Range Of Flow Rate Ratio Research Articles

Numerical and experimental investigation of a flow focusing droplet-based microfluidic device

Droplet-based microfluidic devices are widely used for drug delivery and cell transport. In most of these applications, flow characteristics and geometry strongly affect the droplet generation process. In this study, a flow focusing droplet generator was designed using numerical simulation. Among all the simulation methods, the 2D phase field method was used to predict the process in a newly designed microfluidic device before fabrication. To this end, the numerical method was verified according to previous research, including ten different cases, which demonstrated its applicability in this regard. The deviation of simulations from experiments was less than 5%. The device was then simulated over a wide range of flow rate ratios (φ) and fabricated based on these results. Following fabrication, experiments were conducted for different ratios. The experimental results were compared with the simulations qualitatively and quantitatively. The investigation revealed pre-estimation effectiveness, indicating that the deviation of simulations from experiments was at most 11%. A wide range of φ from 0.25 to 1.05 was applied to study the droplet generation characteristics during the tests. Furthermore, two fundamental modes were introduced, and the flow regime shift from squeezing to dripping was studied over a wide range of φ. Given the droplets’ shape, the transition procedure is divided into three stages: pre-transition, transition, and post-transition. This study’s results can be used to investigate the capabilities of a newly designed microdevice before fabrication and choose the best flow regime that simultaneously reduces shear stress on cells and increases droplet generation frequency and monodispersity. Finally, this passive design can transport cells inside droplets up to 280 samples per second.

Simulation of immiscible liquid\u2013liquid flows in complex microchannel geometries using a front-tracking scheme

The three-dimensional two-phase flow dynamics inside a microfluidic device of complex geometry is simulated using a parallel, hybrid front-tracking/level-set solver. The numerical framework employed circumvents numerous meshing issues normally associated with constructing complex geometries within typical computational fluid dynamics packages. The device considered in the present work is constructed via a module that defines solid objects by means of a static distance function. The construction combines primitive objects, such as a cylinder, a plane, and a torus, for instance, using simple geometrical operations. The numerical solutions predicted encompass dripping and jetting, and transitions in flow patterns are observed featuring the formation of drops, ‘pancakes’, plugs, and jets, over a wide range of flow rate ratios. We demonstrate the fact that vortex formation accompanies the development of certain flow patterns, and elucidate its role in their underlying mechanisms. Experimental visualisation with a high-speed imaging are also carried out. The numerical predictions are in excellent agreement with the experimental data.

Mixing of fluids with dissimilar viscosities in Confined Impinging Jets

An elastic analogue model that describes the equilibrium between opposed jets have been validated for Confined Impinging Jets (CIJs) mixers using fluids with the same physical properties for a wide range of flow rate ratios between the opposed jets. Using this model as a design tool CIJs can efficiently mix fluids at non-unitary flow rate ratios. This paper extends the experimental and numerical characterization of the flow in CIJs for fluids with a viscosity ratio of 1:2 for a range of flow rate ratio up to 1:2. Experimental characterization is made with Planar Laser Induced Fluorescence, and numerical study is based on 3D CFD simulations of the CIJs. Experimental and numerical results coincide and show that the balance of kinetic energy rate of the opposed jets plays the key role on the jets impingement point positioning, although viscosity also has an impact on this. The elastic analogue model makes a good prediction of the impingement point position of the opposed jets. This paper shows that the operation at a flow regime with high mixing efficiency, a chaotic flow regime, depends on impinging the opposed jets at the CIJs mixing chamber axis and operating at Reynolds number larger than 125 for the lower Reynolds number jet.

Characterization of interfacial waves and pressure drop in horizontal oil-water core-annular flows

We study the transportation of highly viscous furnace-oil in a horizontal pipe as core-annular flow (CAF) using experiments. Pressure drop and high-speed images of the fully developed CAF are recorded for a wide range of flow rate combinations. The height profiles (with respect to the centerline of the pipe) of the upper and lower interfaces of the core are obtained using a high-speed camera and image analysis. Time series of the interface height are used to calculate the average holdup of the oil phase, speed of the interface, and the power spectra of the interface profile. We find that the ratio of the effective velocity of the annular fluid to the core velocity, α, shows a large scatter. Using the average value of this ratio (α=0.74) yields a good estimate of the measured holdup for the whole range of flow rate ratios, mainly due to the low sensitivity of the holdup ratio to the velocity ratio. Dimensional analysis implies that, if the thickness of the annular fluid is much smaller than the pipe radius, then, for the given range of parameters in our experiments, the non-dimensional interface shape, as well as the non-dimensional wall shear stress, can depend only on the shear Reynolds number and the velocity ratio. Our experimental data show that, for both lower and upper interfaces, the normalized power spectrum of the interface height has a strong dependence on the shear Reynolds number. Specifically, for low shear Reynolds numbers, interfacial modes with large wavelengths dominate, while, for large shear Reynolds numbers, interfacial modes with small wavelengths dominate. Normalized variance of the interface height is higher at lower shear Reynolds numbers and tends to a constant with increasing shear Reynolds number. Surprisingly, our experimental data also show that the effective wall shear stress is, to a large extent, proportional to the square of the core velocity. Using the implied scalings for the holdup ratio and wall shear stress, we can derive an expression for the pressure drop across the pipe in terms of the flow rates, which agrees well with our experimental measurements.

CO2 absorption using diethanolamine-water solutions in a rotating spiral contactor

Results for mass transfer in a rotating spiral device are presented here for absorption of carbon dioxide from nitrogen carrier gas using mixtures of diethanolamine (DEA) and water. The ability of the device to examine the full range of flow rate ratio for the two phases while controlling the relative thicknesses of the phase layers is applied to surveying absorption performance over a wide range of DEA concentration at 312K and 1.8 bara. Comparisons are made for a fixed 86μm liquid layer thickness, which is shown to fix also the fraction of the liquid accessible by diffusion, while maintaining 90% removal of CO2 from a gas stream of 10% (mole) CO2 in nitrogen. The increasing liquid viscosity with DEA fraction is countered by reducing the liquid flow rate to maintain constant liquid layer thickness and diffusion depth. The allowed gas throughput, while meeting 90% removal, increases with DEA concentration until the increasing viscosity gives sufficient reduction in liquid flow rate to offset the increasing CO2 capacity of the liquid. The maximum gas flow rate has a broad peak centred at a DEA mole fraction of about 0.072 (31% by mass). Utilisation of the amine is increased as DEA concentration increases, apparently as a result of the longer residence time, suggesting an effect of chemical time scales on the order of seconds. For a fixed concentration, full utilisation of the amine is achieved by decreasing the liquid flow rate, which reduces layer thickness and increases diffusion time. The work highlights the use of the rotating spiral for rapid and accurate testing to determine optimum liquid composition of absorbent formulations.

An elastic analog model for controlling the impingement point position in confined impinging jets

Confined impinging jets (CIJs) are highly efficient mixers. The scales of mixing in CIJs are controlled by the opposed jets interaction. A mechanistic model is described here, which accurately predicts the impinging position of the opposed jets for a large range of flow rate ratios. The impinging point position is shown to impact the dynamic properties of the flow and the achieved mixing quality. The opposed jets kinetic energy ratio is shown to have a critical impact on mixing, similar to the Reynolds number. A mixing chamber design relation is proposed and verified for the opposed injectors diameters ratio, , which enables to operate CIJs under optimum mixing conditions for large ranges of flow rate ratios, viscosity and density ratios between the opposed streams. Optimum values have asymptotes for large and small Reynolds number depending on the process stoichiometry, viscosity, and density ratios of the opposed jet streams. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2200–2212, 2016

Liquid–liquid mixing enhancement rules by microbubbles in three typical micro-mixers

Micro-mixers are good platforms to realize liquid–liquid mixing, but they still need intensification when treating extreme flow conditions, such as relatively higher fluid viscosity than water (μ>3mPas) or high flow rate ratio (R>10). In this work, the quantitative mixing enhancement rules with inert gas phase as enhancing medium in three typical micro-mixers, T-junction microchannel, cross-junction microchannel as well as co-flowing microchannel, are experimentally studied using “Villermaux–Dushman” parallel competitive reaction. The results show that the gas makes Xs reduced by 2–5 times in the working systems with various viscosities (0.86mPas≤μ≤11.46mPas) and operated in a wide range of flow rate ratio (1≤R≤25). The mixing enhancement mechanism is the two-phase segmented flow and we find only 0.05–0.1 gas–liquid phase flow rate ratio (Rgas) is enough for the gas phase to develop its enhancing effect. Based on the characteristics of segmented liquid flow, a mathematical model is established to predict the variation of Xs in three micro-mixers.

Dynamic behaviors of double emulsion formation in a flow-focusing device

Double emulsion drop formation in flow-focusing microfluidic device is investigated numerically by computational fluid dynamics simulation using VOF method. The simulation produces two typical drop formation modes, including dripping and jetting, and provides the detailed hydrodynamic information underlying these two modes. In addition, the effects of viscosity ratio, flow rate ratio and interfacial tension ratio on the double emulsion formation by the flow-focusing system are clarified quantitatively. It is indicated that the cooperation of the interfacial tension and viscous drag of the outer fluid induce the detaching of the double emulsion droplet under both dripping and jetting modes, while the large viscous drag from the outer fluid under jetting mode generates a long stretched neck which leads to the different formation behaviors. The drop formation mode transits from dripping to jetting abruptly when the middle fluid is more viscous than other fluids. And the size of the generated drops increases with the rise in viscosity ratio of the middle fluid to the inner one. The flow rate of the outer fluid determines the drop formation mode as well as the generated drop size, while the flow rate of the middle fluid contributes only to the drop size but have no obvious effect on drop formation regime in a wide range of flow rate ratio. In addition, the interfacial tension ratio shows little influence on drop sizes but large effect on the interface shapes.

Design of a New Micromixer for Instant Mixing Based on the Collision of Micro Segments

AbstractA new type of micromixer for rapid mixing was developed based on mixing by both kinetic energy and molecular diffusion of fluids. The rapid mixing was realized by the collision of micro segments, which were divided into several streams in the radial, at the center of the mixer. The excellent performance of the developed micromixer was confirmed by examining the Villermaux/Dushman reaction and the precipitation of polymer particles by phase separation. From these results, it was clarified that the rapid mixing was driven by both the breaking of micro segments under a high shear rate and instant molecular diffusion between micro segments. In addition, the mixer had a high throughput and corresponded to a wide range of flow rate ratios of fluids. Thus, it is expected that the proposed design concept for micro mixing will give us an efficient micro processing route in the future.

FLOW DEVELOPMENT OF CO-FLOWING STREAMS IN RECTANGULAR MICRO-CHANNELS

The diffusion and flow development characteristics of two co-flowing, laminar streams in a high aspect ratio rectangular micro-channel have been examined. A long, thin splitter plate initially separates the two streams such that fully developed flow in each of the two channels is established prior to merging. The co-flowing micro-channel has an aspect ratio of 16 with a width of 1006 μm and a height of 63 μm. Micro-Particle Image Velocimetry (μPIV) was utilized to observe the interaction between the streams for a range of flow rate ratios ranging from one to nine, for Reynolds numbers of one and ten. For flow rate ratios greater than one, a cross-stream pressure gradient exists immediately downstream of the splitter plate, which results in a strong lateral flow of the faster moving fluid into the slower moving fluid. Despite this rapid expansion, the fluids in the two streams do not mix. The two streams eventually recover a fully developed velocity profile across the entire channel. A model is presented to predict this development length based on the pressure imbalance between the two streams. The model is expressed in terms of the flow rate ratio between the streams, which is shown to be a function of channel aspect ratio. An asymptotic condition for the development length is found for high flow rate ratios and high aspect ratio channels. It is shown that existing entrance length relationships greatly underpredict this development length.

DIFFUSION AND FLOW DEVELOPMENT IN CO-FLOWING MICROCHANNEL STREAMS

A study of the momentum diffusion and flow development of two co-flowing streams in a microchannel is presented. The microchannel has a high-aspect-ratio (20) rectangular cross section with a height of 50 w m and a width of 1,000 w m. A splitter plate is used to obtain fully developed flow in each of two channels prior to the flows merging. Experimental results are obtained illustrating the streamwise variation of the interface between the streams for a range of flow rate ratios ranging from 1 to 9. Numerical simulations were used to determine details of the velocity and pressure fields in the region of momentum mixing. When there is a velocity difference between each stream there is a cross-stream pressure gradient just downstream of the splitter plate that causes the faster-moving stream to expand very rapidly into the slower-moving stream. The faster-moving stream is shown to recirculate into the slower-moving stream, resulting in a stagnation region. The two streams do not mix during this process, although mass diffusion is observed. The merged streams, although unmixed, have a fully developed velocity profile within approximately 4 to 14 hydraulic diameters downstream of the end of the splitter plate, depending on the flow rate ratio. This is shown to be significantly further than predicted using existing entrance length relationships in the literature.

Room Temperature Deposition of Silicon Oxynitride Films with Low Stress Using Sputtering-Type Electron Cyclotron Resonance Plasmas

AbstractSilicon oxynitride (SiOxNy) films with low stress were deposited successfully at room temperature using sputtering-type electron cyclotron resonance (ECR) plasmas. Films were deposited for a wide range of flow rate ratio of O2 to N2 at a constant Ar flow rate. Film properties were verified by characterizations of refractive index (ellipsometry), structural properties (Fourier transform infrared and Auger electron spectroscopy), intrinsic stress, and barrier strength of water penetration (thermal desorption spectroscopy). A near-stoichiometric SiOxNy (x = 1.44 and y = 0.41) film with low stress could be formed at the optimum deposition condition, under which the SiOxNy film had a refractive index of 1.54. The results of thermal desorption spectroscopy measurements showed that the SiOx Ny film had a higher barrier against moisture penetration relative to deposited SiOx and SiNy films. The SiOxNy film was directly deposited on the organic EL device and the applicability was shown clearly. These results indicate that this SiOxNy film deposited using a sputtering-type ECR plasma has the potential to be utilized as a passivation layer of organic EL devices, which are required to be formed at low temperature.

The dynamics of magma-mixing during flow in volcanic conduits

The production of mixed magmas (streaky pumice) during flow in a volcanic conduit has been modelled in the laboratory by studying the flow of two miscible fluids of differing viscosity passing concentrically through a vertical pipe. In the experiments reported in this paper, the outermost fluid is the more viscous, as would be the case when two magmas are simultaneously tapped from a zoned chamber in which silicic magma overlies mafic magma. At a Reynolds number (Re) which is much less than that required for turbulence in isoviscous pipe flow, the interface between two liquids of different viscosity can become unstable. Growth of the instability and mixing proceed when Re, based on the properties of the inner, less viscous fluid (Re i), is greater than approximately 3 if between 10% and 90% of the flowing fluid is composed of the more viscous fluid. Outside this range of flow rate ratios, higher Re i and viscosity ratios are required to ensure mixing. When the viscosity ratio U≤10 the unstable flow takes the form of an asymmetric, sinusoidal wave and at higher viscosity ratios axisymmetric, bead-like waves are the dominant instability. Entrainment across the boundaries of these wavy interfaces results in the production of streaky mixtures of the two liquids. The degree of mixing increases with Re 1, U and distance downstream. Application of experimental results to magmatic situations shows that mixing will be possible in eruptions which tap layers of different viscosity from a stratified chamber. If a volcanic feeder is allowed to become lined by silicic magma before a mafic magma layer is drawn up from the chamber then a mixed pumice (or lava) sequence will ensue. Alternatively, if draw-up occurs when the feeder is still propagating away from the chamber, the slower flowing silicic magma may be overtaken by the faster flowing mafic magma. The advancing conduit will then have mafic or hybrid chilled margins enclosing a silicic interior, i.e. the usual arrangement in composite dykes and sills. Simultaneous tapping of silicic and underlying mafic magmas from a chamber can thus lead to magma mixing and to the emplacement of either mixed pumice sequences or composite intrusions, depending on the history of magma withdrawal and the dynamics of flow in the conduit.