Search Results (11)

The Reynolds-averaged Navier–Stokes (RANS)-based stochastic multiple mapping conditioning (MMC) approach has been used to study partially premixed jet flames of dimethyl ether (DME) introduced into a vitiated coflowing oxidizer stream. This study investigates DME flames with varying degrees of partial premixing within a fuel jet across different coflow temperatures, delving into the underlying flame structure and stabilization mechanisms. Employing a turbulence k-$\epsilon$ model with a customized set of constants, the MMC technique utilizes a mixture fraction as the primary scalar, mapped to the reference variable. Solving a set of ordinary differential equations for the evolution of Lagrangian stochastic particles’ position and composition, the molecular mixing of these particles is executed using the modified Curl’s model. The lift-off height (LOH) derived from RANS-MMC simulations are juxtaposed with experimental data for different degrees of partial premixing of fuel jets and various coflow temperatures. The RANS-MMC methodology adeptly captures LOH for pure DME jets but exhibits an underestimation of flame LOH for partially premixed jet scenarios. Notably, as the degree of premixing escalates, a conspicuous underprediction in LOH becomes apparent. Conditional scatter and contour plots of OH and CH₂O unveil that the propagation of partially premixed flames emerges as the dominant mechanism at high coflow temperatures, while autoignition governs flame stabilization at lower coflow temperatures in partially premixed flames. Additionally, for pure DME flames, autoignition remains the primary flame stabilization mechanism across all coflow temperature conditions. The study underscores the importance of considering the degree of premixing in partially premixed jet flames, as it significantly impacts flame stabilization mechanisms and LOH, thereby providing crucial insights into combustion dynamics for various practical applications. Full article

Numerous combustion applications are concerned with the stabilization of diffusion flames formed by injecting gaseous fuels into a co-flowing stream containing an oxidizer. The smooth operation of these devices depends on the attachment and lift-off characteristics of the edge flame at the base of the diffusion flame. In this paper, we address fundamental issues pertinent to the structure and dynamics of edge flames, which have attributes of both premixed and diffusion flames. The adopted configuration is the mixing layer established in the wake of a splitter plate where two streams, one containing fuel and the other oxidizer, merge. The analysis employs a diffusive-thermal model which, although it excludes effects of gas expansion, systematically includes the influences of the overall flow rate, unequal strain rates in the incoming streams, stoichiometry, differential and preferential diffusion, heat loss and gas–solid thermal interaction, and their effect on the edge structure, speed, and temperature. Conditions when the edge flame is anchored to the plate, lifted-off and stabilized in the flow, or blown-off, are identified. Two stable modes of stabilization are observed for lifted flames; the edge flame either remains stationary at a specified location or undergoes spontaneous oscillations along a direction that coincides with the trailing diffusion flame. Full article

Identifying combustion regimes is important for understanding combustion phenomena and the structure of flames. This study proposes a combustion regime identification (CRI) method based on rotated principal component analysis (PCA), clustering analysis and the back-propagation neural network (BPNN) method. The methodology is tested with large-eddy simulation (LES) data of two turbulent non-premixed flames. The rotated PCA computes the principal components of instantaneous multivariate data obtained in LES, including temperature, and mass fractions of chemical species. The frame front results detected using the clustering analysis do not rely on any threshold, indicating the quantitative characteristic given by the unsupervised machine learning provides a perspective towards objective and reliable CRI. The training and the subsequent application of the BPNN rely on the clustering results. Five combustion regimes, including environmental air region, co-flow region, combustion zone, preheat zone and fuel stream are well detected by the BPNN, with an accuracy of more than 98% using 5 scalars as input data. Results showed the computational cost of the trained supervised machine learning was low, and the accuracy was quite satisfactory. For instance, even using the combined data of CH4-T, the method could achieve an accuracy of more than 95% for the entire flame. The methodology is a practical method to identify combustion regime, and can provide support for further analysis of the flame characteristics, e.g., flame lift-off height, flame thickness, etc. Full article

In this study we report the effect of fuel type (biodiesel vs. methane), flame structure and flame height (inner-cone vs. outer-cone), and the percent of oxygen content in the oxidizer stream for the formation of hydrophobic carbon layers using co-flow diffusion flames. It was found that a flame formed using a gaseous fuel (methane) over a vaporized liquid fuel, Canola Methyl Ester (CME), has significant structural differences that enable vastly different deposition behavior of soot layers on the surface of solid substrates. Due to its larger pyrolysis zone (taller inner-cone), the CH₄/air flame has a smaller region that supports uniform soot deposition of hydrophobic carbon layers (C-layers) compared to the CME/air flame. When a solid substrate is placed within the pyrolysis zone (inner-cone) of a flame the resulting layer is non-uniform, hydrophilic, and consists of undeveloped soot. However, when outside the pyrolysis zone, the deposited soot tends to be uniform and mature, ultimately creating a hydrophobic C-layer consisting of the typical microscale interconnected weblike structures formed of spherical soot nanoparticles. The effect of oxygen content (35% and 50% O₂) in the oxidizer stream for the formation of hydrophobic C-layers was also studied in this work. It was found that oxygen enrichment within the CME flame alters the structure of the flame, hence affecting the morphology of the formed C-layer. Under oxygen enrichment the central region of the deposited C-layer is composed of a weblike structure similar to those seen in the air flames; however, this central region is bordered by a region of densely compacted soot that shows signs of significant thermal stress. At 35% O₂ the thermal stress is expressed as multiple microscale cracks while at 50% O₂ this border region shows much larger cracks and macroscale layer peeling. The formed C-layers under the different flame conditions were tested for hydrophobicity by measuring the contact angle of a water droplet. The morphology of the C-layers was analyzed using scanning electron microscopy. Full article

Determination of blood viscosity requires consistent measurement of blood flow rates, which leads to measurement errors and presents several issues when there are continuous changes in hematocrit changes. Instead of blood viscosity, a coflowing channel as a pressure sensor is adopted to quantify the dynamic flow of blood. Information on blood (i.e., hematocrit, flow rate, and viscosity) is not provided in advance. Using a discrete circuit model for the coflowing streams, the analytical expressions for four properties (i.e., pressure, shear stress, and two types of work) are then derived to quantify the flow of the test fluid. The analytical expressions are validated through numerical simulations. To demonstrate the method, the four properties are obtained using the present method by varying the flow patterns (i.e., constant flow rate or sinusoidal flow rate) as well as test fluids (i.e., glycerin solutions and blood). Thereafter, the present method is applied to quantify the dynamic flows of RBC aggregation-enhanced blood with a peristaltic pump, where any information regarding the blood is not specific. The experimental results indicate that the present method can quantify dynamic blood flow consistently, where hematocrit changes continuously over time. Full article

Air compliance has been used effectively to stabilize fluidic instability resulting from a syringe pump. It has also been employed to measure blood viscosity under constant shearing flows. However, due to a longer time delay, it is difficult to quantify the aggregation of red blood cells (RBCs) or blood viscoelasticity. To quantify the mechanical properties of blood samples (blood viscosity, RBC aggregation, and viscoelasticity) effectively, it is necessary to quantify contributions of air compliance to dynamic blood flows in microfluidic channels. In this study, the effect of air compliance on measurement of blood mechanical properties was experimentally quantified with respect to the air cavity in two driving syringes. Under periodic on–off blood flows, three mechanical properties of blood samples were sequentially obtained by quantifying microscopic image intensity (<I>) and interface (α) in a co-flowing channel. Based on a differential equation derived with a fluid circuit model, the time constant was obtained by analyzing the temporal variations of β = 1/(1–α). According to experimental results, the time constant significantly decreased by securing the air cavity in a reference fluid syringe (~0.1 mL). However, the time constant increased substantially by securing the air cavity in a blood sample syringe (~0.1 mL). Given that the air cavity in the blood sample syringe significantly contributed to delaying transient behaviors of blood flows, it hindered the quantification of RBC aggregation and blood viscoelasticity. In addition, it was impossible to obtain the viscosity and time constant when the blood flow rate was not available. Thus, to measure the three aforementioned mechanical properties of blood samples effectively, the air cavity in the blood sample syringe must be minimized (V_{air, R} = 0). Concerning the air cavity in the reference fluid syringe, it must be sufficiently secured about V_{air, R} = 0.1 mL for regulating fluidic instability because it does not affect dynamic blood flows. Full article

Blood flows in microcirculation are determined by the mechanical properties of blood samples, which have been used to screen the status or progress of diseases. To achieve this, it is necessary to measure the viscoelasticity of blood samples under a pulsatile blood condition. In this study, viscoelasticity measurement is demonstrated by quantifying interface variations in coflowing streams. To demonstrate the present method, a T-shaped microfluidic device is designed to have two inlets (a, b), one outlet (a), two guiding channels (blood sample channel, reference fluid channel), and one coflowing channel. Two syringe pumps are employed to infuse a blood sample at a sinusoidal flow rate. The reference fluid is supplied at a constant flow rate. Using a discrete fluidic circuit model, a first-order linear differential equation for the interface is derived by including two approximate factors (F₁ = 1.094, F₂ = 1.1087). The viscosity and compliance are derived analytically as viscoelasticity. The experimental results showed that compliance is influenced substantially by the period. The hematocrit and diluent contributed to the varying viscosity and compliance. The viscoelasticity varied substantially for red blood cells fixed with higher concentrations of glutaraldehyde solution. The experimental results showed that the present method has the ability to monitor the viscoelasticity of blood samples under a sinusoidal flow-rate pattern. Full article

Double emulsions are useful geometries as templates for core-shell particles, hollow sphere capsules, and for the production of biomedical delivery vehicles. In microfluidics, two approaches are currently being pursued for the preparation of microfluidic double emulsion devices. The first approach utilizes soft lithography, where many identical double-flow-focusing channel geometries are produced in a hydrophobic silicone matrix. This technique requires selective surface modification of the respective channel sections to facilitate alternating wetting conditions of the channel walls to obtain monodisperse double emulsion droplets. The second technique relies on tapered glass capillaries, which are coaxially aligned, so that double emulsions are produced after flow focusing of two co-flowing streams. This technique does not require surface modification of the capillaries, as only the continuous phase is in contact with the emulsifying orifice; however, these devices cannot be fabricated in a reproducible manner, which results in polydisperse double emulsion droplets, if these capillary devices were to be parallelized. Here, we present 3D printing as a means to generate four identical and parallelized capillary device architectures, which produce monodisperse double emulsions with droplet diameters in the range of 500 µm. We demonstrate high throughput synthesis of W/O/W and O/W/O double emulsions, without the need for time-consuming surface treatment of the 3D printed microfluidic device architecture. Finally, we show that we can apply this device platform to generate hollow sphere microgels. Full article

In this study, we make use of the AC field-effect flow control on induced-charge electroosmosis (ICEO), to develop an electrokinetic micromixer with 3D electrode layouts, greatly enhancing the device performance compared to its 2D counterpart of coplanar metal strips. A biased AC voltage wave applied to the central gate terminal, i.e., AC field-effect control, endows flow field-effect-transistor of ICEO the capability to produce arbitrary symmetry breaking in the transverse electrokinetic vortex flow pattern, which makes it fascinating for microfluidic mixing. Using the Debye-Huckel approximation, a mathematical model is established to test the feasibility of the new device design in stirring nanoparticle samples carried by co-flowing laminar streams. The effect of various experimental parameters on constructing a viable micromixer is investigated, and an integrated microdevice with a series of gate electrode bars disposed along the centerline of the channel bottom surface is proposed for realizing high-flux mixing. Our physical demonstration on field-effect nonlinear electroosmosis control in 3D electrode configurations provides useful guidelines for electroconvective manipulation of nanoscale objects in modern microfluidic systems. Full article

A simple and robust co-flowing microfluidic device for double emulsion preparation is designed and assembled to visually study the double emulsion formation by the use of a microscope and high-speed camera. Using a visualization system, the transient processes of double emulsion formation in co-flowing stream are observed and recorded for a variety of flow rates. The effects of flow rates of each fluid on flow modes, drop sizes, and polydispersities are examined and analyzed. The results indicate that the detaching of the inner drops accelerates the detaching of the outer drops and speeds up the drop formation process of double emulsions. The manipulation of flow rates is capable to actively control the sizes of the inner and outer drops as well as the number of inner drops encapsulated. Without surface modification, the microfluidic device produces a variety of emulsions, including the single-core and multi-core O/W/O double emulsions as well as binary emulsions of single and double emulsions. The proposed co-flowing microfluidic device is highly desirable in producing double emulsions in an easy and cheap way. Full article

Microfluidic platforms capable of complex on-chip processing and liquid handling enable a wide variety of sensing, cellular, and material-related applications across a spectrum of disciplines in engineering and biology. However, there is a general lack of available active microscale mixing methods capable of dynamically controlling on-chip solute concentrations in real-time. Hence, multiple microfluidic fluid handling steps are often needed for applications that require buffers at varying on-chip concentrations. Here, we present a novel electrokinetic method for actively mixing laminar fluids and controlling on-chip concentrations in microfluidic channels using fluidic dielectrophoresis. Using a microfluidic channel junction, we co-flow three electrolyte streams side-by-side so that two outer conductive streams enclose a low conductive central stream. The tri-laminar flow is driven through an array of electrodes where the outer streams are electrokinetically deflected and forced to mix with the central flow field. This newly mixed central flow is then sent continuously downstream to serve as a concentration boundary condition for a microfluidic gradient chamber. We demonstrate that by actively mixing the upstream fluids, a variable concentration gradient can be formed dynamically downstream with single a fixed inlet concentration. This novel mixing approach offers a useful method for producing variable on-chip concentrations from a single inlet source. Full article