Gas-liquid Bubbly Flow Research Articles

Simultaneous Measurement of Volumetric Flowrates of Gas-Liquid Bubbly Flow Using a Turbine Flowmeter.

The flowrate measurement of the gas-liquid two-phase flow frequently observed in industrial equipment, such as in heat exchangers and reactors, is critical to enable the precise monitoring and operation of the equipment. Furthermore, certain applications, such as oil and natural gas processing plants, require the accurate measurements of the flowrates of each phase simultaneously. This study presents a method that can simultaneously measure the volumetric flowrates of each phase of gas and liquid two-phase mixtures, Qg and Ql, respectively, without separating the phases. The method employs a turbine flowmeter and two pressure sensors connected to the pipes upstream and downstream of the turbine flowmeter. By measuring the rotational speed of the rotor and the pressure loss across the flowmeter, the flowrate of the two-phase mixtures Qtp = (Qg + Ql) and the gas volumetric flowrate ratio β = (Qg/Qtp) are determined. The values of Qg and Ql are calculated as βQtp and (1 - β)Qtp. This study also investigates the measurement accuracies for air-water two-phase flows at 0.67 × 10-3 ≤ Qtp ≤ 1.67 × 10-3 m3/s and β ≤ 0.1, concluding that the full-scale accuracies of Qtp, β, Qg, and Ql are 3.1%, 4.8%, 3.9%, and 3%, respectively. These accuracies either match or improve the accuracies of similar methods reported in the literature, indicating that the proposed method is a viable solution for the determination of phase-specific flowrates in gas-liquid two-phase mixtures.

Multiphysics simulations of concentric-tube internal loop airlift photobioreactors for microalgae cultivation

In this work, a comprehensive multiphysics model is developed in-house to numerically predict microalgae productivity in concentric-tube internal loop airlift photobioreactors (PBRs). The program integrates solar radiation, light attenuation in microalgae suspensions, cell growth kinetics, and hydrodynamics. The simulations of light attenuation and cell growth kinetics are based on experimental data measured in a PBR. The gas–liquid bubbly flow hydrodynamics is simulated using computational fluid dynamics (CFD) (Ansys FLUENT). The program is employed to predict the annual biomass areal and volumetric productivities of PBRs with various geometrical and operating parameters. The governing equations for the model integration are numerically solved using the finite-volume method for spatial discretization and an explicit scheme for time integration with a time step of PBR liquid circulation time. The effects of plant latitude, shading, clearance between adjacent PBRs, PBR dimension, downtime, and harvesting biomass threshold concentration on the annual biomass productivities are investigated. Under the assumptions made in this study, it is found that the 2.5m-high PBRs yield higher annual areal biomass productivity than the 1.5m- and 2m-high PBRs. The highest productivity is found at a plant latitude of approximately 15°. The optimal harvesting threshold concentration for the highest annual areal productivity is in the range of 0.4–0.7gl−1 depending on the downtime. The optimal concentration increases with longer downtime. Small clearances between adjacent PBRs are preferred to offer higher areal productivity. For PBRs positioned with small clearances, PBRs with smaller diameter-to-height ratios are more advantageous since the higher land utilization efficiency outweighs the increased shading loss.

Overlapping bubble detection and tracking method based on convolutional Neural network and Kalman Filter

Gas-liquid bubbly flow is widely applied in chemical process engineering. Geometric and dynamic parameters of bubbles play an essential role in the numerical prediction of mass and heat transfer processes. However, the critical obstacle in bubble detection is the inability of bubble segmentation and reconstruction when the overlapping issue of multiple bubbles is serious under high void fraction conditions. A new detection and tracking technique for overlapping bubbles was proposed in this paper to identify the overlapped bubbles. First, a novel convolutional neural network is used to detect bubbles. Afterward, the relationship between the detected bubbles in two frames is correlated using the Kalman Filter and neural network. The algorithm achieves 85 % accuracy under high overlap rate conditions in a 10 mm narrow rectangular channel with around 0.1 s for an image. In addition, a comparison test was conducted to evaluate the present technique's accuracy and robustness compared with conventional methods.

Wax Deposition Law under a Gas-Liquid Bubbly Flow Pattern.

Under the high-pressure and low-temperature environment of the deep sea, wax deposition will occur in the wellbore. This study discusses the effect of gas–liquid two-phase bubbly flow on wax deposition. In this study, wax deposition experiments of a gas–liquid two-phase flow in vertical pipelines were carried out using waxy white oil and air as the experimental medium. The results show that under the bubble flow pattern, within a short period of time from the start of wax deposition, it rapidly accumulates to the peak, then rapidly cuts off part of it, and then presents a simple harmonic dynamic trend until the final stability. The average wax-deposit thickness decreases first and then increases with the increase of superficial gas velocity, and it increases first and then decreases with the increase of superficial liquid velocity. The average wax-deposit thickness decreases with the increase of oil temperature. The average wax-deposit thickness was substantially constant and then decreases with the increase of ambient temperature. The wax-deposit thickness increases with the increase of wax content in oil flow. This research is useful for people to further understand the changes in the wax deposition process.

NNumerical investigation of bubble interaction mechanisms in gas-liquid bubbly flows: Harmonisation of bubble breakup and coalescence effects

Numerical investigation of the dynamics of gas-liquid bubbly flow has been performed by using the population balance approach. Using the homogeneous MUSIG model coupled with the two-fluid model, the evolution of bubble size distribution in vertical pipes has been simulated with appropriate consideration of bubble interaction mechanisms. In particular, the study focuses on harmonising the imbalance often associated with breakup and coalescence rates by performing extensive numerical investigations for the examination and comparison of different model formulations to harmonise the coalescence calibration factor of Prince and Blanch (1990) model. To avoid the fine-tuning procedure of selecting calibration factors to harmonise the breakup and coalescence effects, a breakup calibration factor of 1.0 (default setting in ANSYS CFX) was used and three different expressions area-averaged and included as the coalescence calibration factor for the model of Prince and Blanch (1990) modelled using CFX Expression Language (CEL). The performance of the three different “cases” (modifications) has been investigated, compared with each other and validated against the experimental data reported by Monrós et al. (2013). In general, the comparison has shown that all the “cases” yielded satisfactory results when the numerical results were compared with five primitive experimental parameters, namely: gas volume fraction, interfacial area concentration, Sauter mean bubble diameter, gas velocity and liquid velocity. The encouraging results obtained clearly show the capability of the “cases” modelled in capturing the evolution of the bubble sizes. The agreement with the gas volume fraction profiles indicates a level of confidence in the interfacial force models used. Similarly, the agreement seen with the interfacial area concentration indicates that the “cases” modelled for the birth and death processes are reasonably adequate to describe the bubble dynamics. Overall, the comparison shows that the model labelled as “case 2” presented the best results in most of the experimental conditions simulated.

Geometric Effect on Gas–Liquid Bubbly Flow in Capillary-Embedded T-Junction Microchannels

Gas–liquid bubbly flows in T-junction microdevices with different microstructures were investigated in this study. Embedded capillaries were used to modify the microstructures of T-junction microch...

A bubble-induced turbulence model for gas-liquid bubbly flows in airlift columns, pipes and bubble columns

The presence of bubbles dramatically changes the liquid turbulence in bubbly flows. Modifications to liquid turbulent transport equations by adding source terms representing the effect of bubble-induced turbulence (BIT) are usually made to promote the reliability of Computational Fluid Dynamics simulations. However, existing closures for BIT fail to give reliable predictions of both mean flow properties and turbulence kinetic energy. In this work, a new BIT model, which embeds both the bubble swarm effects and the physics of bubble-induced vortices, is proposed to modify the Reynold stress model for bubbly flows. The anisotropy nature of BIT is also considered. The new BIT model can predict both mean flows and turbulence kinetic energy in airlift column systems, while investigated existing BIT models from the literature fail. Three simulation test cases for pipes and one for a bubble column have been conducted to show the validity of the proposed BIT model.

Eulerian-Eulerian large eddy simulation of two-phase dilute bubbly flows

An efficient mathematical model is presented for conducting large eddy simulation (LES) of turbulent bubbly flows at low gas hold-ups. The mathematical model is based on the filtered multi-fluid equations derived by application of the component-weighted volume-averaging process of each phase. A two-fluid model is applied to the filtered set of equations, yielding a more computationally tractable set of filtered equations. The filtered equations are formulated in generalized curvilinear coordinates to simulate turbulent gas-liquid bubbly flows in complex geometries. The Germano model is used to account for sub-grid turbulent stresses. The gas and liquid momentum equations are discretized with the hybrid staggered/non-staggered method. The performance of the developed mathematical model is demonstrated by simulating the benchmark test case of a turbulent gas-liquid bubbly flow in a rectangular bubble column. The accuracy of the proposed method is assessed via comparison with the experimental data and other numerical results in the literature.

Modelling bubble induced turbulence for gas-liquid bubbly flow in a vertical pipe

A two-fluid model in the Eulerian-Eulerian framework has been implemented for the turbulent gas-liquid bubbly upward flow in a vertical pipe. The transport equations, i.e. Reynolds-Averaged Navier-Stokes equations for the two-fluid model, are discretized using the finite volume method. The effect of the disperse gas phase on the liquid phase turbulence, referred to as turbulence modulation, was accounted for through source terms in the transport equations for a low Reynolds number k-ε turbulence closure. The model was used to predict the turbulence kinetic energy and its dissipation rate, mean phasic velocities and volume fraction distribution for a set of test cases selected from the available literature. The focus of the present study was on assessing the model predictions using measurements of the turbulence kinetic energy of the liquid phase. A relatively novel aspect of the present analysis is the use of the budgets of the transport equations for the turbulence kinetic energy and dissipation rate to assess the effect of the turbulence modulation on the liquid turbulence field. Although the present model formulation is shown to adequately predict the effect of bubbles in some flows and perform better than some of the other model formulations, the overall conclusion is that the present approaches for incorporating turbulence modulation are at best partially successful and still need further development.

Image processing techniques for the measurement of two-phase bubbly pipe flows using particle image and tracking velocimetry (PIV/PTV)

Particle Image Velocimetry (PIV) measurement in gas-liquid bubbly flows is a challenging task, mainly due to the dispersion of the laser light caused by the gas-liquid interfaces. A common solution adopted is the use of fluorescent seeding particles associated with a bandpass filter for the laser light. Therefore, the camera captures only the light fluoresced by the seeding particles, filtering the laser light dispersed by the gas-liquid interfaces and measuring a velocity field which corresponds to the liquid phase, seeded with the particles. However, even for relatively low gas fractions, the fluoresced light reflected by the interfaces distorts the measurements, as it illuminates particles out from the laser plane. In addition, the fluoresced light also reflects at the interfaces, distorting the measurements and trending to overshoot the measured liquid velocities In this work, PIV measurements of air-water bubbly flows in a small diameter pipe (D = 26.2 mm) are performed with fluorescent tracer particles (PIV/LIF). A new method for the phase discrimination in PIV was developed to overcome the problems caused by the presence of bubbles in the flow, which uses the pixels intensity information of each interrogation window, to identify if that window corresponds to liquid or bubble region. To validate the PIV procedure, a Particle Tracking Velocimetry (PTV) algorithm was also developed to measure the bubbles velocities, based on similar implementations found in literature, but some corrections were also proposed to overcome the bubble overlap phenomena which arises in bubbly pipe flows, when backward illumination is used. The PIV and the PTV methods were tested and validated by a series of distinct gas void fraction and bubble diameters experimental cases, confirming the accuracy and reliability of both methods. The proposed method was used to analyze a set of upward laminar and turbulent bubbly flows, showing that the averaged axial velocity and the dispersed gas bubbles modify the turbulence intensity profiles.

Void fraction measurements of bubbly flow in a horizontal 90° bend using wire mesh sensors

Wire mesh sensors were used to investigate the void fraction distribution of two-phase gas-liquid bubbly flow in a 50.8 mm diameter horizontal test section containing a 90° bend. The 90° bend had a radius of curvature of 76.2 mm, and its inlet was installed at an axial distance of one hundred pipe diameters from the test section's inlet. Wire mesh sensors measurements were recorded at axial locations of 97.5, 112.8, and 130.8 pipe diameters from the air-water inlet, which included axial locations both upstream and downstream of the 90° bend. Deionised water and compressed air were used as the working fluids, with the bubbly flow regime achieved at superficial liquid velocities ranging from 3.50 m/s to 5.42 m/s and superficial gas velocities, referenced at atmospheric conditions, ranging from 0.10 to 1.25 m/s. The effects of the 90° bend, as well as superficial gas and liquid velocities, on the void fraction distribution were parametrically investigated. Extensive measurements of the area-averaged, time-averaged, and bulk-averaged void fraction are reported for a total of 27 individual test cases. Radial profiles of the time-averaged void fraction are presented for all test cases along both vertical and horizontal axes. Results demonstrated the effects of superficial liquid and gas velocities on phase distribution before and after the bend. Statistical techniques were applied to discretize a transitional bubbly-slug flow pattern into constituent small bubble and large bubble flow patterns.

Analysis of the homogeneous turbulence structure in uniformly sheared bubbly flow

Separating “shear induced turbulence” and “bubble induced turbulence” contributions in gas-liquid bubbly flows is still experimentally difficult to characterize. Numerical simulations were investigated in order to isolate the turbulence produced by the mean velocity gradient of the continuous phase which comprises the turbulence generated in bubbles wakes, from the pseudo-turbulence induced by the bubbles’ displacements. Simulations of the uniformly sheared flow over an isolated sphere are carried out to superimpose the homogeneous turbulence with constant shear at equilibrium conditions with the turbulence produced in the bubble wake. The analyses of the turbulence statistics computed on a control volume whose size is set based on void fractions, for different shear rates, are carried out in order to assess the modelling of the transport equation of the “shear induced turbulence”. Simulation results show that for low void fractions, the hypothesis of turbulence equilibrium in the bubble wake is verified in a wide range of shear rate. Numerical results are in satisfactory agreement with the formulations resulting from the reduction of second order turbulence closures in bubbly flow. These formulations are based on a dimensionless number expressed in terms of two time scales which characterizes the bubbles’ effects on turbulence structure in sheared flow.

Experimental investigation into the drag volume fraction correction term for gas-liquid bubbly flows

Estimates have been made using data from an instrumented pilot-scale bubble column of the volume fraction correction term over a range of local volume fractions (0.03–0.38) and mean bubble diameters (4–10mm) for Bubble Size Distributions (BSDs) of varying ‘broadness’. The value of this correction term was found to depend on both the mean bubble diameter and the dispersity of the BSD, not simply on the local volume fraction. Using the available experimental data, no hindered bubble rise was observed. The results from this study should assist in the development of more accurate predictive CFD models of bubble columns and other gas-liquid systems operated at high local volume fractions.

Simulation of a reacting gas–liquid bubbly flow with CFD and PBM: Validation with experiments

In this work we use computational fluid dynamics (CFD) to simulate a reactive gas–liquid bubbly system in a rectangular bubble column, operating at low superficial velocities (i.e. homogeneous regime). The gas bubbles, injected in the column through a sparger, contain one of the reactants, namely CO2, that via mass transfer moves to the continuous liquid phase, where it reacts with NaOH. A key role is played by the bubble size distribution (BSD) and the specific surface area that define the overall mass transfer rate in the CFD model. In order to correctly predict the BSD and the polydispersity of the bubbly system the population balance equation is solved by the quadrature method of moments (QMOM), within the OpenFOAM (v. 2.2.x) two-fluid solver compressibleTwoPhaseEulerFoam. To reduce the computational time and increase stability, a second-order operator-splitting technique for the solution of the chemically reactive species is also implemented, allowing to solve the different processes involved with their own time-scale. To our knowledge this is the first time that QMOM is employed for the simulation of a real reactive bubbly system and predictions are validated against experiments.

Interfacial area transport due to shear collision of bubbly flow in small-diameter pipes

In view of the practical importance of the interfacial area transport equation in flow analysis based on the two-fluid model, especially for flow analysis in a reactor core which has a relatively small channel configuration, the dominant mechanism in the interfacial area transport in small diameter pipes was firstly identified by evaluating the experimental data obtained for pressurized bubbly flows under microgravity, where the source and sink terms of the interfacial area concentration due to bubble expansion and wake entrainment collision are negligible. The sink term of the interfacial area concentration of bubbly flow in small diameter pipe was modeled in terms of the shear collision mechanism. The one-dimensional interfacial area transport equation with the developed sink term was evaluated by using 74 datasets obtained for adiabatic gas-liquid bubbly flows, including upward flows and microgravity flows in pipes with inside diameters of 9.0, 5.0 and 3.0mm. A detailed discussion was provided on the interfacial area transport mechanism in small-diameter pipe. The overall standard deviation of the absolute error between predicted and measured interfacial area concentrations was 11.8%. The present model will reproduce the interfacial area transport of bubbly two-phase flows in small-diameter pipes with inside diameters of 3.0–9.0mm, at superficial liquid and gas velocities over the ranges 〈jf〉=0.073–0.949m/s, and 〈jg〉=0.00434–0.0500m/s, and also over a wide range of axial distances, z/D=5–188.

Gas-liquid bubbly flow structure in a vertical large-diameter square duct

An experimental study was performed on the local structure of atmospheric upward-moving air–water two-phase flow in a vertical large-diameter square duct with a cross-section of 100 mm × 100 mm and a height of 3 m by using a four-sensor probe at the axial position of z/DH = 29.2. The flow regime was simultaneously observed by a high speed camera at z/DH = 12.5. The four-sensor probe method classifying spherical and non-spherical bubbles was applied as a key measurement way to obtain local parameters such as 3-D bubble velocity vector, bubble diameter and interfacial area concentration, which are necessary for people to know the internal flow structures as well as local void fraction change. The local measurements were performed at 66 points within an octant symmetric triangular area of the cross-section, which agreed well with area-averaged void fractions from differential pressure gauges and the superficial gas velocity from air flow meters. Both the local void fraction and interfacial area concentration indicated radial core-peak and wall-peak distributions at low and high liquid flow rates respectively. Main flow component of the bubble velocity vector showed a power-law distribution shape with its center peak increasing with the liquid flow rate. The 2 components of the bubble velocity vector in the cross-section revealed that there exists a rotating secondary flow in the octant symmetric triangular area and the magnitude of the rotating secondary flow increases with the liquid flow rate. Some of constitutive correlations of drift-flux model for rectangular flow channels and large-diameter circular flow channels and interfacial area concentration correlations for small-diameter and large-diameter circular flow channels are selected and reviewed to study their predictabilities against the present experimental data. The comparison study shows that the drift-flux correlations by Hibiki and Ishii (2003) reasonably predict void fraction and the interfacial-area-concentration correlations by Hibiki and Ishii (2002) and Shen and Hibiki (2015) can be recommended for the prediction of interfacial area concentration in the bubbly flows in large-diameter square ducts.

Ultrasonic method for measuring the gas holdup of gas-liquid bubbly flow in a small-diameter pipe

Based on ultrasonic sound pressure attenuation, the ultrasonic pulse transmission method is proposed for measuring gas holdup in gas-liquid two-phase bubbly flows. Two ultrasonic transducers are positioned on opposite sides of a vertical upward pipe with an inner diameter of 20 mm. To obtain the relationship between ultrasonic attenuation and gas holdup, the mean value of the first pulse sequence of ultrasonic signals is first extracted as the measured signal. We used the quick closing valve method to obtain the gas holdup as the set value. Second, the relationship between the gas holdup and measured ultrasonic signals was established. The experiment result shows that the ultrasonic attenuation rate is significantly different at low and high gas holdups, as indicated by the bubble size images with a high-speed camera. We also investigated the ultrasonic field distribution using numerical simulation. The bubble size has an important effect on the ultrasonic attenuation coefficient, which provides a further physical explanation and reference for the experimental phenomena.

Simulation of gas–liquid flow in a helical static mixer

CFD simulations using the Euler–Euler approach are performed to model the gas–liquid bubbly flow in a helical static mixer. The model validation work was based on experiments, which are carried out in a column of diameter 0.08m packed with helical static mixer elements (length 80mm/diameter 80mm). Measurements of gas volume fractions, gas velocities and bubble size distributions at several planes within the mixer elements by in-house developed ultrafast X-ray electron beam tomography were taken (Rabha et al., 2015. Chem. Eng. J., 15, 527–540).The predicted axial and radial gas phase distribution considering different mono-disperse bubble sizes (3, 5.8 and 8mm) is studied and validated against the experimental results. The dependency of non-drag forces on the bubble size was considered. Consequently, the bubble size dependent effects of the non-drag forces on the flow and on the cross–sectional gas volume fraction distribution are shown.Despite obvious shortcomings of the models for this application, some conclusions on the suitability of certain mixer designs for gas–liquid dispersion may be drawn already. The swirling flows created by the twist and turn of the helical mixer elements, which in turn pushes the lighter phase towards the center of the pipe is well predicted and validated. Further investigations have to consider the bubble size distribution e.g. by a population balance model to accurately predict the dispersion of the gas phase within and downstream the helical static mixer.

F233 2×2ロッドバンドル内気液二相気泡流の液相速度分布(OS9 熱・流動(6))

F233 2×2ロッドバンドル内気液二相気泡流の液相速度分布(OS9 熱・流動(6))

CFD Modeling of Gas-Liquid Bubbly Flow in Horizontal Pipes: Influence of Bubble Coalescence and Breakup

Modelling of gas-liquid bubbly flows is achieved by coupling a population balance equation with the three-dimensional, two-fluid, hydrodynamic model. For gas-liquid bubbly flows, an average bubble number density transport equation has been incorporated in the CFD code CFX 5.7 to describe the temporal and spatial evolution of the gas bubbles population. The coalescence and breakage effects of the gas bubbles are modeled. The coalescence by the random collision driven by turbulence and wake entrainment is considered, while for bubble breakage, the impact of turbulent eddies is considered. Local spatial variations of the gas volume fraction, interfacial area concentration, Sauter mean bubble diameter, and liquid velocity are compared against experimental data in a horizontal pipe, covering a range of gas (0.25 to 1.34 m/s) and liquid (3.74 to 5.1 m/s) superficial velocities and average volume fractions (4% to 21%). The predicted local variations are in good agreement with the experimental measurements reported in the literature. Furthermore, the development of the flow pattern was examined at three different axial locations ofL/D= 25, 148, and 253. The first location is close to the entrance region where the flow is still developing, while the second and the third represent nearly fully developed bubbly flow patterns.