Microchannels with curved geometries have been employed for many applications in microfluidic devices in the past decades. The Dean vortices generated in such geometries have been manipulated using different methods to enhance the performance of devices in applications such as mixing, droplet sorting, and particle/cell separation. Understanding the effect of the manipulation method on the Dean vortices in different geometries can provide crucial information to be employed in designing high-efficiency microfluidic devices. In this review, the physics of Dean vortices and the affecting parameters are summarized. Various Dean number calculation methods are collected and represented to minimize the misinterpretation of published information due to the lack of a unified defining formula for the Dean dimensionless number. Consequently, all Dean number values reported in the references are recalculated to the most common method to facilitate comprehension of the phenomena. Based on the converted information gathered from previous numerical and experimental studies, it is concluded that the length of the channel and the channel pathline, e.g., spiral, serpentine, or helix, also affect the flow state. This review also provides a detailed summery on the effect of other geometric parameters, such as cross-section shape, aspect ratio, and radius of curvature, on the Dean vortices’ number and arrangement. Finally, considering the importance of droplet microfluidics, the effect of curved geometry on the shape, trajectory, and internal flow organization of the droplets passing through a curved channel has been reviewed.

Rising bubble systems are used and investigated in a wide variety of industrial applications. However, the influence of strong confinement in rectangular flow regions has received little attention. An experimental study is undertaken here on a flow channel that allows the passage of bubbles from a region that can be modelled as two parallel plates into a region of rectangular confinement. The effect of a co-flow of a water/glycerol mixture on bubble size and rising velocity in the two confined regions for a wide variety of size ranges is investigated using particle shadow velocimetry. In the parallel plate region, as bubbles become larger in size, their terminal velocity increases due to the relatively higher buoyancy force and negligible effects of the confining geometry, compared to smaller bubble sizes. On entering the rectangular confinement, however, bubbles of relatively large size decelerate to a much lower terminal velocity due to the drag force expressed by the confining walls. Available models in the literature for predicting bubble terminal velocity through circular tubes and parallel plates were evaluated and showed poor predictive performance. To address this gap, a semi-empirical model for the bubble terminal velocity in a rectangular geometry is developed, based on the experimental data, to predict this motion. This model includes the effect of bubble size, fluid medium properties, net co-flow, and confinement geometry. The curious phenomenon of the threshold size of a bubble, which maintains a constant velocity through both geometries, is then predicted using the model.

Flow measurement in porous media is a challenging subject, especially when it comes to performing a three-dimensional (3D) velocimetry at the micro scale. Volumetric flow measurement techniques such as defocusing and tomographic imaging generally involve rigorous procedures, complex experimental setups, and multi-part data processing procedures. However, detailed knowledge of the flow pattern at the pore and subpore scales is important in interpreting the phenomena that occur inside the porous media and understanding the macro-scale behaviors. In this work, the flow of an oil inside a porous medium is measured at the pore and subpore scales using refractive index matching (RIM) and shadowgraph imaging techniques. At the macro scale, flow is measured using the particle image velocimetry (PIV) method in two dimensions (2D) to confirm the volumetric nature of the flow and obtain the overall flow pattern in the vicinity of the flow entrance and at the far field. At the micro scale, the three-dimensional (3D) flow within an arbitrary volume of the porous medium was quantified using 2D particle-tracking velocimetry (PTV) utilizing the law of conservation of mass. Using the shadowgraphy method and a single camera makes the flow measurement much less complex than the approaches using laser light sheets or multiple cameras with multiple viewing angles.

Pore-scale velocity measurements can be achieved by using micro particle shadow velocimetry ( µ -PSV). Characteristic properties of a flow are, however, best investigated and described by the pressure distribution in the field. At the pore scale, applying direct pressure measurement techniques comes with significant challenges. By detailed measurement of velocity and applying theoretical relations that suit the flow field under study, the pressure field can be determined. This study demonstrates the application of image-based approaches to investigate the multi-phase flow of a single droplet. Experiments based on µ -PSV are used to determine the velocity field in the flow within the droplet as it passes through a single-pore geometry in the presence of a stationary continuous phase. The results are used to determine the pressure field calculated from a simplified Navier–Stokes expression of the flow, discretised using an Eulerian approach. For the dispersed phase, the theory of the Jamin effect allows the capillary pressure to be related to the observed change in radii of the leading and trailing edges of the droplet. To highlight this approach, two sizes of the glycerol droplets passing through a pore geometry in the presence of a stationary canola oil are investigated. The results show that the velocity and pressure distributions are dictated by the deformation properties of the droplet. The same trends are seen in the distribution of pressure and velocity gradients as a function of location along the channel. The larger of the two droplets showed increased levels of velocity and pressure gradients as it flows through the pore geometry. In general, this work demonstrates the use of the deformation behavior of a dispersed phase to determine the velocity and pressure distributions in a multi-phase flow field.

Centrifugal separators have many applications in the separation industry. Comprehensive information based on performance and energy consumption for a detailed comparison between different types of centrifugal separators is essential to allow design optimization and is the focus of this work. A framework for comparing performance with an updated performance chart for four types of centrifugal separators and a comparison with a continuous gravity settling tank is described. A model and chart for performance and energy consumption, which makes it possible to compare different separators, is a key result of this work which extends current available handbooks and guidelines.