Flow focusing with miscible fluids in microfluidic devices

This study uses a T-junction to examine the effects of different parameters (velocity ratio, viscosity, contact angle, and channel size ratio) on the generation of microdroplets, related rate, and size. More specifically, numerical simulations are exploited to investigate situations with a velocity varying from 0.004 to 1.6 m/s for the continuous phase and from 0.004 to 0.8 m/s for the dispersed phase, viscosity ratios (0.668, 1, 6.689, 10, 66.899), contact angle 80° < θ < 270° and four different canal size ratios (1, 1.5, 2 and 4). The results show that canal size influences droplet size and the generation rate. The contact angle has an impact on the form and the quality of generated droplets. Moreover, the relationship between velocity and viscosity ratios, droplet size, and generation rate is non-monotonic.

Fine-tuning width and aspect ratio of an improved microchannel heat sink for energy-efficient thermal management

Viscous fingering is a type of flow instability that occurs when a less viscous fluid displaces a more viscous fluid, causing instability at the displacement front. Owing to the opaque nature of porous media, experimental studies on the structure of viscous fingering and its development over time have been mostly limited to 2D porous media or Hele–Shaw cells. In this study, we used a micro-focused X-ray computed tomography scanner to investigate the 3D characteristics of viscous fingering in porous media. A low-viscosity iodine brine solution was injected in 3D-printed porous samples initially saturated by a high-viscosity syrup solution. The resulting miscible invasion was monitored by a sequence of X-ray tomography. Seven experiments were conducted to explore a large range of Péclet number and viscosity ratio. It allowed to identify the 3D morphology of fingering patterns and to quantitatively evaluate changes in physical properties such as iodine concentration in miscible fluids. As already identified in 2D experiments, characteristic events such as tip splitting, shielding, and coalescence were observed in 3D viscous fingering. At relatively low Péclet numbers, the fingering of the brine phase was more prone to breakthrough at certain points, forming a slender finger structure. As the Péclet number increased, the instability of the brine phase flow increased, making it easier to form multiple finger structures, and the tips were more prone to splitting into small finger structures. When the viscosity ratio decreased, the brine phase flow became more unstable. After halving the viscosity ratio of the miscible solution, multiple finger structures were formed at lower Péclet numbers, with small finger structures continuously splitting at the tips.

Microscopic particle image velocimetry (microPIV) was used to measure velocities in rectangular microchannels with aspect ratios ranging from 0.97 to 5.69 for 200&amp;lt;Re&amp;lt;3267. Mean velocity profiles, velocity fluctuations, and Reynolds stresses were determined from the microPIV data. Transition to turbulence was observed at Re=1765-2315 for the five aspect ratios studied, agreeing very well with both recent microscale experiments and macroscale duct flow and indicating no evidence of early transition for any of the aspect ratios studied. The onset of fully turbulent flow was observed at Re=2600-3200. For the fully turbulent flow, the ⟨u′⟩∕umax and ⟨v′⟩∕umax fluctuations at the channel centerline were 6% and 3%–3.5% and generally agreed well with macroscale results. As aspect ratio increased, the ⟨u⟩∕umax and ⟨u′⟩∕umax profiles became flatter, with nearly uniform values extending for some distance from the centerline of the channel. This region of uniform ⟨u⟩∕umax and ⟨u′⟩∕umax became larger with increasing aspect ratio. The Reynolds shear stress for fully turbulent flow also displayed a strong dependence on aspect ratio. For the W∕H=0.97 microchannel, ⟨u′v′⟩∕umax2 steadily increased in value moving from the centerline to the wall, but for the higher aspect ratio microchannels, ⟨u′v′⟩∕umax2 remained close to zero in the center region of the microchannel before increasing in value at locations close to the wall, and this region of near zero ⟨u′v′⟩∕umax2 became larger with increasing aspect ratio. This behavior in the Reynolds shear stress is due to the region of uniform velocity and, hence, small mean shear, near the channel centerline of the high aspect ratio microchannels.

The advancements in droplet microfluidics have made it an eminent technology for high-throughput biomedical applications involving biofluids such as blood, saliva, tear film, and synovial fluid, all characterized by their non-Newtonian/viscoelastic properties. However, most existing models for the design and optimization of droplet microfluidic systems are primarily based on Newtonian fluids. Therefore, studies that systematically explore the dynamics underlying biofluid droplet formation and manipulation are critically important, both fundamentally and practically. This study uses artificial tear films to experimentally investigate the effects of operational parameters such as geometric conditions (width ratio Λ = wdwc and aspect ratio h* = hwc), fluid properties (viscosity ratio η = μdμc), and flow rate ratios (ϕ = QdQc) on filament formation during droplet generation in a T-junction geometry. All the experiments were conducted at a constant capillary number of the continuous phase (Ca = μcQcwchγ). The results indicate that filament growth and rupture characteristics are determined by the predominance of two distinct regions: pre-stretch, governed by the interplay between inertial and capillary forces, and elastocapillarity, regulated by elastic and capillary forces. Specifically, filament rupture is influenced by inertia–capillary effects at low flow rates and viscosity ratios. In contrast, the domination of elastic effects is more pronounced at higher flow rates and viscosity ratios. Additionally, reductions in the width (Λ) and aspect (h*) ratio when Λ≥0.5 and h*≥0.4 increase the elastic effects within the filaments while inertial effects dominate for Λ&amp;lt;0.5 and h*&amp;lt;0.4. The relaxation times (λ) are obtained from the exponential thinning curve, with longer times observed for cases with high elastic effects. Finally, the filament collapses into satellite droplets, and their monodispersity increases with elasticity.

A precise knowledge of co-axial flow dynamics and a better understanding of the mechanisms that act to destabilize the interface between the two fluids is of fundamental interest in many industrial applications like lubricated transport, injection devices, atomization and controlled microdroplet production. The flow is in general unstable, since at least two mechanisms act to destabilize the cylindrical interface: shear and capillary instabilities. While these two mechanisms are active in a jet issuing from a tap, their respective influence strongly depends on the Reynolds number, the Ohnesorge number, but also on the viscosity, density, and aspect ratios. In this thesis, the global stability characteristics of two-phase co-axial flow are determined. The stability analysis follows two successive steps. First the steady base flow is determined, via the resolution of the nonlinear Navier-Stokes equations together with the location of the free interface. Second, these equations are linearized around the base flow and the dominant eigenmodes determined. The novelty lies in the formulation of models that can describe both the qualitative and quantitative characteristics of the two-phase flow configuration and the adaptation of the tools of global stability analysis for this configuration. We find that the dripping to jetting regime transition depends on the Capillary number, the degree of the confinement and the viscosity ratio, and we show that, surprisingly, the nozzle geometry does not affect the stability properties of the flow. Finally, the influence of surface viscosity on these coaxial flows has been considered. The governing and constitutive equations describing the continuum mechanics of the surface in the axisymmetric case are derived. With the addition of surface viscosity at the interface, the base flow evolves over a lengthscale which is much larger than the entry length in the Stokes regimes and than the typical unstable wavelength. We show that while the flow becomes eventually more convectively unstable once it reaches the fully developed profile, the surface viscosity creates an absolute region at the inlet, that is expected to promote droplet formation.

Internal crossflow, or internal flow that is perpendicular to the overflowing mainstream, reduces film cooling effectiveness by disrupting the diffusion of coolant at the exit of axial shaped holes. Previous experimental investigations have shown that internal crossflow causes the coolant to bias toward one side of the diffuser and that the severity of the biasing scales with the inlet velocity ratio, VRi, or the ratio of crossflow velocity to the jet velocity in the metering section of the hole. It has been hypothesized and computationally predicted that internal crossflow produces an asymmetric swirling flow within the hole that causes the coolant to bias in the diffuser and that biasing contributes to ingestion of hot mainstream gas into the hole, which is undesirable. However, there are no experimental measurements as of yet to confirm these predictions. In the present study, in and near-hole flow field and thermal field measurements were performed to investigate the flow structures and mainstream ingestion for a standard axial shaped hole fed by internal crossflow. Three different inlet velocity ratios of VRi = 0.24, 0.36, and 0.71 were tested at varying injection rate. Measurements were made in planes normal to the nominal direction of coolant flow at the outlet plane of the hole and at two downstream locations — x/d = 0 and 5. The predicted swirling structure was observed for the highest inlet velocity ratio and flow within the hole was shown to scale with VRi. Ingestion within the diffuser was significant and also scaled with VRi. Downstream flow and thermal fields showed that increased biasing contributed to more severe jet detachment and coolant dispersion away from the surface.

Internal crossflow, or internal flow that is perpendicular to the overflowing mainstream, reduces film cooling effectiveness by disrupting the diffusion of coolant at the exit of axial shaped holes. Previous experimental investigations have shown that internal crossflow causes the coolant to bias toward one side of the diffuser and that the severity of the biasing scales with the inlet velocity ratio, VRi, or the ratio of crossflow velocity to the jet velocity in the metering section of the hole. It has been hypothesized and computationally predicted that internal crossflow produces an asymmetric swirling flow within the hole that causes the coolant to bias in the diffuser and that biasing contributes to ingestion of hot mainstream gas into the hole, which is undesirable. However, there are no experimental measurements as of yet to confirm these predictions. In the present study, in- and near-hole flow field and thermal field measurements were performed to investigate the flow structures and mainstream ingestion for a standard axial shaped hole fed by internal crossflow. Three different inlet velocity ratios of VRi = 0.24, 0.36, and 0.71 were tested at varying injection rates. Measurements were made in planes normal to the nominal direction of coolant flow at the outlet plane of the hole and at two downstream locations—x/d = 0 and 5. The predicted swirling structure was observed for the highest inlet velocity ratio and flow within the hole was shown to scale with VRi. Ingestion within the diffuser was significant and also scaled with VRi. Downstream flow and thermal fields showed that increased biasing contributed to more severe jet detachment and coolant dispersion away from the surface.

Experiments on the Laminar/Turbulent Transition of Liquid Flows in Rectangular Microchannels

The motion and deformation of a neutrally buoyant drop in a rectangular channel experiencing a pressure-driven flow at a low Reynolds number has been investigated both experimentally and numerically. A moving-frame boundary-integral algorithm was used to simulate the drop dynamics, with a focus on steady-state drop velocity and deformation. Results are presented for drops of varying undeformed diameters relative to channel height ( $D/H$ ), drop-to-bulk viscosity ratio ( $\lambda$ ), capillary number ( $Ca$ , ratio of deforming viscous forces to shape-preserving interfacial tension) and initial position in the channel in a parameter space larger than considered previously. The general trend shows that the drop steady-state velocity decreases with increasing drop diameter and viscosity ratio but increases with increasing $Ca$ . An opposite trend is seen for drops with small viscosity ratio, however, where the steady-state velocity increases with increasing $D/H$ and can exceed the maximum background flow velocity. Experimental results verify theoretical predictions. A deformable drop with a size comparable to the channel height when placed off centre migrates towards the centreline and attains a steady state there. In general, a drop with a low viscosity ratio and high capillary number experiences faster cross-stream migration. With increasing aspect ratio, there is a competition between the effect of reduced wall interactions and lower maximum channel centreline velocity at fixed average velocity, with the former helping drops attain higher steady-state velocities at low aspect ratios, but the latter takes over at aspect ratios above approximately 1.5.

The phenomenon of viscous fluid buckling has a long and distinguished history, dating back to Taylor (1968). This paper is concerned with demonstrating that a numerical method, GENSMAC, is capable of simulating this physical instability. A table of the parameter values (e.g. the Reynolds number, the Froude number, inlet width, inlet velocity and aspect ratio) is provided giving details of when buckling occurs and when it does not. This allows the deduction of a possible buckling condition in terms of the Reynolds number and the ratio of height of the jet to the inlet width, modifying a previous hypothesis. Visualization of jet buckling is provided. This work has been motivated by the need of industry to understand jet filling of containers; jet buckling can lead to air entrapment and this is undesirable. Copyright © 1999 John Wiley & Sons, Ltd.

The digital particle image velocimetry (DPIV) technique has been used to investigate the flow fields of an elliptic jet in cross flow (EJICF). Two different jet orientations are considered; one with the major axis of the ellipse aligned with the cross flow (henceforth referred to as a low aspect ratio (AR) jet), and the other with the major axis normal to the cross flow (henceforth referred to as a high aspect ratio jet). Results show that the vortex-pairing phenomenon is prevalent in the low aspect ratio jet when the velocity ratio (VR)≥3, and is absent in the high aspect ratio jet regardless of the velocity ratio. The presence of vortex pairing leads to a substantial increase in the leading-edge peak vorticity compared to the lee-side vorticity, which suggests that vortex pairing may play an important role in the entrainment of ambient fluid into the jet body, at least in the near-field region. In the absence of vortex pairing, both the leading-edge and the lee-side peak vorticity increase monotonically with velocity ratio regardless of the aspect ratio. Moreover, time-averaged velocity fields for both AR=0.5 and AR=2 jets reveal the existence of an “unstable focus” (UF) downstream of the jet, at least for VR≥2. The strength and the location of this focus is a function of both the velocity ratio and aspect ratio. In addition, time-averaged vorticity fields show a consistently higher peak-averaged vorticity in the low aspect ratio jet than in the high aspect ratio jet. This behavior could be due to a higher curvature of the vortex filament facing the cross flow in the low aspect ratio jet, which through mutual interaction may lead to higher vortex stretching, and therefore higher peak-averaged vorticity.

Cylindrical cyclones are a popular choice for oil-water separation and sewage treatment in the petroleum industry. Here, we investigate the cavitation and vibration phenomena in a cylindrical cyclone with a vortex finder by using various cyclone operating parameters and conducting multiphase flow numerical simulations. The lowest point of pressure on the cross section of the main cylinder is defined as the pressure center point and is used to understand the generation of the flow field oscillation, which is shown to exhibit an approximately circular motion. This circular oscillation of the flow field is an important characteristic that influences the overall performance and behavior of the cyclone system. Cavitation analysis results show that an increase in the inlet velocity leads to more intense cavitation and an expansion of the cavitation area. At lower split ratios, cavitation primarily occurs at the junction of the inlet pipe and the main cylinder, while at higher split ratios, the cavitation area gradually extends into the overflow pipe. Additionally, structural response analysis demonstrates that increasing the inlet velocity and overflow split ratio significantly enhances the vibrational degree of the structure, leading to greater stress levels. Adjusting the oil content at the inlet has a relatively minimal impact on the structural response compared with the influence of the inlet velocity and split ratio. Furthermore, dimensional analysis is used to analyze the change of wall pressure, and the wall pressure that induces structural vibration can be calculated using the inlet velocity and the split ratio.

The forced response of a swirl-stabilized, partially premixed flame to inlet velocity and equivalence ratio oscillations was experimentally investigated in a model lean premixed gas turbine combustor. The responses of a premixed flame to three different forcing mechanisms were studied: velocity oscillations, equivalence ratio oscillations, and the combination of velocity and equivalence ratio oscillations. The overall heat release response of the flame was determined from measurements of the CH* chemiluminescence emission intensity from the entire flame, while the response of the spatially distributed heat release was determined from phase-synchronized chemiluminescence images. In addition, simultaneous measurements were made of the inlet velocity and equivalence ratio oscillations using the 2-microphone method and an IR absorption technique, respectively. The results show that in the linear regime, the response of a partially premixed flame to simultaneous velocity and equivalence ratio oscillations can be reconstructed from independent measurements of the flame's response to velocity oscillations and to equivalence ratio oscillations using a vector summation method. This is the first experimental demonstration of a 2-input 1-output model of a swirl-stabilized partially premixed flame. It suggests that the response of a partially premixed flame is governed by four physical parameters (i.e., the oscillation frequency, the amplitude of velocity oscillation, the amplitude of equivalence ratio oscillation, and the phase difference between the two oscillations). As a result, the heat release response of a partially premixed flame can be amplified or damped, depending on the phase difference between the velocity and equivalence ratio oscillations at the combustor inlet.

We present a CFD based model to understand the Taylor bubble behavior in Newtonian and non-Newtonian liquids flowing through a confined co-flow microchannel. Systematic investigation is carried out to explore the influence of surface tension, inlet velocities, and apparent viscosity on the bubble length, shape, velocity, and film thickness around the bubble. Aqueous solutions of carboxymethyl cellulose (CMC) with different concentrations are considered as power-law liquid to address the presence of non-Newtonian continuous phase on Taylor bubble. In all cases, bubble length was found to decrease with increasing Capillary number, inlet gas–liquid velocity ratio, and CMC concentration. However, bubble velocity increased due to increasing liquid film thickness around the bubble. At higher Capillary number and inlet velocity ratio, significant changes in bubble shapes are observed. With increasing CMC concentration, bubble formation frequency and velocity increased, but length decreased.