Gas-liquid Two-phase Slug Flow Research Articles

Characterization of Gas-Liquid Two-Phase Slug Flow Using Distributed Acoustic Sensing in Horizontal Pipes.

This article discusses the use of distributed acoustic sensing (DAS) for monitoring gas-liquid two-phase slug flow in horizontal pipes, using standard telecommunication fiber optics connected to a DAS integrator for data acquisition. The experiments were performed in a 14 m long, 5 cm diameter transparent PVC pipe with a fiber cable helically wrapped around the pipe. Using mineral oil and compressed air, the system captured various flow rates and gas-oil ratios. New algorithms were developed to characterize slug flow using DAS data, including slug frequency, translational velocity, and the lengths of slug body, slug unit, and the liquid film region that had never been discussed previously. This study employed a high-speed camera next to the fiber cable sensing section for validation purposes and achieved a good correlation among the measurements under all conditions tested. Compared to traditional multiphase flow sensors, this technology is non-intrusive and offers continuous, real-time measurement across long distances and in harsh environments, such as subsurface or downhole conditions. It is cost-effective, particularly where multiple measurement points are required. Characterizing slug flow in real time is crucial to many industries that suffer slug-flow-related issues. This research demonstrated the DAS's potential to characterize slug flow quantitively. It will offer the industry a more optimal solution for facility design and operation and ensure safer operational practices.

Experimental and numerical study on the flow characteristics of slug flow in a horizontal elbow

• Phase distribution of gas-liquid two-phase slug flow at the elbow was clarified. • Effects of elbow on flow characteristics at slug flow were analyzed. • A CFD simulation method for slug flow at the elbow was presented. Elbow is one of the most accident-prone locations in multiphase flow lines, and understanding the flow field is the key to manage the risk. In the meantime, the flow characteristics of slug flow are most complicated in the flow field. To clarify the flow characteristics of slug flow at the elbow, the influence and mechanism of the elbow on the flow characteristics of slug flow under different superficial gas-liquid velocities were studied with experiments and numerical simulations. A set of transparent loops with gas-liquid two phase flow was built. High-speed camera, pressure sensor, Doppler velocimeter and wire mesh sensor were used to collect flow pattern, pressure, slug velocity and gas-liquid distribution characteristics. In the meantime, the multi-fluid volume of fluid (VOF) model was used in the CFD simulations, and the simulation results were compared against the experimental results for verification. The results showed that the void fraction is highest at the inlet and lowest at the middle position, and the pressure and flow velocity at the extrados were higher than those at the intrados of the elbow. With the increase of the superficial liquid velocity and gas velocity, the number of bubbles accumulated inside the horizontal elbow increased, the slug flow liquid film velocity of the horizontal elbow increased, and the secondary flow phenomenon weakened.

Experimental slug flow dissipation analysis in pipes with horizontal to inclined downward direction change

This work presents an experimental study on the gas–liquid two-phase slug flow pattern in pipes with slight direction change. On offshore seabed and onshore lands, situations where the pipeline must conform to the terrain are common, thus causing direction changes and slight flow inclinations. The slug flow tends to modify itself by changing its features or even transitioning to another flow pattern. The pipe consisted of a horizontal section followed by a 26-mm ID, 35.6-m long downward inclined section with inclinations of either −3°, −5° or −7° with the horizontal. Air and water were the working fluids. Resistive sensors were installed at three different pipe locations in order to detect both phases and to obtain the slug flow characteristic parameters. A high-speed camera was used in two different locations to visualize and identify the flow phenomena along the pipe, and to obtain slug flow parameters as well. The slug dissipation mechanism occurs because the inclined downward section accelerates the liquid film; simultaneously, the slug upstream fills the space left while it also accelerates, thus generating bubble coalescence. Three different phenomena were observed when the slug dissipation occurs: the gas plug, stratification and wave flooding. It was also observed that the bubble rear velocity (VR) tends to exceed that of its front for several experimental points when those bubbles enter the downward section and the dissipation process begins – a phenomenon poorly explained in the literature. Each phenomena can be classified through the mixture Froude number and the superficial gas and liquid velocities as functions of the inclination angle. Finally, the present work brings a twofold contribution: phenomena so far superficially discussed in the literature were detailed with the aid of average values and probability density function (PDF) and, as a central point, the use of the dimensionless Froude number to classify the occurrence of these phenomena in the dissipation process.

Slug Flow Hydrodynamics Modeling for Gas–Liquid Two-Phase Flow in a Pipe

Gas–liquid flow in a pipeline is a very common. Slug two-phase flow is dominated in the case of slightly upward flow (+0.25°) and considered to be the comprehensive flow configuration, and can be in close contact with all the other flow patterns. The models of different flow patterns can be unified. Precise prediction of the slug flow is crucial for proper design and operation. In this paper, we develop hydrodynamics unified modeling for gas–liquid two-phase slug flow, and the bubble and droplet entrainment is optimized. For the important parameters (wall and interfacial friction factors, slug translational velocity and average slug length), the correlations of these parameters are optimized. Furthermore, the related parameters for liquid droplet and gas bubble entrainment are given. Accounting for the gas–liquid interface shape, hydrodynamics models, i.e., the flat interface model (FIM) and the double interface model (DIM), of liquid film in the slug body are applied and compared with the experimental data. The calculated results show that the predictions for the liquid holdup and pressure gradient of the DIM agree with experimental data better than those of the FIM. A comparison between the available experimental results and Zhang’s model calculations shows that the DIM model correctly describes the slug dynamics in gas–liquid pipe flow.

Prediction of the instability for simply supported pipes conveying gas-liquid two-phase slug flow

The characteristics of the natural frequency for simply supported pipes conveying gas-liquid two-phase slug flow is studied to reveal the critical conditions which would bring out the instability of the piping system. The intermittent features for local flow parameters are emphasized to analyze the dynamics of the piping system. Some correlations are introduced to uncover the inherent regularity for the flow characteristics and the vibration performances of the slug flow piping system. The results show that the variations for the natural frequency could be matched by the correlations which are adopted to research the natural frequency of the single-phase pipes with an optimal appropriate adjustment coefficients and two-phase slug flow parameters. Critical conditions which lead to the instability for the pipelines could be predicted by a dimensionless velocity which is related to the correlations if the optimal appropriate adjustment coefficients are employed.

Rescaled range permutation entropy: a method for quantifying the dynamical complexity of gas–liquid two-phase slug flow

Uncovering dynamic nonlinearity of slug flow with structural mutation constitutes a challenging problem of significant importance. Rescaled range permutation entropy has been recently introduced as a novel measure for the complexity of chaotic time series with extreme volatility by assigning the ratio of range and standard deviation as the weight for each extracted vector. However, previous work is only limited to the analysis of typical chaotic systems. The intrinsic advancement and the potentiality in the practical application of the algorithm still need to be investigated. Therefore, we firstly conduct simulations on characterizing impulsive signal to demonstrate that rescaled range permutation entropy is more appropriate to characterize the extreme volatility of system. In addition, a practical application in experimental signals analysis of gas–liquid two-phase slug flow is discussed in detail. The result shows that the rescaled range permutation entropy has a significant discernibility for the flow structure sudden change and paves a new way on investigating the dynamic complexity of slug flow.

Experimental investigation on gas-liquid two-phase flow distribution characteristics in parallel multiple channels

The maldistribution in the parallel multiple channels commonly leads to a reduction in the efficiency of heat exchangers and even causes the equipment to suffer the risk of failure. The distribution of gas–liquid two-phase slug flow when moving through the parallel multiple channels is still an unresolved problem and limited by the lack of experimental data. This work presents an experimental study on the air–water two-phase flow in the parallel multiple channels characterized by a horizontal header and 2~4 vertically-upward branches under the slug flow pattern conditions and develops a new predictive model. The effects of inlet superficial velocities and branch number have been investigated in the ranges of inlet gas and liquid superficial velocities of 1.4~25.0 and 0.2~1.7 m·s−1, respectively. Results show that the distribution characteristics of two-phase slug flow highly depend on the inlet superficial velocities which manifest in that the peak value of liquid phase flow ratio shifts to the downstream branches sequentially as the inlet superficial velocities increase. Decreasing the branch number is beneficial for the uniform distribution of each phase. A predictive model is proposed for the distribution of air–water slug flow by considering the effects of superficial velocities and branch number. The new model achieves the agreement of 91.3% for prediction distribution within the range of ±25% when compares to present experimental data and shows good agreement with previously reported experimental results of both T-junction and parallel multiple channels. The acquired experimental data and predictive model are crucial for the development and optimal design of the heat exchangers.

Research on the Dynamic Responses of Simply Supported Horizontal Pipes Conveying Gas-Liquid Two-Phase Slug Flow

The dynamic responses of simply supported horizontal pipes conveying gas-liquid two-phase slug flow are explored. The intermittent characteristics of slug flow parameters are mainly considered to analyze the dynamic model of the piping system. The results show that the variations of the midpoint transverse displacement could vary from periodic-like motion to a kind of motion whose amplitude increases as time goes on if increasing the superficial gas velocity. Meanwhile, the dynamic responses have certain relations with the vibration acceleration. By analyzing the parameters in the power spectrum densities of vibration acceleration such as the number of predominant frequencies and the amplitude of each peak frequency, the dynamic behaviors of the piping system like periodicity could be calculated expediently.

Distribution of gas-liquid two-phase slug flow in parallel micro-channels with different branch spacing

An experimental study was conducted in order to investigate the phase distribution of gas-liquid slug flow in six parallel micro-channels in presence of different branch spacing of 0.8 mm(2d), 4 mm(10d) and 12 mm(30d), respectively. The parallel micro-channels was composed of a header with hydraulic diameter of 0.48 mm and six branch channels with hydraulic diameter of 0.40 mm, all with rectangle cross sections. The entire test section was machined by PMMA to facilitate flow visualization. Nitrogen and 0.03 wt% sodium dodecyl sulfate (SDS) solution at ambient pressure and room temperature were used as the test fluids. A flow-regime map of bubbly, slug, slug-annular and annular was generated, covering the range of gas and liquid inlet superficial velocities of 0.28 ≤ JG ≤ 33.3 m/s and 0.008 ≤ JL ≤ 2.52 m/s, respectively. The phase distribution experiments of slug flow were conducted. The fluid dynamics of two-phase flow splitting in parallel micro-channels was captured by high speed recording technique. It was found that the phase distribution characteristics of two-phase flow in parallel channels highly depend on the inlet flow conditions and the distance between channels. Besides, the effect of branch spacing took on distinct characteristics under different inlet flow conditions. At low mass flux and quality, the increase of branch spacing can facilitate the liquid phase to flow into channels at the rear part of the header, while the first three channels are more supplied with liquid as the branch spacing increasing at high mass flux and quality. Specially, an improvement of gas distribution was also observed with the increase of the branch spacing. Finally, a correlation capable of predicting the liquid phase distribution of slug flow in parallel micro-channels was developed.

Experimental Study on the Pressure-Difference Fluctuation Characteristics of Slug Flow in Horizontal T-Junction Tube

In practice, two-phase fluids may be often mixed or separated in T-junction tube, and alternating flow of liquid plug and gas plug may cause its working condition to be complex and changeable, so the study on it needs to be improved at present. Based on air and water, the characteristics of response to differential pressure of gas-liquid two-phase slug flow in two kinds of transparent horizontal T-junction PMMA tubes with different diameters are studied in depth and systematically, and data collection and processing are conducted by LabVIEW platform to obtain the probability density function (PDF) and the power spectral density function (PSD) characteristics of slug flow in different periods and forms. All characteristics provide reliable slug flow identification for flow pattern recognition.

Natural frequency analysis of a cantilevered piping system conveying gas–liquid two-phase slug flow

Intermittent feature of slug flow could lead to variations of flow parameters varying with time and position along the pipe, which may influence dynamic behaviors of the piping system a lot. By considering significance of intermittent characteristic for two-phase slug flow, a dynamic model of a cantilevered horizontal piping system conveying gas–liquid two-phase slug flow is established to analyze natural frequency of the piping system. The functions indicated the variations of flow parameter associated with position along the pipe and time are calculated to analyze various forms of integral equations which are caused by the intermittent characteristic of slug flow to obtain the final matrix equation. The results show that intermittent feature of slug flow could have a crucial impact on natural frequency of the piping system especially when both of superficial gas velocity and pipe length are large enough. Flow parameters in gas bubble zone could play a more significant role for these cases. In addition, it is stated that in the process of predicting natural frequency by the correlation utilized in piping system conveying single-phase flow, it is pivotal to select appropriate forms of liquid holdup and liquid velocity.

Heat transfer correlation for two-component two-phase slug flow in horizontal pipes

Two-component gas-liquid two-phase slug flow in horizontal pipes occurs frequently in many industrial applications. The flow and heat transfer characteristics are complicated due to the intermittent flow structures. In view of the importance of predicting the heat transfer characteristics of the slug flow, this present study aims at developing a semi-theoretical heat transfer correlation for two-component two-phase slug flow based on the concept of Reynolds and Chilton-Colburn analogies. Firstly, an extensive literature review on existing databases and correlations of heat transfer coefficient for two-component two-phase slug flow was conducted. More than 500 experimental data and 8 heat transfer correlations were collected. The comparison between collected database and correlations indicated that none of the correlations could estimate the whole database satisfactorily. The relationship between heat transfer and pressure drop was investigated theoretically and an improved semi-theoretical heat transfer correlation was developed based on Reynolds and Chilton-Colburn analogies and the collected experimental results from 16 sources. The comparison analysis demonstrated that the newly-developed correlation achieved an excellent predictive capability with a wide range of test conditions. The newly developed correlation demonstrated that 91.5% of the data was predicted within ±30% error with the mean absolute relative deviation of 14.0%. In addition, the extension of the new correlation to other horizontal two-phase flow regimes was discussed. The newly-developed semi-theoretical correlation would be useful for predicting heat transfer coefficient of two-component two-phase flow in horizontal pipes.

Study on the natural frequencies of pipes conveying gas-liquid two-phase slug flow

The natural frequencies of pipes conveying gas-liquid two-phase slug flow are explored, considering the significance of the intermittent characteristic of gas-liquid two-phase slug flow particularly. The widely used finite element method is employed to establish the theoretical model. The local flow parameters including local gas and liquid velocities and local liquid holdup varying with time and position along the pipe are calculated to obtain the global matrix equation of motion for the system. An experiment is conducted to verify the theoretical model. The results show that the time domain values of the natural frequency would keep periodicity during the process of slug flow. Root-mean-square of the natural frequency is employed to represent the characteristic of the piping system natural frequency. If the superficial gas velocity is given, root-mean-square of the natural frequency will decrease with the increase of the superficial liquid velocity. Yet if the superficial liquid velocity is given, root-mean-square of the natural frequency will increase firstly and then decrease with the increase of superficial gas velocity when the superficial liquid velocity is relatively large. Thus a critical superficial gas velocity exists for these conditions. The range of superficial gas velocity that could make the system become unstable may be predicted preliminarily by analyzing this critical value. Sufficient analysis of slug parameters and geometrical parameters of the pipes is necessary to obtain the critical superficial gas velocity. Finally, the divergent instability of the piping system is predicted based on the above analysis.

M815 マイクロチューブ内気液二相スラグ流に関する研究 : 管内径と粘度による影響(GS5 流体工学(3),修士研究発表セッション)

M815 マイクロチューブ内気液二相スラグ流に関する研究 : 管内径と粘度による影響(GS5 流体工学(3),修士研究発表セッション)

Slug flow in a vertical narrow rectangular channel – Laminar and turbulent regimes in the main flow and turbulent regime in the wake region of the Taylor bubble

Using a high-speed video-camera system, the hydrodynamics of gas–liquid two-phase slug flow in a vertical narrow rectangular channel with the cross section of 2.2 mm × 43 mm is investigated for laminar and turbulent regimes in the main flow and turbulent regime in the wake region of the Taylor bubble. The Taylor bubble velocity is the function of the liquid slug length ahead of it, and the interaction of Taylor bubbles for turbulent flow in main flow is weaker than that for laminar flow. Thus, two correlations are proposed for laminar and turbulent flows, respectively. The distributions of Taylor bubble velocity, lengths of liquid slug and Taylor bubble are all positively skewed. The lengths of Taylor bubble and the liquid slug decrease with the liquid flow rate. The Taylor bubble length increases with the gas flow rate, whereas the influence of the gas flow rate on the liquid slug length is insignificant.

Measurment of gas-liquid two-phase slug flow with a Venturi meter based on blind source separation

We propose a novel flow measurement method for gas–liquid two-phase slug flow by using the blind source separation technique. The flow measurement model is established based on the fluctuation characteristics of differential pressure (DP) signals measured from a Venturi meter. It is demonstrated that DP signals of two-phase flow are a linear mixture of DP signals of single phase fluids. The measurement model is a combination of throttle relationship and blind source separation model. In addition, we estimate the mixture matrix using the independent component analysis (ICA) technique. The mixture matrix could be described using the variances of two DP signals acquired from two Venturi meters. The validity of the proposed model was tested in the gas–liquid two-phase flow loop facility. Experimental results showed that for most slug flow the relative error is within 10%. We also find that the mixture matrix is beneficial to investigate the flow mechanism of gas–liquid two-phase flow.

Hydrodynamics of slug flow in a vertical narrow rectangular channel under laminar flow condition

The hydrodynamics of gas–liquid two-phase slug flow in a vertical narrow rectangular channel with the cross section of 2.2mm×43mm is investigated using a high speed video camera system. Simultaneous measurements of velocity and duration of Taylor bubble and liquid slug made it possible to determine the length distributions of the liquid slug and Taylor bubble. Taylor bubble velocity is dependent on the length of the liquid slug ahead, and an empirical correlation is proposed based on the experimental data. The length distributions of Taylor bubbles and liquid slugs are positively skewed (log-normal distribution) at all measuring positions for all flow conditions. A modified model based on that for circular tubes is adapted to predict the length distributions in the present narrow rectangular channel. In general, the experimental data is well predicted by the modified model.

Prediction of Gas-Liquid Two-Phase Slug Flow Characteristics in Vertical Small Diameter Pipes by a One-Dimensional Two-Fluids Model

The purpose of this study is to improve a two-fluid model applicable to gas-liquid two-phase slug flows in small diameter pipes. Experimental data on void fraction and frictional pressure drop were obtained for vertical upward slug flow in 5 and 9 mm i.d. circular pipes, and those on the interfacial friction force were obtained by substituting the above data into an equation derived from the one-dimensional two-fluids model. The test liquids were a Poly-oxy-ethylene lauryl ether water solution, a 72 wt% glycerin water solution and a tap water at 30 °C, while the test gas was air at near atmospheric pressure. In order to study the effects of liquid properties, surface tension of the test liquids against air was varied from 0.042 to 0.071 N/m, viscosity was 0.797 to 19.6 mPa·s, and density was 996 to 1184 kg/m3. The range of volumetric flux of the liquid was 0.1 to 2.0 m/s, and that of the gas was 0.1 to 22 m/s. In addition, the drag coefficient data of the large bubble was determined from the data on the bubble diameter, the respective void fractions of the liquid slug section and the large bubble one, the interfacial friction force and the length ratio of the large bubble to the total of the liquid slug and the large bubble. These data were used to validate representative two-fluid model codes. Since the prediction by the codes did not fit well the present data, a new drag coefficient correlation for the large bubble was proposed using 5 dimensionless parameters. The predictions by the new correlation and familiar ones in literatures have been tested against the present data, and the applicability of the new one has been demonstrated.

Application of CIP Method to Two-Phase Flow in Gas-Lift Pumping System to Reduce Numerical Dissipation

ABSTRACT Traditionally, the gas-lift pump has been used to lift corrosive or toxic liquid due to its simple structure and maintenance. It is also considered as a technically feasible system for the development of deep-ocean minerals and as a means to sequestrate CO2 gas into the ocean. Donor-cell scheme or Forward-Time and Backward- Space (FTBS), which is commonly used in solving advection equation such an air-lift system, is very stable but causes numerical diffusion. In order to reduce numerical diffusion or dissipation by the first-order upwind scheme, CIP (Cubic-Interpolated Propagation/Constrained Interpolation Profile) has been applied to bubble and slug flows in a gas-lift pumping system. The GLAD (Gas-Lift Advanced Dissolution) model using CIP method were validated by comparison with gas-liquid two-phase experiments and the effectiveness of the CIP method over the typical one has been demonstrated from the simulation of gas-liquid two-phase slug flow.

MNM-5A-2 T型マイクロ流路混合部における均一スラグ流形成過程(セッション 5A ナノ・マイクロ異相界面センシングと制御)

The detailed processes of gas-liquid two-phase slug flow formation with microchannel T-junctions were investigated for the purpose of improving the uniformity of generated monodisperse flow patterns. Experiments were performed using different types of T-junctions to study how the shapes of the channel cross-sections and the wettabiliry of the channel walls affect the fluid slug formation processes at the mixing part, under conditions of relatively high flow velocities. The three dimensional flow field of the T-junction with cylindrical channels was calculated by a numerical simulation based on the phase-field model and compared with the experimental observation. It was found that the laminar gas conduits developed in rectangular channels have a tendency to take longer time to be broken than in the case of cylindrical channels. This trend was enhanced for the hydrophobic channels and affected the size uniformity of the generated fluid slugs.