Numerical investigation of mixing performance in microchannel T-junction with wavy structure
Introduction
Over the past decade, micro-mixers have gained considerable attention especially in the chemical processes, pharmaceutical industries, reactive polymerization and nanoparticle precipitations. Apparent advantages of microreactors are highly efficient micromixing, a high area-to-volume ratio, efficient heat transfer ability, the avoidance of hot spots by effective temperature control, and high operational safety for highly exothermic or explosive chemical reactions, compact design and simpler process control. The main concept underlying these mixers is the impinging of two reactants streams in a confined space where mixing proceeds without stirring devices, i.e. passive mixing; however, since small scale mixing depends mainly on molecular diffusion, a relatively long channel is necessary to achieve the desired mixing. Here, mixing is a multiscale processes as it involves various scales phenomena, ranging from microchannel length in the order of ∼1 cm, width and height of microchannel ∼1 mm, flow advection ∼10 μm, diffusivity of liquid ∼1 nm, and chemical reaction ∼0.1 nm.
Numerous experimental [1], [2], [3], [4], [5], [6] and numerical investigations [7], [8], [9], [10], [11], [12] on mixing processes in micro-channel T-junction have been conducted and reported. Extensive reviews on the mixing processes in micro-channel have also been published [13], [14], [15], [16]. Mixing in T-junction has been studied regarding factors such as the influence of the aspect ratio of the mixer, fluid speed, flow regimes and approaches to enhance mixing performance. Sultan et al. [6] identified four flow regimes in the T-junction microchannel:
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Segregated flow regime: a steady flow regime for which two parallel streams of fluid flow from inlet to outlet with hardly mixing; typically, it occurs at the very low Reynolds number.
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Vortex flow regime: this is characterized by the existence of vortices inside each jet and their rotation axis is aligned with the mixing channel axis.
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Engulfment flow regime: steady flow where the flow patterns are not parallel; but, there is a rotation of the jets over the channel axis that transports fluid from one half of the channel to the other half.
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Chaotic flow regime: this is characterized by the formation of a vortex street evolving through mixing channel, which promotes fast mixing of the two jets. The chaotic flow regime is a dynamic flow regime where the jets are engulfed by action of vortices that have a diameter close to half of the mixing channel width and are formed immediately downstream the jet impingement point. The fluids from the two inlet jets are engulfed in the vortex street that evolves from the jet impingement point towards the outlet.
Good mixing performance in a micromixer is an important design factor as it is particularly suited for fast complex chemical reaction where the reaction yields is strongly affected by the mixing quality [17]. In order to enhance mixing, two approaches can be implemented, i.e., passive and active mixing enhancement [13], [16]. For the former, we can make use of the energy of the flow itself by, for example, designing the shape of the channel to stretch and fold the flow [18], and generate chaotic advection in cross-channel micromixer [19]. Moreover, several geometries have been proposed by many researchers, e.g. channel of zigzag shape [20], omega shape [17], converging–diverging shape [21], splitting-recombination configurations [22], [23] and SZ shape mixer [24]. In short, the mixing enhancement can be achieved by carefully designing the channel so that the reactants are forced to flow through channels of special shapes and create vortices and disturb the flow stability which, in turn, leads to the chaotic flow regime. These can increase the interfacial area between fluid segments thus created, and by reducing the diffusion length for heat and mass transport without any additional equipment or energy added to the system.
In an active micromixer, on the other hand, external forces and artifact is added to help enhance mixing [25]; this external force can be introduced using mechanical rotation [26], pulsation of the flow [27], vibration or electrical excitation of the flow [28] and microwaves [29]. Most of the proposed approaches, however, add complexities to the design and manufacturing processes/cost which can be impractical for some applications. Tonkovich et al. [30] showed that the most economical method for mass manufacturing of microreactor is stamping method; however, this method is unable to create complex geometry in three-dimensional shapes as proposed by many researchers to enhance mixing performance. There is thus a need for improved yet easy-to-manufacture design for mixing in micro-channel T-junction with the focus of achieving chaotic flow regime at relatively low Reynolds number with a simple geometry modification. Here, we propose to use microchannel T-junction with wavy structure to create chaotic advection which is expected to improve the mixing performance. The objectives of the work presented here are twofold: (i) to evaluate mixing performance of micro-channel T-junctions with wavy structure relatives to conventional straight microchannel T-junction; and (ii) to study the effect of Reynolds and Schmidt number to the mixing performance at microchannel T-junction with wavy structure.
The layout of the paper is as follows. First, the model development is introduced; it comprises conservation equations of mass, momentum and species for mixing. The mathematical model is then solved numerically utilizing finite-volume based CFD software. The sensitivity of numerical solutions to the grid and discretization scheme is evaluated. Fluid flow and mass transfer (mixing) performance of various designs are evaluated in terms of a “mixing index” (MI) and “Performance index” (PI) defined later. Parametric studies are then carried out for the effect of Reynolds, Schimdt number and Dean number (wavy frequency and amplitude). Finally, advantages and limitations of the design are highlighted, and conclusions are drawn based on the results presented.
Section snippets
Model development
The physical model (see Fig. 1) comprises of two micro-channel T-junction designs, e.g., straight T-junction and T-junction with wavy structure. Both channels are of square cross-section. We assume that liquid A enters the channel from the right inlet (red arrow in Fig. 1); whereas liquid B flows from the left inlet (blue arrow in Fig. 1). Liquid A and B mix in the opposing streams in a T-junction. The channel height and width is 500 μm × 500 μm and channel length is 10,000 μm. For comparison
Numerics
The computational domains (see Fig. 1) were created in AutoCAD 2010; the commercial pre-processor software Gambit 2.3.16 was used for meshing, labeling boundary conditions and to determine the computational domain. Four different mesh designs – 5 × 105, 7 × 106, 2 × 107 and 4 × 107 – were implemented and compared in terms of the mixing index to ensure a mesh independent solution. We found that the mesh numbers around 5 × 105 give about 30% deviation compared to a finer mesh size of 7 × 106, especially at
Results and discussion
The numerical simulations were carried out for typical conditions found in micro-channel T-junctions; the base-case conditions together with the physical parameters and geometric parameters are listed in Table 1. In the following, the performance of microchannel T-junction with wavy structure is compared with that of conventional counterpart, effect of Reynolds and Schmidt numbers are also simulated the on mixing enhancement.
Conclusions
A computational study has been conducted to investigate the mixing performance of micro-channel T-junction with wavy structure relatives to the conventional straight micro-channel T-junction. It was found that microchannel with wavy structure yields superior mixing index and performance index than that of conventional counterpart due to the presence of periodically reversed secondary flow generated by curves wavy structure which leads to a chaotic flow regime. It is also found that the wavy
Acknowledgments
NS and JB: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0025650).
References (36)
- et al.
Biodiesel production from waste cooking oil in a microtube reactor
J Ind Eng Chem
(2013) - et al.
Micromixing efficiency of viscous media in micro-channel reactor
Chinese J Chem Eng
(2009) - et al.
Ideal micromixing performance in packed microchannels
Chem Eng Sci
(2011) - et al.
Chem Eng Sci
(2006) - et al.
Transverse momentum micromixer optimization with evolution strategies
Comp Fluids
(2004) - et al.
Computational fluid dynamics (CFD) software tools for microfluidic applications – a case study
Comp Fluids
(2008) - et al.
Mesoscopic simulation of fluid flow in periodically grooved microchannels
Comp Fluids
(2013) - et al.
Dimensionless number for identification of flow patterns inside a T-micromixer
Chem Eng Sci
(2008) - et al.
Analysis of mixing in a curved microchannel with rectangular grooves
Chem Eng J
(2012) - et al.
Mixing of a split and recombine micromixer with tapered curved microchannels
Chem Eng Sci
(2012)
Single-phase fluid flow and mixing in microchannels
Chem Eng Sci
A state-of-the-art review of mixing in microfluidic mixers
Chinese J Chem Eng
Micromixers – a review on passive and active mixing principles
Chemical Eng Sci
Effects of channel geometry on mixing performance of micromixers using collision of fluid segments
Chem Eng J
Hydrogen production in a zig-zag and straight catalytic wall coated micro channel reactor by CFD modeling
Int J Hydrogen Energy
A novel microreactor with 3D rotating flow to boost fluid reaction and mixing of viscous fluids
Sensor Act B-Chem
Gas/liquid dispersion in a sequential split micromixer
J Ind Eng Chem
Pressure drop and mixing in single phase microreactors: simplified designs of micromixers
Chem Eng Process
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