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Numerical analysis of a two-dimensional, axisymmetric, incompressible, laminar liquid-liquid Taylor flow inside a vertical circular microchannel is carried out in the present study. The focus is laid on fluid flow characteristics in the slug flow regime. Although many researchers have performed numerical and experimental studies of two-phase flows in narrow channels, their efforts seem to have been fairly successful in explaining the underlying mechanisms of fluid flow phenomena, especially for slug flow regime. Here, an attempt has been made to explore the hydrodynamics of such flows. In the present study, dodecane and Pd5 have been used as the carrier phases and water is used as the discontinuous phase. The internal diameter of the circular microchannel is 1.5 mm with its wall being insulated. The flow and volume fraction equations are solved by the finite volume approach (FVM). The volume of fluid (VOF) method has been adopted for capturing the interface. The effect of Capillary number on film thickness and interfacial pressure drop is explained. The film thickness is found to increase with Capillary number and is also found to be in a close match with the models available in the literature. The pressure drop per unit length obtained from the CFD study is compared with a standard model available in the literature. The pressure drop across the unit cell is found to be following the phenomenological model. It is observed that the pressure drop at the interface has the highest contribution to the total pressure drop in contrast to the other pressure drops in the channel, with ~50-55% in dodecane-water and ~55-62% in Pd5-water systems. Besides, the distribution of the velocity, axial, and wall pressure fields inside the microcapillary are also discussed.

International audience; Mass transfer in non-reactive gas–liquid Taylor flow has been studied at the unit cell scale with high resolution non-invasive experimental techniques for a large variety of hydrodynamic regimes at high in- ertia (30 ≤ Re b ≤ 1430). The planar laser induced fluorescence with inhibition (PLIF-I) technique has been used to measure the local oxygen concentration fields in different liquid phases (tap water, water and Breox solutions at different concentrations) in order to vary the Schmidt number Sc . The concentration field can be separated into a film region, corresponding to the thin lubrication film extended all along the channel wall, and the remainder of the liquid which makes up the slug region. It has been found that even though the global mass transfer is mainly driven by the rate of transfer in the slug, the film plays a significant role as a source of oxygen, in addition to the bubble caps, to feed the slug. In the investigated circular capillary, fed by means of a T-mixer, two contrasted configurations have been observed in the liquid phase (slugs and films), depending on a critical bubble Reynolds number of ∼300, where the time-averaged concentration fields are found to differ considerably. For large Reynolds number, particle image velocimetry (PIV) measurements have revealed low temporal fluctuations at the rear of the bubble, possibly due to the presence of adsorbed contaminants, that tends to increase mixing in the slug. Despite this difference, the mass transfer dynamics were found to be controlled in all cases by the intensity of the recirculating motion in the slug, which is directly related to the bubble velocity for these cases of thin films. A new scaling law has been proposed for the overall Sherwood number, based on Re b and Sc , which satisfactorily describes the overall mass transfer of the experimental results for Re b > 120 to an accuracy of ±11%.

International audience; Direct Numerical Simulations of mass transfer within Taylor flows are carried out using the periodic unit-cell approach by means of the Level-Set method, under the axisymmetric assumption. The considered cases are based on the experimental study of Butler et al. (2018) (absorption of gaseous species) for bubbles of Reynolds numbers Re_b>200 and capillary numbers Ca>10^{-3}. Firstly, the hydrodynamics of five cases are calculated up to steady state, after which the bubble shape, lubrication film thickness and velocity profiles are compared to experimental and theoretical results. Using these converged hydrodynamics, the transient mass transfer between the gas and liquid phases is then simulated, assuming no change in bubble volume. The Péclet number $Pe$ is varied between 10 and 900 by changing the diffusion coefficient, allowing for new insight into local phenomena of mass transfer. In this way, the maximal transfer fluxes at the interface are observed to be (i) close to the stagnation point at the film entrance, and (ii) at the rear cap where the tangential velocity is greatest. As once as the mass transfer coefficient becomes constant, the fluxes across the part of the interface in contact with the film and around the bubble caps are each characterised by a local Sherwood number. The latter evolves by Pe_{film}^{1/2} across the film and is found to be predictable by a simple model when Pe_{film} > 1, where Pe_{film} is the film Péclet number. Concerning the caps, it evolves by Pe^{1/2} but only in a finite range of Pe, contrary to the common assumption of similarity of transfer around the caps with that around a rising unconfined spherical bubble. Such local analyses could be further used in multizone models of mass transfer for Taylor flows.Finally, a correlation is proposed to scale the global Sherwood number Sh_{\infty} far from channel inlet, defined as a function of both a Péclet number Pe_R based on the relative velocity between the bubble and the two-phase flow and the gas volume fraction in the unit cell. Its predictions are discussed against experimental results at much higher Péclet numbers, after showing that Sh_{\infty} is independent on the initial concentration distribution in the liquid (the latter being sensitive to the injection conditions in experiments).

Taylor flow is a strategy-aimed flow to transfer conventional single-phase into a more efficient two-phase flow resulting in an enhanced momentum/heat/mass transfer rate, as well as a multitude of other advantages. To date, Taylor flow has focused on the processes involving gas–liquid and liquid–liquid two-phase systems in microchannels over a wide range of applications in biomedical, pharmaceutical, industrial, and commercial sectors. Appropriately micro-structured design is, therefore, a key consideration for equipment dealing with transport phenomena. This review paper highlights the hydrodynamic aspects of gas–liquid and liquid–liquid two-phase flows in microchannels. It covers state-of-the-art experimental and numerical methods in the literature for analyzing and simulating slug flows in circular and non-circular microchannels. The review’s main objective is to identify the considerable opportunity for further development of microflows and provide suggestions for researchers in the field. Available correlations proposed for the transition of flow patterns are presented. A review of the literature of flow regime, slug length, and pressure drop is also carried out.

New theoretical approach to model Gas-Liquid and Gas-Liquid-Solid mass transfer for two-phase Taylor flows in milli and microchannels is proposed. The main idea of the proposed mathematical model is to take into account not only diffusion but the convection inside of liquid slugs caused by circulation within Taylor vortices and the convective transport from the film surrounding the bubble to the slug induced by the film movement. For that aim a three-layer model elaborated earlier by Abiev (CEJ, 2013) is used to decompose the liquid slug onto three interacting layers. Diffusion within the film around bubbles and the transit film belonging to the slug, as well as diffusion from the bubble caps was included in the model as well. Experimental data of Bercic & Pintar (CES, 1997), Butler et al. (CHERD, 2016; IJMF, 2018) and Haase et al. (TFCE, 2020) used for proof have shown quite better coincidence with the proposed model than the other available models. Special tests without convective term have demonstrated much less accuracy with the experimental data confirming the huge role of convection in the overall gas-liquid and gas-liquid-solid transport. The correlation between frequency of circulations within Taylor vortices and overall gas-liquid mass transfer coefficient found recently (Abiev et al., CEJ, 2019) was confirmed by use of Bercic & Pintar (CES, 1997) data. In the frame of the moving bubble the velocity of the transit film equals to the absolute value of two-phase velocity. That is why the convective term significantly influences the total mass transfer in the film and overall gas-liquid transport as well.

International audience; A strategy for the scale-up of a monolith reactor dedicated to gas-liquid catalytic reactions is worked out; focus is made on the crucial step of gas-liquid mass transfer modelling via a steady-state numerical study based on a single channel and single unit cell representation, using a frame moving with the bubble and solving the liquid phase only. The relevance of this simplified approach is assessed through a specific case (given bubble shape, channel diameter and fluid flow rates), and hydrodynamics as well as mass transfer results are successfully compared to previously published numerical, semi-analytical and experimental works. Influence of unit cell length and of catalytic surface reaction rate is thoroughly investigated. Inferred overall mass transfer coefficients are found to increase with bubble frequency and resulting higher interfacial area in unit cell and intensified recirculation in slug. Film contribution to mass transfer is proved dominant in the case of short bubbles with reactive wall, and hardly varies with reaction rate. However, this contribution is strongly linked to bubble frequency, and a reliable evaluation of local mass transfer by correlations demands accurate knowledge on the precise dimensions of bubble, slug and film entities.

International audience; This paper deals with the comparison of Eulerian methods to take into account the capillary contribution in the vicinity of a fluid–fluid interface. Eulerian methods are well- known to produce additional vorticity close to the interface that leads to non-physical spurious currents. Numerical equilibrium between pressure gradient and capillary force for the static bubble test case within a VOF framework has been reached in [35] with the height-function technique [14,35]. However, once the bubble is translated in a uniform flow, spurious currents are maintained by slight errors induced by translation schemes. In this work, two main points are investigated: the ability of Volume of Fluid and Level Set methods to accurately calculate the curvature, and the magnitude of spurious currents due to errors in the calculation of the curvature after advection in both translating and rotating flows. The spurious currents source term is expressed from the vorticity equation and used to discuss and compare the methods. Simulations of gas–liquid Taylor flow at low capillary number show that the flow structure and the bubble velocity can be significantly affected by spurious currents.

Gas-liquid two-phase flow in a microfluidic T-junction with nearly square microchannels of 113 mu m hydraulic diameter was investigated experimentally and numerically. Air and water superficial velocities were 0.018-0.791 m/s and 0.042-0.757 m/s, respectively. Three-dimensional modeling was performed with computational fluid dynamics (CFD) software FLUENT and the volume of fluid (VOF) model. Slug flow (snapping/breaking/jetting) and stratified flow were observed experimentally. Numerically predicted void fraction followed a linear relationship with the homogeneous void fraction, while experimental values depended on the superficial velocity ratio U-C/U-L. Higher experimental velocity slip caused by gas inlet pressure build-up and oscillation caused deviation from numerical predictions. Velocity slip was found to depend on the cross-sectional area coverage of the gas slug, the formation of a liquid film and the presence of liquid at the channel corners. Numerical modeling was found to require improvement to treat the contact angle and contact line slip, and could benefit from the use of a dynamic boundary condition to simulate the compressible gas phase inlet reservoir. (C) 2009 Elsevier Ltd. All rights reserved. ispartof: International Journal of Multiphase Flow vol:36 issue:4 pages:314-323 status: published

Many countries in recent year have focused on small satellite launch. Therefore, Solid Rocket Motor (SRM) plays an important role in the development of small satellites. However, vortical flows sometimes occur from an inhibitor in the propellant, which causes the pressure oscillation. The basic research was carried out to compute various types of simplified SRM model. In this study, Cartesian mesh-based CFD solver, Building-Cube Method (BCM), was employed with the implementation of internal flow boundary condition, and applied for two Taylor Flow models, CDV nozzle and Simple SRM model with/without an inhibitor. According to two Taylor flow model result, the BCM was able to analyze the internal flow in the simplified SRM combustion chamber. The BCM is proved to be valid for nozzle model because the result of CDV nozzle was qualitatively agreed with theoretical solution. According to the result of the simple SRM model, the BCM was able to predict flowfield in the SRM combustion chamber, even with the existence of an inhibitor. Throughout the computations, BCM solver was able to reproduce flows in the SRM combustion chamber, thus it is expected the approach has a great potential to apply for the internal flows in the complicated SRM combustion chamber.

The performance of heterogeneous catalysts in multiphase reactions in general is governed by different types of extrinsic and intrinsic structural effects on all length scales, i.e., on the macro- (m to cm), meso- (mm to µm), and microlevel (nm). This PhD research, with a catalysis-engineering approach, focused on several of these multiscale structure-performance relationships of supported palladium (Pd) catalysts applied in, industrially important, multiphase hydrogenations. The structure-performance relationships were studied in various batch and continuous reactors of which most are related to important topics in process intensification such a monolithic reactors and flow chemistry. The performance of monolithic Pd catalysts was enhanced by combining a new type of structured highly porous monoliths with a pressure pulse generating gas-liquid flow (i.e., Taylor flow). This induced a convective flow inside the ‘open’ monolith walls thereby enhancing the mass exchange with the Pd catalyst. This favourable result opens the avenue to higher catalyst loadings without increasing internal mass transfer limitations. Furthermore, a proof of concept study showed that a cheap and readily available gas chromatography capillary, wall-coated with an alumina-supported Pd catalyst and operated in the Taylor flow regime, can be used to synthesize high-value products and to rapidly produce (visual) information about catalytic hydrogenations. This Pd capillary flow device is an excellent alternative for expensive microchip technology and bulky round-bottom flasks. Finally, the intrinsic property of Pd to absorb hydrogen into its crystal lattice was shown to have a strong influence on its performance in the hydrogenation of aromatic nitriles. The transformation into stable Pd ?-hydride above a certain threshold hydrogen pressure induced a persistent change in activity and by-product selectivity.

This article is a comprehensive overview of gas-liquid and liquid-liquid mass transfer in microstructured reactors (MSR). MSR are known to offer high heat and mass transfer rates for two phase systems due to high surface to volume ratio as compared to conventional reactors. The reactions with fast kinetics controlled by mass transfer have been successfully intensified using MSR. The first part of the review deals with the methods of mass transfer characterization. Further, different dimensionless parameters used to analyze mass transfer in MSR are discussed. The literature data with different flow regimes and proposed empirical correlations for both gas-liquid and liquid-liquid systems is also presented. The conventional mass transfer models such as penetration and film theory are analyzed. Finally, the important issues of mass transfer in MSR are summed up. (C) 2011 Elsevier Ltd. All rights reserved.

An innovative Flexible Coupled Level Set (LS) and Volume of Fluid (VOF) algorithm (flexCLV) to simulate two-phase flows at the microscale on unstructured and non-uniform meshes is proposed. The method combines the advantages of the VOF method in terms of mass conservation and the LS method in terms of accuracy of the surface tension implementation and can handle both 2D and 3D domains discretized by either structured hexaedra or unstructured tetrahedral grids with high aspect ratio elements, thus guaranteeing flexibility and robustness. The method is implemented within the VOF-based OpenFOAM's solver interFoam, which is retained as the base algorithm for the interface advection, while the surface tension force is calculated by using the level set function reconstructed from the VOF's fraction. The method is first validated in static flow conditions by simulating a circular bubble at equilibrium and then in dynamic flow conditions by studying a freely bubble rising in both 2D and 3D domains discretized by both structured and unstructured meshes. The proposed flexCLV algorithm is then used to simulate the dynamics of confined bubbles in circular microchannels in the low capillary number regime. 2D and 3D mesh grids with high aspect ratio elements are utilized to discretized the liquid film at the tube's walls. The numerical results are compared with the available literature and simulations performed with the original interFoam solver in terms of bubble shape and velocity, thickness of the liquid film and amplitude of the bubble tail oscillations. Results compare very well with the experimental measurements and demonstrate the superior accuracy of the coupled flexCLV method with respect to the original VOF method when surface tension and accurate interface representation play a fundamental role. Importantly, the present study also provides a precious insight on the time-dependent patterns appearing on the bubble surface in the visco-inertial regime, which could be here investigated in detail. (C) 2017 Elsevier Ltd. All rights reserved.