Mixing Enhancement Research Articles

Fabrication and characteristic study on mixing enhancement of a magnetofluidic mixer

The needs for fast mixing of two miscible fluids in microfluidic device is increasing recently and has been widely used for DNA synthesis, clinic diagnostics and chemical reaction. However, the mixing at microscale relies heavily on slow molecular diffusion because of its low Reynolds number and thus laminar flow. In this work, therefore, a magnetofluidic mixer with the integration of magnetic field is presented for rapid mixing between ferrofluid and distilled water with no harmful Joule heat generated. Parametric study of experiment and simulation was performed to discuss the effect of flow rate, placement angle of magnet and width of magnet on the mixing efficiency between two fluids. It was found that the mixing performance of 0.76 was the most homogeneous when the angle α = 1/12π because of the acceleration-deceleration couples (ADC) and mass conservation law. It was also found that the mixing performance increases from 0.13 to 0.49 as the magnet width increases from wm=50μm to wm=250μm while decreases when magnet width is wider than wm=250μm due to local reduced Peclet number. The developed micromixer and mixing mechanism will be of great potentials and insight for future quick optimization of biomedical and chemical mixing devices.

Mixing and transport enhancement in microchannels by electrokinetic flows with charged surface heterogeneity

Electrokinetic flow in a microchannel driven by charged surface heterogeneity in the presence of an external electric field is investigated by three-dimensional simulations. A computational framework is developed coupling a two-relaxation-time lattice Boltzmann solver for the transport equations of fluids, charged species, and passive tracing scalars and a fast Poisson solver for the electric potential. The two-relaxation-time lattice Boltzmann method is used to resolve the spatiotemporal distribution of flow field, ion concentration, and two passive tracing scalars. The fast Poisson solver is used to solve the electric potential at every time step. Three charged surface patterns together with various external electric fields are investigated. The induced electrokinetic vortices contribute to the mixing and transport enhancement of the passive scalars, depending on the surface patterns and the external electric field. The transport enhancement is found to follow a power law with respect to the magnitude of the external electric field.

Kinematic Measurements of Novel Chaotic Micromixers to Enhance Mixing Performances at Low Reynolds Numbers: Comparative Study.

In this work, a comparative investigation of chaotic flow behavior inside multi-layer crossing channels was numerically carried out to select suitable micromixers. New micromixers were proposed and compared with an efficient passive mixer called a Two-Layer Crossing Channel Micromixer (TLCCM), which was investigated recently. The computational evaluation was a concern to the mixing enhancement and kinematic measurements, such as vorticity, deformation, stretching, and folding rates for various low Reynolds number regimes. The 3D continuity, momentum, and species transport equations were solved by a Fluent ANSYS CFD code. For various cases of fluid regimes (0.1 to 25 values of Reynolds number), the new configuration displayed a mixing enhancement of 40%–60% relative to that obtained in the older TLCCM in terms of kinematic measurement, which was studied recently. The results revealed that all proposed micromixers have a strong secondary flow, which significantly enhances the fluid kinematic performances at low Reynolds numbers. The visualization of mass fraction and path-lines presents that the TLCCM configuration is inefficient at low Reynolds numbers, while the new designs exhibit rapid mixing with lower pressure losses. Thus, it can be used to enhance the homogenization in several microfluidic systems.

Capacitance Effects of a Hydrophobic-Coated Ion Gel Dielectric on AC Electrowetting.

We present experimental studies of alternating current (AC) electrowetting dominantly influenced by several unique characteristics of an ion gel dielectric in its capacitance. At a high-frequency region above 1 kHz, the droplet undergoes the contact angle modification. Due to its high-capacitance characteristic, the ion gel allows the contact angle change as large as Δθ = 26.4°, more than 2-fold improvement, compared to conventional dielectrics when f = 1 kHz. At the frequency range from 1 to 15 kHz, the capacitive response of the gel layer dominates and results in a nominal variation in the angle change as θ ≈ 90.9°. Above 15 kHz, such a capacitive response of the gel layer sharply decreases and leads to the drastic increase in the contact angle. At a low-frequency region below a few hundred Hz, the droplet’s oscillation relying on the AC frequency applied was mainly observed and oscillation performance was maximized at corresponding resonance frequencies. With the high-capacitance feature, the ion gel significantly enlarges the oscillation performance by 73.8% at the 1st resonance mode. The study herein on the ion gel dielectric will help for various AC electrowetting applications with the benefits of mixing enhancement, large contact angle modification, and frequency-independent control.

Jet mixing enhancement of a supersonic twin jet nozzle using rectangular tabs

Computational study on the development of mean flow and mixing capability of a rectangular tab placed at the exit of a Mach 1.6 single and twin jet nozzles has been presented. Placing two identical conical nozzles side by side separated by a distance of 1.5 times exit diameters makes the twin jet configuration. The mitigation technique for twin jet cross coupling effects by rectangular tabs for the enhancement of aerodynamic and acoustic characteristics is validating. The results are relevant to situations wherever shock cell structures, potential core length, jet mixing related noise are of concern. Centre line pressure decay characteristics shows that there is an abrupt reduction in the core length and suppression of shock cell structure at off design conditions. The results are in good agreement with the experimental results.

Effect of tab parameters on the near-field mixing characteristics of a Mach 1.5 elliptic jet

The effect of delta-tabs with its apex leaning downstream on the jet mixing characteristics of a Mach 1.5 elliptic jet of aspect ratio (AR) 3.37 is reported in this paper. The delta-tabs were placed along the minor and major axes of the elliptic nozzle, referred to as tab configuration C1 and C2, respectively. For each tab configuration, four different orientation angles varying from θ= 90∘ to 150∘ were studied, where θ= 90∘ is the tab positioned normal to the nozzle exit and θ= 150∘ is the tab oriented in the downstream direction at the nozzle exit. The tab effects on jet manipulation were investigated via shadowgraph visualization, and the mean Pitot pressure was measured along the jet centerline and along the orthogonal planes of the jet. The non-dimensonalized supersonic core length Lc*, average shock-cell spacing Ls¯*, and the axis-switching location xas* were examined in detail to understand the jet characteristics of the natural and manipulated jets. The mixing capability of the manipulated jets quantified based on the reduction in supersonic core length ΔLc* depends strongly on the ideally expanded jet Mach number Mj, tab configuration (C1 and C2), and θ. As such, the ΔLc* for the manipulated jet at θ= 135∘ is the largest, revealing the optimal tab orientation for jet mixing enhancement, as also observed for the circular jet [Zaman et al., “Control of an axisymmetric jet using vortex generators,” Phys. Fluids 6(2), 778–793 (1994)]. The relationship between ΔLc* and ΔLs* is presented to understand the correlation between the jet mixing and noise characteristics of the manipulated jet. Discussion is conducted based on the dependence of ΔLc* and ΔLs* with θ, which provides useful insight into the jet flow physics. With suitable placement of tabs C1 and C2, the axis switching could be either stopped or augmented. The superior mixing characteristics of an elliptic jet relative to a circular jet, which were found in previous works on the supersonic over-expanded jet [Kumar and Rathakrishnan, “Characteristics of a supersonic elliptic jet,” Aeronaut. J. 120(1225), 495 (2016)] prevail in the supersonic jet under-expanded jet as well.

Performance of corrugated actuator-tabs of aspect ratio 2.0 on supersonic jet mixing enhancement

The present experimental investigation explores the ability of the corrugated actuator-tabs (or simply, the corrugated actuators) of aspect ratio 2 in encouraging the mixing of Mach 1.73 circular jet at different levels of expansion conditions. Essentially, rectangular, triangular, and semi-circular corrugated actuators are deployed at the opposite positions across the diameter of the nozzle outlet and compared with the plain or simple actuators located at the same positions. Quantitative and qualitative observations were conducted by pitot pressure measurements and shadowgraph flow visualization. Interestingly, the characteristic decays are higher for the semi-circular and triangular corrugated actuators, exhibiting superior mixing than the rectangular corrugated actuator in the far-field. However, the core length reduction for rectangular corrugation is greater than for triangular and semicircular corrugation geometry. As high as 96.5 % core length reduction was obtained for the rectangular corrugated actuators operating at the overexpansion condition. The pressure profiles along and perpendicular to these actuators confirmed that the deployment of the corrugation geometry significantly reduces the flow asymmetry. The shadowgraphic flow visualizations reveal that the plain and corrugated actuators weaken the waves and shorten the cells when compared to the uncontrolled or free jet.

Two-phase model for mixed convection and flow enhancement of a nanofluid in an inclined channel patterned with heated slip stripes

PurposeThe purpose of this study is to analyze the heat transfer and flow enhancement of an Al2O3-water nanofluid filling an inclined channel whose lower wall is embedded with periodically placed discrete hydrophobic heat sources. Formation of a thin depletion layer of low viscosity over each hydrophobic heated patch leads to the velocity slip and temperature jump condition at the interface of the hydrophobic patch.Design/methodology/approachThe mixed convection of the nanofluid is analysed based on the two-phase non-homogeneous model. The governing equations are solved numerically through a control volume approach. A periodic boundary condition is adopted along the longitudinal direction of the modulated channel. A velocity slip and temperature jump condition are imposed along with the hydrophobic heated stripes. The paper has validated the present non-homogeneous model with existing experimental and numerical results for particular cases. The impact of temperature jump condition and slip velocity on the flow and thermal field of the nanofluid in mixed convection is analysed for a wide range of governing parameters, namely, Reynolds number (50 ≤ Re ≤ 150), Grashof number (), nanoparticle bulk volume fraction (5), nanoparticle diameter () and the angle of inclination ().FindingsThe presence of the thin depletion layer above the heated stripes reduces the heat transfer and augments the volume flow rate. Consideration of the nanofluid as a coolant enhances the rate of heat transfer, as well as the entropy generation and friction factor compared to the clear fluid. However, the rate of increment in heat transfer suppresses by a significant margin of the loss due to enhanced entropy generation and friction factor. Heat transfer performance of the channel diminishes as the channel inclination angle with the horizontal is increased. The paper has also compared the non-homogeneous model with the corresponding homogeneous model. In the non-homogeneous formulation, the nanoparticle distribution is directly affected by the slip conditions by virtue of the no-normal flux of nanoparticles on the slip planes. For this, the slip stripes augment the impact of nanoparticle volume fraction compared to the no-slip case.Originality/valueThis paper finds that the periodically arranged hydrophobic heat sources on the lower wall of the channel create a significant augmentation in the volume flow rate, which may be crucial to augment the transport process in mini- or micro-channels. This type of configuration has not been addressed in the existing literature.

A new stochastic ocean physics package and its application to hybrid‐covariance data assimilation

AbstractGenerating optimal perturbations is a key requirement of several data assimilation schemes. Here, we present a newly developed stochastic physics package for ocean models, implemented in the NEMO ocean general circulation model. The package includes three schemes applied simultaneously: stochastically perturbed parametrization tendencies (SPPT), stochastically perturbed parameters (SPP) and stochastic kinetic energy backscatter (SKEB) schemes. The three schemes allow for different temporal and spatial perturbation scales. Within a limited‐area ocean model configuration, ensemble free‐running simulations were performed to assess the impact and reliability of the schemes. They prove complementary in increasing the ensemble spread at different scales and for different diagnostics. The ensemble spread appears reliable; for instance, it proves consistent with the root‐mean‐square differences with respect to higher‐resolution (sub‐mesoscale) simulations that here represent the “truth” (in the sense that it includes “unresolved physics”). Interestingly, both the SPPT and the SKEB schemes lead to an increase of eddy kinetic energy at small spatial scales (2–10 km), and contribute to modify the ensemble mean state, mitigating warm biases near the thermocline due to the enhancement of the upper ocean vertical mixing. As an application of the stochastic packages, the ensemble anomaly covariances coming from the ensemble free‐running simulations are used to feed large‐scale anisotropic covariances that complement smaller‐scale ones in a hybrid‐covariance regional analysis and forecast system in the Mediterranean Sea. Ensemble‐derived covariances are formulated as slowly varying three‐dimensional low‐resolution empirical orthogonal functions (EOFs). The improvements due to the addition of such covariances to the stationary ones are found significant in real‐data experiments, within verification skill scores against glider profile data, remotely sensed observations and current speed measurements from drifters, radar and moorings.

Observational Study on the Variability of Mixed Layer Depth in the Bering Sea and the Chukchi Sea in the Summer of 2019

Abstract. Based on the high-resolution CTD data from 58 stations in the Bering Sea and the Chukchi Sea in the summer of 2019, the mixed layer depth (MLD) was obtained according to the density difference threshold method. It was verified that the MLD could be estimated more accurately by using a criterion of 0.125 kg/m3 in this region. The MLD in the Bering Sea basin was larger than that in the Bering Sea shelf, and both of them were smaller than that in the Bering Sea slope. The MLD increased northward both in the Chukchi Sea shelf and the Chukchi Sea slope. The farther northward, the greater the difference between the MLD calculated from temperature (MLDt) and the MLD calculated from density (MLDd) was, and the more important the role of salinity was in determining the MLD. The larger MLD (refer to MLDd specifically) in the Bering Sea slope might be due to the enhancement of mixing caused by the Bering Slope Current (BSC) and eddies. The horizontal advection of the Bering Sea Anadyr Water and the Alaska Coastal Water in the Bering Sea shelf led to the shallower MLD in the central transition zone. The northward increase of the MLD in the Chukchi Sea might be related to the low-salinity seawater resulting from the melting of sea ice in summer. The spatial variation of MLD was more closely related to the surface momentum flux than the sea surface buoyancy flux, and the wave had little effect.

Experimental investigation on mixing characteristics of high speed co-flow jets by using tabbed chevron nozzle

Abstract The present study investigates the effect of chevron with tab (Tabbed chevron) fixed at the exit of the co-flowing primary nozzle on the mixing characteristics in subsonic and sonic jets. The experiment was conducted for the jet Mach number 0.6 and 0.8. The centerline Mach number decay was calculated for all these jet Mach numbers for the co-flow baseline nozzle, nozzle with chevrons and tabbed chevron (primary flow) nozzle respectively. It was found that tabbed chevron was effective in reducing the potential core length by 88.23% as compared to the chevron nozzle with baseline nozzle. The radial profile also shows that the mixing enhancement by tabbed chevron is better than chevron nozzle.

Experimental investigation on mixing characteristics of high speed co-flow jets by using tabbed chevron nozzle

Abstract The present study investigates the effect of chevron with tab (Tabbed chevron) fixed at the exit of the co-flowing primary nozzle on the mixing characteristics in subsonic and sonic jets. The experiment was conducted for the jet Mach number 0.6 and 0.8. The centerline Mach number decay was calculated for all these jet Mach numbers for the co-flow baseline nozzle, nozzle with chevrons and tabbed chevron (primary flow) nozzle respectively. It was found that tabbed chevron was effective in reducing the potential core length by 88.23% as compared to the chevron nozzle with baseline nozzle. The radial profile also shows that the mixing enhancement by tabbed chevron is better than chevron nozzle.

Effect of a revolved wedge strut induced mixing enhancement for a hydrogen fueled scramjet combustor

The mixing concept of fuel and air is the burning issue for hypersonic vehicles (scramjet) due to the less resident time of supersonic air in the combustion chamber. So far, significant research has been done for mixing enhancement and introduced different technologies; still, there is a lack of research for mixing improvement. Shock wave and shear mixing layer are the main parameters for investigating mixing criteria at supersonic speed. In this research, an innovative fuel injection strut has been designed to develop mixing enhancement by elevating multiple interactions between the shock wave and shear mixing layer. This new strut has been designed with the reference of the DLR scramjet combustor. From the reference of a wedge-shaped strut, a revolved (wedge shape – circular 3D) wedge strut has been modeled with the same fuel injection base points. This new strut's performance has been analyzed for mixing enhancement by visualizing the development of shock wave, shear-mixing layer, and their interactions. Three-dimensional numerical analysis has been carried out by solving the Reynolds-Averaged and Navier-Stokes (RANS) equations. A comparison of results has been made for the basic wedge and new strut and identified the increase in multiple interactions of the shock wave and shear layer, which leads to an increase in mixing enhancement. For the new strut, complete mixing has been achieved within a distance of 0.180 m with an average increase in mixing efficiency of 9% and increased pressure losses of 12%.

Effective Chemical Delivery Through Multi‐Screen Wells to Enhance Mixing and Reaction of Solute Plumes in Porous Media

AbstractIn‐situ groundwater remediation is usually limited by the incomplete mixing of reactive species due to creeping flow characteristics and limited dispersion in porous media. In this study, we propose a novel approach to deliver chemicals through a multi‐screen well (MSW) to enhance the in‐situ remediation efficiency without extra energy or labor costs. The basic idea is to separate the injected solution into several plumes to enlarge the contact area between injected chemical compounds and ambient contaminants and enhance the mass exchange flux and reactive mixing. We perform laboratory experiments and numerical simulations of steady‐state instantaneous reactive transport in two‐dimensional porous media. We consider three separate screens in an MSW for the injection of treatment solutions and compare its plume distributions with a classical single‐screen well system. The experimental results confirm our conjecture and demonstrate the reliability of the numerical model. Furthermore, we derive an optimal injection interval of the MSW system by comparing analytical and numerical methods and demonstrate its capability to achieve effective transverse mixing and reaction enhancement. An interval that is smaller than the optimal injection interval leads to the exhaustion of contaminants between the injected plumes and the coalescence of the separate injected plumes at the downgradient portion along the travel distance, which reduces the effectiveness of the MSW system. We also investigate the sensitivity of the injected concentration, individual screen length, transverse dispersivity, and seepage rate on the determination of the optimal injection interval and provide suggestions about the design of an MSW system.

Mixing Enhancement by Pulsed Energy Deposition in Jet/Shock Wave Interaction

The mechanism of the supersonic round jet mixing enhancement by a pulsed energy deposition is studied numerically by solving the three-dimensional unsteady Reynolds-averaged Navier–Stokes equations with the second-order scheme in space and time. A round jet at different Mach numbers (, 1.76, 3.00) is parallelly injected into a coflowing air stream of Mach number 2.5, interacting with the oblique shock generated by a compression ramp of 20°. The density ratio between the jet and external flow is 0.5. The laser pulse with at 20 kHz is modeled as an ellipsoid for the energy addition, and proposed to put above or inside the jet for jet mixing control. It is found that the energy deposition inside the jet is much more effective for the jet mixing amplification compared with that above the jet. The jet mixing enhancement is achieved through the large-scale vortices, initiated by the energy deposition zone interacting with the oblique shock wave. It is shown that the jet cross-sectional area is doubled, and the turbulent kinetic energy is increased, even reaching 10 times the original level at the downstream location. However, a slight decrease about 0.3% of the total pressure recovery coefficient is observed far downstream.

Design exploration on the mixing augmentation induced by the oblique shock wave and a novel step in a supersonic flow

The mixing process in the supersonic flow is very important for the engineering implementation of the scramjet engine, and many novel mixing augmentation schemes have been proposed and investigated in the past few years. In this paper, the combination between the oblique shock wave and a novel step has been proposed to promote the mixing process between the supersonic airflow and the fuel, and the oblique shock wave has been induced by a ramp located on the upper wall of the channel. The impacts of the jet location and the jet pressure ratio have been both evaluated in order to explore their influences on the mixing enhancement mechanism. The obtained results show that the oblique shock wave is beneficial to prolong the residence time of the hydrogen in the supersonic flow, and the hydrogen distribution moves upstream with the oblique shock wave. At the same time, when the jet pressure ratio is large enough, the recirculation zone formed downstream of the step would increase, and this is beneficial for the mixing process between the fuel and the air as well. This may imply that the proposed combination scheme is a good candidate for the fuel mixing augmentation strategy, and its physical mechanism should be explored much deeper in the future, especially for its three-dimensional effect.

Mixed distortion image enhancement method based on joint of deep residuals learning and reinforcement learning

The images distortion leads to the loss of image information and the degradation of perceptual quality. To solve this problem, we investigate a novel mixed distortion image enhancement method based on the parallel network combines deep residual and reinforcement learning. The no-reference image quality assessment algorithm is used to determine the type and level of distorted images accurately. According to the type of distortion, the mixed distortion images enter one of the subsequent parallel joint learning networks automatically. In the joint learning framework, we prepare different residual networks to handle specialized restoration assignments including deblurring, denoising, or JPEG compression. Simultaneously, reinforcement learning then learns a policy to select the next best restoration tasks to progressively restore the quality of a corrupted image. Our method is capable of restoring images corrupted with complex mixed distortions in a more parameter-efficient manner in comparison to conventional networks. The extensive experiments on synthetic and real-world images validate the superior performances of the proposed method.

Behavior of mixing enhancement in microscale shock cylindrical bubble interaction

Behavior of mixing enhancement in microscale shock cylindrical bubble interaction

Asian Food Image Classification Based on Deep Learning

To improve Asian food image classification accuracy, a method that combined Convolutional Block Attention Module (CBAM) with the Mobile NetV2, VGG16, and ResNet50 was proposed for Asian food image classification. Additionally, we proposed to use a mixed data enhancement algorithm (Mixup) to have a smoother discrimination ability. The effects of introducing the attention mechanism (CBAM) and using the mixed data enhancement algorithm (Mixup) were shown respectively through experimental comparison. The combination of these two and the final test set Top-1 accuracy rate reached 87.33%. Moreover, the information emphasized by CBAM was reflected through the visualization of the heat map. The results confirmed the classification method’s effectiveness and provided new ideas that improved Asian food image classification accuracy.

Wave mixing and high-harmonic generation enhancement by a two-color field driven dielectric metasurface [Invited

High-harmonic generation in metasurfaces, driven by strong laser fields, has been widely studied. Compared to plasma, all-dielectric nanoscale metasurfaces possess larger nonlinearity response and higher damage threshold. Additionally, it can strongly localize the driven field, greatly enhancing its peak amplitude. In this work, we adopt a Fano resonant micro-nano structure with transmission peaks at different wavelengths and explore its nonlinear response by both single and two-color pump fields. Compared to the high-order harmonics induced by the first resonant wavelength, the intensity of the high-harmonic radiation results is enhanced by one order of magnitude, when the metasurface is driven by various resonant and non-resonant wavelength combinations of a two-color field.