Gas-liquid Two-phase Flow Research Articles

An artificial neural network model for the prediction of entrained droplet fraction in annular gas-liquid two-phase flow in vertical pipes

An artificial neural network model for the prediction of entrained droplet fraction in annular gas-liquid two-phase flow in vertical pipes

Simulation Study on Dynamic Characteristics of Bubbles in Transformer Oil under Extremely Nonuniform Field

Bubbles in transformer oil seriously threaten the deterioration of internal insulation performance during its movement, leading to severe transformer failure. A gas-liquid two-phase flow simulation model is established based on the phase field method to study the effect of bubbles on the insulation performance of transformer oil. The dynamic characteristics of bubbles in transformer oil under extremely non-uniform electric fields are studied. The results show that the electric field force has a pushing effect on the bubble. As the voltage increases, the pushing effect is more significant, and the displacement distance of the bubble in the horizontal direction is larger. The velocity curve of a single bubble in the vertical direction shows an oscillating attenuation trend. The pushing effect of electric field force on the bubble causes the bubble velocity near the needle electrode to decrease, but the stretching effect increases the average bubble velocity. When the electric field strength is further improved due to the enhanced push-off impact of the electric field force on the bubbles, the bubbles will gather under the needle electrode to form cluster bubbles, further reducing the insulation performance of the transformer oil. This needs to be paid attention to in the design of the insulation structure to avoid the accumulation of bubbles caused by too high an electric field intensity at a particular place.

MFCC-LSTM framework for leak detection and leak size identification in gas-liquid two-phase flow pipelines based on acoustic emission

Two-phase gas–liquid flows are crucial to the pipeline system. Due to their complicated flow state, existing leak detection techniques are unsuitable for two-phase flow pipelines. To avoid accidents caused by a leak of pipelines, we present a framework for combining the Mel-frequency cepstral coefficient and long short-term memory (MFCC-LSTM) based on acoustic emission (AE). A series of experiments are performed considering 1152 operating conditions, including flow pattern, leak size, direction, and location. In addition, the detection capability of the different features of AE signals combined with LSTM is discussed. The results show that the recognition accuracy of the MFCC-LSTM framework reaches 98.4%. Then, we further perform leak size identification under different flow patterns and found that the MFCC-LSTM framework still exhibits excellent performance. The proposed MFCC-LSTM framework provides a promising solution to identify the leak state and size in two-phase flow pipelines based on the AE technique.

Research on Liquid Flow Rate Detection of Mixed Fluid Based on Vibration Signal Characteristic Analysis of Gas Liquid Two-Phase Flow

The online detection of liquid flow rate in gas-liquid two-phase flow has become an important factor in ensuring the safe operation of gas wells. In this paper, the real-time measurement of liquid holdup in gas-liquid two-phase flow is carried out by analyzing the characteristics of vibration signals excited by gas-liquid two-phase flow impacting on the pipe wall. Firstly, an acceleration sensing detection and processing system is constructed to obtain the vibration signals excited by gas-liquid two-phase flow impacting on the pipe wall. Then, the pure airflow vibration signals at different flow velocities and the gas-liquid two-phase flow excitation vibration signals at different liquid flow rates are tested, respectively, and the time-frequency characteristics analysis based on STFT is implemented. The practice shows the following: firstly, the frequency band of 6.5−15 Hz is identified as the characteristic frequency band of liquid flow rate in gas-liquid two-phase flow. Secondly, the liquid holdup is positively correlated with the vibration energy in its characteristic frequency band. Thirdly, a mathematical model of the relationship between liquid flow rate and vibration energy is constructed. Finally, the measurement error of liquid holdup is within 10%. This research method has laid a good foundation for the subsequent detection of characteristic parameters of each phase in gas-solid-liquid complex multiphase flow fluids, and it has certain application and promotion value.

Study on prediction model of liquid holdup based on back propagation neural network optimized by tuna swarm algorithm

ABSTRACT It is unavoidable that there will be liquid accumulation in the low-lying areas of the pipelines during the operation of wet gas pipelines. The existence of liquid accumulation can generate a variety of safety issues and, in extreme circumstances, accidents. The accurate calculation of liquid holdup in gas-liquid two-phase flow is of great significance for the study of flow pattern identification, pressure drop calculation, pigging cycle determination, hydrate prediction, wax deposition prediction, pipeline corrosion evaluation and prediction, and transportation efficiency calculation of gas pipelines. Therefore, it is crucial to predict the liquid holdup of wet gas pipelines. 2141 independent experimental data samples were collected and screened out from literatures. Based on the gray theory, gray relation analysis was carried out on the influencing factors of liquid holdup, and the factors with greater influence were selected as the influencing variables; the liquid holdup prediction model based on tuna swarm algorithm optimized BP neural network was established, with pipe diameter, inclination angle, apparent gas velocity, apparent liquid velocity, average temperature, average pressure, and liquid viscosity as input parameters, and liquid holdup as output parameter. Liquid holdup was predicted for upward inclined, downward inclined, and horizontal pipelines respectively. The results show that the prediction model of liquid holdup established in this paper has high accuracy, with the MAPE value of 5.3223%, RMSE value of 0.0213, and R 2 value of 0.9924 for upward inclined pipelines; the MAPE value of 10.1859%, RMSE value of 0.0174, and R 2 value of 0.9922 for downward inclined pipelines; the MAPE value of 4.8037%, RMSE value of 0.0113, and R 2 value of 0.9974 for horizontal pipelines. The predicted results are generally stable and have a wider scope of application, providing a new idea and approach for predicting the liquid holdup of wet gas pipelines.

Mass transfer mechanism and relationship of gas–liquid annular flow in a microfluidic cross-junction device

Mass transfer performance of gas–liquid two-phase flow at microscale is the basis of application of microreactor in gas–liquid reaction systems. At present, few researches on the mass transfer property of annular flow have been reported. Therefore, the mass transfer mechanism and relationship of gas–liquid annular flow in a microfluidic cross-junction device are studied in the present study. We find that the main factors, i.e., flow pattern, liquid film thickness, liquid hydraulic retention time, phase interface fluctuation, and gas flow vorticity, which influence the flow mass transfer property, are directly affected both by gas and liquid flow velocities. But the influences of gas and liquid velocities on different mass transfer influencing factors are different. Thereout, the fitting relationships between gas and liquid flow velocities and mass transfer influencing factors are established. By comparing the results from calculations using fitting equations and simulations, it shows that the fitting equations have relatively high degrees of accuracy. Finally, the Pareto front, namely the Pareto optimal solution set, of gas and liquid velocity conditions for the best flow mass transfer property is obtained using the method of multi-objective particle swarm optimization. It is proved that the mass transfer property of the gas–liquid two-phase flow can be obviously enhanced under the guidance of the obtained Pareto optimal solution set through experimental verification.

Migration of Leaked Oil Vapor in Underground Water-Sealed Oil Storage Cavern Considering the Influence of Fractures

During the operation of underground water-sealed oil storage caverns, a large amount of oil vapor is generated due to volatilization. Oil vapor can easily leak into the surrounding rock, and fractures in the surrounding rock are usually the dominant channels for oil vapor leakage. To study the influence of fractures on oil vapor leakage and migration in underground water-sealed oil storage caverns during the oil storage period, a gas–liquid two-phase flow model of the fracture–pore dual medium in fractured rock mass was established. The program was implemented on the COMSOL platform by using weak-form PDE (partial differential equation). Then, taking an underground water-sealed cavern of an oil reserve as an example, the influence of the characteristic parameters of a single fracture on the evolution process of oil vapor leakage and migration during the oil storage period of the underground water-sealed oil storage cavern was studied. The results were further applied to the Huangdao underground oil depot project. The results show that the spatial distribution of oil vapor leakage is mainly affected by fractures. Through parameter sensitivity analysis, it was found that the geometric characteristic parameters of fractures will have a certain impact on the migration field of oil vapor leakage in underground caverns. Specifically, fracture permeability (kf), fracture width (df), and fracture inclination (θ) are positively correlated with oil vapor leakage parameters (oil vapor leakage range and leakage volume), while the distance between the fracture and the middle cavern (s) is negatively correlated with oil vapor leakage parameters (oil vapor leakage range and leakage volume). The relative influence of fracture geometry parameters on the migration process of oil vapor leakage during the oil storage period of the underground water-sealed oil storage cavern is in the following order: kf> df> s > θ. Engineering application shows that the existence of fractures affects the spatial distribution of oil vapor leakage and migration, and the relationship between oil vapor leakage parameters and oil storage operation time is a positive power function. The gas–liquid two-phase flow model of the fracture–pore dual medium in fractured rock mass developed in this study could offer a numerical simulation tool to assess and mitigate the risk of oil vapor leakage. The research conclusions can provide some references for related problems encountered in similar projects.

Flow pattern identification of gas-liquid two-phase flow based on integrating mechanism analysis and data mining

The flow pattern identification of gas-liquid two-phase flow is very important to gas field gathering pipeline. Due to the complexity of gas-liquid two-phase flow, it is difficult to accurately and quickly identify the flow pattern by mechanism model. Machine learning (ML) model has high accuracy, but poor generalization and physical significance. In order to improve the deficiencies of ML model and mechanism model, a new physically guided neural network (PGNN) is proposed. Dimensionless numbers with physical significance are used as the input of PGNN. The structure of PGNN is designed according to the physical modeling process of flow pattern identification. The physical intermediate variables that affect flow pattern transformation are added to the loss function. 1812 experimental data in two literatures are used for training and testing different models, and 88 experimental data in one literature are used as an additional test dataset to evaluate the generalization of different models. The average accuracy of PGNN on test set and additional test data set is 95.2% and 95.9%, which are higher than other models. This study proves that the integration of mechanism can improve the accuracy and generalization of ML model, and provides a new perspective for the study of gas-liquid two-phase flow.

Experimental study of the effect of fastening bolts on PEMEC performance

The fastening bolt is one of the important parts of the stacked structure proton exchange membrane electrolyzer (PEMEC), and the bolt tightening behavior has a significant impact on the electrochemical performance of the PEMEC. In this paper, we use a transparent visualization system to explore the effect of tightening bolt behavior on the performance of PEMEC. Pressure sensitive paper was used to record the contact pressure distribution map of PEMEC under different bolt tightening methods, and a high-speed camera was used to record the state of gas-liquid two-phase flow inside PEMEC under different bolt tightening methods. Both of them are used to verify the effect of bolt tightening behavior on the performance of PEMEC. The experimental results show that the contact pressure of PEMEC increases with the increase of bolt torque, and the electrochemical performance first increases and then decreases with the increase of bolt torque. The optimum bolt torque is 3.5 N m for a square PEMEC with an end plate size of 100 mm × 100 mm and a membrane size of 50 mm × 50 mm × 0.17 mm. The PEMEC performance increases with the increase of bolt tightening steps, and the optimal tightening step for the system is three steps. In addition, symmetrical tightening of bolts is beneficial for improving electrochemical performance, and there is an optimal tightening sequence in the system. This paper illustrates the relationship between tightened bolts and PEMEC electrochemical performance, showing the relationship between bolt tightening and the gas-liquid two-phase flow state inside PEMEC. The research results can provide a reference for optimizing the assembly and installation process of PEMEC.

Influence of vibration on transient flow characteristics of gas-liquid two-phase flow in inclined pipes

As an advanced method of transportation globally, pipelines play a significant role in the energy sector of many nations. Given the rapid development of offshore oil and gas engineering, it is critical for researchers to focus on the dynamics of pipelines and the flow characteristics of internal fluids under oceanic conditions. In this paper, a gas-liquid two-phase flow numerical model was developed to simulate transient flow characteristics in an inclined pipe at different vibration conditions based on CFD dynamic mesh technology. The CFD model was verified against experimental data, showing that the numerical simulation predicts average pressure drop within 10 % of experimental data. Parametric studies were conducted based on the CFD analysis, including vibration direction (horizontal, vertical, and inclined), vibration amplitude (0.5D∼1.5D) and flow velocities (jsg=0.5∼1.0m/s, jsl=0.1∼1.0m/s). Meanwhile, the downhill and uphill sections of the pipeline were considered. The results indicated that inclined vibration had the most substantial impact on flow, followed by horizontal vibration and vertical vibration. The changes of average liquid holdup were closely related to flow pattern under vibration conditions. These vibration conditions significantly effected pressure fluctuations and secondary flow. The impact of vibration on transient flow heightened with increased vibration amplitude. In addition, transient flow behaviors showed periodicity, and the evolution period was consistent with the vibration period. These findings are helpful to understand the transient flow behaviors in inclined pipe under vibration conditions.

Numerical study of transient flow characteristics of gas-liquid two-phase flow in inclined upward tube under periodic vibration

The transient flow characteristics of gas-liquid two-phase flow in inclined upward tubes exhibit a high degree of complexity. The tube vibration caused by multiple uncertain factors, such as seawater impacts or subsea earthquakes, poses more significant challenges for flow safety assurance. Thus, it becomes increasingly crucial to explore the transient flow characteristics of gas-liquid two-phase flow in inclined upward tubes under various vibration conditions. In this paper, the transient flow characteristics of gas-liquid two-phase flow in an inclined upward tube under different vibration conditions are numerically studied based on CFD dynamic mesh technique. In the current study, the studied parameters include vibration direction (heaving, transverse and coupling), vibration amplitudes (0.5D, 1.0D and 1.5D), and inlet velocity (jsg=0.5∼1.0m/s; jsl=0.1∼1.0m/s). The transient flow structure, cross-sectional void fraction and pressure drop are analyzed. The findings indicate that the transient flow structure of the slug flow demonstrates periodic variation characteristics under transverse and coupling vibrations, and the cross-sectional void fraction increase. For transition flow, the cross-sectional void fraction increases under transverse and coupling vibrations, while displays an increase initially and subsequently a decrease significant with an increase in the heaving vibration amplitude. Additionally, all considered vibration conditions intensify the pressure drop of the transition flow and decrease the cross-sectional void fraction of the turbulent flow.

Effect of high pressure on severe slugging and multiphase flow pattern transition in a long pipeline-riser system

Severe slugging poses an enormous threat to the safe operation of high-pressure long petroleum pipelines. However, most published studies are conducted in short pipelines under atmospheric pressure. In this work, the gas–liquid two-phase flow experiment is conducted in the pipeline-riser system of 380 m long and 75 mm inner diameter under the separator pressure from atmospheric pressure to high pressure (10 MPa), and flow pattern maps are established. Under high-pressure conditions, liquid fallback and second blowout of severe slugging disappear. In the flow pattern maps, flow pattern types reduce, and severe slugging region shrinks. The boundary between stable flow and severe slugging appears on the left side of the flow pattern map and moves in the direction of higher superficial gas velocity. Increasing separator pressure leads to the increase in the severe slugging period. A correlation equation applicable to predicting the severe slugging period for different pressures (0, 1 and 5 MPa) is proposed, and the average error is less than ± 7.9%. Due to the inhibition of high pressure, the amplitude of the riser differential pressure decreases at most working points and increases only at a few working points where flow pattern transition occurs.

Interconnected ordinal pattern complex network for characterizing the spatial coupling behavior of gas-liquid two-phase flow.

The complex phase interactions of the two-phase flow are a key factor in understanding the flow pattern evolutional mechanisms, yet these complex flow behaviors have not been well understood. In this paper, we employ a series of gas-liquid two-phase flow multivariate fluctuation signals as observations and propose a novel interconnected ordinal pattern network to investigate the spatial coupling behaviors of the gas-liquid two-phase flow patterns. In addition, we use two network indices, which are the global subnetwork mutual information (I) and the global subnetwork clustering coefficient (C), to quantitatively measure the spatial coupling strength of different gas-liquid flow patterns. The gas-liquid two-phase flow pattern evolutionary behaviors are further characterized by calculating the two proposed coupling indices under different flow conditions. The proposed interconnected ordinal pattern network provides a novel tool for a deeper understanding of the evolutional mechanisms of the multi-phase flow system, and it can also be used to investigate the coupling behaviors of other complex systems with multiple observations.

Locating leakage in pipelines based on the adjoint equation of inversion modeling

This paper presents an adjoint method for locating potential leakage in a single-phase fluid pipeline based on the analytic solution of inversion modeling. By studying the mechanism of pipeline leakage pressure, the adjoint equation based on the governing equation of transient flow is established in the single-liquid phase aspect using inverse adjoint theory and sensitivity analysis method. The inverse transient adjoint equation is primarily derived from the single linear fluid pipeline in the semi-infinite domain. The Laplace method is then used to obtain an analytical solution that determines the location of pipeline leakage. The experimental results indicate that the analytic solution can quickly and accurately judge the leakage location of the pipeline. Furthermore, it presents a new approach to engineering applications, such as gas-liquid two-phase flow complex pipe networks, etc.

An improved void fraction prediction model for gas-liquid two-phase flows in pipeline-riser systems

This paper introduces the application of void fraction prediction models (25 models total) from a simple horizontal pipe to a complex, real pipeline. The flow mechanism in the pipeline-riser system is explained based on the upstream phase interface and differential pressure in the downstream riser. The transition boundaries of “stratified to intermittent flow” and “stratified smooth to stratified wavy flow” in the horizontal section were predicted well by the models based on simple pipes. Electrical capacitance volume tomography was used to obtain the upstream void fraction just before the downstream gas-liquid eruption, α1, or during multiple cycles, α2. The difference between α1 and α2 are ranging between −10 % and 5 %. Finally, an improved correlation is proposed based on the model found to be best (Lockhart and Martinelli, with error values (root mean square error (RMSE) and mean absolute percentage error (MAPE)) of about 12 % and 18 %); the corresponding error levels (RMSE and MAPE) amount roughly to 5 % and 10 %, respectively. The improved model also delivers a good prediction for α1 when separating in two flow regimes. Overall, the finally proposed correlation reads and is found superior in terms of accuracy to all previously published correlations.

Numerical investigation on effects of rib shapes on oscillating flow and heat transfer in heat exchanger channels

Ribs are straightforward and cost-effective to improve the heat exchange efficiency, and the impact of cooling medium driven by the movement of the channel itself can also enhance the heat exchange capacity. This paper investigates the flow patterns of the gas–liquid two-phase flows in a smooth cubic and a cubic with semi-circular ribs by using a high-speed camera under different low-frequency oscillations. A numerical simulation method was used to study the impact of different ribs on each wall at various frequencies. The microscopic flow and heat transfer characteristics of the ribs were analyzed. Reflux occurred near ribs, and the fluid accumulated near the wall under the ribs with a liquid coverage ratio of approximately 79%. After the gas–liquid two-phase flow fully oscillated in the channel, the heat transfer contribution of the top wall to the entire channel reached the greatest value of approximately 40%, and the left and right walls contributed to the entire channel with 21% and 27%, respectively. An additional inertial force of the two-phase flow was sustained in the oscillating environment to form a periodic impingement flow on the rib and hence improve heat transfer. Due to the reflux near the rib, the local minimum values of the heat transfer coefficient appeared on the downwind of the rib. The difference in rib design affected the fluid motion and caused different heat transfer distributions on the top surface. These findings can support the application design of ribs in dynamic heat exchange channels for optimizing heat transfer.

Hydrodynamics and Heat Mass Transfer in Two-Phase Dispersed Flows in Pipes or Ducts

Two-phase gas-liquid and gas-dispersed flows are frequently encountered in energy, nuclear, chemical, geothermal, oil and gas and refrigeration industries [...]

Two-dimensional Euler grid approximation method for multi-droplet motions

Multi-droplet motion in gas flow is a significant two-phase flow in nuclear power plant, chemical industry and environmental engineering. In contrast to droplet generation and collision in microscopic view, multi-droplet motion presents both macroscopic interaction with gas flow and microscopic single droplet motion characteristic. To simulate the multi-droplet motion, we presented the Euler grid approximation method to update the multi-droplet imformation in all nodes under each calculation step, and constructed a criterion about droplet velocity and direction to ensure droplet motion limited on grid nodes without loss of accuracy. The novelty of this method lies in its description of the virtual streamlines of multiple particles in the same state, as well as the discontinuous flow field composed of these virtual streamlines, furthermore, this method is independent of the time step and can simultaneously calculate droplet information of the entire field. Meanwhile, this paper focuses on the two-phase gas-liquid flow in the gravity separation space of the rotary vane separator, and the motion behavior of multiple droplets in the flow field is simulated based on the parameters of the actual steam-water separator.The result shows that the droplets tend to gather towards the center of the main stream, which is the focus area for droplet separation, and the position with higher density may be the point where the droplets collide. The model can be adopted to calculate the separation efficiency in the process of multi-droplet motion in the gravity separation space, rotary vane separator and other steam separation devices, and the results will help guide the optimal design of the separation device structure.

A New Contactless Cross-Correlation Velocity Measurement System for Gas–Liquid Two-Phase Flow

Based on the principle of Contactless Conductivity Detection (CCD), a new contactless cross-correlation velocity measurement system with a three-electrode construction is developed in this work and applied to the contactless velocity measurement of gas–liquid two-phase flow in small channels. To achieve a compact design and to reduce the influence of the slug/bubble deformation and the relative position change on the velocity measurement, an electrode of the upstream sensor is reused as an electrode of the downstream sensor. Meanwhile, a switching unit is introduced to ensure the independence and consistency of the upstream sensor and the downstream sensor. To further improve the synchronization of the upstream sensor and the downstream sensor, fast switching and time compensation are also introduced. Finally, with the obtained upstream and downstream conductance signals, the velocity measurement is achieved by the principle of cross-correlation velocity measurement. To test the measurement performance of the developed system, experiments are carried out on a prototype with a small channel of 2.5 mm. The experimental results show that the compact design (three-electrode construction) is successful, and its measurement performance is satisfactory. The velocity range for the bubble flow is 0.312–0.816 m/s, and the maximum relative error of the flow rate measurement is 4.54%. The velocity range for the slug flow is 0.161 m/s–1.250 m/s, and the maximum relative error of the flow rate measurement is 3.70%.

Multi-Physical Field Coupling Simulation and Experiments with Pulse Electrochemical Machining of Large Size TiAl Intermetallic Blade

Large size TiAl alloy blade is one of the important parts to reduce the weight of advanced aero-engines. However, the precision manufacturing of such blades is a challenge due to their large size, low ductility at room temperature, and high hardness of the TiAl alloy. Electrochemical machining (ECM) is a very promising method for the precision manufacturing of such blades, considering its unique advantages. In this study, a very comprehensive multi-physical field coupling simulation and pulse ECM experiments on large size TiAl alloy blades are carried out. Geometric and theoretical models involving electric fields, gas-liquid two-phase flow, heat transfer, and anodic dissolution are developed. The variation of bubble, temperature, electrolyte flow rate, and electrical conductivity at the outlet and the different areas on the blade surface with the processing time and distribution along the flow channel in the machining gap are revealed by simulation. It is found that the influence of electrolyte temperature on electrical conductivity is more dominant than that of bubble concentration. Finally, the experiments of pulse ECM on large size TiAl alloy blade are carried out, and the experimental results are analyzed in detail. The high efficiency and high surface quality of large size TiAl alloy blades are realized. The surface roughness and machining accuracy of the blade are about Ra 0.9 μm and 0.18 mm, respectively.