Search

The T-shaped micro-junction is among the most used geometry in microfluidic applications, and many design modifications of the channel walls have been proposed to enhance mixing. In this work, we investigate through numerical simulations the introduction of one pair of small rectangular cavities in the lateral walls of the mixing channel just downstream of the confluence region. The aim is to preserve the simple geometry that has contributed to spread the practical use of the T-shaped micro-junction while suggesting a modification that should, in principle, work jointly with the vortical structures present in the mixing channel, further enhancing their efficiency in mixing without significant additional pressure drops. The performance is analyzed in the different flow regimes occurring by increasing the Reynolds number. The cavities are effective in the two highly-mixed flow regimes, viz., the steady engulfment and the periodic asymmetric regimes. This presence does not interfere with the formation of the vortical structures that promote mixing by convection in these two regimes, but it further enhances the mixing of the inlet streams in the near-wall region of the mixing channel without any additional cost, leading to better performance than the classical configuration. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Ghost Particle Velocimetry and micron-scale Particle Image Velocimetry are compared. • GPV performance are comparable to μPIV up to Reynolds number 250. • Complex flow structures at a T-junction are analysed by GPV and μPIV. • GPV is a reliable, easier and cheaper alternative to μPIV for micro/milli-fluidic devices. Abstract Ghost Particle Velocimetry (GPV) has only been recently introduced and has already been proven useful in small scale phenomena investigations, such as the study of the flow field during single droplets generation in microfluidic devices. In this work, GPV was used to experimentally investigate fluid flow close to a T-shaped branched junction in a millimetre sized device. The experimental setup allowed for the first time, the study of complex fluid dynamic structures such as vortices and recirculation zones. Several experiments were performed to exploit the capability of GPV in carrying out flow field measurements, at different Reynolds numbers within the laminar flow regime and for two channel sizes. The results were validated by verifying the steady state and stability conditions and by comparing them with results obtained using the well-established micron-scale Particle Image Velocimetry (μPIV). Differences between these two velocimetry techniques were analysed in terms of qualitative and quantitative parameters, to attain a performance comparison and understand the strengths and weaknesses of each respective method. [ABSTRACT FROM AUTHOR]

The process of laminar mixing in a T-shaped micro-device is studied by direct numerical simulation for a model binary mixture, composed of two fluids having the same density and the same viscosity, yet presenting a strong fluidity of mixing effect, i.e. the viscosity of the mixture is a function of its composition. In all cases, the inlet streams remain separated up to a critical Reynolds number, corresponding to the transition from a vortex flow regime, with a double mirror symmetry, to an engulfment flow regime, with a point central symmetry. In the case of a positive fluidity of mixing, the onset of the engulfment regime is accompanied by a sharp increase of the degree of mixing, with the critical Re decreasing as the fluidity of mixing increases. On the contrary, when the fluid mixture has a larger viscosity than that of its pure components, a viscous layer forms at the confluence of the inlet flows, which tends to keep the two streams separated. Therefore, in this case, no sudden increase of the degree of mixing is observed at the onset of the engulfment regime. [ABSTRACT FROM AUTHOR]

Abstract: Numerical simulations were performed to study the mixing dynamics of two miscible liquids within a T-shaped micromixer, comparing the case where the two inlet fluids are both water (W–W) with that where they consist of water and ethanol (W–E). We showed that at smaller Reynolds number, Re<100, there is no vortex formation for either cases; therefore, mixing water and ethanol is slightly easier than mixing water with water because the residence time of the fluid occupying the interfacial region in the W–E case is larger than that of the W–W case. On the other hand, at larger Reynolds numbers, mixing water and ethanol may take considerably longer, as the onset of engulfment is retarded and occurs at larger Reynolds number. All these behaviors are due to the fact that, as a W–E mixture has a viscosity that is up to almost three times larger than that of water, at the confluence of the T-mixer the two streams are separated by a viscous layer of a W–E mixture, that hampers vortex formation and retards mixing. [Copyright &y& Elsevier]

Abstract: The effect of the inlet flow conditions on the degree of mixing in a T-shaped micro-mixer is investigated numerically. Specifically, it is observed that if the flows at the micro-mixer confluence are not fully developed, engulfment occurs at larger Reynolds number, with a different flow pattern and with much lower mixing efficiencies, compared with the case where the inlet flows are fully developed. Thus, for a given micro-mixer geometry, two different engulfment flows may be found as the Reynolds number is increased, corresponding to successive symmetry breaking processes. This work highlights also the importance of both a fine discretization of the micro-mixer as well as of suited convergence criteria for the setting of the numerical simulations. [Copyright &y& Elsevier]

Abstract Despite the very simple geometry, T-shaped micro-mixers are characterized by different flow regimes when operated for liquid mixing. In this work, experiments and direct numerical simulations are employed jointly to investigate such flow regimes. First, a novel methodology is proposed to provide a quantitative comparison with numerical predictions starting from simple flow visualization experiments that use non-collimated light. This method is applied and validated in the steady flow regimes. Moreover, flow visualizations provide for the first time an experimental support to the flow dynamics scenarios for the periodic unsteady flow regimes previously proposed in the literature on the basis of numerical simulations. In particular, at a Reynolds number, based on the bulk velocity and on the hydraulic diameter of the mixing channel, of approximately 225, the flow passes from a steady engulfment regime to an unsteady asymmetric regime, which is characterized by a periodic dynamics of the three-dimensional structures present in the mixer. In this regime, the flow in the mixing channel always remains asymmetric promoting, thus, the mixing between the two streams. By further increasing the Reynolds number, another change in flow dynamics is observed. In the new regime, called periodic symmetric regime, the flow always maintains a double mirror symmetry in the mixing channel and this leads to a dramatic decrease of mixing. The flow unsteadiness is mainly due to a periodic shift of the vortical structures in the top part of the mixer along the direction parallel to the incoming flow. [ABSTRACT FROM AUTHOR]

[Display omitted] • Engulfment regime occurs at Re ≈ 50 , promoting mixing for a wide range of velocities. • The mixing degree grows monotonically in the engulfment regime. • X-shaped reactors enable a superior mixing compared to T-, Y-, and Arrow-shaped ones. Microreactors are very attractive for intensifying chemical and biochemical reactions, as they enable a large heat transfer with high control of the operating conditions. These features allow improving the achievable reaction yield and selectivity. However, the mixing of reactants should be ensured, especially in simple designs, as the flow regime is laminar. This work aims at characterizing the flow regimes and degree of mixing in a X-shaped micromixer with two opposite inlets and at investigating how these regimes are affected by fluid properties. To this purpose we carry out numerical simulations of two different mixtures, i.e. Water-Water (W-W) and Water-Ethanol (W-E), for Reynolds numbers Re ⩽ 200 , to gain insight into the complex three-dimensional flow structures stemming from the impinging jets at the confluence. Besides, experimental flow visualizations from the W-W case provide a validation of the numerical model. The X-shaped reactor is observed to enable a superior mixing performance compared to T-, Y-, and Arrow-shaped microreactors, which are extensively employed. [ABSTRACT FROM AUTHOR]

• Experimental and numerical investigation of a chemical reaction in a T-junction. • Quantification of reaction yield vs. Reynolds and Damköhler numbers. • Decrease of reaction yield with Reynolds number in the segregated regime. • Increase of reaction yield with Reynolds number in vortex and segregated regimes. The effect of flow regimes on the reaction yield of a chemical reaction occurring in a T-shaped micro-reactor is investigated both experimentally and numerically by spanning different Reynolds and Damköhler numbers. The experimental set-up is arranged in order to get in-line information on both fluid dynamics and reaction progress along the mixing channel. Numerical simulations are performed using mesh-adaption cycles to well resolve the mixing between reactants; in this manner, the agreement between experiments and numerical simulations is very satisfactory. For the considered reaction involving two reagents different from water, despite flow regimes are coherent with the ones observed in case of water mixing, significant differences arise because stratification occurs due to the different density of the fluid streams. The dependency of reaction yield on both Reynolds and Damköhler numbers is strongly affected by the flow regimes. In the stratified regime, the reaction yield decreases with the Reynolds number, because the reduced residence time hampers the reaction progress, while in the engulfment regime, the Reynolds number has a positive effect on the yield, thanks to the enhanced mixing. [ABSTRACT FROM AUTHOR]