1,720,974 research outputs found
One-dimensional model of wet gas flow in near-horizontal pipes
No full textA one-dimensional model able to predict the film distribution around the pipe wall under conditions typical of wet gas flow in near-horizontal pipes is presented. The model is based on the assumption that i) liquid droplets can only be entrained by die gas from the thick liquid layer flowing at pipe bottom and ii) the deposition of smaller droplets is related to an eddy diffusivity mechanism, while larger droplets deposit by gravitational settling mainly on the pipe bottom. The presence of a thin liquid film all around the pipe wall significantly affects the pressure gradient along the pipe. The present model is a new component of MAST (Multiphase flow Analysis and Simulation of Transitions), a transient. 1-D flow simulator developed for advanced flow assurance studies
Prediction of the liquid film distribution in stratified-dispersed gas-liquid flow
No full textA mathematical model for predicting the circumferential liquid film distribution in stratified-dispersed flow is presented. Objective of the model is to describe the typical flow conditions of wet gas transportation in long, near-horizontal pipelines. In these applications, depending on the gas velocity and pipe diameter, a large asymmetry of the liquid film distribution may arise. The model is based on the assumption that in stratified-dispersed flow, liquid droplets can only be entrained by the gas from the thick liquid layer flowing at pipe bottom. It is also assumed that the deposition of smaller droplets is related to an eddy diffusivity mechanism and regards the entire pipe circumference, while larger droplets deposit by gravitational settling on the pipe bottom. These assumptions explain the formation of a thin, non-atomizing film in the upper part of the pipe. The presence and flow structure of this film appreciably affect the pressure gradient and the liquid hold-up in the pipe and are of great importance in flow assurance studies. The model has been validated against i) the experimental observations recently published by Pitton et al. (2014), the data collected by ii) Laurinat (1982), iii) Dallman (1978), and iv) the predictions of three-dimensional CFD simulations conducted by Verdin et al. (2014). It is shown that the relevant mechanisms which are responsible for the liquid film distribution are the gravitational film drainage, droplet entrainment/deposition and wave spreading. In particular, at high gas velocities and/or small pipe diameters, the asymmetry of the liquid film diminishes owing to the wetting mechanism of wave spreading which makes the distribution of the film more uniform in the circumferential direction. As the gas velocity diminishes and/or for larger pipe diameters, wave spreading is less effective and for these flow conditions only gravitational drainage and droplet entrainment/deposition are responsible for the more asymmetric shape of the liquid film
Is it possible to reduce the cost (and increase the accuracy) of multiphase flow meters?
No full textIn the present paper, the development of a low cost multiphase flow meter, designed for an oil field where it seemed advisable to avoid the use of radioactive sources, is illustrated. This meter combines the measurements of the pressure differential through a multiphase oritice and in a vertical tube. When required, the composition of the liquid- liquid mixture can be determined with a semi-continuous sampling method. Before installation, a set of tests, performed in a laboratory where it is possible to operate at actual field conditions, allowed the development of two semi-empirical equations that enable an accurate evaluation of the gas and liquid flow rates
Motion of elongated gas bubbles over a horizontal liquid layer
No full textThe drift velocity of a gas bubble penetrating into a quiescent viscous liquid which initially fills a short pispe has been determined both with a commercial CFD software and with a 1-D transient flow simulator. The results obtained agree fairly well with the limited published data and show that the phenomenon under investigation is more complex than expected. In particular it has been found that for large liquid viscosity, the bubble drift velocity decreases along the pipe. This result, which can be easily justified on a theoretical ground, poses some clear limitations to the use of the standard closure equations used to model gas-liquid slug flow in pipes
Flow regime independent, high resolution multi-field modelling of near-horizontal gas-liquid flows in pipelines
No full textFor fully-developed two-phase flows, maps that correlate experimental and semi-empirical expressions for flow regimes are widely used. For calculations of the various important two-phase flow parameters, this in turn requires correlations for various interfacial and wall interaction effects that are flow regime dependent. For many systems of practical interest, however, the evolution of flow regimes (such as slug flow in oil-gas pipelines) is of interest because the development lengths are long and flow regimes may change in regions where pipeline inclination changes due to the terrain. It is shown here that for slow transients in near-horizontal pipes, the one-dimensional multi-field model, when solved with sufficient resolution, does not require flow regimes to be specified or flow regime dependent closure relationships. The formulation predicts the development of flow regimes and various flow parameters without the need for maps, or the need to change closure relationships. To accomplish this, the model includes four fields, i.e. continuous and dispersed liquid, continuous and dispersed gas, as well as a set of appropriate closure relationships from the literature. For the main application considered here, i.e. slow transients in oil-gas pipelines, order of magnitude analyses indicate that certain inertial terms in the model are very small and can be neglected in comparison to the others. Advantage is taken of this to simplify both the structure of the mathematical problem and the solution procedure, which is sufficiently accurate that mass is conserved for each of the four fields. Furthermore, the calculations require high spatial resolution, so a fast, easily-parallelizable numerical procedure has been applied. The results indicate that the development of certain flow regimes, including transitions from bubbly to stratified flow and vice versa, slug flow including slug frequency and length, and the evolution of these parameters along a pipeline are well predicted by the model when compared to experimental data. As part of the validation it is also shown that the model predicts, without need to change closure relationships, flow regimes in fully-developed near-horizontal two-phase flows in good agreement with existing flow regime maps. This suggests that for slow transients in flows for which one-dimensional effects dominate, predictions can be made without requirements for flow regime maps and closure relationships that depend on them. (C) 2008 Elsevier Ltd. All rights reserved
Prediction of the transition from stratified to slug flow or roll-waves in gas-liquid horizontal pipes
No full textIn stratified gas-liquid horizontal pipe flow, growing long wavelength waves may reach the top of the pipe and form a slug flow, or evolve into roll-waves. At certain flow conditions, slugs may grow to become extremely long, e.g. 500 pipe diameter. The existence of long slugs may cause operational upsets and a reduction in the flow efficiency. Therefore, predicting the flow conditions at which the long slugs appear contributes to a better design and management of the flow to maximize the flow efficiency. In this paper, we introduce a wave transition model from stratified flow to slug flow or roll-wave regimes. The model tracks the wave crest along the pipe. If the crest overtakes the downstream wave end before hitting the top of the pipe, a roll-wave is formed, otherwise a slug. For model validation we performed measurements in air-water horizontal pipe flow facilities with internal diameters of 0.052 and 0.06 m. Furthermore, we made numerical calculations using a transient one-dimensional multiphase flow simulator (MAST) which adopts a four-field model. The model presented in this paper successfully predicts the evolution of waves and their transition into either slugs or roll-waves. It also predicts the formation time of slugs and roll-waves with a satisfactory agreement. (C) 2009 Elsevier Ltd. All rights reserved
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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