Oil-water two-phase flow in microchannels: flow patterns and pressure drop measurements

Canadian Journal of Chemical Engineering, Dec, 2008 by Abdelkader Salim, Mostafa Fourar, Jacques Pironon, Judith Sausse

INTRODUCTION

Understanding and modelling two-phase flows in Microchannels--those with hydraulic diameters smaller than 1 mm--are of considerable interest in several industrial processes involving complex microsystems. Areas of application include chemical engineering (Tice et al., 2003; Shui et al., 2007), biomedical science (Shui et al., 2007), petroleum science (Lenormand et al., 1983) and so on. Significant attention has been focused on gas-liquid two-phase flows in microchannels. However, the flow patterns and corresponding pressure drops of two immiscible liquids in microchannels are still not well understood (Zhao et al., 2006). A number of studies, however, have used two immiscible liquids flowing in microchannels in order to form droplets or plugs (Tice et al., 2003). Droplets have been used in microchannels to carry out and enhance two-phase chemical reactions (Tice et al., 2003). Song et al. (2003) showed that it is possible to form droplets of multiple aqueous reagents in a flow of water-immiscible carrier fluid, transport the droplets through microchannels without dispersion, and mix the contents of the droplets by chaotic advection in winding channels.

Oil-water two-phase flows in microfluidic devices have been successfully employed in creating emulsions commonly used in the chemical and textile industries, food, and many other domains. In these applications, it is necessary to precisely control the droplet size and the polydispersity (Tan et al., in press). Therefore, most investigations of the liquid-liquid flow in microsystems have focused on the droplet size versus liquid physical properties and microsystem geometry.

Recently, the effect of various operating conditions on the flow patterns, slug size, interfacial area, and pressure drops was investigated by Kashid and Agar (2007). Experiments were carried out using different Y -junction mixing elements with various downstream capillaries. The authors showed that the capillary Y-junction dimensions had a significant effect on slug size and thus interfacial area, which increases with decreasing dimensions. Three flow regimes were observed as a function of liquid flow rate: slug flow, droplet flow of water in organic phase, and deformed interface flow. The latter regime is unstable and was observed only at a high ratio of water to cyclohexane flow rate. Water forms long slugs while cyclohexane is present as small droplets. A theoretical prediction of the pressure drop along a slug flow capillary was developed based on the capillary pressure and hydrodynamic pressure drop of the two individual phases.

In the case of pipes of large hydraulic diameter, several studies have been devoted to two-phase flows of immiscible liquids, where it appears that, as for gas-liquid flow, pressure drop is strongly dependent on flow patterns.

The simplest pattern that can be modelled easily is annular flow (Brauner, 1991; Rovinsky et al., 1997; Bannwart, 2001). In this case, the pressure drop differs according to whether the less viscous fluid is in contact with the pipe wall or flows in the centre of the pipe (lubrification effect). However, due to density effects, liquid-liquid patterns in pipes are seldom annular, and other flow patterns are frequently encountered (Fujii et al., 1994; Beretta et al., 1997; Bannwart et al., 2004), namely dispersed, slug, bubbly, and plug flows.

Angeli and Hewitt (2000) studied the effect of wettability on flow patterns. Two horizontal test sections were used, one made of stainless steel and another of acrylic resin. The authors observed a substantial difference in flow patterns and phases distribution between these two tubes. The flow patterns were classified as follows: stratified flow, stratified flow with a layer of drops or three-layer flow, stratified/mixed flow, and dispersed flow (droplets of oil in water or vice versa). The effect of pipe inclination on liquid-liquid two-phase flow patterns, phase holdups, and pressure drops was also studied recently by Rodriguez and Oliemans (2006) and Lum et al. (2006).

In this paper, we present experimental results of two-phase oil-water flows in horizontal microchannels. The following section describes the experimental set-up. Experimental Results Section presents experimental observations of two-phase flow patterns and two-phase pressure drops. The results are interpreted in Discussion Section using the homogeneous and Lockhart-Martinelli models.

EXPERIMENTAL SET-UP AND PROCEDURE

This section presents the experimental set-up used in this study, the wettability characteristics of the microchannels, and the determination of their hydraulic diameters.

Experimental Set-Up

A schematic view of the experimental set-up is shown in Figure 1. The 120-mm-long horizontal microchannel constitutes the test section. Two fluids are introduced separately through a T-junction using two calibrated pumps. The mixture is evacuated via a second T-junction. This last one was used for a phase distribution study to be presented in a future report. In the present paper, only the results obtained in the central channel are presented.