Experimental analysis of flow regimes and pressure drop reduction in oil–water mixtures

doi:10.1016/j.ijmultiphaseflow.2008.06.001

International Journal of Multiphase Flow

Volume 34, Issue 12, December 2008, Pages 1161-1174

https://doi.org/10.1016/j.ijmultiphaseflow.2008.06.001 Get rights and content

Abstract

The physical understanding of two-phase flow characteristics in horizontal pipes is of importance in the petroleum industry since significant savings in pumping power can be derived from the water-lubricated transportation of crude oil.

An experimental study of water continuous oil–water flow in horizontal pipes is performed using mineral oil and tap water of viscosity ratio about 900 and density ratio 0.9. A set of seven different pipes of Pyrex and Plexiglas where used, with diameters ranging between 21 and 40 mm. Pressure drop measurements, flow pattern maps and clear pictures of the oil–water flow are reported in this article together with comprehensive comments. The results obtained are compared to empirical laws, theoretical findings and experimental results by different authors in the literature.

In order to identify the regions with operational conditions that are suitable for applications, a novel criterion for the location of the annular/stratified transition is proposed which is based only on experimental observations.

Introduction

The widespread occurrence of multiphase flows in pipes has motivated extensive research in the field; a number of practical applications in the petroleum industry involve oil–water two-phase flow phenomena. Significant savings in the pumping power required for oil transportation can be attained when water flows in the pipeline together with the oil, especially when the highly viscous phase is surrounded by a water annulus, giving place to the core annular flow configuration. As the establishment of a particular flow regime depends upon the interaction of gravitational, inertial and surface tension forces, annular flow is observed only under particular combinations of the oil and water flow rates.

A large body of literature is presently available on the two-phase flow of oil and water in pipes. A thorough report of industrial applications and annular flow studies made before 1997 is provided in the review by Joseph et al. (1997). The same group published a book on the topic (Joseph and Renardy, 1993) and a number of papers ranging from the linear stability analysis for the pipe flow of two liquids (Joseph et al., 1984) to an experimental study including results from a 0.6-m diameter, 1000-m-long pipeline (Joesph et al., 1999). Measurements of pressure drop and holdup for water lubricated transportation of oil in a 15.9 mm glass pipeline are presented by Arney et al. (1993) together with a friction factor formula based on concentric cylindrical core annular flow theory and an empirical equation for the holdup volume fraction.

Ooms and co-workers analyzed the case of a wavy, eccentric core annular flow with a rigid core and a thin annulus; they interpreted the balancing of buoyancy with laminar (Ooms, 1972, Ooms and Beckers, 1972, Oliemans and Ooms, 1986, Ooms and Poesio, 2003) and turbulent (Oliemans et al., 1987) hydrodynamic lifting. The theory requires the knowledge of the wavy interface shape and is not easily applied for practical purposes. A two-fluid model which is instead commonly used to predict pressure gradient and holdup in liquid–liquid annular flows was devised by Brauner (1991). The following year Brauner and Maron (1992a) proposed a set of criteria for the transition between stratified, annular and dispersed flow patterns and tackled (1992b) the stability of stratified liquid–liquid flow. Recently Brauner developed a model for the dispersed flow regime (2001).

Angeli and Hewitt (1998) performed measurements on the flow of a low viscosity oil and tap water in two 25.4-mm pipes of stainless steel and acrylic resin; they noticed a large difference between pressure gradients measured in the two pipes and ascribed the effect to different wettability characteristics of the pipe walls. Also flow patterns were found to be substantially dependent on the pipe material, the flow structure being investigated through video recording and the use of a high frequency impedance probe (Angeli and Hewitt, 2000). The flow pattern maps provided by Angeli and Hewitt, 2000, Lovick and Angeli, 2004 and the transition criteria introduced by Brauner and Maron (1992a) are compared by Chakrabarti et al. (2007) to their own map. This was compiled using quantitative indications coming from a newly designed, non-intrusive optical probe in a 25.4-mm Perspex pipe. The comparisons between maps in the papers by Angeli and Hewitt, 2000, Chakrabarti et al., 2007 reveal a marked sensitivity of liquid–liquid flow results to the many experimental parameters.

When annular flow is used for the lubricated transportation of a highly viscous oil, the liquid–liquid system is more conveniently operated far from the transition boundary with the stratified flow. The reasons are twofold, first the pressure drop reduction is larger for higher oil flow rates and second, the undesired occurrence of the transition to the stratified configuration would lead to a sharp increase in the pressure drop. The accurate and reliable prediction of the transitional boundaries between annular flow and the stratified configuration is therefore of recognized importance for applications. Bannwart (2001) reports two necessary requirements for the occurrence of annular flow. The first comes from the paper by Joseph et al. (1984) but was derived under very specific conditions for density-matched liquids, the other was suggested by the Eötvös number classification of liquid–liquid systems but fails even to follow the trends observed experimentally. On the other hand the equations for the identification of annular/stratified transitional boundaries provided by Brauner and Maron (1992b) are not straightforward to use for engineering applications because only implicitly defined functions are provided. Moreover while the holdup ratio of the flow is needed to perform the required calculations, the holdup itself depends on the flow pattern.

This paper presents an experimental study of water continuous oil–water flows; the oil used in the experiments is very viscous, with properties similar to crude oil transported in industrial pipelines. The liquid–liquid pressure drop behavior is related to the superficial velocities of the two phases and their configuration; results are compared with empirical and theoretical models in the literature. Discussion is made of the influence on the two-phase flow features of the inlet mixer, the pipe material and its history; flow pattern maps are compiled for two different pipe diameters displaying results from a large number of experiments, together with clear pictures of the wide variety of flow configurations. A criterion for the location of boundaries between annular and stratified flow is provided. In contrast with other criteria for the annular/stratified transition (Joseph et al., 1984, Brauner and Maron, 1992b, Bannwart, 2001), the condition presented here arises only from experimental observation and does not include indications for the transition of different fluids over a very wide range of diameters. However the use of the suggested condition in applications is viable, requiring only a preliminary test to be performed using the same fluids. This would ensure an accurate indication of the most suitable flow parameters for lubricated transportation of highly viscous liquids over a wide range of conditions.

Section snippets

Experimental apparatus

The experiments reported in this paper were performed in the two-phase thermo-fluid dynamics laboratory of the Politecnico di Milano, in the liquid–liquid flow facility shown schematically in Fig. 1.

Governing parameters

The superficial velocity $J$ (m/s) is defined as the ratio between the volumetric flow rate of one single phase and the pipe section $J_{o} \equiv \frac{{\dot{V}}_{o}}{S}; J_{w} \equiv \frac{{\dot{V}}_{w}}{S}$ the mixture superficial velocity equals the average velocity of the mixture, $J_{ow} \equiv J_{o} + J_{w}$ .

In order to assess the pumping power economy relative to the single phase flow of oil, a pressure drop reduction factor, is defined as $R \equiv \frac{Δ p_{o}}{Δ p_{ow}}$ where $Δ p_{ow}$ is the measured pressure drop of the two-fluid flow and $Δ p_{o}$ is the pressure drop calculated using the

Flow regime observations

Fig. 5, Fig. 6, Fig. 7, Fig. 8 display the main stages of four experimental runs that are representative of most observations. Each picture series follows the progress of flow patterns when the oil superficial velocity $J_{o}$ is held constant while $J_{w}$ is decreased in steps. Fig. 5, Fig. 6, are relative to the 26-mm Plexiglas pipe. Pictures taken during experiments in the 40-mm Pyrex pipe are displayed in Fig. 7, Fig. 8.

In Fig. 5 the evolution from dispersed to wavy stratified regime is shown for $J_{o} =$

Conclusions

Water continuous oil–water flow is experimentally studied in seven different pipes of Plexiglas and Pyrex with diameter between 21 to 40 mm. The influence is discussed of the inlet mixer shape, the pipe material and its history on the flow features. The pressure drop behavior is evaluated in terms of the oil–water interface shape and its modifications caused by variations of the flow rates. Results in perfect annular flow are shown to compare well with two widely used fully developed flow models

Acknowledgments

The authors thank prof. M. Collins of the Brunel University for his careful proof-reading of the manuscript. The authors also thank prof. P. K. Das of the Indian Institute of Technology, Kharagpur, for the provision of the flow pattern map constructed experimentally and recently published (Chakrabarti et al., 2007). The map was not used here because of the too large difference in physical properties of the oil.

Financial support for this research was provided by the Italian MUR – Ministero

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Experimental analysis of flow regimes and pressure drop reduction in oil–water mixtures

Abstract

Introduction

Section snippets

Experimental apparatus

Governing parameters

Flow regime observations

Conclusions

Acknowledgments

Int. J. Multiphase Flow

Int. J. Multiphase Flow

J. Pet. Sci. Eng.

Int. J. Multiphase Flow

Int. J. Multiphase Flow

Int. J. Multiphase Flow

Chem. Eng. Sci.

J. Petrol. Sci. Eng.

Int. J. Multiphase Flow

Int. J. Multiphase Flow

Pressure gradient in horizontal liquid–liquid flows

Int. J. Multiphase Flow

Flow patterns in heavy crude oil–water flow

ASME J. Energy Resources Technol.