gap by providing an insight on the role that steady flow regimes play on the performance of a chemical reaction in a T-shaped microreactor. More specifically, we considered different kinetic constants to comprehend the interplay between flow and kinetics.…”