Showing papers on "Slug flow published in 2002"

An Expermentally Validated Model for Two-Phase Pressure Drop in the Intermittent Flow Regime for Noncircular Microchannels

Abstract: We report the development of an experimentally validated model for pressure drop during intermittent flow of condensing refrigerant R134a in horizontal microchannels. Two-phase pressure drops were measured in five circular channels ranging in hydraulic diameter from 0.5 mm to 4.91 mm. For each tube under consideration, pressure drop measurements were first taken over the entire range of qualities from 100% vapor to 100% liquid. In addition, the tests for each tube were conducted for five different refrigerant mass fluxes between 150 kg/m 2 -s and 750 kg/m 2 -s. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were then used to identify data that corresponded to the intermittent flow regime. A pressure drop model was developed for a unit cell in the channel based on the observed slug/bubble flow pattern for these conditions. The unit cell comprises a liquid slug followed by a vapor bubble that is surrounded by a thin, annular liquid film. Contributions of the liquid slug, the vapor bubble, and the flow of liquid between the film and slug to the pressure drop were included

Single tube downhole densitometer

Abstract: A measurement device is provided that determines fluid properties from vibration frequencies of a sample cavity. In one embodiment, the measurement device includes a sample flow tube, vibration source and detector mounted on the tube, and a measurement module. The sample flow tube receives a flow of sample fluid for characterization. The measurement module employs the vibration sources to generate vibrations in the tube. The measurement module combines the signals from the vibration detector on the tube to determine properties of the sample fluid, such as density, viscosity, compressibility, water fraction, and bubble size. The measurement module may further detect certain flow patterns such as slug flow, for example. To measure the sample fluid density, the measurement module determines the resonant frequency of the sample flow tube. The density can then be calculated according to a formula that compensates for the temperature and pressure of the system. Other fluid properties may be determined from the sample tube's resonance peak amplitude, peak width and/or peak shape. Variation of the density measurements may be used to detect and characterize multiple phase fluid flow. The use of a vibration source and detector are arranged in a manner that greatly enhances the accuracy and reliability of the measurement device over a range of temperatures, pressures, and shock accelerations such as those that may be found downhole in a well.

Active Feedback Control as a Solution to Severe Slugging

Abstract: Severe slugging in multiphase pipelines can cause serious and troublesome operational problems for downstream receiving production facilities. Recent results demonstrating the feasibility and the potential of applying dynamic feedback control to unstable multiphase flow like severe slugging and casing heading have been published. 1 - 5 This paper summarizes our findings on terrain-induced slug flow.5 Results from field tests as well as those from dynamic multiphase flow simulations are presented. The simulations were performed with the pipeline code OLGA2000.* The controllers applied to all of these cases aim to stabilize the flow conditions by applying feedback control rather than coping with slug flow in the downstream processing unit. The results from simulations with feedback control show stable process conditions at both the pipeline inlet and outlet in all cases, whereas without control, severe slug flow is experienced. Pipeline profile plots of the liquid volume fraction through a typical slug flow cycle are compared against corresponding plots with feedback control applied. The comparison is used to justify the internal stability of the pipeline. In many cases, feedback control enables a reduced pipeline inlet pressure, which, again, means an increased production rate. This paper summarizes the experience gained with active feedback control applied to severe slugging. The focus is on extracting similarities and differences between the cases. The main contribution is demonstrating that dynamic feedback control can be a solution to the severe slugging problem.

Novel method for non-intrusive measurement of velocity and slug length in two- and three-phase slug flow in capillaries

Abstract: A method was developed for the measurement of velocity and slug length of two and three-phase flow in capillaries. The method consists of the combination of an impedance meter and two IR sensors. Non-intrusive measurement of the velocity can be performed with the IR sensors in gas-liquid and gas-liquid-liquid flow using the intrinsic absorbance of the fluids. In liquid-liquid (1-1) flow the velocity is measured using a combination of reflectance and refraction of the IR light on the 1-1 interfaces. The three phases, gas, organic liquid and water, can be discriminated and their slug lengths determined with the combination of the signal from the impedance meter and the IR sensor.

Fluid mechanics of Taylor bubbles and slug flows in vertical channels

Abstract: Fluid mechanics of Taylor bubbles and slug flows is investigated in vertical, circular channels using detailed, three-dimensional computational fluid dynamics simulations. The Volume of Fluid model with the interface-sharpening algorithm, implemented in the commercial CFX4 code, is used to predict the shape and velocity of Taylor bubbles moving along a vertical channel. Several cases are investigated, including both a single Taylor bubble and a train of bubbles rising in water. It is shown that the potential flow solution underpredicts the water film thickness around Taylor bubbles. Furthermore, the computer simulations that are performed reveal the importance of properly modeling the three-dimensional nature of phenomena governing the motion of Taylor bubbles. Based on the present results, a new formula for the evaluation of bubble shape is derived. Both the shape of Taylor bubbles and the bubble rise velocity predicted by the proposed model agree well with experimental observations. Furthermore, the present model shows good promise in predicting the coalescence of Taylor bubbles.

Multiphase fluid conveyance system

Abstract: A mixture of fluids is conveyed from a hydrocarbon reservoir and comprises gas and slugs of liquid. The mixture is passed through a slug catcher vessel (12) which temporarily retains the slugs of liquid. The gas is passed through a gas compressor (18) and the resulting pressurised gas is then conveyed to a remote location. When a liquid slug is detected in the slug catcher vessel (12), liquid is drawn from the vessel by an injector device (24) and entrained into the flow of gas downstream of the gas compressor (18) until the level of the interface between the gas and liquid in the vessel reaches a sufficiently low level. The system may alternatively be configured to accommodate a flow of liquid containing slugs of gas.

Fast wire-mesh sensors for gas-liquid flows - Visualisation with up to 10 000 frames per second

Abstract: A wire-mesh sensor developed by the Forschungszentrum Rossendorf produces sequences of instantaneous gas fraction distributions in a cross section at a rate of up to 10 000 frames per second and a spatial resolution of about 2-3 mm. This sensor was applied to an upwards air-liquid flow in a vertical pipe of 51.2 mm diameter. After a brief introduction of the functioning of the sensor, the paper presents results obtained in a at vertical pipe operated with an air-water mixture. Two wire-mesh sensors with a measuring matrix of 24 x 24 points (resolution 2 mm) were placed in a small axial distance behind each other. They were used to study the flow structure in the transition region from bubble to slug flow at an imaging frequency of 2 500 Hz. The two available measuring planes allowed to obtain velocity profiles of the gaseous phase. A sensor with 16 x 16 points (resolution 3 mm) was applied to visualize the transition from bubbly via churn turbulent to annular flow with 10 000 frames per second. In the churn flow region, periodic plug-like structures were found. In the annular flow the sensor is able to resolve wispy structures. (authors)

Fluid slug launcher

Abstract: Apparatus for projecting a slug of liquid through the air. The invention provides means to accelerate liquid molecules to substantially equal velocity, and then to release them with minimal turbulence.

The Stability of Fluid Production From a Flexible Riser

Abstract: The prediction of slug formation and slug size has a direct impact on the design of topsides separation facilities and flow assurance to the process. The characteristics and stability of liquid production from an S-shaped riser have been studied over a range of pressures, focusing on the implications for flow management. Severe slugging in an S-shaped riser gives a period of no liquid production, followed by a period of steady production and finally a large production spike, many times larger than the steady production. In contrast with classical severe slugging in a vertical or catenary riser the transient spike is broken into two parts as a direct consequence of the bend in the line. Little attention has been paid to transition-type flows (between stable and unstable flow) previously. Data collected to date has shown transition flows have production peaks that are larger than those of classical severe slugging.

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

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

Analytical solution for conjugated heat transfer in pipes and ducts

Abstract: The conjugated transient forced convection heat transfer for laminar, thermally developing, steady slug flow of a Newtonian fluid of constant thermal properties in semi-infinite pipes and ducts is considered. An analytical solution for three cases, namely (1) steady state, (2) transient with negligible axial conduction in the wall and (3) early stages of time, is obtained.

An experimental technique for the analysis of slug flows in pneumatic pipelines using pressure measurements

Abstract: An experimental technique has been developed to measure the flow characteristics of slugs in dense phase pneumatic conveying using pressure measurements. This method is based on the unique characteristics of slug flows in pipes, i.e., an axial pressure fluctuation along the pipeline and a pressure difference in the radial direction at the back of a slug. Standard differential pressure transducers were used in this study and the influence of the finite response time of these transducers was considered. Experiments were conducted over a range of gas-solids flow conditions and experimental data were analyzed to describe the behavior of solids slugs through pipes. The calculated slug velocity and length using axial pressure measurements were confirmed by video recordings, and the synthesis between axial and radial pressure signals showed reasonable agreement in flow pattern analysis. This relatively simple measuring technique has been found effective in detecting solids slugs traveling through horizontal pipes and will distinguish various flow regimes. It provides a useful and easily applied tool for system optimizing and benchmarking in industrial applications.