The present work was movitated by an earlier study on pumps based on a Wankel geometry (Wankel pumps) which revealed a flow structure, rich in three dimensional vortices (including Taylor like counter-rotating ones), in the pump chamber, suggesting the possibility of a Wankel pump of also functioning as a mixer. To this end, numerical mixing experiments were run using the general scalar transport equation to model the evolution of species concentration. Interestingly, it was observed that species dispersion occurs predominantly sideways (laterally) and that vertical dispersion is almost non-existent. In other words, for binary mixing of two species with a Wankel pump, they must be introduced side by side to ensure effective mixing.

Previous work about the oblique fin channels shows an abnormal variation on the static pressure drop. Thus numerical work was performed to investigate the influence of the lengthwise distance and the parallelogrammic fin section shape separately using two groups of fin mesh. The results showed that the influence of the parallelogrammic fin section shape was similar to the three-dimensional oblique fin channels. Besides, when air flow is askew to the fin, there is a vortex near the leeward side. The pressure had a rapid change in the entrance and vortex region and then decreased linearly along the flow length. The static pressure drop in the vortex region is determined by both the fin slope angle and the inlet angle. Thus the variation law of oblique fin channels is different from the straight fin channels.

Advancements in the field of microfluidics has led to an increasing interest to study laminar flow in microchannel and its potential applications. Understanding mixing at a microscale can be useful in various biological, heating and industrial applications due to the space and time reduction that micro mixing permits. This work aims to study mixing enhancement due to curved microchannel and the influence of varying microchannel cross sectional shape through numerical and experimental investigations. Unlike prior studies which use channel dimensions in the lower microscale range, this work has been conducted on channels with dimensions in the higher end of micrometer range. Using a cross sectional hydraulic diameter of 600 μm enables introduction of flow into the curved channel at a Reynolds Number ranging from 0.15 to 75, the findings of which show considerable improvement in the mixing performance as compared to that of equivalent straight channels, due to the development of secondary flows known as Dean Vortices.

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilise the enhanced fluid mixing characteristics of Dean vortices, and thus transferring heat efficiently. Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimising the heat exchanger design based on the parameters of maximising heat transfer whilst minimising pressure drop and unit cost. A range of cross-sectional geometries for the curved channels were compared, showing significantly higher Nusselt Numbers than equivalent straight channels throughout, and finding superior performance factors for square, circular and symmetrical trapezoidal profiles. Due the difficulty and expense in manufacturing circular microchannels, the relatively simple to fabricate square and symmetrical trapezoidal channels are put forward as the most advantageous designs. These results take into account both constant wall temperature and constant heat flux conditions. For a given set of channel dimensions, an optimal input flow rate condition is also determined.

Microchannel heat exchangers have become widely employed in modern systems, found within aerospace applications, waste heat recovery, water treatment processes, air conditioning, biomedical treatments and various industrial process applications. The microchannels increase the ratio of heat transfer surface to volume, thus improving the heat transfer performance significantly whilst reducing the overall weight and size. Moreover, by utilizing secondary flow from Dean Vortices induced by curved microfluidic channels, the fluid flow and heat transfer performance can be enhanced even further beyond conventional straight channels. However, since pressure drops found in microchannels are often quite high, channel lengths must be kept relatively short to balance the friction loss and energy consumption. Due to this, the developing region length at the microchannel entrance area has a greater impact than for macroscale channels, in terms of hydrodynamic and thermal performance over the remaining full developed region. The thermo-hydraulic design for heat transfer microchannel surfaces is strongly dependent on several dimensionless performance indicators, namely Nusselt number ‘Nu’ for heat transfer, and Poiseuille number ‘Po’, which is the product of Fanning friction factor ‘f’ and Reynolds number ‘Re’. These parameters are used to characterize and optimize the performance of microchannel surfaces and heat exchangers in general, also can be used to determine both the thermal and hydraulic developing region lengths at the channel entrance area. Whilst many such studies exist for theoretical analysis and experimental verifications, currently there is little literature on the developing region lengths and impacts researched through the method of Computational Fluid Dynamics (CFD). As such, this paper identifies and explores via quantitative analysis the hydraulic and thermal performance changes created by the relevant developing region lengths at the entrance area of spiral microchannels, as well as determinations and comparisons of these effects over straight channels. The numerical results, generated via COMSOL Multiphysics and contrasted with previous literature on the subject, also compared with the effect of the developing region on the effectiveness and efficiency of both spiral and straight microchannels, finding an improved heat transfer performance but an increased impact of hydraulic friction as well for spiral channels against straight counterpart. Furthermore, significant differences between thermal developing region length and hydraulic developing region length can be observed throughout, which illustrates high challenge and the need for compromise in microchannel design. In this way, implications for the configuration and design of industrial microchannels and micro heat exchangers are self-evident. All the key factors given in this paper are dimensionless, and thus the generated results can be utilized for a variety of flow conditions. Hence, this work should permit an increased understanding for and boost the curved microchannel and micro heat exchanger designs subsequently, through reducing the required numbers of tests and experiments and expediting the development for similar applications followed.

The effect of ethanol in the binary solution sessile droplet is investigated on the flow field, nanoparticle motion and nanoparticle deposition pattern. It is found that the droplets with ethanol exhibited three distinct flow regimes through the Particle Image Velocimetry (PIV) analysis on the flow field of droplets suspended with fluorescent microspheres. Regime I features furious flows and vortices which transport particles to the liquid-vapor interface and make them aggregate. In regime II, the aggregates of particles move towards the central area of the droplet dominated by Marangoni flow led by non-uniformity of ethanol along the droplet surface. As the droplet enters regime III, most ethanol has evaporated and it is dominated by the drying of the remaining water. The loading of ethanol in the solution prolongs the relative durations of regimes I and II, resulting in the variety of the final drying pattern of nanoparticles.

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilise the enhanced fluid mixing characteristics of Dean vortices, and thus transferring heat efficiently. Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimising the heat exchanger design based on the parameters of maximising heat transfer whilst minimising pressure drop and unit cost. These studies examined the variation of Nusselt Number over the length of the channel, for a range of different curvatures (and hence Dean numbers). The results showed significantly higher heat transfer occurring in curved channels, especially in areas where the generated Dean vortices are strongest, with the variation in Nusselt Number forming the shape of an ‘arc’. In this way, a relationship between the Dean Number and the Nusselt Number is characterised and discussed, leading to suggestions regarding optimal microfluidic heat transfer design.

An incompressible electrically conducting viscous fluid flow influenced by a local external magnetic field may develop vortical structures and eventually instabilities similar to those observed in flows around bluff bodies (such as circular cylinder), denominated magnetic obstacle. The present investigation analyses numerically the three-dimensional flow and heat transfer around row of magnetic obstacles. The vortex structures of magnetic obstacles, heat transfer behaviors in the wake of magnetic obstacles and flow resistance are analyzed at different Reynolds numbers. It shows that the flow behind magnetic obstacles contains four different regimes: (1) one pair of magnetic vortices, (2) three pairs namely, magnetic, connecting, and attached vortices, (3) smaller vortex shedding from the in-between magnetic obstacles, i.e. quasi-static and (4) regular vortex shedding from the row of magnetic obstacles. Furthermore, downstream cross-stream mixing induced by the unstable wakes can enhance wall-heat transfer, and the maximum value of percentage heat transfer increment (HI) is equal to about 35%. In this case, the thermal performance factor is more than one.

The Wavy channels are supposed to enhance performance of microchannel heat sink through chaotic advection. The change in boundary layer thickness (thinning) and the macroscopic mixing due to the formation of Dean’s vortices have been found to be main reasons for enhanced heat transfer in wavy microchannel. Present study carries out a detailed numerical investigation for flow and heat transfer in wavy channel. A 3D geometry for a single loop of wavy channel is modeled in GAMBIT and simulated in CFD software FLUENT. The basic dimensions were 0.15 mm width, 0.3 mm height and 1.5 mm length. The formation of Dean vortices are shown. In parametric study, the effect of Re number on the flow and heat transfer performance is shown. Heat transfer was found to be increased with Re. The effect of Aspect ratio is shown. The channel with the aspect ratio of 0.5 is found to be best among the channels studied including wavy and straight microchannels. A novel concept of secondary branches is introduced to wavy microchannel to take advantage of high pressure zone at crust. The branched wavy microchannel encouraged the secondary flow thus enhanced the macroscopic mixing. Due to disrupt of boundary layer development and its re-initialization, an improved thermal performance was achieved.

The heat transfer characteristic and flow structure of fluid in the rectangular channel with different height vortex generators in small scale are investigated with numerical simulation. Meantime, the properties of heat transfer and flow of fluid in the rectangular channel are compared with the channel which located small scale vortex generator. The variation law of local heat transfer and flow structure in channel is obtained. The mechanism of heat transfer enhancement of small scale vortex generators is discussed in detail. It is found that the influence of vortex generator on heat transfer is not in proportion to the size of vortex generator. What is more, turbulent flow structure near the wall, which influences the temperature distribution near the wall, induces the variety of local heat transfer. The fluid movement towards to the wall causes the heat transfer enhanced. On the contrary, the fluid movement away from the wall decreases the local heat transfer.

In this study, a parametric investigation on mixing of two fluids in a modified Tesla microchannel, has been preformed. Modified Tesla micromixer applies both flow separation and vortices string principles to enhance the mixing. The fluid stream splits into two sub-streams and one of them mixes with the other again at the exit of the Tesla unit. Analyses of mixing and flow field have been carried out for a wide range of Reynolds number from 0.05 to 40. Mixing performance and pressure drop characteristics with two geometrical parameters, i.e, ratio of the diffuser gap to channel width ( h/w ) and ratio of the curved gap to the channel width ( s/w ), have been analyzed at six different Reynolds numbers. The vortical structure of the flow has been analyzed to explain mixing performance. The sensitivity analysis reveals that mixing is more sensitive s/w , than the h/w .

In this paper, a passive interdigital micromixer with the circular-sector obstacles is proposed and the mixing performance is estimated by numerical simulation. The tested Reynolds numbers range from 0.01 to 10. Flow recirculation or vortices seems impossible to generate to enhance the mixing at such low Reynolds numbers. Hence, molecular diffusion is the dominant mixing mechanism. Based on the diffusion principle, enlarging the mixing length, reducing the diffusion length and increasing the surface area between species are major methods to obtain mixing enhancement. In order to achieve rapid mixing, shortening the mixing length is necessary. However, the reduced mixing length induces the decreased mixing time which the species take to mix. The circular-section obstacles are placed in the straight microchannels to enlarge the contact surface area between species. The flow path is distorted after passing the obstacles so that the real mixing length increases compared with traditional T-shape micromixers. Furthermore, flow advection takes a part role in mixing since the velocity direction is no longer perpendicular to diffusion direction. Different geometries and layouts of obstacles are analyzed for optimization. The results of optimal design show the worst mixing efficiency, around 50%, occurs at Re = 1. In order to improve the lower limitation of mixing efficiency, the duplicated layouts of obstacles in lateral direction with interdigital inlet are applied to reduce the diffusion path and increase the interface area so that the mixing efficiency could be enhanced. The results show that the mixing efficiency could achieve 85% at Re ≤ 1 with a low pressure drop of 100 Pa. It has the potential to be used in applications with low Reynolds numbers.

A microfluidic device based on Marangoni effect on a volatile meniscus is proposed for cell enrichment and collection. The device consists of a main channel in which cells are flowing with the mainstream, and an open-outlet channel which is connected with the main channel. A meniscus is sustained in the opening channel and keeps evaporating since the outlet is open to the atmosphere. Marangoni vortexes are generated near the meniscus due to the non-uniform evaporation on the meniscus. The vortexes influence the flow field and cell dynamics in the main channel, which can be used to trap the cells. A comprehensive numerical simulation is adopted in present work to reveal how the heat and mass transport going on in such a system. The evaporation and vapor diffusion at the liquid-air interface, and the vapor diffusion in air are considered. The convections due to the evaporation and Marangoni effect are simulated.

The governing equations of electroosmotic flow, including the Navier-Stokes (N-S) equations, Laplace equation and Poisson-Boltzmann equation, are set up in a straight microchannel. The meshless method is employed as a discrete scheme for the solution domain. The semi-implicit multistep (SIMS) method is used to solve the Navier-Stokes equations. The simulation results demonstrated that different patterns of the zeta potential over the channel surface could induce different flow profiles for the vortex. The rotational direction of the vortex is determined by the electroosmotic driving force.

In this work, we found experimentally that there exist fairly strong fluid flows in AC electrowetting, which can be utilized as a means to mix the fluids in EWOD-based micro-devices. We visualized the internal flow. There may exist two distinct flow-generation mechanisms; one is the droplet oscillation, and the other is the electrohydrodynamic flow. The flow pattern is significantly dependent on the applied AC frequency. At low frequencies (represented here by 1 kHz), the center of the vortices is located somewhat randomly and the flow directs upward near the symmetric axis. At high frequencies (represented by 128 kHz), however, a pair of vortices having quite a regular structure is clearly visible and the flow directs downward near the symmetric axis. The flow patterns are strongly dependent on the position of the point electrode. The droplet surface undergoes a periodic oscillation (visualized by a high-speed camera) with a frequency exactly twice the frequency of applied electrical signal. The oscillating interface can generate a steady streaming. However the numerical results show that there exists no electric field at low AC frequencies. On the contrary, there exists quite a strong electric field inside the droplet at high frequencies. It means the electrohydrodynamic flow cannot be generated at the low frequency region, and the droplet oscillation might cause the flow generation at low frequencies. We also demonstrated the flow can be beneficially utilized as a mixing method.

Microchannels are an integral part of various micro devices i.e. micro chemical reactors, fuel cell and electronic packaging etc. There are two important criteria for design of microchannels. The first criterion is the minimization of pressure drop across the microchannel such that the parasitic power requirement is minimized during the operation of these devices. The second criterion is the mixing enhancement due to stronger wall-normal motion of the near wall fluid particles leading to reduction in velocity, temperature and concentration gradient at the wall. Wall surface roughness can be suitably modified for achieving the above objectives in macro-scale flows. Riblets on the channel surface are effective in reducing the frictional resistance for macro flow situations. Various ribs or turbulator geometries have proved to be effective for near wall mixing in heat exchangers and gas turbine blade cooling ducts. However, no systematic study on mixing enhancement due to flow past repeated micro ribs inside microchannel is available in literature. The present study presents the detailed flow past repeated micro-ribs inside a microchannel from micro-PIV (Particle Image Velocimetry) measurements. The microchannel made of Polydimethylsiloxane (PDMS) has been fabricated by PDMS replica moulding. The channel is 50μm × 120μm cross section. The width and height of ribs are 30μm and 10μm respectively. The pitch between the multiple ribs is equal to 100μm. The presence of flow separation at the edge of the rib with generation of vortex structure is not observed at the flow rate set in the present study contrary to the macro flow situations. However, the shear layer is deflected towards the wall with increase in wall-normal velocity indicating possible effectiveness of micro ribs for mixing enhancement in the microchannels.