Slug Body Research Articles

Slug flow identification using ultrasound Doppler velocimetry

An identification method of slug flow using velocity information was developed in this study. Quantitative analysis of two-phase (gas–liquid) flow in horizontal pipes is essential to ensure the stability of, or to accurately estimate the thermal performance of, various engineering applications. The spatial and temporal distribution of liquid velocity was measured using the ultrasound Doppler velocimetry method under various superficial velocity conditions. From the results, we proposed a novel identification method of the slug flow based on two parameters, the maximum velocity and maximum velocity difference ratio. In addition, the ratio of liquid velocities between the slug body and the bubble area is a reliable parameter for separating the slug flow from the plug flow. Finally, we investigated the changes in moving velocity, slug frequency, and slug length along the pipeline under a wide range of superficial velocities. The results show that the liquid and gas superficial velocity have the opposite effect on the slug frequency, except in the downstream region. It was found that the slug body length increased continually along the pipe and the average slug body length in the downstream region is invariant regardless of the gas superficial velocity.

Experimentally investigating the flow characteristics of airlift pumps operating in gas-liquid-solid flow

Experiment were conducted to investigated the flow characteristics of airlift pumps operating in gas-liquid-solid three phase flow by employing a high-speed camera. The results showed that an intermittent flow consisted of film and liquid slug appeared in airlift pumps with a flow frequency of 1.5–2.6 Hz. The portion of slug body directly determined the performance of airlift pumps due to a large particle concentration. In practical engineering, some methods for increasing the portion of slug body could be used for better pump performance.

Slug Translational Velocity for Highly Viscous Oil and Gas Flows in Horizontal Pipes

Slug translational velocity, described as the velocity of slug units, is the summation of the maximum mixture velocity in the slug body and the drift velocity. Existing prediction models in literature were developed based on observation from low viscosity liquids, neglecting the effects of fluid properties (i.e., viscosity). However, slug translational velocity is expected to be affected by the fluid viscosity. Here, we investigate the influence of high liquid viscosity on slug translational velocity in a horizontal pipeline of 76.2-mm internal diameter. Air and mineral oil with viscosities within the range of 1.0–5.5 Pa·s were used in this investigation. Measurement was by means of a pair of gamma densitometer with fast sampling frequencies (up to 250 Hz). The results obtained show that slug translational velocity increases with increase in liquid viscosity. Existing slug translational velocity prediction models in literature were assessed based on the present high viscosity data for which statistical analysis revealed discrepancies. In view of this, a new empirical correlation for the calculation of slug translational velocity in highly viscous two-phase flow is proposed. A comparison study and validation of the new correlation showed an improved prediction performance.

Hessian-based topology of two-phase slug flow

Experimental slug flow is analyzed through a topological method and proper orthogonal decomposition (POD). Local phase fractions of a pipe cross-section, acquired through X-ray tomographic reconstruction, are analyzed by extracting critical points via eigenvalues associated with the Hessian matrix. Based on the sign of the eigenvalues, three types of critical points are classified: local maxima, minima and saddle points. Reduced order descriptions (ROD), obtained as results of the POD, are examined to investigate the flow dynamics associated with the primary eigenfunctions with respect to critical point placement and frequency. Voronoï mapping is employed for visualization of the critical points as a function of cross-sectional location, and quantification of the spatial distribution of the critical points. For each classification, the sum of the respective critical point over the cross-section correlates to the temporal holdup of liquid within the pipe. The local maxima and minima display an increase in their occurrence that relates to the liquid slug passage while the local saddle points follow the profile of the liquid holdup as the bubble is forming after the passage of the liquid slug. The total number of local maxima per snapshot as a function of time peaks prior to the liquid holdup peak, displaying the instabilities present in the flow field preceding the liquid slug reaching the top of the pipe. Applying critical point theory to the RODs reveals that the first two modes capture the liquid-dominated slug body region and the Taylor bubble/liquid film region. The observed time lag between the total maxima and the liquid holdup provides opportunity for implementation of predictive control monitoring in fields where instabilities influence system mechanics.

Study of viscous oil-water-gas slug flow in a horizontal pipe

Slug flow characteristics for viscous oil-water-air are experimentally conducted in a horizontal pipe with a 40 mm ID and 12 m long. Results of experimental pressure gradient are presented, taking into account μo = 0.83 Pa s. Water and air superficial velocities ranged 1.02–2.1 m s−1 and 0.22–1.9 ms−1, respectively. Flow patterns were observed and images captured by using a video camera. New data-sets on such flow that include translational bubble velocity, slug and elongated bubble length, total slug unit length and frequency are experimentally provided. Translational velocity of slug units is measured by means of optical probes and video camera, and compared with modified Nicklin (1962) correlation. Moreover, slug body, elongated bubble and slug unit lengths are measured by optical probe. Statistical analysis was adopted based on measured data of slug length to develop probability density function (PDFs). As a result, a new expression to compute slug unit length in terms of liquid, gas superficial velocities, and pipe diameter was proposed.

Estimating slug liquid holdup in high viscosity oil-gas two-phase flow

Slug flow is one of the most critical and often encountered flow patterns in the oil and gas industry. It is characterised by intermittency which results in large fluctuations in liquid holdup and pressure gradient. A proper understanding of its parameters (such as slug holdup) is essential in the design of transport facilities (e.g. pipelines) and process equipment (slug catchers, separators etc.). In this paper, experimental investigation of slug liquid holdup (defined as the liquid volume fraction in the slug body of a slug unit) is performed. Mineral oil with viscosity, μ=−0.0043T3+0.0389T2−1.4174T+18.141 and air were used as test fluids. A 0.0254 m and 0.0762 m pipe internal diameters facilities with pipe lengths of 5.5 and 17 m respectively were used in the study. Electrical Capacitance Tomography was used for slug holdup measurements. Results obtained in the study shows that slug liquid holdup varied directly as the viscosity and inversely as the gas input fraction. Existing slug holdup correlations and models in literature did not sufficiently predict present experimental results. A new empirical predictive correlation for estimating slug liquid holdup was derived from present experimental databank and from data obtained in literature. The databank's liquid viscosity ranges from 0.189 to 8.0 Pa s. Statistical analysis of the new correlation vis-à-vis existing ones showed that the present correlation gave the best performance with an average percent error, E1; absolute average percent error, E2 and standard deviation, E3 of 0.001, 0.05 and 0.07 respectively, when tested on the high viscosity liquid–gas databank.

CFD modeling of air and highly viscous liquid two-phase slug flow in horizontal pipes

Computational Fluid Dynamics (CFD) approach, encoded on STAR-CCM+ was used to simulate air and highly viscous liquid two-phase slug flow in a 50.8-mm (2-in.) ID and 18.9-m (62ft.) long horizontal pipe. The Volume of Fluid (VOF) method, the Continuum Surface Force (CSF) model and the High Resolution Interface Capturing (HRIC) scheme were utilized. The liquid viscosity varied from 161 to 567mPas based on the experimental data. A sensitivity analysis of the sharpening factor, the angle factor, and the Interface Momentum Dissipation (IMD) model was carried out to obtain the best combination of parameters. Besides, comparison with experimental measurements concerning slug frequency, average liquid holdup and velocity profiles in the liquid region was carried out. The comparison shows a fair correspondence in terms of behavior and magnitude with the experimental results obtained by Particle Image Velocimetry (PIV). It was observed that the velocity profile is not fully developed near the bottom of slug body. Finally, extrapolation to the extended velocity conditions was performed. The reduction of translational velocity at higher mixture velocity conditions was investigated. A certain amount of gas passing from the tail to the front of the liquid slug body was visualized by three-dimensional velocity profiles. This phenomenon supports the reduction of translational velocity above the certain level of mixture velocity.

Detailed flow field measurements and analysis in highly viscous slug flow in horizontal pipes

The flow field in the liquid phase of high viscous oil and air two-phase slug flow were experimentally studied under laminar flow conditions. The horizontal 50.8-mm (2-in.) inner diameter (ID) pipe and high viscous fluid properties (nominal viscosity values of 0.51, 0.68 and 0.96 Pa·s) yielded mixture Reynolds numbers less than 100. Particle Image Velocimetry (PIV) was used to analyze the flow field in the liquid phase. Slug characteristics were measured with a set of capacitance probes (CPs). Based on velocity field measurements, the liquid phase mass is shown to be conserved within the slug body despite local spatial variations in volumetric flow rate – flow accelerates at slug front and decelerates at slug tail. A low momentum region inside the liquid phase is identified beneath the mixing zone and close to the lower sections of the pipe. Furthermore, based on a frame of reference moving with the slug front, a pair of vortices is identified at the slug front. Accordingly, a continuous increase in velocity is observed at lower sections of the pipe. From CPs data, the flow coefficient is estimated to be between 2.11 and 2.20 for the elongated bubble flow, and 2.26 and 2.38 for the slug flow in laminar conditions investigated. From PIV data, the flow coefficient is estimated to be between 2.28 and 2.34 for the elongated bubble, and 2.31 and 2.44 for the slug flow, respectively. These slight differences in the obtained values of flow coefficient indicate the possible misunderstands of 1.13 multiplier.

Characterization of flow dynamics and reduced-order description of experimental two-phase pipe flow

Two-phase dispersed and slug flows in a pipe are investigated using proper orthogonal decomposition (POD). The data are acquired through tomographic reconstruction of X-ray measurements, where holdup, cross-sectional phase distributions and phase interface characteristics are obtained. Instantaneous phase fractions of the flow fields are analyzed and reduced-order descriptions of the flow are achieved. The dispersed flow displays coherent features for the first few modes near the center of the pipe, representing the liquid-liquid interface location while the slug flow case shows coherent features that correspond to the cyclical formation of the slug in the first ten modes. For slug flow, the first two modes capture the liquid-dominated slug body region and the Taylor bubble/liquid film region, respectively. The reconstructions of the fields indicate that main features are observed in the low order descriptions utilizing less than one percent of the degrees of freedom of the full order descriptions. POD temporal coefficients a1, a2 and a3 show interdependence for the slug flow case. The coefficients also describe the phase fraction holdup as a function of time for both dispersed and slug flow.

Gas–Liquid Two-Phase Flow Velocity Measurement With Continuous Wave Ultrasonic Doppler and Conductance Sensor

Flow velocity is an important process parameter that quantifies the volume or mass flow rate as well as monitors the process safety. To nonintrusively measure the flow velocity of horizontal gas–liquid two-phase flow, an ultrasonic Doppler sensor and a conductance sensor with dedicated measurement models are presented. The air superficial flow velocity can be directly obtained and the water superficial flow velocity can be calculated through a two-fluid model for bubble flow and plug flow. For slug flow, a correlation between the phase velocity in slug body and overall superficial flow velocity was built based on a slug closure model. In order to eliminate the influence of the changing velocity profile in the fluid, the sample volume was designed to cover the whole pipe cross section by installing a two-chip piezoelectric transducer with 1-MHz center frequency at the bottom of the pipe. The conductance sensor provided water holdup estimate to compensate the velocity measurement model. Experiments were carried out in a 50-mm inner diameter pipe to verify the proposed sensor and model. Three flow patterns (bubble flow, plug flow, and slug flow) were generated by adjusting the inlet flow rate of the air and the water. The results show that the mean relative error can achieve within 5%.

Visualization of gas-liquid multiphase pseudo-slug flow using Wire-Mesh Sensor

Intermittent two-phase flows are commonly encountered in the petroleum industry. Much attention has been focused by several researchers on intermittent flows existing at low superficial gas velocities (<10 m/s). There is limited work performed on intermittent structures persisting at higher superficial gas velocities (pseudo-slug flows). In the present experimental study, a conductivity-based Wire-Mesh Sensor (WMS) was used to visualize and characterize pseudo-slug flow. Experiments were performed in a 76.2 mm horizontal pipe with air and water as the working fluids at atmospheric conditions. The superficial gas and liquid velocities ranged from 9 m/s to 35 m/s and 0.45 m/s to 0.76 m/s, respectively. A 16 × 16 WMS was placed 17 m away from the pipe inlet to measure spatio-temporal void-fraction distribution. The WMS data acquisition frequency was set to 10 kHz.From the void-fraction time series data, the periodic pseudo-slug structures were visualized. The visualization suggested that unlike slug flow where the liquid structures fill the pipe cross-section, the pseudo-slugs were extremely aerated structures (high gas-liquid mixing) formed due to the gas penetration into the liquid slug body. This paper also presents the measurements of important hydrodynamic characteristics such as cross-sectional averaged void-fraction time series and mean void fraction. The effect of liquid viscosity on the visualized structures is also presented.

Statistical assessment of experimental observation on the slug body length and slug translational velocity in a horizontal pipe

In the present paper, the flow was examined experimentally using air and water as the working fluids. Various sets of experimental data were conducted by varying the superficial velocities within ranges of 0.7–3.84m/s and 0.7–1.33m/s for air and water, respectively. The slug characteristics were acquired and computed using non-intrusive optical based technique. Statistical analysis for the translational slug velocity and slug body length was conducted. It was observed that for fixed water velocity, the slug length was increasing with increasing the superficial air velocity while the slug frequency decreased. However, for fixed superficial air velocity, the slug length was decreasing with increasing the water velocity whiles the slug frequency increased. Also, it was found that when the superficial water velocity increased the slugs were formed and initiated far downstream from the pipe inlet section. Furthermore, the obtained results were found in a good agreement with the previous empirical correlation.

Positive frictional pressure gradient in vertical gas-high viscosity oil slug flow

The present study describes the wall shear stress and the falling liquid film behavior in upward vertical slug flow of air and high viscosity oil. The frictional pressure gradient is directly related to the wall shear stress, and it is usually negative (opposite to the overall flow direction). However, in vertical slug flow, the average total wall shear stress of a slug unit may be negative (in the same direction of the overall flow), resulting in a positive frictional pressure gradient. However, this does not mean, by any way, generation of additional energy or violation of the second law of thermodynamics.The positive frictional pressure gradient phenomenon, reasons and required conditions were explained in this paper. A simplified model was developed and validated against recent experimental data of air-high viscosity oil slug flow in a 50.8mm ID vertical pipe. The oil viscosity was in the range of 127mPas to 580mPas. Positive frictional pressure gradient appears when the liquid film wall shear stress supersede the wall shear stress in the slug body. The rate of increase of both wall shear stresses (with respect to the mixture Reynolds number) depend, not only, on the mixture Reynolds number but also, highly, on the liquid viscosity.

Investigation and prediction of slug flow characteristics in highly viscous liquid and gas flows in horizontal pipes

Slug flow characteristics in highly viscous liquid and gas flow are studied experimentally in a horizontal pipe with 0.074m ID and 17m length. Results of flow regime map, liquid holdup and pressure gradient are discussed and liquid viscosity effects are investigated. Applicable correlations which are developed to predict liquid holdup in slug body for low viscosity flow are assessed with high viscosity liquids. Furthermore, a mechanistic model is developed for predicting the characteristics of slug flows of highly viscous liquid in horizontal pipes. A control volume is drawn around the slug body and slug film in a slug unit. Momentum equations with a momentum source term representing the significant momentum exchange between film zone and slug body are applied. Liquid viscosity effects are considered in closure relations. The mechanistic model is validated by comparing available pressure gradient and mean slug liquid holdup data produced in the present study and those obtained from literature, showing satisfactory capabilities over a large range of liquid viscosity.

Two-Fluid Model for 1D Gas-Liquid Slug Flows: Realizable Mean Slug Characteristics

A two-fluid model is used to predict hydrodynamic characteristics of gas water two phase slug flows in horizontal pipes. Mathematical models are averaged based forms of balance laws with algebraic source terms. Predictions of transient solutions of such systems rely on accurate evaluations of speeds and scales of discontinuities generated by the flow conditions. Slug flows are characterized through slug frequencies, slug lengths and slug translational velocities. These are the fundamental quantities that one has to estimate in order to assess the validity of the model and the numerical method in use. A two fluid 1D model for slug flows along with an AUSMDV* numerical scheme is able to predict correct translational speeds, lengths scales and frequencies of slugs. An adaptive mesh refinement AMR procedure based on Kelvin-Helmholtz stability condition as an indicator for refinement is used to speed up simulations. The marginal Kelvin-Helmholtz stability condition is also introduced to estimate slug body length.

Evaluating the effects of high liquid viscosity and flow variables on horizontal oil–gas slug flows by gamma radiation method

Hydrodynamic slug fl ow is the commonest fl ow regime observed in high viscosity liquid–gas horizontal pipelines over a wide range of different fl ow conditions. Hydrodynamic slugging tends to generate large vibrations that may impose structural instability or even damage oil production pipelines. For that reason, there is a need to investigate high viscosity slug fl ow regime to understand its complex characteristics. This is pertinent when considering that existing slug fl ow models used in the petroleum industry to design production pipelines are not suitable for predicting the behaviour of high viscosity oil–gas fl ow. In this study, the effects of liquid viscosity and fl ow variables on slug fl ow regime were investigated experimentally through the analysis of two key parameters—slug frequency and slug body liquid holdup, both measured with a gamma densitometer. Comparison of the measured slug parameters to existing correlations revealed that slug body liquid holdup correlations were in close agreement with high viscosity experimental data. However, none of the existing slug frequency correlations used was able to produce accurate predictions. A new empirical correlation for slug frequency was proposed. Compared with existing correlations, the newly proposed correlation performed much better in predicting slug frequency of high viscosity liquid–gas fl ows.

A Mechanistic Slug-Liquid-Holdup Model for Different Oil Viscosities and Pipe-Inclination Angles

Summary Slug liquid holdup is one of the most important parameters of slug flow. It is closely related to the average liquid holdup and pressure gradient of slug flow in wells and pipelines. The mechanistic models of Barnea and Brauner (1985) and of Zhang et al. (2003a) are based on the turbulent liquid-slug assumption for low-viscosity oils. However, for high-viscosity oil, the liquid slug is laminar because of the low slug Reynolds number. In this study, a mechanistic slug-liquid-holdup model is developed for low- and high-oil-viscosity slug flows. The model is based on two shear mixings: shear mixing between the slug front and pipe wall and shear mixing between the slug body and liquid film. The model uses slug-flow characteristics that can be calculated by solving the continuity and momentum equations of slug flow. A data bank consisting of 418 slug-liquid-holdup measurements that were obtained from various authors is used to analyze and validate the model. In the data bank, liquid viscosity ranges from 0.0016 to 0.589 Pa·s (1.6 to 589 cp). Pipe-inclination angle ranges from −30° to upward vertical. Pipe inside diameter varies from 5.08 to 10 cm. Statistical evaluations are conducted and compared with predictions of other models, and significant improvement is observed in the performance of the new model.

Study on hydrodynamic slug flow mitigation with wavy pipe using a 3D–1D coupling approach

Objective: A wavy pipe, constructed by connecting standard piping bends in series in one plane, was proposed to mitigate hydrodynamic slug flows in horizontal pipelines.Methods: A newly developed 3D–1D co-simulation method, i.e. STAR–OLGA coupling, was applied to model the wavy-pipe systems experiencing hydrodynamic slug flows. The flow characteristics of slug flow in the wavy-pipe systems were investigated, through which the effectiveness of the wavy pipe on slug mitigation and how the geometrical parameters of the wavy pipe affecting its performance were examined.Results: It has been found that the upstream liquid slug degenerates into a ‘liquid dense zone’ of a greater length downstream of the wavy pipe due to the gas penetration into the slug body. A reduction of the effective density of the ‘liquid dense zone’ from that of the upstream slug body is resultant showing the effectiveness of the wavy pipe on mitigating the adverse impacts of slug flow. The amount of gas penetration increases then decreases with the increasing wavy-pipe amplitude indicating that excessively higher amplitude is even less effective on introducing the gas phase into the slug body. Differently a wavy pipe with the same amplitude but a greater length, i.e. more bends, is more effective.Conclusions: It is concluded that the wavy pipe works by reducing the effective density of the liquid slug through introducing the gas phase into the slug body in the push-out process in each bend of the wavy pipe and the geometrical parameters need to be designed properly in specific applications.

Expression of Asn-d-Trp-Phe-NH2 in the brain of the terrestrial slug Limax valentianus

The tripeptide Asn-d-Trp-Phe-NH(2) (NdWFamide) is a D-amino acid-containing cardioexcitatory peptide initially isolated from Aplysia. Previously we detected NdWFamide immunoreactivity in the visceral giant cells, the largest neurons in the brain of the terrestrial slug Limax located at the dorsal surface of the visceral ganglia. In the present study, we further analyzed the morphological features of these neurons by an intracellular injection of Lucifer yellow, and found that these neurons extend neurites out of the brain through at least 5 nerve bundles. We then isolated a gene and a cDNA clone potentially encoding a NdWFamide precursor, and investigated expression at the levels of mRNA and protein in Limax. The NdWFamide gene consists of 5 exons spanning at least 17 kb of the genome, and its open reading frame extends over 3 exons. The spatial expression pattern of NdWFamide mRNA was almost identical to that of the NdWFamide peptide, with some minor discrepancies in between. Although the most remarkable expression was evident in the visceral giant cells, we also found the expression of NdWFamide mRNA and peptide in the cerebral and pedal ganglia. These results suggest the involvement of NdWFamide in the regulation of a broad area of the slug's body.

Internal representation and memory formation of odor preference based on oscillatory activities in a terrestrial slug

The terrestrial slug Limax exhibits a highly developed ability to learn odors with a small nervous system. When a fluorescent dye, Lucifer Yellow (LY), is injected into the slug's body cavity after odor-taste associative conditioning, a group of neurons in the procerebral (PC) lobe, an olfactory center of the slug, is labeled by LY. We examined the relationships between conditioning strategies and LY labeling. The positions of LY-labeled neurons in the PC lobe after appetitive conditioning were more apical than those after aversive conditioning and did not depend on the conditioned odor, suggesting that the biological value of odors affected the position of LY-labeled neural clusters. A simple computational model consisting of two layers of oscillators with electrical synaptic interaction was constructed based on physiological features of the PC lobe, and showed that the oscillatory phase difference between the layers contributed to determination of the positions of LY-labeled neurons, suggesting that phase difference in oscillatory activity plays a role in the association of odors and the preference for them.