An experimental investigation of oil-water flow in a serpentine channel
Introduction
Co-current flows of two immiscible liquids are common in food, pharmaceutical, personal and health care products, as well as in many industrial formulations. In particular, 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, controlling the size distribution and polydispersity of the dispersed phase can improve significantly the characteristics of the final product (Tan et al., 2008).
Early studies on liquid-liquid flows focused mainly on oil-water systems in straight pipes of circular cross section, with diameters ranging between 1 cm and 10 cm (Brauner, 2003). Later on, there was a growing interest in capillary tubes of millimetric diameter to investigate the flow behaviour in micro-gravity conditions (Beretta et al., 1997). More recently, investigations of the hydrodynamics and mass transfer aspects of liquid-liquid two phase flow have been extended to micro-channels with diameters < 1 mm, for their relevance in bio- and micro-fluidic applications (Burns, Ramshaw, 2001, Kashid, Agar, 2007). Experimental parameters include the inlet geometry (e.g. T-type and Y-type), the micro-channel cross-sections (e.g. square, rectangular and circular), flow rates and the flow ratio, and the physical properties of the fluids. The majority of works was limited to low flow velocities where surface forces dominate, in which intermittent flow patterns are generally observed.
The first step in the study of a liquid-liquid flow system requires the identification of flow patterns, which can be classified into four basic morphologies: (i) stratified layers with either smooth or wavy interface; (ii) large slugs, elongated or spherical, of one liquid in the other; (iii) a dispersion of relatively fine drops of one liquid in the other; (iv) annular flow, where one of the liquids forms the core and the other liquid flows in the annulus. In many cases, however, the flow pattern consists of a combination of these basic morphologies (Dessimoz et al., 2007). The identification of the flow pattern is usually based on visual observations, photographic/imaging techniques, or on abrupt changes in the average system pressure drop, sometimes combined with conductivity measurements or high frequency impedance probes for local holdup sampling, or local pressure fluctuations and average holdup measurements (Bertola, 2003).
Oil-in-water flow patterns and slug hydrodynamics were experimentally studied in rectangular and square glass micro-channels of different hydraulic diameters ranging between 200 µm and 600 µm (Cao et al., 2018). The three main flow patterns observed were annular flow, slug flow and droplet flow, and general flow pattern transition criteria were proposed based on the Reynolds and the Weber numbers. In addition, a scaling law was proposed to predict the slug length, while the slug velocity was shown to be a linear function of the bulk velocity of the two phases. Flow pattern maps for the same systems were produced as a function of the flow rates ratio and the Capillary number of the dispersed phase (Wu et al., 2017). Alternatively, flow pattern maps can be reported as a function of the Reynolds and the Capillary numbers (Kashid and Kiwi-Minsker, 2011).
Experiments on liquid-liquid flow of different fluids in a square microchannel (200 µm hydraulic diameter) concluded that flow pattern maps based on Weber numbers cannot be generalised, and propose to introduce the product of the Weber and Ohnesorge numbers as a universal parameter to generalise flow maps (Yagodnitsyna et al., 2016).
Fluid mixing can be enhanced by fluid breakup in chaotic liquid-liquid flows (Muzzio et al., 1991). Chaotic fluid mixing can be obtained, for example, by means of secondary flow induced by curved streamlines (Castelain et al., 2001). Thus, one can build a simple, continuous flow chaotic mixer using a long serpentine channel. Adding complexity to the flow field has potential to increase the amount of mixing also in microchannels with zigzag geometry (Branebjerg et al., 1995) and three-dimensional serpentine channels designed to introduce chaotic advection into the system and further enhance mixing over a 2-D serpentine channel (Liu et al., 2000). Whilst these studies concern single-phase flows, the literature about liquid-liquid flows in serpentine or coiled channels is very limited (Sarkar, Singh, Shenoy, Sinha, Rao, Ghosh, 2012, Wu, Sundén, 2018).
The present work investigates experimentally the co-current oil-water flow in a polypropylene serpentine mini-channel of millimetric size with a square cross-section. Fluidic devices built with polymer materials have an important role in emerging and perspective applications such as portable electronics, chemical reactors, soft robotics and deployable systems, where the use of metallic channels is limited due to cost, weight and mechanical flexibility constraints (Der, Edwardson, Marengo, Bertola, 2019, Der, Marengo, Bertola, 2019, Maleki, Bertola, 2019). Flow patterns were identified by high-speed imaging, and flow pattern maps were constructed with respect to different flow parameters and compared with those reported in the literature for straight channels.
Section snippets
Test section design and fabrication
The test section consisted of a polypropylene serpentine channel having a square cross section with a width mm, consisting of a sequence of 46 circular half turns, with inner and outer radii of 1 mm and 4 mm, respectively, as shown in Fig. 1a.
The channel had two inlets, respectively for oil and water, merging together through a Y-junction, and one outlet. The channel shape was cut out into the black polypropylene sheet (3 mm thickness) using a LS1290 PRO Laser Cutter (maximum power: 80 W;
Flow patterns
The typical flow patterns observed during the oil/water flow are displayed in Fig. 4. Since the polypropylene channel walls are moderately hydrophobic (), oil was the continuous phase in all the flow conditions considered (water-in-oil dispersion). In contrast, most of the works reported in the existing literature use channels built with highly wettable materials, such as glass or metals (Garstecki, Fuerstman, Stone, Whitesides, 2006, Wu, Cao, Sundén, 2017, Cao, Wu, Sundén, 2018),
Conclusions
The two-phase flow of oil and water flow in a horizontal serpentine channel was investigated experimentally by high-speed imaging. Different flow regimes were identified depending on the superficial velocities of the two fluids, and sorted into an empirical flow pattern map. Flow pattern transitions were found to be a function of the product of the Weber and Ohnesorge numbers, in agreement with dimensional analysis arguments for straight channels.
The serpentine geometry enhances the fluids
CRediT authorship contribution statement
Oguzhan Der: Writing - original draft. Volfango Bertola: Writing - original draft.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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2021, Process Safety and Environmental ProtectionCitation Excerpt :The problem of oil-water interface two-phase flow has always been one of the important problems in the field of multiphase flow (Halim et al., 2020; Kumar et al., 2016; Munirasu et al., 2016). Numerical and experimental studies are both often proposed to solve oil/water two-phase flow problems (Baig et al., 2019; Jamsaz and Goharshadi, 2020; Wang et al., 2011; Der and Bertola, 2020; Dehkordi et al., 2018). LBM has achieved some success in the fields of micro-scale flow and heat transfer, porous medium flow and heat transfer, multiphase flow and, so on.