Steady flow regimes and mixing performance in arrow-shaped micro-mixers

Abstract

The different flow regimes occurring for increasing Reynolds number (Re) in a arrow-shaped micro-mixer operated for liquid mixing are investigated by the synergic use of experimental flow visualizations and direct numerical simulations. The tilting angle of the arrow-mixer inlet channels with respect to those of the T-shaped mixer is α = 20 • . Consistently with previous studies in the literature, it is found that the onset of the engulfment regime, and consequently the sharp increase of the degree of mixing between the two flow streams, occurs at a lower Reynolds number than for a T-mixer operating in the same conditions. However, in contrast to what observed for T-mixers, the degree of mixing does not increase monotonically with Re. In fact, at approximately Re = 150, based on the inlet bulk velocity and the hydraulic diameter of the mixing channel, there is a drop of the mixing degree. This is due to a change in the topology of the vorticity field, and, in particular to the presence of a unique vortical structure in the mixing channel instead of two co-rotating ones typical of the engulfment regime. By further increasing the Reynolds number, the so originated vortical structure in the mixing channel grows in size, and eventually starts to oscillate, leading again to an increase of the degree of mixing, starting from Re 170. Additional simulations have been carried in order to investigate the sensitivity to the tilting angle of the inlet channels, namely for α = 10 • , 15 • , 25 • . As expected, it is found that the value of Re at which the engulfment regime occurs decreases with increasing α. Then, for α = 10 • and 15 • , the degree of mixing monotonically increases with the Reynolds numbers and the flow topology in the whole engulfment regime is practically the same as for T-mixers. Conversely, the configuration with α = 25 • shows a behavior of mixing and of flow features with increasing Reynolds number very similar to those observed for α = 20 • . therefore, a careful control of the operating conditions is needed for these configurations.

Unsteady flow regimes in arrow-shaped micro-mixers with different tilting angles

“…The anticipation of the engulfment regime onset was found to become more pronounced by increasing the tilting angle α. However, confirming the clues in the work of Galletti et al, 26 a significant drop of mixing was observed within the steady engulfment regime for α larger than some critical value (between 15 ○ and 20 ○ for the configuration investigated in the work of Mariotti et al 27 ). This is not present for T-mixers and low tilting angles.…”

“…The anticipation of the engulfment onset in arrow-shaped mixers is confirmed by the simulations in the work of You et al 22 On the other hand, in the numerical simulations of Galletti et al, 26 a non-monotonic behavior of the degree of mixing with Re was found in arrow-shaped mixers within the engulfment regime, which is conversely not observed for T-mixers. In a recently published paper, 27 a systematic investigation on the steady regimes occurring in arrow-shaped micro-mixers fed by liquids and with varying the tilting angle α was carried out. Experimental flow visualizations and numerical simulations were used together to study mixers having square inlet sections and a rectangular 2:1 outlet.…”

“…The present work is motivated by the results of Mariotti et al, 27 and it is aimed at completing the investigation of the mixing behavior of arrow-mixers for larger Reynolds numbers, for which the unsteady periodic flow is usually present in classical T-mixers. 10,[28][29][30][31] For T-mixers having inlet channel sections with aspect ratio equal or close to one, the first periodic regime occurring by increasing the Reynolds number after the steady engulfment one is the so-called "unsteady asymmetric regime," 29,30 which is interesting from a practical viewpoint because it is characterized by better mixing than the engulfment one.…”

“…In particular, two values of the tilting angle, α, viz. α = 10 ○ ("small" tilting angle) and α = 20 ○ ("large" tilting angle), which were found in the work of Mariotti et al 27 to be characterized by noticeably different flow behavior and mixing in the steady engulfment regime, are considered. A detailed and systematic analysis of the flow behavior and the mixing performance is carried out by considering Re from the ending of the engulfment regime up to the onset of chaotic flow.…”

Mixing Improvement in a T-Shaped Micro-Junction through Small Rectangular Cavities

“…As for the first strategy, arrow-shaped junctions, in which the axis of the inlet channels is tilted downward with respect to the mixing channel, allow us to achieve the onset of the engulfment regime at significantly lower Reynolds numbers compared to T-mixers. However, the mixing performance does not monotonically increase with Reynolds [45]. On the contrary, the onset of the engulfment regime occurs at a larger Reynolds number if Y-shaped junctions are used and, thus, this device appears to be less efficient in mixing [46][47][48].…”