Vertical Gas-liquid Two-phase Flow Research Articles

Study on characteristics of phase-isolation caused by different classic axial flow swirlers in upward vertical gas-liquid two-phase flow

The characteristics of phase-isolation of annular flow caused by axial flow swirler are very critical for separation efficiency of axial inlet separator and accuracy of phase-isolation based measurement method. However, there are lack of comparative study among different types of swirlers and research focusing on characteristics of phase-isolation. In this paper, the phase-isolation characteristics caused by four different classic axial flow swirlers with the same main structural parameters were firstly experimentally studied in upward vertical gas-liquid two-phase flow. Five flow patterns were proposed after obvious evolution in phase distribution at downstream of the swirler, including gas-column flow, swirling annular flow, central bubble flow, central bubble-column flow, and variable diameter gas-column flow. According to the different phase-isolation characteristics, flow patterns downstream of the swirler were divided into stable phase-isolation, quasi-phase-isolation, and non-effective phase-isolation. The corresponding flow pattern maps were also obtained respectively. The experimental results also show that the swirler with good phase-isolation performance generally requires a larger pressure drop as the cost. Finally, the flow in the swirlers was studied numerically. It is found that although the outlet angle of the swirlers is the same, different types of swirlers will result in flow with different deflection angle, and eventually cause the different phase-isolation performance.

Performance model evaluation of turbine flow meter in vertical gas-liquid two-phase flows

Aiming at the need for flow measurement of gas-liquid flows in domestic gas well production, this paper proposes a measurement method based on the combination of the turbine flow meter (TFM) and a rotating electric field conductance sensor (REFCS). In experiments, the REFCS is used for the measurement of the gas holdup. To verify the applicability of the TFM models investigated in the previous study, for the modeling part, the mass, momentum and torque models are evaluated in vertical upward gas-liquid two-phase flows. In our model test, the meter factor model of TFM considers the effects of the slip ratio between the gas and liquid phases and flow patterns. In particular, the gas holdup involved in calculating the slip ratio in the model evaluation is obtained from the REFCS measurements. Model test results show the torque model has better volumetric flow rate prediction accuracy than the mass and momentum models. In the present study, the ranges of the liquid and gas phases are Qw = 2–30 m3/d and Qg = 1–16 m3/d, it was found that the average absolute deviation (AAD) in the predicted volume flow rate is equal to 1.23 m3/d and the average absolute percentage deviation (AAPD) is equal to 7.69%. The evaluated results presented in this paper will allow better estimates of the volumetric flow rates of gas-liquid flows based on the combined TMF and REFCS measurements during the monitoring of gas well production.

Comparative Analysis of Riser Base and Flowline Gas Injection on Vertical Gas-Liquid Two-Phase Flow

Gas injection is a frequently used method for artificial lift and flow regime rectification in offshore production and transportation flowlines. The flow behaviour in such flowlines is complex and a better understanding of flow characteristics, such as flow patterns, void fraction/hold up distributions and pressure gradient is always required for efficient and optimal design of downstream handling facilities. Injection method and location have been shown to strongly affect downstream fluid behaviour that can have important implications for pumping and downstream facility design, especially if the development length between pipeline and downstream facility is less than L/D = 50 as reported by many investigators. In this article, we provide the results of an experimental investigation into the effects of the gas injection position on the characteristics of the downstream upwards vertical gas flow using a vertical riser with an internal diameter of 52 mm and a length of 10.5 m. A horizontal 40-m-long section connected at the bottom provides options for riser base or horizontal flow line injection of gas. The flowline gas injection is performed 40 m upstream of the riser base. A 16 by 16 capacitance wire mesh sensor and a gamma densitometer were used to measure the gas-liquid phase cross-sectional distribution at the riser top. A detailed analysis of the flow characteristics is carried out based on the measurements. These include flow regimes, cross-sectional liquid holdup distributions and peaking patterns as well as analysis of the time series data. Our findings show that flow behaviours differences due to different gas injection locations were persisting after a development length of 180D in the riser. More specifically, core-peaking liquid holdup occurred at the lower gas injection rates through the flowline, while wall-peaking holdup profiles were established at the same flow conditions for riser base injection. Wall peaking was associated with dispersed bubbly flows and hence non-pulsating as against core-peaking was associated with Taylor bubbles and slug flows. Furthermore, it was found that the riser base injection generally produced lower holdups. It was noted that the circumferential injector used at the riser base promoted high void fraction and hence low liquid holdups. Due to the bubbly flow structure, the slip velocity is smaller than for larger cap bubbles and hence the void fraction is higher. The measurements and observations presented in the paper provides valuable knowledge on riser base/flowline gas introduction that can directly feed into the design of downstream facilities such as storage tanks, slug catchers and separators.

Reconstruction of Taylor Bubbles in Slug Flow Using a Direct-Image Multielectrode Conductance Sensor

Gas-liquid two-phase flows are widely encountered in industrial processes. To realize the dynamic detection of gas-liquid slug flows, a direct-image multi-electrode conductance sensor (DMCS) is optimized using the finite element method to access high measurement sensitivity. Through carrying out an experiment of vertical gas-liquid two-phase flows, the gas-liquid interfaces in slug flows are detected by the DMCS and compared to the results from a planar laser-induced fluorescence (PLIF) system. The interfaces detected by the DMCS and the PLIF system indicate a good agreement. By selecting an optimal threshold, the liquid film thicknesses around Taylor bubbles (TBs) are derived, and the asymmetry of circumferential film thickness distribution is investigated. Besides, the local cross-correlation velocities at circumferential pipe wall are measured by the DMCS. Finally, the TBs in slug flows are reconstructed based on the thickness and the velocity of the TBs. The reconstructed TBs allow for a clear indication of gas-liquid slug flow structures.

Experimental study on radial evolution of droplets in vertical gas-liquid two-phase annular flow

In vertical gas-liquid two-phase annular flow, the radial evolution of droplets in the gas core plays a key role in affecting the characteristic of gas carrying liquids. In this paper, the droplets size and velocity at 11 radial positions were measured with the phase Doppler anemometry and then analyzed based on the recognition method of droplet role to figure out the radial evolution characteristics of droplets flow rate, size, velocity, and momentum. The results show that the droplet quantity flow rate decreases with the entrained droplets moving from the liquid film-gas core interface to the centerline, due to the fact that the droplet coalescence has a greater effect than the droplet breakup. The proportion of small droplets decreases while the proportion of relatively large droplets increases. The overall distribution characteristics of droplet diameter remain unchanged. The overall distribution characteristics of droplet axial velocity, radial velocity, and velocity size remain unchanged, although the droplets undergo the process of axial acceleration and radial deceleration during the radial migration. The distribution of droplet momentum varies greatly in the process of droplet radial migration. The contribution of small droplets to the total droplet momentum gradually decreases, while the contribution of large droplets to the total droplet momentum gradually increases. Although the number of large droplets is small, their contribution to the total droplet momentum cannot be neglected.

Multiresolution Multiplex Network for Analyzing Multichannel Fluid Flow Signals

Gas-liquid two-phase flow widely exists in chemical industries and natural gas industries. But characterizing the nonlinear flow behaviors underlying such flow remains a challenge of great importance. In this brief, we first carry out the vertical gas-liquid two-phase flow experiment in a 50mm-inner-diameter pipe to acquire signals via the four-sector conductance measurement system. Then, we develop a multiresolution multiplex network (MMN) to analyze multichannel measurements. In particular, we employ average edge overlap to quantitatively characterize the derived network corresponding to different flow patterns. Results show that our method allows to effectively uncover the evolution from bubble flow to slug flow. In addition, we calculate the approximate entropy of different flow patterns to verify the findings from our network analysis. All the results demonstrate that our proposed method allows characterizing the nonlinear flow behaviors from multichannel measurements and also provides a novel way for multivariate time series analysis.

Complex Network Analysis of Wire-Mesh Sensor Measurements for Characterizing Vertical Gas–Liquid Two-Phase Flows

Gas–liquid two-phase flows widely exist in industrial production, where understanding the dynamics underlying flow patterns is a challenging problem of great significance. In this brief, we propose a complex network framework for analyzing the wire-mesh sensor measurements, aiming to characterize the flow behavior in the transition from bubble flow to slug flow in a 50 mm-inner-diameter vertical pipe. In particular, we design a wire-mesh sensor system and carry out the gas–liquid two-phase flow experiments to acquire the local flow signals from different flow conditions. We infer a complex network from wire-mesh sensor measurements in terms of the mutual information between time series, and then calculate some network measures to quantitatively characterize the network topological features associated with flow behavior. We find that the networks corresponding to different flow patterns show different topological features, which allows us to characterize the transition of flow behavior. Our results suggest that the proposed analytical framework can effectively reveal the complex dynamical behavior of gas–liquid two-phase flows.

Measurement of vertical gas-liquid two-phase flow by electromagnetic flowmeter and image processing based on the phase-isolation

In this paper, the electromagnetic flowmeter and the image processing are used to measure vertical gas-liquid two-phase flow. In order to reduce the effect of randomly distributed bubbles on the weight function of the electromagnetic flowmeter and the bubbles overlap on the image processing in raw two-phase flow, a special implementation method of phase-isolation for vertical upward flow is developed to achieve a stable and uniform swirling core-annular flow. For this flow pattern, the behavior of the electromagnetic flowmeter is analyzed and an automatic image-processing algorithm is developed. Both the simulated gas column experiment and dynamic air-water two-phase flow experiment were carried out to verify the theoretical equation of liquid flowrate. The results demonstrate that the instrument constant of electromagnetic flowmeter does change in the annular flow cross-section. A cross-section shape coefficient kα is needed in the equation of liquid flowrate, and kα is proved to be only related to the void fraction, independent of the type of electromagnetic flowmeter and the properties of central insulated column. Moreover, the swirl has no effect on the electromagnetic flowmeter. The overall average error of the liquid flow rate is 2.6%, and the relative error is within ±5% in bubble and slug flow condition. The measurement result is immune to the gas velocity.

Multi-scale cross-correlation characteristics of void fraction wave propagation for gas-liquid two-phase flows in small diameter pipe

The void fraction wave is a special physical phenomenon in a gas-liquid two-phase flow system. Understanding the propagation of the void fraction wave is of great significance for uncovering the physical mechanisms in both flow pattern transition and the fluid velocity measurement. In this study, detrended cross-correlation analysis (DCCA) is used to investigate the multi-scale cross-correlation characteristics of the coupled ARFIMA processes. It is found that the DCCA can effectively reveal the multi-scale cross-correlation dynamical behaviors of complex system. Then, we carry out the experimental test in a vertical gas-liquid two-phase flow pipe with small inner diameter. The DCCA is used to detect the cross-correlation characteristics of the void fraction wave on multiple time scales, and the growth rate of the cross-correlation level for the void fraction wave is observed on low time scales. Additionally, the spatial attenuation factor (SAF) of the void fraction wave is calculated to investigate the instability of the wave propagation. The SAF is close to zero under the transitional flow patterns, which means that the void fraction wave is in a stable propagating state. For bubble flows, the void fraction wave presents the attenuation characteristics, whilst the void fraction wave shows the amplification characteristics under the slug and churn flow patterns. Interestingly, the instability behaviors of the void fraction wave are always associated with its multi-scale cross-correlation characteristics. Specifically, the increasing rate of the wave cross-correlation level on low scales is much higher for transitional flow patterns, which is corresponding to the stable propagating characteristic of the void fraction wave. However, when the void fraction wave exhibits attenuation or amplification characteristics under other flow patterns, the increasing rate of the wave cross-correlation level on low scales is much lower.

Numerical investigation of models for drag, lift, wall lubrication and turbulent dispersion forces for the simulation of gas–liquid two-phase flow

In order to qualify CFD codes for accurate numerical predictions of transient evolution of flow regimes in a vertical gas–liquid two-phase flow in a pipe, suitable closure models (inter-phase forces) for the momentum exchange between the continuous and dispersed phases are needed. In this study, under the assumption of monodisperse bubbles, a consistent set of inter-phase force models have been investigated. The effect of Drag Force, Lift Force, Wall Lubrication Force and Turbulent Dispersion Force has been assessed. The predicted local radial distributions of four primitive variables: gas volume fraction, interfacial area concentration, gas velocity and liquid velocity, are validated against experimental data of Monrós-Andreu et al. (2013, EPJ Web Conf. 45, 01105). New parameters have been introduced in the wall lubrication force models of Antal et al. (1991, Int. J. Multiphase Flow 7, 635) and Frank et al., (2004, Proc. of the Third Int. Symposium on Two-Phase Modelling and Experimentation, Pisa, Italy, 2008, Nucl. Eng. and Des. 238, 647) as well as implementing additional drag coefficient models using CFX expression language (CEL). In general, the predictions from the sets of inter-phase closure models presented in this paper yielded satisfactory agreement with the experimental results. Based on the result of the validation of different inter-phase force models, a set of Grace drag coefficient model, Tomiyama lift coefficient model, Antal et al.’s wall force model, and Favre averaged turbulent dispersion force was found to provide the best agreement with the experimental data.

Effect of surfactant additive on vertical two-phase flow

Abstract The effect of surfactant on vertical gas–liquid two-phase flow was experimentally simulated in a low pressure apparatus. Liquid holdup reduction, pressure drop reduction and drag reduction caused by a surfactant additive were investigated. The flow patterns cover bubbly, slug, churn and annular-mist flows. The results show that the maximum liquid holdup reduction induced by the surfactant additive is high up to 88.6% which appears in churn flow. The maximum pressure drop reduction induced by the surfactant additive is high up to 96.5% which occurs in slug flow. Drag reduction caused by the surfactant additive is unexpectedly beyond 100% in some cases of vertical two-phase flow at high gas–liquid ratios. The frictional pressure drops are found to be negative for some vertical two-phase pipe flows at high gas–liquid ratios, challenging the general sense that the frictional pressure drop should be positive in pipe flow. The surfactant additive does not have significant effect on the transition of two-phase flow pattern. But the surfactant additive makes considerable impact on the detailed configurations of two-phase flow. The surfactant-assisted flow improvement is dependent on gas–liquid ratio, gas velocity and two-phase flow pattern.

Velocities Effects on the Void Fraction Distribution in a Vertical Gas-Liquid Two-Phase Flow Channel

Measurement of void fraction in a vertically arranged gas-liquid two-phase flow channel has been conducted for wide range of flow conditions using a specially developed sensor based on electric conductance method. The effects of velocities of both phases were examined with variety of combinations using an industrial scale two-phase flow loop. The results show that at a constant rate of liquid superficial velocities the void fractions increases with the increasing gas superficial velocities and at higher velocities of liquid phase, the value of void fraction becoming lower. The average void fractions were also compared with other scholars results.

A new method for evaluating drag reduction in gas–liquid two-phase flow based on energy dissipation

The energy dissipation is generally regarded as only contributed by the frictional pressure drop in gas–liquid two-phase flow. Based on energy dissipation analysis, a new term of energy dissipation is found in inclined gas–liquid two-phase flow. This new term is contributed by the combined effect of gravity, liquid holdup and pipe inclination. The change of liquid holdup is an indication of mechanical energy dissipation for inclined or vertical gas–liquid flows. As a result, a new method is proposed for the evaluation of drag reduction in gas–liquid two-phase flow. An experimental setup with a pipeline was established to examine the different drag reductions in gas–liquid flows by adding small amount of drag reducing agent (DRA). This pipeline has five different test segments with different inclinations. One of them was designed for horizontal single-phase liquid flow. Four of them were designed for horizontal, inclined upward, inclined downward and vertical gas–liquid two-phase flows. The inside diameter of the pipeline is 0.04m. The DRA used here is a water-soluble polymer. The experiments were conducted slightly above atmospheric pressure, using air and the water-based solution of the polymeric DRA. For horizontal gas–liquid flow, the drag reductions evaluated by this new method are identical with the results evaluated by a method usually used in multiphase flow. But for vertical and inclined gas–liquid flows, the deviations between these two methods are obvious. The maximum relative deviation is as high as 3.9 which occurs in inclined downward flow. It is found that the polymeric DRA has the maximum effect in horizontal gas–liquid flow rather than in inclined and vertical cases from the point of view of energy dissipation. The findings here provide new insights into the drag reduction occurring in gas–liquid two-phase flow, especially in inhomogeneous inclined and vertical cases.

Multiscale entropy and dynamic characteristics of two-phase flow patterns

Multiscale entropy is a new method to analyze nonlinear time series on multiple temporal and spatial scales. Firstly，multiscale entropy characteristics of several typical nonlinear series were studied，and then based on this，the fluctuant conductance signals of 144 two-phase flow conditions were analyzed，which were collected by using array conductance sensors in upward vertical gas-liquid two-phase flow. The results indicated that the changing rate of sample entropy at small scales could be used to classify the three typical flow patterns （bubble flow，slug flow and churn flow），and the fluctuation of sample entropy of large scales reflected the dynamic characteristics of each flow pattern. The stochastic characteristic of bubble flow was shown as higher and oscillating sample entropy at large scales; the intermittence of gas slug and liquid slug of slug flow was represented as lower and stable sample entropy of large scales; the unstable and oscillating characteristics of churn flow behaved as the entropy between that of bubble flow and slug flow，and the entropy closed to that of bubble at larger scales. The multiscale entropy analysis of two-phase flow is helpful for understanding the dynamic characteristics of flow pattern transition，and the rate of multiscale entropy is a new indicator of flow pattern identification.

Gas–liquid phase distribution and void fraction measurements using MRI

We used a permanent-magnet magnetic resonance imaging (MRI) system to estimate the integral, spatially- and temporally-resolved void fraction distributions and flow patterns in vertical gas–liquid two-phase flow. Air was introduced at the bottom of the stagnant liquid column using an accurate and programmable syringe pump. The air flow rate was in upward vertical direction against the gravity vector. Air flow rates were varied between 1 and 200 ml/min. Bubble formation and detachment at the inlet of the vertical column was non-uniform. The cylindrical non-conducting test tube in which two-phase flow is characterized was placed in a 0.26 T permanent-magnet MRI unit. A roughly linear relationship has been observed for the integral void fraction versus volume flow rate of air through vertical stagnant liquid column. Integral or sample-averaged void fraction was obtained by volume-averaging of the spatially-resolved signals. Spatial averaging was performed in a radial or longitudinal axis of the column. The time-averaged spatially-resolved void fraction has also been obtained for the quasi-steady flow of air in a stagnant liquid column. No great accuracy is claimed as this was an exploratory proof-of-concept type of experiment. Preliminary results show that MRI can indeed provide qualitative and quantitative data and is especially well suited for characterization of averaged transport processes in adiabatic and diabatic multi-phase and/or multi-component flows. Better and faster MRI sequences could improve spatial and temporal resolutions significantly.

The mechanism for the formation of slug flow in vertical gas–liquid two-phase flow

This paper studies the mechanism for the formation of a slug flow in vertical gas–liquid two-phase flow. By analyzing void fraction waves and their instability, it is proved that the formation of a slug flow regime is due to the increase of void fraction waves, which causes the conglomeration of gas bubbles and the coalescence of bubble clusters in unstable bubbly flow. Experiments and analysis show that intense turbulence can restrain the formation of Taylor bubbles. Therefore, in a large diameter vertical pipe, a Taylor bubble can form under a condition of low continuous volume flux due to the action of void fraction waves. However, the coalescence effect of void fraction waves as it affects bubbles is suppressed in high continuous volume flux, and therefore, a slug flow regime cannot be observed in the evolution of flow patterns. Under a condition of high continuous volume flux ( V L=0.15 m/s) described in the paper, the flow pattern evolution is from cap bubbly flow to cap churn flow, and then gradually to churn flow with the increase of void fraction.

The instability of void fraction waves in vertical gas-liquid two-phase flow

The measuring and analyzing results of void fraction waves in different flow regimes show that the propagating velocity of void fraction waves depends on flow regimes and mean void fraction. The disturbance at some frequencies can enhance the void fraction wave velocity. Non-linear analysis show that the instability process of bubbly flow is a chaotic process. Before the bubbly flow transits to cap-bubbly flow the growth rate of void fraction waves becomes the maximum value when the disturbance frequency is around the main frequency of void fraction waves.

Statistical Characteristics of Void Fraction Fluctuations and Flow Regime Identification in Vertical Gas-Liquid Two-Phase Flow

In this investigation, an objective and quantitative method is developed for the identification and classification of flow regimes in vertical gas-liquid two-phase flow. It is clarified that the features of probability density function (PDF) and the parameters of the PDF expected according to the maximum entropy principle (MEP) can be used as objective flow regime indicators and quantitative flow regime discriminators, respectively. In this experiment, the void fraction fluctuations are measured by the constant current technique. The PDFs are classified into three major types and correspond to three dominant flow patterns: bubbly, intermittent and annular. That is, a uni-modal PDF with a low void fraction corresponds to a bubbly flow pattern; a bimodal PDF corresponds to an intermittent flow pattern; and a uni-modal PDF with a high void fraction corresponds to an annular flow pattern. Furthermore, the relationship between the statistical parameters of the void fraction fluctuations (the mean value, the standard deviation, and the coefficients of skewness and kurtosis) and the flow patterns is discussed. It is shown that classification of uni-modal and bimodal PDFs can be made according to the coefficients of skewness and kurtosis.

Characteritic of Low-Mass-Velocity Vertical Gas-Liquid Two-Phase Flow.

Low-mass-velocity two-phase flow in a vertical pipe shows lower void fraction than high-mass-velocity two-phase flow even though their qualities are the same. In order to clarify the flow characteristics of the low-mass-velocity two-phase flow, air, water two phase flow experiments were conducted under the froth or annular flow conditions. Experimental results show that wall shear stress is positive even though both gas and liquid superficial velocities are positive. Measured water film average velocity is negative under this condition. These results indicate that local flow reversal should exist along the channel wall. This local flow reversal gives rise to the low void fraction in low-mass-velocity two-phase flow. It is also clarified that the drift flux model can be applied to the low-mass-velocity two-phase flow with local flow reversal.

Effects of System Pressure on Flow Regime Transitions in Vertical Upward Gas-Liquid Two-Phase Flow.

The purpose of this study is to investigate the pressure effects on the flow regime transitions in vertical gas-liquid two-phase flow. The following five flow conditions were tested to examine the effects of system pressure; the flow conditions in the transitional area in which the plug flow changes to huge wave flow; the flow conditions of the central gas core in the tube; the flow conditions in the area where the liquid slug disappears; the flow conditions in the transitional area in which the bubble flow changes to plug flow; and the flow conditions in the transitional area in which the time averaged wall shear stress changes from the negative to the positive value. New correlations to predict the flow conditions for the five above-mentioned flow conditions are presented. System pressures tested, range from 0.3 to 20 MPa.