Flow regimes in T-shaped micro-mixers

The different flow regimes occurring in T-mixers are investigated by means of direct numerical simulations. Three different values of the aspect ratio of the inlet channels, ki, that is their width to height ratio, are considered, namely ki= 0.75, 1 and 2. For the configurations with ki= 0.75 and 1, the same behavior as previously described in the literature, is found. In particular, as the Reynolds number is increased, the flow evolves from vortical to engulfment steady regimes, then to unsteady asymmetric and symmetric periodic regimes, until, finally, it becomes chaotic. All the critical values of the Reynolds number, at which the transitions between the different regimes occur, are found to be very similar for ki= 0.75 and 1, while some differences are highlighted in the vorticity dynamics and characteristic frequencies of the unsteady regimes. The observed scenario is completely different for ki= 2. Indeed, in this case, the flow evolves directly from the vortical regime to an unsteady symmetric behavior, with a vorticity dynamics that is significantly different from those observed for the other aspect ratios

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

Two arrow-shaped micro-mixers, obtained from the classical T-shaped geometry by tilting downward the inlet channels, are considered herein. The two configurations, having different tilting angle values, have been chosen since they show significantly different flow topologies and mixing performances at low Reynolds numbers. In the present paper, we use both experimental flow visualizations and direct numerical simulations to shed light on the mixing behavior of the two configurations for larger Reynolds numbers, for which the mixers present unsteady periodic flows, although in laminar flow conditions. The tilting angle influences the flow dynamics also in the unsteady regimes and has a significant impact on mixing. The configuration characterized by the lower tilting angle, i.e., α = 10°, ensures a better global mixing performance than the one with the larger angle, i.e., α = 20°

Mixing in a T-type micromixer at high Reynolds numbers

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Flow regimes and mixing performance in a T-type micromixer at high Reynolds numbers were studied by numerical solution of the Navier–Stokes equations. The Reynolds number was varied from one to one thousand. The cross section of the mixing channel was 100 μm×200 μm, and its length was 1400 μm. The transverse inlet channels were symmetric about the mixing channel, and their cross-section was 100 μm×100 μm, and the total length was 800 μm. Five different flow regimes were identified: (i) stationary vortex-free flow (Re 400). Maximum mixing efficiency was obtained for stationary asymmetric vortex flow. In this case, an S-shaped vortex structure formed in the flow field. The effect of the slip conditions on the flow pattern and mixing efficiency are studied.The Russian Foundation for Basic Research (Grant No. 10-01-00074) and the Federal Special Program “Scientific and scientific-pedagogical personnel of innovative Russia in 2009-2013” (projects No. P230, No. 14.740.11.0579 and No. 14.740.11.0103)

An Overview of Flow Features and Mixing in Micro T and Arrow Mixers

An overview of the mixing performances of micro T mixers operating with a single fluid is presented. The focus is on the relationship between the flow features and mixing. Indeed, T mixers are characterized by a variety of regimes for increasing Reynolds numbers; they are briefly described, in particular in terms of the three-dimensional vorticity field, which can explain the different mixing performances. The effects of changes in the aspect ratio of the channels are also reviewed. The role of instability and sensitivity analyses in highlighting the mechanisms of the onsets of the different regimes is then described. These analyses also suggest possible geometrical modifications to promote mixing. We focus on that consisting of the downward tilting of the inlet channels (arrow mixers). Arrow mixers are interesting because the onset of the engulfment regime is anticipated at lower Reynolds numbers. Hence, the mixing performances of arrow mixers with varying Reynolds number are described

Micromixing and microchannel design: Vortex shape and entropy

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.In very recent years microdevices, due to their potency in replacing large-scale conventional laboratory instrumentation, are becoming a fast and low cost technology for the treatment of several chemical and biological processes. In particular microfluidics has been massively investigated, aiming at improving the performance of chemical reactors. This is because of the fact that reaction is often an interface phenomenon where the greater the surface to volume ratio, the higher the reaction speed, and microscale mixing increases the interfacial area (in terms of mixing-induced-by-vortices generation). However, microfluidic systems suffer from the limitation that they are characterized mostly by very low Reynolds numbers, with the consequence that (i) they cannot take advantage from the turbulence mixing support, and (ii) viscosity hampers proper vortex detection. Therefore, the proper design of micro-channels (MCs) becomes essential. In this framework, several geometries have been proposed to induce mixing vortices in MCs. However a quantitative comparison between proposed geometries in terms of their passive mixing potency can be done only after proper definition of vortex formation (topology, size) and mixing performance. The objective of this study is to test the ability of different fluid dynamic metrics in vortex detection and mixing effectiveness in micromixers. This is done numerically solving different conditions for the flow in a classic passive mixer, a ring shaped MC. We speculate that MCs design could take advantage from fluidic metrics able to rank properly flow related mixing

Investigation of the symmetry-breaking instability in a T-mixer with circular cross section

This paper investigates the laminar flow inside a T-mixer composed of three pipes with a circular cross section. The flow enters the mixer symmetrically from the two aligned pipes and leaves the device from the third pipe. In similar devices, but involving rectangular channels instead of pipes, an important regime for mixing has been identified, denoted as engulfment. Despite the symmetries of the flow and of the geometry, engulfment is an asymmetric steady regime, which is observed above a critical value (Rec) of the flow Reynolds number. Conversely, for Reynolds numbers lower than Rec, the flow regime is steady and symmetric, and it is usually denoted as the vortex regime. In this paper, both the vortex and the engulfment regimes are identified for the considered geometry, and they are characterized in detail by dedicated direct numerical simulations (DNSs). Despite an apparent similitude with the behavior of T-mixers employing rectangular channels, which are the most investigated T-mixers in the literature, substantial differences are observed and highlighted here concerning both regimes, i.e., the vortex and the engulfment ones, and concerning transition between the two. Global stability analysis is finally used in synergy with DNS to investigate the onset of the engulfment regime, which is shown to be related to a symmetry-breaking bifurcation of the vortex regime

Evaluation of flow characteristics that give higher mixing performance in the 3-D T-mixer versus the typical T-mixer

This document is the Accepted Manuscript of the following article: Cesar Augusto Cortes-Quiroz, Alireza Azarbadegan, and Mehrdad Zangeneh, ‘Evaluation of flow characteristics that give higher mixing performance in the 3-D T-mixer versus the typical T-mixer’, Sensors and Actuators B: Chemical, Vol. 202: 1209-1219, October 2014, DOI: https://doi.org/10.1016/j.snb.2014.06.042, made available under the the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License CC BY NC-ND 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/).A 3-D configuration of a T-mixer is evaluated under normal operating conditions of the called convective micromixers. The design has been called 3-D T-mixer in our previous work [1] as it adopts a three-dimensional structure at the T-junction. This design feature has been found that it exerts a strong effect on the flow characteristics in the device downstream in the mixing channel. A numerical study has been carried out in the 3-D T-mixer and the typical T-mixer, being these modelled with equal dimensions of channel lengths and cross sections and operated with the same flow rates. The flow analysis in the 3-D T-mixer reveals the quick formation of vortical flow structures composed of intertwined fluid filaments which increase drastically the fluids interface to enhance mixing. The flow patterns in the mixing channel vary with Reynolds number (Re) in the range 100-500. This study shows that the 3-D T-mixer provides a significant enhancement of mixing and presents lower pressure loss and similar level of shear stress compared to a typical T-mixer, in the whole range of Re used to characterize the flow. It has a simple channel configuration which is easy to fabricate and effective for mixing of continuous fluid and potentially particles. The 3-D T-mixer is called to be tested and applied for improving the efficiency of systems which have a T-junction in their design and require fast mixing with high throughput.Peer reviewedFinal Accepted Versio

Accessing the Influence of Hess-Murray Law on Suspension Flow through Ramified Structures

The present study focuses on fluid flow and particle transport in symmetric T-shaped structures formed by tubes with circular and square cross-section. The performances of optimized structures (i.e., structures designed based on constructal allometric laws for minimum flow resistance) and not optimized structures were studied. Flow resistance and particle penetration efficiency were studied both for laminar and turbulent flow regimes, and for micrometer and submicrometer particles. Optimized structures have been proven to perform better for fluid flow but they have a similar performance for particle transport

Mixing sensitivity to the inclination of the lateral walls in a T-mixer

One of the simplest geometries for micro-mixers has a T-shape, i.e., the two inlets join perpendicularly the mixing channel. The cross-sections of the channels are usually square/rectangular, as straight walls facilitate experimental and modeling analysis. On the contrary, this work investigates through Computational Fluid Dynamics the effect of a cross-section with lateral walls inclined of an angle α as such an inclination may stem from different microfabrication techniques. Considering water as operating fluid, the same mixing performance as square/rectangular cross-sections is obtained for inclinations α≤3°; this indicates the maximum admissible error on the perpendicularity of the walls in the manufacturing process. Above this value, the presence of inclined walls delays the onset of the engulfment regime at higher Reynolds numbers, and for α≥23°the mixing is hampered dramatically, as the flow is unable to break the mirror symmetry and enter in the engulfment regime. At low Reynolds numbers, the mixing is moderately improved for α≥10°, because the vortex regime presents a lower degree of symmetry than that of T-mixers with straight walls

Mixing enhancement due to time periodic flows in a T-shaped micro-mixer

A Direct Numerical Simulation of the mixing process between two streams having the same flow rate in a T-shaped micro-device indicates that for a certain range of Reynolds numbers, Re, the flow regime might be unsteady, with time-periodic structures. Using a rectangular cross section with a 3:2 aspect ratio at the outlet and assuming fully developed flow at the inlet, pulsating flows are observed when 230<410; for example, when Re= 360, the time periodic structures were characterized by a Strouhal number, St 0.25, in good agreement with recent experimental results. Predictably, these pulsating flows strongly enhance the degree of mixing, that can be as large as twice the one that is measured in the steady engulfment regime. Then, when we increase the Reynolds number beyond the pulsating range, the flow is no more unsteady, with a consequent reduction of the mixing efficiency. In addition, we also observe that for our investigated geometry the unsteady, periodic behavior does not occur when the inlet flow profiles are blunt and not fully developed