Liquid - liquid or gas - liquid multi phase flow patterns are of great of importance in a wide range of technological applications, e.g. heating and cooling systems, distillation processes, steam generation, oil recovery, food manufacturing, cosmetic production, nuclear power plant cooling and fire- fighting are encountered multi phase flow behavior and its governing factors. However, some these of applications require continuous flow of phases; and some others require dispersed flow of them. For example, the recent technical applications such as, the design of membrane less fuel cells and the liquid membranes (Choban et al 2004, Choban et al 2005, Maruyama et al 2004) are achieved by proper control of the flow parameters to generate a simultaneous flow of phases with stable interface. The droplet based reactors are explored in several chemical and biochemical applications with the simplified technology for the continuous synthesis of micro/nano droplets (Wagner & Ko1hler 2005, Washizu 1998, Torkkeli et al 2001, Vykoukal et al 2001, Günther & Khan 2004). So, a background of flow characteristics is necessary for optimization of design and operation of multiphase flow systems. However, the study on multi phase flow is mostly limited to straight channels of circular cross sections in which the commonly observed flow patterns are slug, bubble, stratified and annular flow. But, in many practical applications for example, designing a heat transfer system, one has to consider different possible geometries. In fact, the ratio between circumference and cross-section is more advantageous for non circular tubes and this allows increasing of transfer rates. Hence, it is equally important and necessary to extend the investigations on flow characteristics through non circular cross sections. Importantly, for fluid flow through such geometries, a so-called turbulent secondary flow occurs (Nikuradse et al 1933) and that results in steeper and non - uniform radial distribution of fluid phases and/or velocities unlike in circular cross sections. It has been shown that these non - uniform radial distributions can affect the flow patterns and their transitions (Wolk et al 2000, Chung et al 2004, Chena et al 2006). For example, for air - water two phase flow, the transition boundaries between different flow patterns i.e. from slug to bubble, slug to churn (frothy flow) and bubble to churn shift down towards lower fluid velocities in the sequence of change in shape of cross section from circular to rectangular, rhombic and equilateral respectively (Wolk et al 2000).
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The above general discussion on multiphase flow pertains to channels or pipes whose equivalent diameters are in the order of millimeters or even more. Specifically, at which the inertial forces are more deterministic than the fluid properties in controlling the flow characteristics. However, at micro scale, the physical factors like viscosity, interfacial tension and wetting of channel gets inherently integrated and become crucial in settling the flow dynamics apart from the fluid velocities. While the effect of most of these parameters have been studied to some detail (Chen et al 2009, Tan et al 2006, Lertnuwat and Bunyajitradulya 2007, Umbanhowar et al 2000, Cubaud et al 2006, Huh et al, 2009, Huh et al 2003, Choi et al 2003, Bird et al 2009, Hibara et al 2005), the effect of geometry of the channel, importantly its three dimensional orientation on multi phase flow has not been investigated. In this context, curved and helical geometries are of great importance and known to generate radial pressure gradients that can perturb the axial flow into the transverse plane. Such perturbations in fluid flow result in enhancing the phase transfer rates (Gehlert et al 1998, Moulin et al 1996, Levy et al 1998). However, several theoretical studies have been carried out to understand fluid flow through these channels at different geometrical parameters and at various flow regimes of Reynolds number (Dean 1927 & 1928, Germano 1982, Wang 1981, Germano & Oggiano 1987, So et al 1991, White 1929, Yamamoto et al 1994). But, the experimental investigations especially in helical micro channels are inadequate due to the difficulty in fabrication of such channels. Our novel method of fabrication of micro channels helped us to study the multiphase flow in multi helical micro channels.
1.2 Main Objectives
The main objectives of the present work are
To explore the multi helical micro channel geometry and flow of miscible and immiscible fluids through it.
To investigate and explore the observed oil - water and air -water two phase flow patterns in the above mentioned channels.
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To study the effect of geometrical, physical and flow parameters on flow regimes of the observed flow patterns.
Application of mentioned flow patterns to determine the dynamic interfacial tension.
Derivation of scaling laws to quantify the above mentioned flow patterns and checking their compatibility with the experimental results.
Overview of the thesis
The constitution of the thesis begins with a theoretical back ground on fluid flow through curved and helical pipes and the aroused modifications to Reynolds number in chapter 2. The interfacial phenomena of two miscible liquids and the enhancement of liquid - liquid mixing in multi helical micro channels have been shown before we move to multiphase flow. In chapter 3, a systematic experimental study on oil - water two phase flow and the novelty in the observed flow patterns i.e. in parallel and wavy annular flow have been presented. Further, the effect of fluid velocities and geometrical parameters in each flow regime has been discussed. A scaling law for the quantification of wavy annular flow from the basics of fluid mechanics has been derived and its compatibility with the experimental results is checked. Chapter 4, basically it gives an application of wavy annular flow to measure the dynamic interfacial tension. The method is tested for aqueous solutions of both anionic and cationic surfactants, and for water - methanol system in the continuous phase of paraffin oil. A hypothesis has been given to address the fluctuations in the interfacial tension data around the critical micelle concentration of both the surfactants. The effect of viscosity on wavy annular flow in quadruple helical micro channels is also presented. In chapter 5, gas - liquid two phase flow of air and water in triple helical channels has been presented. A brief discussion on the experimental method has been given as it is mostly similar with liquid - liquid two phase flow. The effect of channel geometry, interfacial tension and fluid velocities in each flow regime has been discussed. Description of novel flow patterns like oscillatory annular flow, and mixed flow patterns like simultaneous occurrence of slug and parallel flow has been given. A pressure profile for change in fluid velocities, at the entrance of air into the channel has been presented. Finally in chapter 6, we conclude by summarizing all our experimental findings and we present a scope of future work with some preliminary experimental results.