Stratified Gas-liquid Flow Research Articles

Multiple holdup equilibria and hydraulic gradients in stratified gas–liquid flows

Experimental ramp-up tests for a two-phase flow with very low liquid loading have been conducted for 2°, 3° and 2° + 3° straight pipe configurations. The liquid draining process resulting from the ramp-up operation is explained by multiple holdup equilibria. For the conditions of the experiments, the multiple holdup equilibria region extends down to a gas oil volume flow rate ratio of about 1000. The flow of a pipe segment operating in the multiple holdup equilibria region will obtain the low holdup equilibrium if the holdup at the end of the pipe is sufficiently low. Otherwise, the flow of the pipe segment will obtain the high holdup equilibrium. Liquid accumulation can be minimized by avoiding steep inclination angles at the culmination of uphill sections in low liquid loading gas condensate systems. Ensuring low holdup equilibria throughout the pipeline, where multiple equilibria exist, minimizes liquid accumulation and reduces minimum flow rate limits during operating conditions.

Experimental investigation of different sub-regimes in horizontal stratified and intermittent gas-liquid two-phase flow: Flow map and analysis of pressure drop fluctuations

Horizontal stratified and intermittent gas-liquid two-phase flows exhibit several sub-regimes. Identifying these correctly would enable the development of more robust predictive models. This study reports on experimental investigation, based on observations and the collection of time series of pressure drop in a 40 mm ID pipe. Nine different sub-regimes (SS, 2D wave, 3D wave, RW, ED+RW, PS+RW, plug, LAS, HAS) were revealed. An original flow pattern map for this diameter is proposed including transitory regions between the sub-regime. Comparison of experimental observations with existing flow maps has highlighted the effect of pipe diameter on transition of sub-regimes. As reported in literature the sub-regimes, could be identified by direct visualization of the pressure drop time series obtained from differential pressure sensor a swell as using Probability Density Function. Furthermore, statistical analysis of standard deviation as function of gas superficial velocity enabled detection of the transition of various sub-regimes. Finally, a space feature based on standard deviation and mixture Froude number is proposed as an identification tool between the different sub-regimes investigated.

Study on velocity profile of gas–liquid two-phase stratified flow in pipelines based on transfer component analysis-back propagation neural network

The measurement of cross-sectional velocity profile is a challenge in the field of two-phase flow. In this paper, the stereoscopic particle image velocimetry (SPIV) technique is employed to obtain the cross-sectional velocity profile of gas and liquid phase in stratified flow. Interface velocity profile is obtained through numerical simulation. By leveraging the concept of transfer learning, we propose to construct a transfer component analysis-back propagation network using stereo particle image velocimetry and numerical simulation and to predict the velocity profile of the gas–liquid interface in stratified flow. The research indicates that the cross-sectional velocity profile of the gas–liquid stratified flow is similar to the “mushroom” shape. The velocity profile of the gas–liquid interface changes from an M-type to the N-type, and the gas–liquid velocity slip affects the transformation process. With the increase in the gas-phase velocity, the distance between the two peaks of the M-type velocity profile increases and the gap between gas–liquid velocity peaks increases accordingly.

Measurement method of gas holdup in horizontal gas-liquid two-phase flow based on fiber-optic probe array

Aiming at the problem of gas holdup measurement in horizontal shale gas wells with low flow rates, a fiber-optic probe array sensor for horizontal gas-liquid stratified flow is designed and developed. The Fluent fluid volume method is used to establish a simulation model of gas-liquid two-phase flow in the horizontal collecting pipe, and a study on the flow characteristics and flow patterns in the gas-liquid two-phase flow in the horizontal collecting pipe with low gas flow is carried out. Based on the acquisition of the fluid flow pattern is stratified flow characteristics, the design of the fiber-optic probe array structure with different numbers and positions of the probes, and the fiber-optic probe array sensor is modeled using the ZEMAX ray tracing software, simulation analysis of its response characteristics in different flow patterns and the array structure selection. Finally, based on the above research, the fiber-optic probe array sensor for gas holdup measurement is designed and developed, and the platform for gas-liquid two-phase flow is constructed and experimentally verified. The experimental results show that the developed fiber-optic probe array sensor has good applicability for gas holdup measurement under stratified flow, and the error of the measurement does not exceed 0.1.

Measurement of interfacial shear stress in gas–liquid two-phase stratified flow

Gas–liquid two-phase stratified flow exists in many industrial processes. Although the flow pattern is simple, the interfacial shear prediction of stratified flow is still the focus of the study. The calculation of the shear stress at the gas–liquid interface is closely related to the measurement of the void fraction and pressure drop of the stratified flow. In this study, a new method for the calculation of interfacial shear stress of gas–liquid two-phase stratified flow is proposed. Differential pressure measurement and planar laser-induced fluorescence technology are combined to obtain important parameters for stratified flow under low-speed flow conditions (Ql = 0.10–0.25 m3/h, Qg = 0.35–1.00 m3/h). The interfacial shear stress is successfully calculated using macroparameters. The uncertainty associated with the calculated parameters using the proposed method is 2.67%, and this study verifies the accuracy of the linear relationship. The method provides a new way to obtain the interfacial shear stress of gas–liquid stratified flow.

Investigation of the Effect of Side Arm Orientation of the T-Junction on Gas–Liquid Stratified Flow

T-junctions are important structures used in a number of industries to separate gas and liquid. This work studied the effect of the orientation of the side arm on the separation efficiency using a computational fluid dynamics (CFD) approach, and a new mechanical model is developed based on force analysis to predict the liquid carryout threshold. Laboratory experiments from published works are used to verify the CFD simulation and the new model. In this work, the angle of the side arm to the horizontal plane, α, and the angle of the side arm to the main arm’s axial direction, β, are investigated. The results show that with increasing β, the liquid carryover threshold increases accordingly, demonstrating that the liquid can be more easily carried to the side arm, while the liquid-carrying performance in the side arm is not sensitive to the inclination angle, β. Hence, in the new model, the inclination angle of is ignored. Experimental data are collected to validate the new model. The results show that this model can accurately predict the liquid carryover threshold, and the relative error is 4.16%.

Three-dimensional interface structures and characteristics in a stratified gas–liquid pipe flow

In this study, the air–water flow in a pipe with a diameter of 100 mm was tested to investigate the three-dimensional (3D) interfacial characteristics of gas–liquid stratified flow in horizontal and slightly inclined pipes. The gas–liquid stratified flow interface exhibited a nonflat phenomenon (concave or convex). In particular, this phenomenon was verified by the transformation of the interface shape when the gas–liquid ratio exceeded 200 and 5 in the horizontal and inclined pipes, respectively. Compared with the flow in the horizontal pipe, the liquid holdup gradually decreased with an increase in the inclination angle in the downward stratified flow, and liquid height changes at the center was slight; however, the convex interface shape was obvious. In addition, the interface exhibited regular fluctuations in the flow direction under fixed operating conditions. Therefore, the hypothesis of a nonflat interface for stratified flow was proposed based on the experimental phenomena, and the interface shape function and flow equations were constructed to predict the degree of interface bending and critical conditions for gas–liquid stratified flow by analyzing the theoretical and influencing factors. The acceleration pressure drop was introduced to quantitatively characterize interfacial fluctuations in the flow direction; therefore, a modified model for nonflat stratified flow was developed. The proposed model could characterize the interface features in both the cross-sectional and flow directions of the pipe and provided a solution for the 3D interface of stratified flow. A comparison with the experimental results revealed that the proposed model performed the prediction satisfactorily. By solving the proposed model, the interfacial information in the pipe cross section and flow direction can be effectively predicted.

Pressure Drop Prediction Model of the Gas-Liquid Stratified Flow Development Section in the Horizontal Pipeline

Abstract The gas phase accelerating beyond the liquid phase caused by gas-liquid slippage cannot be ignored in short horizontal pipelines with undulation and inflow, and there is no method to calculate it. Therefore, a pressure drop prediction model for variable liquid holdup was developed in this paper. The theoretical model calculation results were validated using computational fluid dynamics. The effectiveness of the pressure drop prediction model has been demonstrated. The various pressure drop, liquid holdup, and development length laws were then examined. The findings indicate that: the pressure drop in the developed section of stratified flow is not only the friction pressure drop but also the acceleration pressure drop; the length of the stratified flow development section and pipeline pressure drop are more easily affected by the flowrate than the liquid holdup in the pipe inlet. Using the relevant data from coalbed methane horizontal wells as an example, the L/D of the development section is approximately 40–85 when the inlet flowrate is 0.8–1 m/s, and the inlet liquid holdup is 0.3–0.5. The pressure drop characteristics in the gas-liquid stratified flow development section are obviously different from those in the stable section. The development of a pressure drop prediction model for the stratified flow development section lays the theoretical groundwork for the investigation of gas-liquid two-phase flow in horizontal pipelines with short or undulating and inflow conditions.

Two-phase stratified flow in horizontal pipes: A CFD study to improve prediction of pressure gradient and void fraction

Improved understanding of flow regime effects on design-influencing engineering quantities is of primary importance. This work is focused on the numerical prediction of pressure gradient and void fraction in a horizontal pipe where gas-liquid stratified flow is present in different operating conditions. The problem was modeled with unsteady, multiphase CFD (computational fluid dynamics) simulations. Volume Of Fluid (VOF) method was used as multiphase model. To define the numerical methodology, this study provides details on the influence of discretization grid and turbulence model on the simulation accuracy. It shows that mesh density on pipe cross-section is the most important grid parameter to focus on. Different turbulence models are required depending on the gas velocity and on its turbulence flow regime. Transition SST is able to model all the operating conditions but Realizable k-ε is adopted to further increase the accuracy of the results. A general underestimation of the pressure gradient is reported with an average error of − 6.43 % and − 16.21 % for a liquid superficial velocity of 0.04 m/s and 0.06 m/s respectively. Comparison with two-fluid 1D models shows that CFD simulations are the most accurate tools for predicting the pressure gradient at gas superficial velocities lower than 1.3 m/s. The implementation of a drift flux model shows a good agreement between experimental results and CFD simulations concerning void fraction estimation. CFD results are also used to underline the physical phenomena limiting the performance of 1D models.

Characterization of interfacial waves in stratified turbulent gas-liquid pipe flow using Particle Image Velocimetry and controlled disturbances

The work reports an experimental investigation on stratified gas-liquid pipe flow characteristics in the presence of controlled interfacial waves. Studies of this flow regime with controlled interfacial waves are scarce in the literature. Here, the disturbances are excited at the liquid interface by an oscillating paddle. The waves are synchronized with image acquisitions, enabling the utilization of phase-locked measurements and ensemble averaging techniques. Off-axis Particle Image Velocimetry (PIV) and Shadowgraph techniques were applied to provide information about mean and wave-induced modifications on the velocity fields. Results show that mean flow velocities in the liquid and gas phases close to the pipe walls adhere well to the single-phase flow log-law profile. In the liquid layer, this agreement was observed up to half of the water depth. Controlled disturbances enabled the estimation of the wave amplitude thresholds for the appearance of relevant nonlinear wave effects on the flow field. Results suggest that such a threshold can be fairly represented by a constant value of the non-dimensional parameter proposed in the work of Kirby (2008). Within the non-linear wave regimes investigated, noticeable changes in the flow field were observed close to the interface. However, near the wall the flow was weakly affected by the presence of waves, suggesting that interfacial wave effects are weakly coupled with near-wall disturbances and might be modelled independently. Moreover, contributions to interfacial shear stress due to the presence of waves were obtained experimentally. The results presented here are useful for validation and improvement of models used to predict flow characteristics in stratified flows. In addition, they contribute to shed further light on the physical mechanisms involved in the phenomenon.

Experimental and Prenemilary Numerical Evaluation of Pressure Drops under the Conditions of the Stratified Gas-Liquid Flow in a Horizontal Pipe Filled with Metal Foam

The paper reports the results of experimental tests and numerical simulations related to the pressure drop during two-phase air-water mixture flow through a pipe containing metal foam packing. Aluminium foam with 40 PPI open cells was used in the tests. A horizontal pipe with an internal diameter of 10 mm was used, and the foam only occupied a section of the pipe length equal to 240 mm. In the section of the pipe upwards of the foam, stratified flow pattern was generated, i.e., the most characteristic type for the gas-liquid flow. The results of the experimental research were compared with the values derived on the basis of the empirical method, which was developed for several different metal foams and two-phase systems. The values derived from measurements and calculations were subsequently applied to validate one numerical simulation method that is known to be particularly applicable for two-phase gas-liquid flow through metal foams. As a final result, the phenomena resulting from the presence of foam in the stratified flow through a gas-liquid system, the deficiencies of the methods applied in calculating pressure drops and modeling their values in accordance with the adopted numerical procedure were indicated. All research and modelling were carried out with the purpose of testing the potential of metal foam use in pipes dedicated to heat exchanger design, particularly ones intended to improve energy efficiency.

The effect of the liquid physical properties on the wave frequency and wave velocity of co-current gas-liquid stratified two-phase flow in a horizontal pipe

The aim of the present study was to investigate the effect of the liquid physical properties on the liquid film thickness, wave frequency, and wave velocity in co-current gas-liquid stratified two-phase flow in a horizontal pipe. The present experimental study was conducted using an acrylic pipe with an inner diameter of 26 mm and a length of 9500 mm. The superficial velocity of liquid (JL) and gas (JG) were 0.02 m/s to 0.1 m/s and 4 m/s to 16 m/s, respectively. A highspeed video camera was used to take the visual data which is then processed by the developed image processing techniques to obtain the time series data of the liquid film thickness. The probability distribution function (PDF) and discrete wavelet transform (DWT) were used to analyze the time series of the liquid film thickness. In addition, the data were also analyzed using the power spectral density (PSD) in order to obtain the wave frequency and the wave velocity. Next, the experimental correlations to predict the wave frequency and the wave velocity were developed using the dimensional analysis. From the time series of the liquid film thickness, the effect of the viscosity and surface tension on the shifting of scale and frequency of the wavelet decomposition level, as well as the frequency and velocity of the waves were clarified. In addition, a correlation to predict the frequency and velocity of waves has been successfully developed, in which the Martinelli parameter, the ratio of gas and liquid Reynold numbers, and the Weber number play the important roles in the correlation.

Investigation on propagation mechanism of leakage acoustic waves in gas-liquid stratified flow

Propagation speed of acoustic waves is of vital importance in pipeline leakage detection using acoustic method. However, the wave front shape and thermodynamic process of leakage acoustic waves in gas-liquid stratified flow remain unclear, which restricts the proper calculation of sound speed in this flow pattern. This paper presents a simulation study using CFD software Fluent to investigate the propagation process of leakage acoustic waves in stratified flow. The leakage acoustic waves are generated in different fluid phases and small time-step simulation is implemented to study the pressure behavior during the wave propagation. The results show that leakage acoustic waves propagate as single plane wave in gas-liquid stratified flow and the propagation process is nearly adiabatic. The value of polytropic exponent when calculating the sound speed in gas phase should be 1.4. Further, experimental study is conducted to verify the simulation results. Influencing factors including pressure and void fraction are considered. The present results contribute to the proper selection of sound speed calculation model in stratified flow, which can improve the leakage detection accuracy in gas-liquid stratified flow pipelines.

Investigation on leakage detection and localization in gas-liquid stratified flow pipelines based on acoustic method

Leakage detection technology is of vital importance to the safe operation and flow insurance of multiphase flow pipelines. This paper for the first time applies acoustic based leakage detection method to stratified flow pipelines and verifies the feasibility of this method in multiphase flow pipelines. Gas-liquid stratified flow pipeline leakage detection and localization experiment is conducted to investigate the effectiveness of different filtering methods when calculating the time difference (delay) between arrival times of leakage acoustic signals at different sensors using cross-correlation algorithm. Propagation characteristics of the leakage acoustic wave in stratified flow are also investigated. The results show that the 5-level Symlets wavelet filter can effectively improve the accuracy of time delay estimation. Moreover, the value of sound speed can be set as 346 m/s when carrying out leakage localization in stratified flow pipelines, which is nearly irrelevant with pressure, void fraction, superficial velocity of gas phase and the structure of pipe. The research in this paper lays a foundation for the application of acoustic based leakage detection technology in multiphase flow pipelines.

Liquid holdup measurement for gas-liquid stratified flows by means of resistive probes and image processing

Flow patterns exert a fundamental influence on the behaviour of multiphase flows, and they must be brought into play when dealing with their modelling. This is usually done by means of summarizing quantities as the phase holdups and the interfacial area concentration. Many techniques have been designed during the years to measure them, among which the use of probes relying on electrical resistance is one of the simplest and less expensive. While having these points of strength, resistive probes are intrusive devices. This work is therefore devoted to a comparison between liquid height (and derived quantities) measurements – for stratified and stratified-wavy air-water flows – performed using a conventional resistive probe and by means of an image-based technique. Validation of the latter was performed using computer-generated flow images. Then, an experimental campaign was carried out for flows with liquid superficial velocities in the range 0.03 ÷ 0.06 m/s and gas superficial velocities in the range 0.77 ÷ 2.31 m/s. Results showed that the two methods give answers within very few percent of difference, which is more than satisfactory in this field. The results are also in good agreement with some of the most credited literature models and correlations.

Hydraulic jump induced flooding and slugging in stratified gas-liquid flow – An experimental appraisal

The paper presents an experimental approach to investigate the role of planar hydraulic jump in termination of stratified gas-liquid flow in a narrow rectangular conduit. When the Froude number of the incoming liquid Fr is greater than unity (supercritical flow), stratified flow is terminated via formation of “natural” planar jump in both co and countercurrent flow. While oscillatory jump induces flooding in countercurrent flow, undular jump results in slugging during co-current flow at higher phase velocities. The superficial gas and liquid velocities at onset of flooding confirm to the criterion proposed by Gargallo et al. (2005) and in co-current flow, the superficial gas and liquid velocities at the onset of large waves downstream of jump are well correlated by the criterion of Taitel and Dukler (1976). The jump types are displayed in a phase diagram that also depicts the domain of stable stratified flow bounded by slugging for co-current and flooding for counter current flow.

Statistical characterization of the interfacial behavior of the sub-regimes in gas-liquid stratified two-phase flow in a horizontal pipe

The aim of the present study was to identify the interfacial behavior of the sub-regime of gas-liquid stratified two phase flow by using the pressure differential signal data. Here, the probability distribution function (PDF), power spectral density (PSD), kolmogorov entropy and discrete wavelet transform (DWT) were used to analyze the differential pressure signals. The indicators of each flow sub regime were analyzed on the basis of the quantitative values of the statistical curves, which were also validated by visual observation of the video images. The results indicated that there are six identified sub-regimes of stratified flow namely stratified smooth (S), two-dimensional (2-D) wave, three-dimensional (3-D) wave, roll wave (RW), entrained droplet + disturbance wave (ED + DW) and pseudo slug (PS). Next, the increase of liquid viscosity will shift the transition line from the RW to ED + DW to a lower both of JL and JG. The increase of the liquid viscosity provides a stabilizing effect to reduce the chaos of the pressure gradient fluctuation. For the RW and the ED + DW sub-regime, the increase of the liquid viscosity shifts the wavelet energy to a larger scale and lower frequency. For the PS sub-regime, the increase of liquid viscosity shifts the wavelet energy to a smaller scale with a higher frequency. For the RW sub-regime, the increase of JG will increase the wavelet energy at the small-scale and high-frequency decomposition levels.

Effect of two-phase gas-liquid flow patterns on cuttings transport efficiency

Effective cuttings transport and accurate drilling hydraulics prediction remain issues of concern during drilling operations of horizontal, extended reach and multilateral wells. While several studies have adopted a two- or three-layered modelling approach to evaluate cuttings transport efficiency, they have neglected the effect of the gas-liquid fluid flow pattern within the annulus on cuttings transport. An experimental and theoretical study was carried out to evaluate the interplay between the two-phase gas-liquid flow patterns and the major drilling parameters and investigate its influence on the cuttings and fluid flow dynamics in a horizontal and inclined drilling wellbore. Several mathematical flow pattern dependent multi-layered models valid for any level of wellbore eccentricity were developed for the different cuttings transport mechanisms in the bubble, dispersed bubble, stratified and slug gas-liquid flow patterns, thereby providing a method to evaluate cuttings transport efficiency and perform wellbore hydraulics calculations for underbalanced drilling operations. Experimental results show that both fluid flow pattern and the drilling fluid flowrate are the most influential controllable parameters that affect the cuttings transport efficiency. Moreover, the hole cleaning requirements for an eccentric annulus is higher than that required for the concentric annulus of both single-phase and two-phase Newtonian or non-Newtonian fluids. Inclination angle was also found to influence hole cleaning and the degree of its effect is highly dependent on the fluid properties, the cutting transport mechanism and prevailing gas-liquid flow pattern. In the horizontal and inclined eccentric annuli, drillpipe rotation can improve cuttings transport for both single-phase and two-phase flows, but generally the effect of the drillpipe rotation on two-phase flow for cutting transport is much less than that of the single-phase flow. Overall, a good match was found between the mathematical flow pattern dependent multi-layered models and the experimental data. The findings of this study serve as a guide in the prediction of the wellbore dynamics for underbalanced drilling operations and provides a tool that can be applied for wellbore pressure management and the evaluation of hole cleaning based upon the specified flow conditions.

A new model for predicting the critical liquid-carrying velocity in inclined gas wells

Based on the assumption of gas-liquid stratified flow pattern in inclined gas wells, considering the influence of wettability and surface tension on the circumferential distribution of liquid film along the wellbore wall, the influence of the change of the gas-liquid interface configuration on the potential energy, kinetic energy and surface free energy of the two-phase system per unit length of the tube is investigated, and a new model for calculating the gas-liquid distribution at critical conditions is developed by using the principle of minimum energy. Considering the influence of the inclination angle, the calculation model of interfacial friction factor is established, and finally closed the governing equations. The interface shape is more vulnerable to wettability and surface tension at a low liquid holdup, resulting in a curved interface configuration. The interface is more curved when the smaller is the pipe diameter, or the smaller the liquid holdup, or the smaller the deviation angle, or the greater gas velocity, or the greater the gas density. The critical liquid-carrying velocity increases nonlinearly and then decreases with the increase of inclination angle. The inclination corresponding to the maximum critical liquid-carrying velocity increases with the increase of the diameter of the wellbore, and it is also affected by the fluid properties of the gas phase and liquid phase. The mean relative errors for critical liquid-carrying velocity and critical pressure gradient are 1.19% and 3.02%, respectively, and the misclassification rate is 2.38% in the field trial, implying the new model can provide a valid judgement on the liquid loading in inclined gas wells.

Mechanistic modeling of flow and heat transfer in turbulent–laminar/turbulent gas–liquid stratified flow

Two-phase gas–liquid stratified flow is characterized by a structure in which the gas and liquid phases are separated from each other by a continuous interface. Adequately understanding its flow mechanism and heat transfer is important for analyzing two-phase stratified flow. This paper develops a mechanistic model of flow and heat transfer in turbulent–laminar/turbulent two-phase stratified flow in horizontal and slightly inclined pipes. First, a hydrodynamic model of two-phase stratified flow is developed by using the concept of two-fluid model. Second, a mechanistic model of heat transfer is derived based on the hydrodynamic model. The overall heat transfer coefficient is integrated by using the coefficients of local heat transfer of the liquid film and the gas core. Third, the effect of such flow geometries and parameters as the superficial Reynolds numbers for liquid and superficial gas, void fraction, pressure drop, and inclination angle of the pipe on heat transfer in two-phase stratified flow is comprehensively investigated. Finally, the relationships between the two-phase heat transfer multiplier and the overall void fraction and pressure drop multipliers are quantified. A simple correlation of the heat transfer multiplier for two-phase stratified flow is developed by using the void fraction as the input parameter serving as a quick but rough prediction of the heat transfer multiplier in two-phase stratified flow.