Gas liquid flow in microchannels

1. 1
Chapter 1
Introduction
With the advancements in microfabrication techniques, the use of miniaturized devices has
become promising for a variety of industrial applications over the past few decades. These
applications include the fields of chemical process engineering, electronics cooling, automotive
applications, aerospace applications, inkjet printing, micro-electro-mechanical devices (MEMS),
Lab-on-a-chip devices, nuclear and biomedical industries. Flow, heat and mass transfer in these
miniaturized devices has assumed greater technological importance in the past decade as part of
this general trend towards miniaturization. The interest in transport processes in micro-structured
devices can be understood from the fact that seven monographs or books (Ehrfeld et al., 2000;
Hessel et al., 2004; Kandlikar et al., 2006; Kockmann, 2007; Ghiaasiaan, 2008; Hessel et al.,
2009; Yarin et al., 2009) have been published on the topic in the last decade. Several journals,
such as Microfluidics and Nanofluidics, Lab on a Chip, Nanoscale and Microscale
Thermophysical Engineering, Journal of Micromechanics and Microengineering, along with
microfluidics specific sections in various fluid mechanics journals have also been started to
cover a wide range of research topic related to micro-structured devices.
The flow in microchannels is generally laminar due to the small length and velocity scales. The
absence of turbulence results in poor mixing and consequently low rates of heat and mass
transfer. Therefore, an additional mechanism is required to enhance heat and mass transfer rates
at this scale. For example, a tortuous flow path causes secondary Dean vortices generated by the
centrifugal force and thereby enhance the rate of heat transfer significantly (Geyer et al., 2007;
Rosaguti et al., 2007; Gupta et al., 2008). Obliquely oriented grooves on the channel wall have
also been used to generate transverse flow component and thus enhance mixing (Stone et al.,
2004). Multiphase flow can also help in enhancing the mixing and thereby increasing transport
rates (Günther et al., 2004).

2. 1. Introduction
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2
Multiphase flow provides several mechanisms for enhancing the performance of single-phase
microfluidic systems (Günther and Jensen, 2006). Evaporative cooling can provide better heat
removal capability in compact heat exchangers. The large interfacial areas associated with
multiphase microflows provide efficient mass transfer between two immiscible fluids. A
multiphase system can also prevent a liquid from directly contacting the channel wall and hence
reduce clogging of the channel wall. Therefore, there has been a growing interest in the study of
adiabatic and non-adiabatic gas-liquid flow in fine passages over the last few decades.
While it has been established now by careful experimental work (Hetsroni et al., 2005; Morini et
al., 2007; Park and Punch, 2008) that the single-phase flow behaviour can be predicted well by
the correlations extrapolated from macro-scale systems, the same is not true for multiphase flow
in microchannels and the hydrodynamics of multiphase flow in microchannels is far from well-
understood (Thome, 2004).
In gas-liquid flow in microchannels, different flow patterns, such as bubbly flow, slug or Taylor
flow, wavy-annular flow, annular flow and churn flow occur. Amongst these flow patterns,
Taylor and annular flow occur over a wide range of gas and liquid flow rates and are important
in various industrial applications. These flow patterns have a well-defined gas-liquid interface
and provide large specific interfacial area for mass transfer. Taylor flow also has a special flow
characteristic of internal recirculation (as viewed by an observer moving with the bubble) in the
liquid slug and therefore it can enhance heat and mass transfer rates, as well as reduce axial
dispersion. While the hydrodynamics and mass transfer in Taylor flow have been studied
extensively, heat transfer is Taylor flow is poorly understood. There is almost nothing in the
literature on annular flow in microchannel.
The experimental investigations of microchannel flow require non-intrusive measurement
techniques. The dynamic nature of the flow also requires the time-resolution of flow
characterisation techniques. Numerical modelling of two-phase flow phenomena require no
heuristic modelling of turbulence and provides detailed information of the transport processes.

3. 1. Introduction
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3
In this work two-phase flow and heat transfer in the Taylor flow regime is studied using
computational fluid dynamics (CFD). The challenges in the numerical modelling of annular flow
are identified and a preliminary study of annular flow is carried out.
1.1 Thesis Structure
The chapters that make up this thesis are organised as follows:
In Chapter 2 a detailed review of the existing knowledge on the flow and heat transfer in the
Taylor and annular flow regimes in microchannels is carried out and the gaps in the
knowledgebase are identified.
In Chapter 3 existing techniques for the modelling of multiphase flow and various strategies
employed for the modelling of Taylor flow are described.
In Chapter 4 a methodology for the CFD modelling of the Taylor flow in a laboratory frame of
reference using the Volume-of-Fluid (VOF) method with geometric reconstruction is developed.
The importance of capturing the liquid film around the gas bubble is discussed and a set of
guidelines are developed to ensure that simulations will capture the physics properly.
In Chapter 5 the results of the CFD modelling of Taylor flow and heat transfer using this
methodology are compared with those obtained from simulations using the level-set technique
and with some experimental data.
In Chapter 6 fully-developed Taylor flow and heat transfer is studied and the effects of changing
the mixture velocity and the homogeneous void fraction on it are explored.
In Chapter 7 a series of simple phenomenological models developed to understand the
phenomena determining the hydrodynamics and heat transfer in the Taylor flow regime are
presented.
In Chapter 8 the effect of gravity on the flow and the distribution of the liquid around the gas
bubble in Taylor flow in horizontal microchannels is studied.
In Chapter 9 the causes of instability in two-phase gas-liquid annular flow are discussed; the
challenges in CFD modelling of annular flow are identified. Preliminary CFD simulations for
gas-liquid annular flow in microchannels are performed.
In Chapter 10 the main conclusions of this work are summarised and recommendations for
future work are made.

4. 1. Introduction
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4
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