Experimental investigation of the effect of 90° standard elbow on horizontal gas–liquid stratified and annular flow characteristics using dual wire-mesh sensors

https://doi.org/10.1016/j.expthermflusci.2014.08.001Get rights and content

Highlights

  • The effect of the 90° bend was to slightly increase the cross-sectional time averaged void fraction value.

  • The contour plots show that the cross-sectional distribution of the void fraction changes after the elbow.

  • The velocities of the periodic structures were obtained.

  • Probability density function was used to discriminate non-stable stratified flows from stratified wavy flow.

Abstract

Fluid flowing through pipelines often encounters fittings such as elbows. Although it is true that two-phase flow patterns observed in elbows are qualitatively the same as those seen in straight pipes, the presence of a pipe elbow can modify relative positions and local velocities of the two phases as they are subjected to forces in addition to those encountered in a straight pipe. That redistribution can affect pressure drop values, chemical inhibitor concentration and distribution to the top of the pipe, as well as the erosion pattern occurring from solid particles such as sand that is entrained in oil and gas transportation pipelines. In this work, a wire-mesh sensor technique based on conductance measurements of void fraction was applied to investigate two-phase pipe flow through a standard elbow. The horizontal flow test section, consisting of a 76.2 mm ID, 18 m long pipe, was employed to generate stratified-wavy and annular flow conditions. Two 16 × 16 wire-mesh configuration sensors were positioned either 0.9 m upstream or 0.6 m downstream of the bend. The experiments were conducted at superficial liquid velocities equal to 0.03 m/s and 0.2 m/s and superficial gas velocities that ranged from 9 m/s to 34 m/s. The effects of liquid viscosity on the measured parameters are also investigated using two different viscosities of 1 and 10 cP. Stratified–slug transition, stratified wavy and annular flow patterns were observed visually in the clear section placed upstream of the wire-mesh sensors. Analysis of time series void fraction data from the dual wire-mesh sensors allows the determination of mean void fraction, local time average void fraction distribution, liquid phase distribution around the tube periphery, interfacial structure velocities, as well as probability density function characteristic signatures within the cross-section of pipe before and after the elbow. The results indicate that the distribution of gas and liquid phases and interfacial velocities are significantly altered even 20 diameters downstream of the elbow.

Introduction

Pipe fittings such as valves, bends, elbows and tees significantly affect multiphase flow distribution including generation of secondary flows, fluctuations in void fractions and pressure losses and velocities of gas and liquid phases. Elbows are often used in oil and gas production systems, and they cause redistribution of gas and liquid which can affect distribution of corrosion inhibitors within and downstream of the bends. Elbows are also a location susceptible to the impact of particles along the outer radius. The requirements for optimal design and evaluation of Computational Fluid Dynamics (CFD) codes and two-phase flow models such as VOF (volume of fluid) that are being developed to predict details of multiphase flows force the need for quantitative information about flow through and downstream of elbows.

Previous research available in literature concerning gas–liquid flows through elbows, only dealt with an accurate calculation of pressure drop [1], [2]. However, in applications such as erosion-corrosion of pipe bends, more detailed information of multiphase flow such as velocities of liquid and gas phases and interfaces are typically required. Computational Fluid Dynamics (CFD) codes are successfully used to predict pipe erosion and even corrosion through elbows and piping systems involving a single-phase carrier fluid. Erosion is a gradual removal of the material from the pipe wall due to the repeated impacts of solid particles entrained in the production fluids. Previous investigations have proved that erosion is more prominent in pipe elbows than straight pipes [3], [4]. Also, it is known that the mechanism of erosion in multiphase flows depends on the flow pattern [5], [6] and distribution of gas and liquid phases.

Similarly, flowing fluids can significantly affect corrosion rates in a number of ways including accelerated mass transfer of reactants and corrosion products [7], distribution of corrosion inhibitors, fluid shear or impingement of solid particles in the fluid which disrupt protective layers [8]. Also, if CO2 is present in the transported fluids, steel pipelines can be corroded as this process is probably enhanced by slug flow turbulence [9]. The ability to model and predict pipeline degradation processes allows service intervals to be better timed so as to reduce unnecessary checks while not being subject to costly downtime due to equipment failure. Also, modeling ability can be applied at the design stage to reduce the susceptibility of parts to wear.

The problem of determining the flow characteristics in the downstream section of an elbow also is important in design and hydrodynamic analysis of the fluid transportation systems. When flow enters the curved portion, the heavier density phase is subjected to a large centrifugal force, which causes the liquid to move away from the center of curvature. This redistribution can significantly affect erosion/corrosion processes. In order to predict complex erosion and corrosion patterns within pipe bends, predictions of gas and liquid velocities are becoming a growing target for many investigators dealing with erosion and corrosion issues. This along with relative success of CFD codes to predict details of multiphase flow, including gas and liquid velocities have motivated investigators to conduct local measurements in multiphase flow. Thus, these investigation efforts were concentrated to obtain experimental data that are required for evaluation of CFD models as they are being used to calculate details of multiphase flows. Reliable CFD calculations can help investigators to predict erosion and corrosion in multiphase flow through bends and other geometries as data and more information become available.

In this experimental study, a dual wire-mesh sensor (WMS) technique based on conductance measurements has been utilized to gather data and investigate details of two-phase flows in a pipe before and after a 90° horizontal-to-horizontal elbow. The wire-mesh sensor allows detailed measurement of the two-phase flow due to its outstanding spatial and temporal resolution. From the measurements, specific parameters of interest such as cross-section time-averaged void fraction, local time-averaged void fraction distribution, gas–liquid interfacial characteristics, probability density functions and periodic structure of interface velocities have been extracted that can be used by investigators to evaluate and improve capabilities of CFD codes to predict details of multiphase flows.

Section snippets

Background on characterization of multiphase flows in elbows

In horizontal pipes carrying gas–liquid two-phase flows, gravity introduces asymmetry to the flow regimes generated. The density difference between the phases causes the liquid to travel preferentially along the bottom of the tube. In stratified flow, the liquid travels along the bottom of the pipe while the gas passes over it. At low velocities, the interface between the gas and liquid is smooth. At higher gas velocities, the shearing action of the gas at the interface generates small

Experimental facility, program and data analysis procedures

A dual wire-mesh sensor (Fig. 1) is used to take cross-sectional phase distribution measurements. The sensor and the corresponding electronics were supplied by the HZDR Dresden, Germany. In this study, a dual 16 × 16 wire configuration sensor was used as depicted in Fig. 2.

Each wire-mesh sensor consists of two parallel wire layers perpendicular to the pipe axis. The wires are made of stainless steel 316L. The spacing between the layers is 1.5 mm; each layer is built of sixteen 0.125 mm diameter

Sequence of cross-section images

Fig. 7, Fig. 9, and Fig. 8, Fig. 10 show the sequence of instantaneous cross-sectional void fraction for stratified–slug transition, stratified wavy and annular flows, respectively, obtained by the WMS raw data upstream and downstream the elbow. The experimental conditions are shown in the flow pattern map previously described (Fig. 5). Fig. 7 shows that in stratified–slug transition flows, waves can grow very fast, and some of them can fill the tube generating pseudo-slugs. Fig. 7d shows that

Conclusions

A dual wire-mesh sensor was used to detect the local instantaneous cross-sectional distribution of the phases in gas–liquid stratified and annular flows in the upstream and downstream locations of a standard elbow. Data were obtained for a wide range of flow rates in the horizontal orientation. Detailed information of the void fraction distribution and interfacial structures are obtained. The signals of the sensor, that are proportional to the liquid conductivity, are processed to obtain void

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