Analyses of liquid film models applied to horizontal and near horizontal gas–liquid slug flows

An analysis of liquid film models for horizontal and near horizontal gas–liquid slug flows is developed. The models’ formulations employ the one dimensional separated phase momentum equations. The formulations differ among themselves, by neglecting some terms on the momentum balance and also on the closure relations. A comparative analysis discloses the differences amongst the formulations. The sensitivity of the liquid film models to the changes on the bubble velocity, liquid slug holdup and liquid viscosity is accessed through a series of parametric runs. Finally, the model is tested against experimental data taken for continuous horizontal slug flow. The tests were designed to check if the models are able to capture the stochastic film properties provided the properly closure relations.     

Measurement of real-time flow structures in gas–liquid and gas–liquid–solid flow systems using electrical capacitance tomography (ECT)

The real-time cross-sectional distributions of the gas holdups in gas–liquid and gas–liquid–solid systems are measured using electrical capacitance tomography. For the gas–liquid system, air as the gas phase and both Norpar 15 (paraffin) and Paratherm as the liquid phases are used. Polystyrene beads whose permittivity is similar to that of Paratherm are used as the solid phase in the gas–liquid–solid system. The three-phase system is essentially a dielectrically two-phase system enabling measurement of the gas holdup in the gas–liquid–solid system independent of the other two phases. A new reconstruction algorithm based on a modified Hopfield dynamic neural network optimization technique developed by the authors is used to reconstruct the tomographic data to obtain the cross-sectional distribution of the gas holdup. The real-time flow structure and bubbles flow behavior in the two- and three-phase systems are discussed along with the effects of the gas velocity and the solid particles.     

Hydrodynamic forces acting on pipe bends in gas–liquid slug flow

In this paper, a one-dimensional, transient theoretical model, the Piston Flow Model (PFM), based on momentum analysis, is proposed to predict the time dependent forces acting on horizontal pipe bends in slug flow. Our experimental apparatus is described and results there from are presented. The PFM has been validated by comparing its predictions with our experimental results for air–water slug flow. The pressure traces, force traces and maximum force predicted agree well with our measurements.     

Hydrodynamics and local mass transfer characterization under gas–liquid–liquid slug flow in a rectangular microchannel

Gas–liquid–liquid three-phase slug flow was generated in a glass microreactor with rectangular microchannel, where aqueous slugs were distinguished by relative positions to air bubbles and organic droplets. Oxygen from bubbles reacted with resazurin in slugs, leading to prominent color changes, which was used to quantify mass transfer performance. The development of slug length indicated a film flow through the corner between bubbles and the channel wall, where the aqueous phase was saturated with oxygen transferred from bubble body. This film flow results in the highest equivalent oxygen concentration within the slug led by a bubble and followed by a droplet. The three-phase slug flow subregime with alternate bubble and droplet was found to benefit the overall mass transfer performance most. These results provide insights into a precise manipulation of gas–liquid–liquid slug flow in microreactors and the relevant mass transfer behavior thereof.     

Modeling of pressure losses for the slug flow of a gas–liquid mixture in mini- and microchannels

In addition to the previously constructed model of the hydrodynamics of a gas-liquid slug flow, a mathematical model is developed that describes pressure losses taking into account the rearrangement of a velocity profile in liquid slugs and energy losses on the formation and renewal of interfacial area during the motion of bubbles. The contribution of different forms of pressure losses in capillaries is analyzed. It is shown that in microchannels tangential stresses on the surface of a bubble substantially affect the total pressure losses. It is found that the length of bubbles does not affect the rate of surface formation and respective pressure losses if bubbles have the same velocity. The results of modeling are in satisfactory agreement with experimental data of other researchers.     

Mass transfer behavior of liquid–liquid slug flow in circular cross-section microchannel

An alkaline hydrolysis reaction was used as the model reaction to investigate the performance of liquid–liquid slug flow microchannel. The specific interfacial area was determined through the photographic snapshot method physically by means of measuring the lengths of relevant slugs. The overall volumetric mass transfer coefficients were calculated through the Danckwerts’ model chemically. The influences of various operating conditions on the slug length, the overall volumetric extraction rate and the mass transfer coefficient were investigated quantitatively. A decreasing trend of volumetric mass transfer coefficients along the channel length was found. The linear dependence of the volumetric extraction rate on the volumetric mass transfer coefficient indicates that the overall rate of the process is determined by the mass transfer process. In addition, the volumetric mass transfer coefficients were correlated for different channel lengths.     

Characteristics of liquid slugs in gas–liquid Taylor flow in microchannels

The hydrodynamics of liquid slugs in gas–liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (L_s/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity U_TP. The product of the liquid slug residence time and the recirculation rate is independent of U_TP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug.     

Analysis of the flow pattern and periodicity of gas–liquid–liquid three-phase flow in a countercurrent mixer-settler

In contrast to the concurrent mixer-settler, the interaction between the mixing and settling chambers have to be taken into account in the simulation of the countercurrent mixer-settler, and no work has been reported for this equipment. In this work, a three-phase flow model based on the Eulerian multiphase model, coupled with a sliding mesh model is proposed for a countercurrent mixer-settler. Based on this, the dispersed phase distribution, flow pattern, and pressure distribution are investigated, which can help to fill the gap in the operation mechanism. In addition, the velocity vector distribution at the phase port shows an intriguing phenomenon that two types of vectors with opposite directions are distributed on the left and right sides of the same plane, which indicates that the material exchange in the mixing and settling chambers is simultaneous. Analysis of this variation at this location by a fast Fourier transform (FFT) method reveals that it is mainly influenced by the mixing chamber and is consistent with the main period of the outlet flow fluctuations. Therefore, by monitoring the fluctuation of the outlet flow and then analyzing it by the FFT method, the state of the whole tank can be determined, which makes it promising for the design of control systems for countercurrent mixer-settlers.     

Influence of slug flow on flow fields in a gas–liquid cylindrical cyclone separator:A simulation study

A simulation method for slug flow based on the VOF multiphase flow model was implemented in ANSYS? Fluent via a user-defined function(UDF) and applied to the dissipation of liquid slugs in the inlet pipe of a gas–liquid cylindrical cyclone(GLCC) separator while varying the expanding diameter ratio and angle of inclination. The dissipation of liquid slug in inlet pipe is analyzed under different expanding diameter ratios and inclination angles.In the inlet pipe, it is found that increasing expanding diameter ratio and inclination angle can reduce the liquid slug stability and enhancing the effect of gravity, which is beneficial to slug flow dissipation. In the cylinder, increasing the expanding diameter ratio can significantly reduce the liquid carrying depth of the gas phase but result in a slightly increase of the gas content in the liquid phase space. Moreover, increasing the inclination angle results in a decrease in the carrying depth of liquid in the vapor phase, but enhances gas–liquid mixing and increases the gas-carrying depth in the liquid phase. Taking into consideration the dual effects of slug dissipation in the inlet pipe and carrying capacity of gas/liquid spaces in the cylinder, the optimal expanding diameter ratio and inclination angle values can be determined.     

Direct numerical simulation of reactive absorption in gas–liquid flow on structured packing using interface capturing method

Direct numerical simulations are performed in order to study physical and reactive absorption in gas–liquid flow on structured packing. The volume of fluid method is used to capture the gas-liquid interface motion. The mass transport is computed by additional chemical species concentration transport equation with adequate modelling of solubility and chemical reaction. The numerical difficulties arise in imposing jump discontinuity for chemical concentrations at the interface due to different solubility. These difficulties are solved by an original method using a continuum mechanical modelling of two phases flow and Henry's law with constant coefficient. The present study shows how the mass transfer is affected by the complex geometry considered here and by the flow conditions. The results show firstly that the liquid side mass transfer is well predicted by the Higbie theory and the exposure time of a typical element of volume near the interface corresponds to ratio between characteristic length and velocity of the interface provided that the real velocity of the interface is used. For the considered geometry, the transfer is found to be increased compared to the transfer of a plane liquid film. Finally, for the case where the mass transfer is accompanied by second order irreversible chemical reaction in the liquid phase, the numerical results are compared to approximate solution presented by Brian et al. [1961. Penetration theory for gas absorption accompanied by a second order chemical reaction. A.I.Ch.E. J. 7, 226–231] and good agreement is observed.     

Mixing and recirculation characteristics of gas–liquid Taylor flow in microreactors

The effects of operating parameters (capillary and Reynolds numbers) and microchannel aspect ratio (α=w/h=[1;2.5;4]$\alpha =w/h=[1;2.5;4]$) on the recirculation characteristics of the liquid slug in gas–liquid Taylor flow in microchannels have been investigated using 3-dimensional VOF simulations. The results show a decrease in the recirculation volume in the slug and an increase in recirculation time with increasing capillary number, which is in good agreement with previous results obtained in circular and square geometries (Thulasidas et al., 1997). In addition, increasing the aspect ratio of the channel leads to a slight decrease in recirculating volumes but also a significant increase in recirculation times.     

Studies of gas–liquid flow through reactors internals using VOF simulations

Two-phase flow through reactor internals have been experimentally and numerically studied. Experiments have been carried out with a setup running under ambient pressure for two configurations. The first configuration consists of a mixing box orifice inlet through which liquid flows as a film sheared by a gas flow. The liquid height at orifice inlet is documented over a wide range of liquid and gas flowrates. The second configuration consists of the two-phase flow through a downcomer of a distributing tray. Two and three dimensional computational fluid dynamic (CFD) simulations using the volume of fluid approach have been used to compute both flows for similar flow conditions as used in the experiments. It is shown that the agreement between experiments and calculations is very good. Based on this good agreement, it is finally discussed how CFD can be used to achieve better design rules for gas liquid reactor internals via simulations carried out for industrial process conditions.     

Effect of ultrasonic waveforms on gas–liquid mass transfer in microreactors

This study aims to investigate the effect of ultrasonic waveforms on the gas–liquid mass transfer process. For a given load power (P), continuous rectangular wave yielded stronger bubble oscillation and higher mass transfer coefficient (k_La) than continuous triangular and sinusoidal wave. For pulsed ultrasound, the k_La decreased monotonically with decreasing duty ratio (D), resulting in weak enhancement at low D (≤33%). For a given average load power (P_A), concentrating the P for a shorter period resulted in a higher k_La due to stronger cavitation behavior. For a given P_A and D, decreasing the pulse period (T) led to an increase in k_La, which reached a constant high level when the T fell below a critical value. By optimizing the D and T, a k_La equivalent to 92% of that under continuous ultrasound was obtained under pulsed ultrasound at a D of 67%, saving 33% in power consumption.     

Hydrodynamics and velocity measurements in gas–liquid swirling flows in cylindrical cyclones

The gas–liquid swirl flow in a gas–liquid cylindrical cyclone separator has been characterized first qualitatively by flow visualizations. The emerged findings were then confirmed quantitatively by Laser Doppler Velocimetry measurements. The vortex core presents a very complex hydrodynamics, characterized by an alternation between a laminar and a turbulent state. The laminar regime is associated with velocities pointing in the same direction as the mean flow, while the turbulent state induces velocities in the opposite direction, i.e. a flow reversal. These observations give a first understanding of the origin of the double flow reversal regime that is encountered in swirl flows. It is shown that this flow structure appears for high swirl intensities, and results from a frequent laminarization of the vortex core. Results show that, contrary to the commonly assumed hypothesis, this flow structure is associated with good separation performance of the cyclone. Accordingly, we propose the use of multiple tangential inlets to generate the swirl motion in the cyclone, which is supposed to favor the double flow reversal regime, and thus, improve the separation efficiency.     

Numerical simulations of gas–liquid flow in thermal sorption processes

Thermal storage systems, used, e.g., for domestic heating, must be able to compensate the mismatch between supply and demand. The most efficient techniques for thermal storage are based on sorption storage processes. Usually in sorption, the adsorption process occurs in combination with a solid state adsorbent, whereas absorption takes place in a liquid/gas system. During such sorption processes the flow behavior of the carrier medium is crucial for the efficiency of a falling film absorber. In this work the hydrodynamics of the falling liquid film in two geometrical setups, namely on an inclined plane and over two horizontal parallel tubes, is studied. For the simulation the Eulerian–Eulerian model of the software ANSYS CFX and the interFoam application of the open source software OpenFOAM were used. The numerical results of the two geometries were compared with each other and also with existing data from literature to predict the performance of a sorption storage regarding the specific wetted area and the needed height for gravity driven film absorption.     

Modeling of Rayleigh convection in gas–liquid interfacial mass transfer using lattice Boltzmann method

Interfacial Rayleigh convection can be generated by concentration gradient near the interface in mass transfer processes. In the present study, a 2D time-dependent lattice Boltzmann method (LBM) with a double distribution model was established for simulating the liquid-phase Rayleigh convection in the mass transfer process of CO₂ absorption into various solvents. Two random parameters P and C_D denoting respectively the possibility and the magnitude of concentration perturbation at interface were introduced to model the interfacial disturbance, which is known as one of the necessary conditions of onset of Rayleigh convection. The values of the parameters were identified (0.05 ≤ P < 0.3 and 0 < C_D ≤ 10⁻⁹ kg m⁻³) by comparing simulated critical onset times of the Rayleigh convection with the experimental result from Blair and Quinn (1969) and theoretical predictions proposed by Kim et al. (2006) and 0245 and 0250. The maximum penetration depths, maximum transient Rayleigh numbers, and critical times for the onset of Rayleigh convection were obtained by the proposed model. The simulations captured the detailed information of the onset and the temporal–spatial evolution of Rayleigh convection, and gave the concentration contours of typical plume convection patterns which were well consistent with literatures. Enhancement of mass transfer by the Rayleigh convection was also demonstrated by comparing the simulated instantaneous mass flux across the interface with that predicted by penetration theory.     

Computer simulation of gas generation and transport in landfills. IV: Modeling of liquid–gas flow

In the first three parts of this series a three-dimensional (3D) model was developed for transport and reaction of gaseous mixtures in a landfill. An optimization technique was also utilized in order to determine a landfill's spatial distributions of the permeability, porosity, the tortuosity factors, and the total gas generation potential, given a limited amount of experimental data. In the present paper we develop the model further by including the flow of both the leachate and the gases. A 3D dynamic model is developed that accounts for the generation of the four main gases of a landfill, along with the dissolved organic acids and the carbon in the presence of the leachate. The model is then utilized, through extensive numerical simulations, to study the effect of the various factors on the concentrations of the gases and the micro-organism. In particular, we demonstrate the strong effect of the heterogeneities of a landfill, represented by the spatial distribution of the local porosities, as well as an anisotropic distribution of the local permeabilities, on the behavior of a landfill, and in particular pressure buildup in it.     

Nature and characteristics of gas–liquid flow regimes in a micro-packed bed reactor

Micro-packed bed reactors (μPBRs) have the advantages of high heat and mass transfer efficiency and excellent safety, and they have been successfully applied to hydrogenation and oxidation reactions. However, the study of gas–liquid flow regimes in the μPBR, which is essential for the mass transfer modeling and reactor scale-up, is still insufficient due to the limitation of micro-scale and complexity of capillary force. In this work, the flow regimes in the two-dimensional μPBR were systematically studied by visual method utilizing a high-performance camera. Four typical flow regimes and characteristics were captured, and flow regime transition was revealed. Effects of gas and liquid superficial velocities, liquid physical properties, and particle sizes on liquid spreading areal fraction and pressure drop were investigated. Flow regime transition correlation of churn flow and pseudo-static flow in the μPBR was provided for the first time based on the summary of the current and previous published results.     

Modeling of gas–liquid flow in a rotating packed bed using an Eulerian multi-fluid approach

Rotating packed beds (RPBs) are ideal candidates for CO₂ removal from offshore natural gas due to their good mass transfer performance and significant volume savings. This article proposes an Eulerian multi-fluid approach to simulate the gas–liquid flow in RPBs. Three new multiphase drag force models are constructed based on single-phase drag force models for wire mesh packings. Based on the Eulerian multi-fluid approach, a new RPB simulation framework is developed. The predicted results using the new simulation framework with the new drag force models are compared with the experimental data. When using the Kołodziej model and the modified Kołodziej model, the predicted overall liquid holdup shows good agreement with the experimental data with errors less than 20%. In addition, the pressure drop predicted by these three models are reasonable compared with the experimental data. This work lays a foundation for RPB simulation of gas–liquid flow using Eulerian multi-fluid approach.