Characteristics of horizontal liquid–liquid flows in a circular pipe using simultaneous high-speed laser-induced fluorescence and particle velocimetry

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Abstract

This paper describes a set of experiments on liquid–liquid flows in a horizontal circular tube. The liquids used in the experiments were an aliphatic hydrocarbon oil (Exxsol D80) and an aqueous solution of glycerol. The concentration of glycerol in the solution was adjusted so that the two liquids had the same refractive index, and optical distortions due to the curvature of the (transparent) circular tube test section were corrected for with the use of a graticule technique. The test section was far downstream of an inlet section that established an initially stratified co-current flow of the two immiscible liquids, with the Exxsol D80 oil flowing over the glycerol/water solution. The flows were investigated at the test section with the application of laser-based optical diagnostic methods, which included high-speed simultaneous Planar Laser Induced Fluorescence (PLIF), Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV). These techniques allowed the reliable evaluation of the nature of the investigated horizontal liquid–liquid flows (i.e., the flow patterns from phase distribution information), together with the detailed spatiotemporally resolved measurement of key flow characteristics such as phase and velocity distributions, and also of important parameters such as droplet size. The resulting PLIF images provide a clear indication of the distribution of the phases within a plane that passed through the channel centreline, and are used to obtain qualitative information about the arising flow patterns. The images were also used quantitatively to generate data on phase distribution, in situ phase fraction, interface level and droplet size distribution. Much of the PLIF data on in situ phase fraction and interface level agrees well with predictions from a simple stratified laminar–laminar flow model. The particle velocimetry methods (PIV and PTV) provide data on the velocity profiles in the investigated flows. Over the range of superficial velocity conditions investigated, the velocity profile in the lower (heavier and more viscous) glycerol/water solution phase was typically characteristic of laminar flow, whereas in the upper (lighter and less viscous) Exxsol D80 oil phase the profile often showed a shape characteristic of turbulent flow.

Highlights

► Initially-stratified co-current horizontal liquid–liquid pipe flows have been studied. ► High-speed PLIF and PIV/PTV have been used for characterisation of the liquid–liquid flows. ► Eight flow regimes were identified from observations. ► Phase distribution, phase fraction, interface level, drop size and velocity profiles were measured. ► Measurements were compared with square cross-section results and simple laminar models.

Introduction

The co-current flow of two immiscible liquids is encountered in wide variety of industrial applications. The investigation described here arose in the context of liquid–liquid flows in subsea pipelines in petroleum production facilities, where the fluids are oil and water. The water can either occur naturally in the reservoir (this is known as “connate water”) or result from water injection into the reservoir to increase pressure and in turn enhance oil recovery (EOR).

An ability to characterise liquid–liquid flow behaviour accurately is of fundamental importance. For example, the accurate prediction of the in situ phase fraction in two-phase flows allows the determination of numerous other flow parameters, such as the two-phase density and viscosity, which are key requirements for the closure of multiphase models for predicting the flow behaviour, particularly the pressure drop and flow pattern transitions, both of which are dependent on the in situ phase fraction.

The importance of liquid–liquid flows led to a number of studies of such flows (see for example Russell and Charles, 1959, Arirachakaran et al., 1989, Soleimani, 1999). However, these studies concentrated on measurement of overall parameters such as pressure gradient and phase holdup; the objective of the study described here was to obtain much more detailed information about the liquid–liquid flow behaviour using modern optical techniques. By matching the refractive index of the two liquids and adding a fluorescent dyestuff to one of the phases, it is possible to determine the phase distribution using planar laser-induced fluorescence (PLIF). Liu (2005) reports a study of vertical downflow of liquid–liquid flow mixtures using PLIF and, more recently, Morgan et al. (2012) reported the application of the technique to horizontal liquid–liquid flows. Though the studies of Liu (2005) and Morgan et al. (2012) revealed a number of interesting new phenomena, they were both carried out with channels which had a square cross-section in order to avoid optical distortion of the image by the curved (transparent) tube wall; this is clearly not typical of the real applications in which tubes of circular cross-section are the norm. In the present study, circular tubes were used together with an automated method for correcting the images for distortion.

One of the most powerful tools for the study of fluid flow and mixing is laser-induced fluorescence (LIF). Liu (2005) employed PLIF to visualise co-current liquid–liquid downward flows and produced images with a strong and clear distinction between the two phases. Liu (2005) used these images to study the phase inversion phenomena that arose in the investigated flows and found an ambivalent range over which the identity of the continuous phase alternates in time between the one fluid and the other. This was extended to the more practical case of horizontal liquid–liquid flows in an original study by the present authors (Morgan et al., 2012). However, phase information on its own lacks the ability to provide a full insight into the numerous facets of these multiphase flows. The current work is a development of that described in Morgan et al. (2012), not only in the use of a circular cross-section tube (a square cross-section tube was used in the work described in Morgan et al. (2012), and also in the work by Liu (2005)), but also in the extension of the laser measurement techniques to include, in addition to PLIF, Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV). To the best of the author’s knowledge, the results described in the present paper are the first obtained in liquid–liquid systems using simultaneous PLIF and PIV/PTV. This enhanced measurement capability, and specifically the use of PIV/PTV, enables the detailed diagnostic inspection of the co-current liquid–liquid flow velocity profiles. In addition, the use of a tube with a circular cross-section is more representative of industrial pipeline systems, although it is optically more challenging. The provision of detailed data in the circular pipe section has been made feasible by the utilisation of an image correction technique involving the use of a graticule (printed target screen), as described in detail in Section 2.3.1.

The analysis performed on the data (i.e., images) generated by the laser-based techniques, and the associated results presented in this paper, are similar to those used to characterise the square section flows presented in Morgan et al. (2012). However, herein the quantitative analysis is extended to include results for the velocity profiles in the flow. A qualitative analysis of the results, including images of the flow regimes observed and a flow regime map constructed from the flow regime classifications, is presented first in Section 3.1. Following this, the subsequent sections present the results from the quantitative analyses of the flow images, as follows: (i) vertical phase distribution profiles (in Section 3.2); (ii) in situ phase fractions (in Section 3.3); (iii) interface level data (in Section 3.4); (iv) droplet size distribution results (in Section 3.5), and (v) velocity profiles (in Section 3.6). Finally, the main conclusions from this work can be found in Section 4.

Section snippets

Experimental methods

This section presents the experimental flow facility, test liquids, measurement procedure and post-processing analysis, and includes a description of the apparatus (inlet, main test section, visualisation section), optical measurements (laser system, camera, synchronisation system) and the image processing methodology used in the present study. Specifically, the flow facility, related experimental procedures and flow conditions are presented in Section 2.1. Section 2.2 discusses the rationale

Results

The PLIF and PIV/PTV measurements presented in this section have provided a wealth of new insights into the flow structure of liquid–liquid horizontal flows, as well as unique information concerning interfacial behaviour and the underlying velocity fields in the investigated flows. Characteristic images from the flow regimes observed are presented in Section 3.1, which also includes a flow regime map constructed from the observations. In the succeeding sections the results of the quantitative

Conclusions

The experimental campaign that is the subject of this paper is a development of the square duct section study presented in Morgan et al. (2012). However, the current investigation makes use of a test section with a geometry more representative of practical/industrial pipeline systems (i.e., with a circular cross-section). In addition, the characterisation of the investigated flows has been developed herein to include both phase information and velocity profiles.

The following main conclusions

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