SUMMARYThis paper presents the results of a numerical study on the flow characteristics and heat transfer over two equal square cylinders in a tandem arrangement. Spacing between the cylinders is five widths of the cylinder and the Reynolds number ranges from 1 to 200, Pr = 0.71. Both steady and unsteady incompressible laminar flow in the 2D regime are performed with a finite volume code based on the SIMPLEC algorithm and non-staggered grid. A study of the effects of spatial resolution and blockage on the results is provided. In this study, the instantaneous and mean streamlines, vorticity and isotherm patterns for different Reynolds numbers are presented and discussed. In addition, the global quantities such as pressure and viscous drag coefficients, RMS lift and drag coefficients, recirculation length, Strouhal number and Nusselt number are determined and discussed for various Reynolds numbers.

Taylor flow is a strategy-aimed flow to transfer conventional single-phase into a more efficient two-phase flow resulting in an enhanced momentum/heat/mass transfer rate, as well as a multitude of other advantages. To date, Taylor flow has focused on the processes involving gas–liquid and liquid–liquid two-phase systems in microchannels over a wide range of applications in biomedical, pharmaceutical, industrial, and commercial sectors. Appropriately micro-structured design is, therefore, a key consideration for equipment dealing with transport phenomena. This review paper highlights the hydrodynamic aspects of gas–liquid and liquid–liquid two-phase flows in microchannels. It covers state-of-the-art experimental and numerical methods in the literature for analyzing and simulating slug flows in circular and non-circular microchannels. The review’s main objective is to identify the considerable opportunity for further development of microflows and provide suggestions for researchers in the field. Available correlations proposed for the transition of flow patterns are presented. A review of the literature of flow regime, slug length, and pressure drop is also carried out.

Two-phase Taylor flow has received considerable attention from researchers in recent decades due to its potential use in a wide variety of industrial and medical applications. A large number of experimental, analytical, and numerical efforts have been taken by researchers to understand the fundamental characteristics of two-phase flows and the relevant transport phenomena. This paper presents a comprehensive review of the hydrodynamics, flow pattern, and liquid film thickness in two-phase flows through mini-and microchannels with different cross-sectional geometry. This paper also reviews correlations that predict liquid film thickness in microchannels for gas-liquid and liquid-liquid flows. The variations of liquid film thickness are plotted over a wide range of capillary numbers for both experimental and computational studies. This study shows that the effects of cross-sectional area on the flow patterns and flow characteristics have not been sufficiently investigated by researchers, particularly for rectangular cross-sectional areas with different aspect ratios.

Experimental flow visualization is a valuable tool for analyzing microfluidics and nanofluidics in a wide variety of applications. Since the late 1990s, considerable advances in optical methods and image postprocessing techniques have improved direct optical measurements, resulting in an accurate qualitative and quantitative understanding of transport phenomena in lab-on-a-chip capillaries. In this study, a comparison of different optical measurement techniques is presented. The state-of-the-art development of particle image velocimetry to date, particularly in microscale applications, is reviewed here in detail. This study reviews novel approaches for estimating velocity field measurements with high precision within interrogation windows. Different regularization terms are discussed to demonstrate their capability for particle displacement optimization. The discussion shows how single- and multi-camera optical techniques provide two-dimensional and three-component velocity fields. The performance of each method is compared by highlighting its advantages and limitations. Finally, the feasibility of micro resolution particle image velocimetry in bioapplications is overviewed.

This paper investigates a CFD‐based analysis for gas–liquid and liquid–liquid Taylor flows through a circular axisymmetric microchannel with a sudden enlargement. A series of simulations are conducted by exploring the influence of different superficial velocity ratios, apparent viscosities, and channel expansion on the hydrodynamics of slug flow. A concentric junction introduces dispersed airflow into a continuous flow of water for gas–liquid flow, and the junction introduces dispersed water into a continuous flow of dodecane for liquid–liquid flow. The air‐bubble and water‐slug evolution processes, slug breakup, and slug expansion are investigated. In all cases, the lengths of air bubbles and water slugs increase with increasing superficial velocity ratio, particularly before the expansion. For gas–liquid flow, the apparent viscosity ratio causes a fluctuating interface over the uniform film region. However, the water slug length is shorter and the film region is slightly thicker in liquid–liquid compared to gas–liquid flow. The numerical analysis developed in this paper is in good agreement with the existing correlations and experimental data in the literature.