Churn-turbulent Flow Regime Research Articles

Column flotation hydrodynamics operating in the heterogeneous regime estimated by electrical resistance tomography and multi-phase CFD model

ABSTRACT This paper investigates the two-phase flow hydrodynamics of a column flotation operated from a homogeneous to a heterogeneous flow regimes using experimental and computational fluid dynamics (CFD) techniques. Experiments were conducted in a lab scale column flotation of 0.1 m internal diameter and 2.5 m length at various superficial gas velocities using a dual plane electrical resistance tomography (ERT). The mean gas holdup increased from 2 to 18% with the increase in superficial gas velocity from 0.6 to 7.2 cm/s. The gas holdup-based bubble swarm velocity method identified four distinct flow regimes, i.e. homogeneous bubbling regime, narrow discrete bubbling regime, a sustained helical flow regime and churn turbulent flow regime in the column. Further, transient simulations were conducted using a two-fluid model coupled with a population balance model to assess the flow behavior inside the column. Virtual mass, lift and drag forces were incorporated into the model to account for the interphase forces. Numerical simulations predicted mean gas holdup was matching with the experimental data at low gas velocities (<2.4 cm/s), and a marginal deviation was noted at higher gas velocities (>2.4 cm/s). The predicted radial gas holdup profiles were found to change from flatter to parabolic with the increase in gas velocities, i.e. similar to the experiments. The effect of particle size on the rate constant and recovery was estimated with Luttrell and Yoon’s performance model using the particle floatability input data and the two-phase experimental data. The developed CFD model can be useful to optimize the operating and design parameters for efficient operation of the column flotation process.

Identification of Local Isotropic Turbulence Conditions in Various Bubble Columns Based on Several Reliable Parameters

Bubble columns (BCs) are widely used in the chemical industry. In many industrial applications, these important gas-liquid contactors operate in a churn-turbulent flow regime. In principle, it is essential to determine the operating conditions in every BC reactor, in which local isotropic turbulence is established. In this work, it was demonstrated that several different parameters (Kolmogorov entropy, correlation dimension and novel hybrid index) follow a monotonic decreasing trend. This finding could be explained by the constantly increasing coalesced bubble size, which brings more order into the gas-liquid system and thus any entropic or chaotic parameter should decrease with the increase in the superficial gas velocity Ug. The profiles of the new parameters in various gas-liquid systems were studied. They were extracted from different pressure signals (gauge or absolute). In this research, BCs of different diameter and equipped with different gas distributors were used. It was demonstrated that the studied parameters could be successfully correlated with the length scale of the micro eddies and thus the Ug range of applicability of the local isotropic turbulence theory under various operating conditions was indirectly determined. The overall gas holdup profiles were analyzed and, based on the exponent of the Ug value, it was found that in the aqueous solutions of alcohols studied, the conditions in the bubble bed (BB) are homogeneous, whereas in the air-tap water system aerated in different BCs, the conditions in the BB are heterogeneous. This result implies that the local isotropic turbulence conditions predominate mainly around the corresponding measurement positions.

Temperature profiles of a direct contact heat transfer in a slurry bubble column

Slurry bubble columns (SBCs) are used industrially in various applications because of their advantages. In spite of the simple construction of the SBCs, their scale-up analyses are complex due to the effect of various parameters on the hydrodynamic and heat transfer rates in these columns. In SBCs, Direct Contact Heat Transfer (DCHT) includes a highly complicated process of the gas flow through the slurry. In this paper, the slurry and gas temperature profiles of the DCHT process in a SBC with a high temperature gas, is investigated by using Computational Fluid Dynamics (CFD) simulations. The case study that is examined in this paper, is injecting a helium gas at 363 K through a slurry of water liquid and alumina solid particles at 295 K. The CFD simulations of the temperature distributions are performed for churn-turbulent flow regime only. In this paper, different factors that can affect the temperature profiles of the slurry and gas are studied, such as: the superficial gas velocity (Ugs), static liquid height (H), and solid particles concentration (Cs). From the results of this paper, it is found that the average slurry temperature decreases by decreasing Ugs and increases by decreasing H and/or Cs at any given Ugs. In addition, it is noted that the decrease rate of the slurry temperature with increasing the solid concentration is insignificant. The results also show that the gas temperature decreases significantly near the bottom of the column, and the effects of Ugs, H and Cs on gas temperature are negligible.

CFD modeling of the hydrodynamic characteristics of a bubble column in different flow regimes

Gas-liquid two-phase flows are used in industrial systems over a range of flow regimes that have different bubble size distributions. In this work, the Eulerian-Eulerian (E-E) Computational Fluid Dynamics (CFD) model using the discrete bubble population balance model (PBM) with 1, 2 and 3 bubble phases was used to simulate the gas-liquid two-phase flows in different flow regimes in bubble columns. Furthermore, a modified air inlet boundary condition was proposed based on commonly observed dual-bubble size distributions in the churn-turbulent regime. A comparison was made with experimental results from the literature to evaluate the suitability of the proposed numerical model. Different interfacial force models were also tested. In the homogeneous flow regime, the homogeneous discrete population balance model was found to be sufficient. In the churn-turbulent regime, the model with the higher number of bubble phases was found to give better results. The model with modified inlet boundary conditions was able to predict higher bubble number density as expected in the churn-turbulent flow regime and the predicted gas-holdup and axial liquid velocity agree with the experimental data. The model also was used to simulate other geometries from literature.

A correlation for pool entrainment phenomenon

The present work aims to study the phenomena of droplet entrainment from water pool in a confined atmosphere, which could be either a consequence of boiling or depressurization. Eventually, these droplets are entrained by the streaming gas (superficial gas velocity) or settling down due to gravity.The number of correlations developed so far quantify the entrainment based on empirical, semi-empirical and theoretical approaches, but they have been generally limited to a specific flow-regime in the water pool, thermal hydraulic conditions or even to a specific geometry. In this context, the present work aims to develop an empirical correlation to cover the flow regimes from bubbly to churn turbulent, and could be applied to a wide range of geometries. The present correlation shows an increase until a maximum entrainment of about 2.10-4, corresponding to superficial gas velocity of 0.05 m/s for bubbly flow regime, and then a slight decrease to 2·10−5, for the transition regime corresponding superficial gas velocity of 0.1 m/s, and a sharp increase in the churn turbulent flow regime as the gas velocity goes up to 5 m/s.The experimental database used to develop the present empirical correlation covers a broader range of boundary conditions, namely pressure [1 bar–15 bar], water pool thermal condition [subcooled – boiling], vessel diameter [0.19 m- 3.2 m], pool diameter [0.1 m–1.4 m], superficial gas velocity up to 5.0 m/s Therefore, the proposed empirical correlation aims to constitute an important tool to transfer the experimental results to reactor application.

Investigation of air-water two-phase flow characteristics in a 25.4 mm diameter circular pipe

High-quality two-phase flow data are essential for further developing various two-phase flow models and validating two-phase flow codes. The work presented in this paper aims to perform detailed measurements of air-water two-phase flow in a 25.4 mm inner diameter pipe using several techniques developed recently. The gas-phase information is obtained by an integrated method synergizing conductivity probe, high-speed imaging system, and X-ray densitometry system while the Particle Image Velocimetry and Planar Laser-Induced Fluorescence (PIV-PLIF) technique are utilized to measure liquid-phase velocity and turbulence characteristics. Selected conditions in the bubbly, slug, and churn-turbulent flow regimes with the gas and liquid superficial velocities ranging from 0.05 to 4.99 m/s and from 0.10 to 3.88 m/s, respectively, are investigated in this work. The two-phase flow interfacial structures are visualized by both high-speed optical and X-ray images. In fully developed bubbly flows, “wall-peak” void profiles are observed, which is due to the transverse lift force effect. The gas-phase velocities share similar profiles as the liquid-phase velocity but have slightly higher magnitudes due to the buoyancy force effect. In this work, flow development in different flow regimes is investigated as well. As bubbly flows develop along the axial direction, the void profiles may change, and the discrepancy between gas-phase and liquid phase velocities will diminish in the process.

Phase distribution in Fischer-Tropsch mimicked slurry bubble column via computed tomography

This study aims to investigate the phases distribution and hydrodynamics in an air-Therminol LT-glass beads slurry bubble column reactor in bubbly and churn-turbulent flow regimes by implementing a gamma-ray computed tomography. It is found that gas holdup increases with increasing superficial gas velocity and operating pressure while increasing solids loading has an opposite effect on gas holdup measurements. It is noticed that the pressure effect is more significant at high solids loading and the solids loading is relatively more substantial at lower pressure. Solids loading has the most significant a stronger effect on the solids holdup profile than on the gas holdup profile. Current results were compared with the Sedimentation-Dispersion Model (SDM) predictions. The findings of this study advance the knowledge of the true behavior of phases distribution in FT mimicked slurry bubble columns which are pertinent in designing such reactors.

A recirculation cell approach for hydrodynamic and mass transfer modeling in bubble columns with and without internals

An advanced recirculation cell model is proposed, which describes fluid dynamics and mass transfer in bubble columns with and without internals. The new model incorporates the cell approach of Shimizu et al. [Chem. Eng. J.2000, 78, 21–28.] with latest breakup and coalescence kernels. Additionally, the gas flow structure is divided according to the two-bubble class assignment with fast-rising large bubbles in the column center and descending small bubbles near the wall following the liquid circulation pattern within the column’s cross-section. The effect of internals is considered dividing the column further into ‘sub-columns’ derived from the internals’ radial profile, which physically refines the liquid circulation pattern. The model was validated with experimental data of Möller et al. [Chem. Eng. Sci.2018, 179, 265–283; Chem. Eng. Res. Des.2018, 134; Chem. Eng. Sci. 2019] for narrow (0.1 m diameter) and pilot-scale (0.39 m diameter) columns, respectively, with and without internals operated up to the well-developed churn-turbulent flow regime. Predictions for bubble size distribution, total gas holdup, Sauter mean diameter as well as interfacial area and volumetric mass transfer coefficients agree well with the experiments.

The effects of internals and low aspect ratio on the fully developed flow region and bubble properties in a pilot-plant bubble column

This work investigates, for the first time, the effects of the presence of internals and low dynamic liquid levels on the bubble dynamics in industrial-size pilot plant bubble columns. Experimental work that conducted in a bubble column of 0.6 m inner diameter and 3.9 m height with an air-water system was utilizing our advanced four-point fiber optical probe technique to measure the radial profiles of the bubble properties. The superficial gas velocity varied from 0.2 to 0.45 m/s to cover the churn turbulent flow regime. PVC pipe of 0.06 m diameter used to represent the heat exchanging internals, occupying 24% of the column cross-section area, and three different aspect ratios (H/D = 3, 4, and 5) were used. Data obtained show that the presence of internals slightly increases the overall gas holdup and significantly affects the radial profiles and distribution of the bubble properties, particularly in the fully developed flow region. However, the presence of internals increases the local gas holdup, the bubble pass frequency, and the interfacial bubble area, especially in the wall region, while decreases the bubble chord length and the bubble rise velocity. The variation in the aspect ratio (H/D) and the presence of internals exhibited a slight impact on the bubble dynamics in the sparger region, whereas the effect of internal on the local gas holdup was concentrated in the wall region. Meanwhile, the axial location (Z), where the fully developed flow region occurs, appears a high sensitivity toward the internals existence and the variations in the aspect ratio and the superficial gas velocity. However, the presence of internals and the increase in aspect ratio both show that the fully developed flow region begins at lower axial locations, while an increase of the superficial gas velocity delays the transition to fully developed flow to a higher axial location.

Impact of heat exchanging internals configurations on the gas holdup and bubble properties in a bubble column

The effect of the vertical heat-exchanging tube bundle configurations on local gas holdup and bubble dynamics, including specific gas-liquid interfacial area, bubble chord length, bubble rise velocity, and bubble passage frequency have been studied using the 4-point optical fiber probe technique. Two different tube bundle configurations were investigated, circular tube bundle and hexagonal tube bundle. 30 internal tubes, each with a diameter of 0.5 inches (0.013 m), were used in each configuration occupying 25% of the column cross-section area to represent the heat exchanging tubes utilized in the Fischer–Tropsch process. The experimental work was performed in a 0.14 m inner diameter Plexiglas bubble column using an air-water system. The applied superficial gas velocities were based on the free cross-sectional area of the column available for fluid flow and were in the range of 0.02 to 0.45 m/s covering bubbly, transition, and churn turbulent flow regimes. Although the size and the number of the tubes in both configurations were similar, their effects on the hydrodynamics were found to be different. When compared to bubble column without internals, the circular tube bundle showed a significant increase in the local gas holdup in the core region and a decrease in the wall regions. Simultaneously, a substantial increase in the bubble chord length and bubble velocity was seen. Another important observation was the decrease in the interfacial area while using circular tube bundle. A distinct asymmetrical effect on the radial profiles of gas holdup and the specific interfacial area was observed when the hexagonal configuration was used. The gas holdup and interfacial area significantly increased on one side of the column and decreased on the other side. The bubble chord length and bubble rise velocity decreased, exhibiting a narrower distribution with smaller values, in comparison to the bubble column without internals.

Impact of heat-exchanging tube configurations on the gas holdup distribution in bubble columns using gamma-ray computed tomography

An advanced gamma-ray computed tomography (CT) technique was used for the first time to visualize and quantify the impacts of the presence of heat-exchanging tubes and their configurations on the gas-liquid distributions and their profiles in a 6-inch (0.1524 m O.D.) Plexiglas® bubble column in an air-water reactor. Two superficial gas velocities (i.e., 0.2 and 0.45 m/s) were employed to simulate the churn turbulent flow regime. To investigate the impact of vertical internals configurations, three arrangements (i.e., hexagonal, circular without a central internal, and circular with a central internal) were employed in addition to the column with no internals. Using the same sized vertical internals and the same occluded cross-sectional area (CSA), it was found that the configuration of the vertical internals significantly impacted the gas holdup distribution over the CSA of the column. All studied superficial gas velocities resulted in symmetrical gas holdup distributions over the CSA of the bubble columns without vertical internals; however, the columns equipped densely with vertical internals did not have symmetrical gas holdup distributions. The presence of an extra central tube in the circular configuration played a key role in the gas-liquid distribution over the CSA of the bubble column. The hexagonal configuration had the advantage of providing the best spread of the gas phase over the entire CSA of the column. Gas holdup values at the wall region of the bubble column increased with the addition of vertical tubes in all investigated configurations. However, a remarkable increase in the gas holdup values was obtained with the hexagonal configuration. The experimental data (i.e., gas holdup distributions and their diametrical profiles) can help to evaluate and validate three-dimensional (3-D) computational fluid dynamics (CFD) simulations to better predict the hydrodynamic parameters involved in these types of reactors.

Assessment of the impact of bubble size modelling in CFD simulations of alternative bubble column configurations operating in the heterogeneous regime

Modelling bubble columns operating in the churn turbulent flow regime is a considerable challenge due to their complex and transient behaviour. A key modelling decision is how best to model the bubble size distribution, with a range of approaches existing from the relatively simple option of using a single, representative bubble size to more complex population balance approaches. In this comparative study, we have examined various approaches (i.e. constant and variable single bubble size models, dual bubble size models, and both homogeneous and inhomogeneous MUSIG models). Model predictions were compared with experimental data from three different configurations of a pilot-scale column (0.39 m in diameter and 2 m in height) operated at industrially relevant superficial gas velocities of 0.16 and 0.25 m s−1. The single bubble size models gave reasonable agreement with experimental data for the ‘symmetrical’ configuration, with the variable bubble size model having the major advantage of providing acceptable predictions of the mean bubble size (without this having to be provided as a model input). Implementation of multiple bubble size models did not lead to any substantial improvement in the modelling fit with the experimental data, leading to the inescapable conclusion that the standard bubble breakup and coalescence kernels proposed in the literature do not adequately describe bubble transformations taking place under heterogeneous operating conditions.

Influence of the size of heat exchanging internals on the gas holdup distribution in a bubble column using gamma-ray computed tomography

The effects of the presence of the vertical internals of different sizes at a wide range of superficial gas velocity on the overall, local gas holdup distributions and their profiles have been studied and quantified in a 6-in. (0.14 m) Plexiglas® bubble column with air-water system using a non-invasive advanced gamma-ray computed tomography (CT) technique. In this study, two sizes of Plexiglas® vertical internals, having the same occupying area (∼25%) of the column's cross-sectional area (CSA) that represents those used in Fischer-Tropsch synthesis, have been used within a range of superficial gas velocities that cover bubbly and churn turbulent flow regimes (0.05–0.45 m/s). The reconstructed CT scan images revealed that the bubble columns equipped with or without internals displayed a uniform cross-sectional gas holdup distribution (symmetric) for all studied superficial gas velocities. However, the bubble column equipped with 1-in. vertical internals exhibited more uniform gas holdup distribution than the column with 0.5-in. internals. Also, the visualization of the gas-liquid distributions for bubble columns with and without internals reveal that the well-known phenomenon of the core-annular liquid circulation pattern that observed in the bubble column without internals still exists in bubble column packed densely with vertical internals. Moreover, a remarkable increase in the gas holdup values at the wall region was achieved in the churn turbulent flow regime based on the insertion of the vertical internals inside the column as compared with using a bubble column without obstacles. Furthermore, the values of the gas holdup in the core region of the bubble column with vertical internals are similar to those of the bubble column without vertical internals when they are operated at high superficial gas velocity (churn turbulent flow regime), based on the free cross-sectional area (CSA) for the flow. In general, the magnitude of the gas holdup increased significantly with increasing superficial gas velocity for the bubble columns with and without internals. However, the gas holdup profile was shaped like a wavy line in the bubble column with vertical internals, whereas it exhibited a parabolic gas holdup profile in the bubble column without obstacles.

Novel Approach and Correlation for Bubble Size Distribution in a Slurry Bubble Column Reactor Operating in the Churn–Turbulent Flow Regime

A novel approach to obtain bubble size distributions and Sauter mean bubble diameters (d32) in three-phase slurry bubble column reactors (SBCRs) is presented. This approach is based on 720 experimental runs using the dynamic gas disengagement (DGD) technique for various gas–liquid–solid systems inside a pilot-scale-SBCR (0.3 m i.d./3 m height), in a wide range of pressures (1–30 bar), temperatures (298–530 K) and solid concentrations (0–17 vol %). The bubble size distributions were obtained using an iterative energy balance algorithm based on transient experimental pressure drops and forces exerted on bubbles in the disengagement cell. Bubble size distributions were described using log-normal distribution functions and the mean and variance were determined for the DGD runs. The mean and variance were correlated as functions of the system’s physical properties and operating conditions, and were used to predict d32 values with an absolute average relative error (AARE) of 11.72%. The approach was also applie...

Eulerian–Eulerian Modeling of Convective Heat Transfer Enhancement in Upward Vertical Channel Flows by Gas Injection

Two-phase bubbly flows by gas injection had been shown to enhance convective heat transfer in channel flows as compared with that of single-phase flows. The present work explores the effect of gas phase distribution such as inlet air volume fraction and bubble size on the convective heat transfer in upward vertical channel flows numerically. A two-dimensional (2D) channel flow of 10 cm wide × 100 cm high at 0.2 and 1.0 m/s inlet water and air superficial velocities in churn-turbulent flow regime, respectively, is simulated. Numerical simulations are performed using the commercial computational fluid dynamics (CFD) code ANSYS fluent. The bubble size is characterized by the Eötvös number. The inlet air volume fraction is fixed at 10%, whereas the Eötvös number is maintained at 1.0 to perform parametric studies, respectively, in order to investigate the effect of gas phase distribution on average Nusselt number of the two-phase flows. All simulations are compared with a single-phase flow condition. To enhance heat transfer, it is determined that the optimum Eötvös number for the channel with a 10% inlet air volume fraction has an Eötvös number of 0.2, which is equivalent to a bubble diameter of 1.219 mm. Likewise, it is determined that the optimum volume fraction peaks at 30% inlet air volume fraction using an Eötvös number of 1.0.

Calcium alginate beads motion in a foaming three-phase bubble column

Calcium alginate beads are frequently used to immobilize enzymes or microorganisms for fermentations carried out in agitated or pneumatic reactors. In this work, the well-known Radioactive Particle Tracking (RPT) technique is used to non-invasively extract relevant information of the motion of calcium alginate beads within a three phase bubble column under foaming conditions, which frequently appear in bioreactors operation. Special care is taken to produce a radioactive tracer that perfectly matches the features of the other particles in density and size. In addition, the tracer has the same texture and wettability since the adherence of gas to particles in foaming systems is crucial in determining the solid motion. Particles distribution, solid residence time, velocity fields, dispersion coefficients, shear stress and turbulence kinetic energy are determined from the radioactive tracer trajectories. Compared to previous works in non-foaming systems with denser particles, a relatively strong inward flow and less definite descending motion of the solid near the column wall is found. Turbulence intensities and shear stress are high in the disengagement zone, particularly for the churn-turbulent flow regime. However, since the biocatalyst damage would also depend on the actual exposure to harsh regions, the frequency of visit at different location was calculated to estimate maps of exposure risks as the product of turbulence stresses and these frequencies. Considering the particles motion, the region of largest risk for hydrodynamic damage is close to the gas entrance zone.

Study the effect of dense internals on the liquid velocity field and turbulent parameters in bubble column for Fischer–Tropsch (FT) synthesis by using Radioactive Particle Tracking (RPT) technique

In this study, the effects of the dense vertical internals on the liquid velocity field and turbulence parameters (Reynolds stresses, turbulent kinetic energy, and turbulent eddy diffusivities) are experimentally investigated for the first time by using advanced Radioactive Particle Tracking (RPT) technique. The experimental work was carried out in a Plexiglas bubble column with 5.5in. (0.14m) and a height of 72in. (1.83m) for the air-water system. In this work, thirty vertical Plexiglass internals of 0.5in. (0.0127m) outer diameter were used which covered ∼25% (typical for Fischer–Tropsch processes) of the total cross-sectional area of the column where they arranged as the triangular pitch of 0.84in. (0.0214m). The superficial gas velocities based on both total cross-sectional area and free cross-sectional area available for the flow were utilized (0.08, 0.2, and 0.45m/s), which covered the transition and churn-turbulent flow regime to meet the industrial applications of FT synthesis. The experimental data show that the presence of the internals at a given superficial gas velocity causes an increase in the axial centerline liquid velocity and a sharp decrease in turbulence parameters while the increase in superficial gas velocity in the presence of internals causes an increase in axial centerline liquid velocity and turbulent parameters. The obtained data are reliable as a benchmark data for validation computational fluid dynamics (CFD) simulation, and models.

Prediction of interfacial area transport in a scaled 8×8 BWR rod bundle

In the two-fluid model, it is important to give an accurate prediction for the interfacial area concentration. In order to achieve this goal, the interfacial area transport equation has been developed. This study focuses on the benchmark of IATE performance in a rod bundle geometry. A set of interfacial area concentration source and sink term models are proposed for a rod bundle geometry based on the confined channel IATE model. This model was selected as a basis because of the relative similarity of the two geometries. Benchmarking of the new model with interfacial area concentration data in an 8×8 rod bundle test section which has been scaled from an actual BWR fuel bundle is performed. The model shows good agreement in bubbly and cap-bubbly flows, which are similar in many types of geometries, while it shows some discrepancy in churn-turbulent flow regime. This discrepancy may be due to the geometrical differences between the actual rod bundle test facility and the facility used to collect the data which benchmarked the original source and sink models.

CFD investigation of nucleate boiling in non-circular geometries at high pressure

Boiling flows are commonplace in the nuclear industry. Computational Fluid Dynamics (CFD) is slowly beginning to be used to deliver the relevant two-phase thermal hydraulic analyses required for nuclear applications. This paper presents a blind assessment of the capabilities of the commercial CFD code STAR-CCM+ against measurements for a vertically upward mildly subcooled boiling flow approaching saturation in a rectangular channel at a pressure of 41bar. The available measurements comprised transverse distributions and cross-sectional area averages of void fraction at numerous axial positions along the channel. The predictive ability of several combinations of turbulence, wall heat flux partitioning, interfacial area transport and lift force models was tested. In general, good agreement was obtained for the area-averaged void, with the most mechanistic modelling combination reproducing the measurements accurately. Reasonable agreement was also observed for the distributions of transverse void, however this agreement could not be maintained beyond the channel entrance. The transition from near-wall to core void peaking exhibited in the experiments, attributable presumably to a bubbly to churn-turbulent flow regime transition, could not be reproduced accurately with any of the modelling combinations used, and the basic qualitative trend was captured only in part. Suggestions for future investigation are outlined subsequently.

Investigation of the radial effect on the transition velocities in a bubble column based on the modified Shannon entropy

A new parameter for flow regime identification in a bubble column based on the calculation of the Shannon entropy (SE) from different parts of the signal was developed. The bubble column (0.15m in ID) was equipped with a perforated plate distributor (14 holes, Ø 4×10−3m) and operated with an air-deionized water system at ambient conditions. The newly introduced dimensionless ratio of minimum SE to maximum SE was capable of identifying the main transition velocities Utrans at three different dimensionless radial positions (r/R): 0.0 (core), 0.63 (inversion point of axial liquid velocity) and 0.88 (annulus).In the column’s core the new dimensionless SE ratio identified successfully three Utrans values at 0.034, 0.089 and 0.134m/s. They marked the end of the gas maldistribution regime, the onset and the end of the churn-turbulent flow regime, respectively. Three Utrans values (at 0.045, 0.089 and 0.124m/s) were also identified in the annulus. However, the second Utrans value identified the boundary between the first and second transition sub-regimes. The third Utrans value distinguished the onset of the churn-turbulent flow regime. It was found that in the core both the transition and churn-turbulent flow regimes started earlier.At r/R=0.63 the end of the gas maldistribution regime was shifted to a somewhat higher Utrans value (0.067m/s). The second transition sub-regime began at 0.101m/s, whereas the onset of the churn-turbulent regime occurred at 0.124m/s.