Coaxial Jet Research Articles

Numerical study of the injection conditions effect on the behavior of hydrogen-air diffusion flame

In order to respond to the increased demand for clean energy without harming the atmosphere through polluting emissions, Energy production from the hydrogen combustion become largely used. This work presents a numerical study of the injection conditions effect on the structure of the H2-Air diffusion flame. The aim is to reproduce a practical case of non-polluting combustion and resulting in very high temperatures. The configuration is composed of two axisymmetric coaxial jets, as can be found in the diffusion burners. A presumed probability density function (PDF) approach is used to describe the chemistry-turbulence interaction. K-epsilon model of turbulence is used. Particular attention is given to phenomena anchoring or blowout of the flame.

Combining responsiveness and durability in liquid crystal-functionalised electrospun fibres with crosslinked sheath

ABSTRACT Responsive functional composite fibre mats that are mechanically stable and impervious to water exposure are produced by coaxial electrospinning of thermotropic liquid crystal (LC) core inside a water-based solution of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) forming the sheath. Because thermotropic LCs usually cannot be spun inside water-based solutions due to excessive interfacial tension , a n enabling step is the addition of ethanol or dioxane to the LC as a co-solvent compatible with both core and sheath fluids. This reduces sufficiently that coaxial jet spinning is possible. After spinning, thermal cross-linking of the PVA+PAA sheath yields LC-functionalised fibres that can be manipulated by hand and remain intact even upon full immersion in water. The LC core retains its behaviour, nematics showing well-aligned birefringence and transitioning to isotropic upon heating above the clearing point, and cholesterics showing selective reflection which is even enhanced upon water immersion due to the removal of sheath scattering. Our results pave the way to producing LC-functionalised responsive fibre mats using durable polymer sheaths, thereby enabling numerous innovative applications in wearable technology, and they also open new opportunities to study LCs in confinement, without visible impact of the container walls.

Coaxial electrospray of uniform polylactide core-shell microparticles for long-acting contraceptive

Despite its high efficacy and good patient compliance, the only long-acting injectable (LAI) contraceptive currently available in the US, depot medroxyprogesterone acetate (DMPA), is limited by significant side effects and a delayed return to fertility for up to 10 months after its intended duration of action. To overcome these limitations, we sought to develop an injectable poly(D,l-lactide) (PLA) microparticle for sustained release of contraceptive hormone, etonogestrel (ENG). A one-step technique, coaxial electrospray method was applied to prepare uniform ENG loaded core-shell structured and slow-degrading PLA microparticles (ENG-cs-MPs) to provide release control while minimizing polymer content. By adjusting voltage, polymer concentration and flow rate of the coaxial jetting solution, the prepared ENG-cs-MPs exhibited uniformly small particle size with volume mean diameter of 14.7 ± 0.5 μm and a shell thickness of 2.5 ± 0.1 μm, high drug loading of ~54%, high encapsulation efficiency of ~99%, and initial 1-day burst release of just ~10%. Long-term in vitro release of ENG was continuous for more than 3 months without change of the shell structure in 6 months. In PK studies, ENG-cs-MPs achieved a steady and continuous drug release for approximately 3 months and then quickly tapered off within 3 weeks. Hence, ENG-cs-MPs prepared by the coaxial electrospray method may be useful as a LAI contraceptive with an improved PK profile relative to DMPA.

Spray dispersion regimes following atomization in a turbulent co-axial gas jet

A canonical co-axial round-jet two-fluid atomizer where atomization occurs over a wide range of momentum ratios: $M=1.9 - 376.4$ is studied. The near field of the spray, where the droplet formation process takes place, is characterized and linked to droplet dispersion in the far field of the jet. Counterintuitively, our results indicate that in the low-momentum regime, increasing the momentum in the gas phase leads to less droplet dispersion. A critical momentum ratio of the order of $M_c=50$ , that separates this regime from a high-momentum one with less dispersion, is found in both the near and far fields. A phenomenological model is proposed that determines the susceptibility of droplets to disperse beyond the nominal extent of the gas phase based on a critical Stokes number, $St=\tau _p/T_E=1.9$ , formulated based on the local Eulerian large scale eddy turnover time, $T_E$ , and the droplets’ response time, $\tau _p$ . A two-dimensional phase space summarizes the extent of these different regimes in the context of spray characteristics found in the literature.

Instability of coaxial viscoelastic jets under a radial electric field

In this paper we investigate temporal linear instability and absolute/convective instability of coaxial viscoelastic jets in the presence of a radial electric field. With a leaky dielectric model to describe the finite conductivity of the outer liquid, both axisymmetric instability and nonaxisymmetric instability are studied and compared to determine the predominant mode in the jet instability. For relatively small wavenumbers, the electric field suppresses the axisymmetric instability, while it enhances axisymmetric instability for relatively large wavenumbers. As the electric field increases, the instability mode tends to be a short wave. The elasticity of the inner and outer liquid is found to contribute to the coaxial jet instability, and the increase of the inner-liquid elasticity is discovered to allow the para-sinuous mode to predominate over the helical mode. Furthermore, the spatiotemporal instability analysis indicates that under weaker electric fields the axisymmetric mode is convectively unstable, but becomes absolutely unstable when the electric field is relatively strong, when the outer liquid has a certain degree of elasticity. While the nonaxisymmetric mode is found to be always absolutely unstable. A boundary line between absolute and convective instability is given on the parameter space of the Deborah number (relative relaxation time) of the outer liquid and the electrical Euler number. Our results in temporal and spatiotemporal instability analysis can be useful for understanding the origin of the different modes and desired mode control in coaxial electrospinning.

Prediction of combustion instability by combining transfer functions in a model rocket combustor

The flame transfer function (FTF) and acoustic transfer function (ATF) are estimated experimentally and numerically, respectively, and both are combined to predict combustion instability of a model rocket combustor with a single coaxial jet–swirl injector. FTF of the injector is measured experimentally by imposing external velocity excitation on the injector. ATF, i.e., model-chamber response to heat-release fluctuation from flames, is difficult to measure, and alternatively, an idea of numerical evaluation method is suggested by applying an equivalence-ratio perturbation to flames. Then, ATF is calculated by numerical simulations with a three-dimensional model and compared with ATF from the one-dimensional analytic equation. Results show that ATF estimated by the 3-D numerical model improves accuracy in predicting resonant frequencies and growth rates of pressure oscillations in the model chamber. Finally, a closed-loop system transfer function describing dynamic behaviors of the model chamber is formulated by combining FTF with ATF and it is applied to predict combustion instability of the chamber. This prediction is compared with experimental observation, which shows good agreement between them.

Experimental Study on the Effect of Locally Convergent Configurations on the Flow Pattern in PMMA Microchips

The geometries of micro-channel play a key role in forming of digital droplets, and can be real-time or effective controlling methodologies. Local convergence regions are designed in the rectangular cross-section channels on PMMA microchips, in which two-phase coaxial jets are introduced by inserting a syringe needle. The two-phase flow (lubricating oil (continuous phase, flow rate Q c)/deionized water (dispersed phase, flow rate Q d)) is considered. Two geometric control variables, the relative position (needle displacement x) and tapering characteristics (convergence angle α), are naturally adopted to discribe such geometry configurations. The micro-flow under the change of these two parameters is mainly studied in this paper. Four kinds of characteristic flow patterns, namely, sausages, slug, dripping and jetting, are found in the experiment, and their occurring parameters and developing dynamic characteristics are discussed. The experiment shows that the increase of inner needle displacement x can produce higher frequency and finer droplets, which is in consistent with our previous results obtained in round tube experiments and simulations. While increasing the convergence angle α, contrarily, takes opposite effects.

GRB 101225A as Orphan Dipole Radiation of a Newborn Magnetar with Precession Rotation in an Off-axis Gamma-ray Burst

Abstract The unusual multiwavelength lightcurves of GRB 101225A are revisited by assuming that they are from an off-axis GRB powered by a newborn magnetar. We show that GRB 101225A’s optical afterglow lightcurve is fitted with the forward shock model by parameterizing its jet structure as a Gaussian function with a half-opening angle of the jet core as 1.°67. The derived initial Lorentz factor (Γ0) is 120, and the viewing angle to the jet axis is θ v = 3.°7. Tentative QPO signatures of P = 488 s and P = 250 ∼ 300 s are found with a confidence level of 90% by analyzing its X-ray flares observed in the time interval of [4900, 7500] s. Its global gamma-ray/X-ray lightcurve and the QPO signatures are represented with the magnetar dipole radiation (DR) model by considering the magnetar precession motion, assuming that the magnetar spindown is dominated by GW emission. The bulk Lorentz factor of the DR ejecta is limited to 8, being much lower than Γ0. Comparing GRB 101225A with the extremely off-axis GRB 170817A, we suspect that the nature of the two-component jet in GRB 170817A is a combination of a co-axial GRB jet and a DR ejecta. GRB 101225A would be among the brightest ones of the CDF-S XT2-like X-ray transient population driven by newborn magnetars. A discussion of the detectability of its gravitational wave emission is also presented.

Simultaneous measurements of velocity, gas concentration, and temperature by way of thermal-anemometry-based probes

Many natural and engineering flows transport more than one scalar. Moreover, to study the scalar mixing therein, knowledge of the velocity field is also essential. For this reason, the present work describes the development of a three-wire thermal-anemometry-based probe to simultaneously measure velocity, helium concentration, and temperature in turbulent flows. It is first demonstrated, both theoretically and experimentally, that the temperature measured by a cold-wire thermometer is effectively insensitive to helium concentration. Then, building on recent work by Hewes and Mydlarski (2021 Meas. Sci. Technol. 32 105305), which pertains to the design of interference probes (i.e. thermal-anemometry-based probes used to measure velocity and gas concentration), a novel temperature compensation technique is proposed to extend their use to non-isothermal flows. The performance of the compensation technique is validated in turbulent coaxial jets by combining the cold-wire thermometer and interference probe to form a three-wire probe. Given that the three-wire probe can be employed to obtain simultaneous measurements of velocity and multiple scalars, it can therefore be used investigate phenomena such as multi-scalar mixing, including differential diffusion.

Numerical Investigation on Combustion-Enhancement Strategy in Shock–Fuel Jet Interaction

Multidimensional numerical simulations are performed to investigate the evolution and formation of unburned fuels for a shock–fuel jet interaction scenario. A full set of Navier–Stokes equations with detailed chemical mechanisms are solved, and the results are analyzed through the Lagrangian method with the goal of improving combustion efficiency in supersonic flows. The flame morphology of a two-dimensional (2-D) non-premixed reactive shock–bubble interaction is first simulated and studied, in which unburned hydrogen is found to prevent efficient combustion. By applying the Lagrangian particle tracking method, most of the unburned hydrogen wrapped into the primary vortex turns out to be initially located upon the symmetry line of the bubble. Motivated by the idea of breaking the primary vortex, this study designs a novel geometry of a concentric bubble, which improves combustion efficiency to 94.4% in contrast to a solid fuel bubble (74%) due to multivortex interaction and a thick bridge structure. With the consistency between qualitative and quantitative 2-D and three-dimensional (3-D) flow dynamics, the idea of a 2-D concentric-bubble configuration is effectively extended to a 3-D coaxial jet interacting with oblique shock despite the existence of Kelvin–Helmholtz instability.

Controlled engineering of multifunctional porous structures using tri-needle co-axial electrohydrodynamic flow and sacrificial media

Three-dimensional porous architectures have gained popularity as advanced materials in emerging therapeutics, drug delivery systems and tissue regeneration. This is largely due to their porosity, inter-connectivity, and mechanical properties. However, current approaches to derive such three-dimensional structures are stochastic and therefore density uniformity varies, with innermost components exhibiting dense attributes. This also limits the encapsulation ability and quality of compounds. In this study, multifunctional structures comprising multi-layered particles are developed. Polycaprolactone (PCL)/ethyl cellulose (EC)/ganoderma lucidum polysaccharide (GLP) is the outer layer; silicone oil (SO) is the intermediate layer; PCL and magnetic Fe3O4 nanoparticles (MNP) are the inner-most (core) component. Particles are initially formed using single step tri-needle co-axial electrostatic jetting, followed by sacrificial template removal to enable multifunctional porous structures. Using this approach, deployment of layered bodies enables the formation of homogeneous particles (150 µm) with tunable pore sizes of 100 s of nanometers to 350 s of microns. The process is versatile, and results indicate multi-scale pore size and hybrid pore connectivity are regulated through manipulation of the sacrificial agent. These structures are therefore promising for several multi-faceted healthcare applications e.g. targeted drug delivery, 3D cell culture, and real-time MRI imaging.

The Relevancy of Electrical Measurements to Determine Plasma Region Extension in a Coaxial SDBD Plasma Jet

A coaxial electrode configuration with surface dielectric barrier discharge is designed to generate a cold plasma jet emerging into the ambient air. A technique to determine the length of plasma extension based on the measurements of average values of current and applied voltage is proposed. The capacity of the discharge cell calculated using current and voltage values agrees with the capacity obtained from the integral length of a plasma layer along a dielectric surface. The average charge deposited on the dielectric surface is determined.

Entropy image analysis of primary atomization for a coaxial airwater jet submitted to acoustics at variable ambient pressure.

Entropy image analysis of primary atomization for a coaxial airwater jet submitted to acoustics at variable ambient pressure.

Surface microfabrication using coaxial waterjet assisted laser-induced plasma micromachining

In this study, a novel coaxial water assisted laser-induced plasma micromachining (CW-LIPM) method was studied to process microstructures on the workpiece surface. In CW-LIPM, a pulsed laser beam with a wavelength 532 nm was focused within a coaxial water jet, leading to the optical breakdown and the subsequent formation of the localized plasma plume. Compared with conventional laser processing, a cleaner surface could be acquired by CW-LIPM, while maintaining the machining resolution and processing accuracy. The mechanism of CW-LIPM and the effects of microbubble explosions on the surface roughness of the workpiece in proximity to the entrance of the processing area have been studied. With the developed experimental setup, the influence of single laser pulse energy, pulse repetition frequency, and water-jet velocity on the plasma distribution was also researched. Moreover, the effects of waterjet speed on the distribution of cavitation bubbles and the relative position on the machining quality of the processed microchannels were also studied, considering the explosions of the micro cavitation bubbles near the workpiece surface due to the pressure difference. Compared with laser micromachining in the air, CW-LIPM could process microstructures with no protrusions and better precision. High-quality surface microstructures could be obtained by combining the higher-velocity waterjet, the lower single pulse energy, multiple scanning passes, and the processing positions 2–3 mm down from the plasma center and 4–5 mm above the bottom. The laser-induced plasma distribution was asymmetric, and the lower half plasma was more suitable for precision processing due to its focused shape, which is preferred to enhance the localization of the proposed CW-LIPM process.

Investigation of three-scalar subgrid-scale mixing in turbulent coaxial jets

Abstract

Role of convective acceleration in the interfacial instability of liquid-gas coaxial jets

When a high-speed gas flow destabilizes a liquid jet into a spray (atomization), the gas-liquid interface undergoes instabilities that play a key role in the breakup events. We use high-speed backlit imaging to track the maxima of such interfacial perturbations as they get stretched and accelerated by the gas phase. The spatial gradients of the local ensemble-averaged velocity statistics exhibit two regimes with respect to the gas Reynolds numbers that are in good agreement with changes in atomization regimes.

Continuous processing of paclitaxel polymeric micelles

A continuous polymeric micelle processing platform was successfully developed, which eliminated batch-to-batch variation in critical quality attributes (for example, size and polydispersity that are typically associated with batch processing). A continuous precipitation process was achieved via coaxial turbulent jet in co-flow technology allowing precise control of particle size with average particle size in the range 15 to 70 nm and low polydispersity. Critical relationships between material attributes (e.g., block copolymer design), process parameters (e.g., polymer concentration, organic to aqueous flow rate ratios, and temperature), and critical quality attributes (e.g., size and polydispersity) of the polymeric micelles were realized via multiple designs of experiments studies. Both polymer molecular weight and concentration were shown to influence the micelle polydispersity index. Notably, higher molecular weight polymer required higher processing temperatures to produce monodispersed particles and were generally of larger size. Using optimized conditions, paclitaxel polymeric micelles that are qualitatively and quantitatively equivalent to commercial Genexol PM were produced, exhibiting comparable quality attributes including particle size, size distribution, morphology, drug loading, release characteristics, and stability. Lastly, a dynamic light scattering method was adapted to determine the critical micelle concentration and aggregation number of the block copolymers, providing useful information about the raw material.

Peak-locking full characterization: PIV error assessment and velocity ensemble measurement correction

Concluding a few previous approaches, the last work by the authors presented a complete analytic model for peak-locking error. That work was devoted to the mathematical foundation of the model. This paper separately focuses on the procedure for calibrating a given particle-image velocimetry measurement setup using the model. In addition to error assessment, it allows for the measurement correction of statistical ensembles such as: (a) the local velocity average and (b) the local velocity root mean square (RMS). This calibration is done by means of an inexpensive multiple Δt strategy, consisting of measuring the same flow field with different Δt subsets. This paper offers a comparison with previous empirical models produced by the authors, showing the advantages of the robust theoretical foundation established for this model. A real case corresponding to a hot high-speed coaxial jet, representative of the turbofan engine propulsion systems used for civil transport aircraft, is used for this comparison. The discrepancy plots between the different Δt subsets indicate that the peak-locking error description and correction are improved by 30% for the local velocity average. For the local velocity RMS, the results are even more striking, as the peak locking error can be almost completely corrected in situations where previous models only provided the order of magnitude of the error. Together with a detailed explanation of the calibration procedure, the paper offers guidelines to avoid getting into an ill-conditioned mathematical problem.

Combustion Characteristics of Multi-Element Swirl Coaxial Jet Injectors under Varying Momentum Ratios

The combustion characteristics of a staged combustion cycle engine with an oxidizer-rich preburner were experimentally studied at different momentum ratios of multi-element injectors. Propellants were simultaneously supplied as a liquid–liquid–liquid system, and an injector was designed in which a swirl coaxial jet is sprayed. The injector burned the propellants in the inner chamber which had a temperature greater than 2000 K. To cool the combustion gas, a liquid oxidizer was supplied to the cooling channel outside the injector. To prevent the turbine blades from melting, the temperature of the combustion gas was maintained below 700 K. To confirm the combustion characteristics at different momentum ratios of the high-temperature combustion gas inside the injector and the low-temperature liquid oxidizer outside the injector, three types of injectors were designed and manufactured with different momentum ratios: MR 3.0, MR 3.3, and MR 3.7. In this study, the results of the combustion test for each type were compared for 30 s. For ORPB-A, a combustion pressure of 18.5 MPaA, fuel mass flow rate of 0.26 kg/s, oxidizer mass flow rate of 15.3 kg/s, and turbine inlet temperature of 686 K were obtained in the combustion stability period of 29.0–29.5 s. The combustion efficiency was 98% for MR 3.0 (ORPB-A), which was superior to that for other momentum ratios. In addition, during the combustion test for MR 3.0, the fluctuations in the characteristic velocity, combustion pressure, and propellant mass flow rate were low, indicating that combustion was stable. The three types of combustion instability were all less than 0.8%, thus confirming that the combustion stability was excellent.

Evaluation of the energy-efficient operation of a two-level air-jet screen with coaxial counter jets with ejection feeding in combination with rotating flows

The use of local exhaust ventilation systems continues to be the most reliable way to capture pollutants in the technological processes of various industries. The main purpose of the application of local exhaust ventilation systems is the effective capture of pollutants in the places of their formation. Improving the efficiency of local exhausts and, accordingly, reducing the load of general ventilation can be achieved through the use of air screens. For baths filled with liquids most often used activated on-board suction pumps that operate on the scheme of "jet-suction", which are structurally located on the same axis of the supply nozzle and slotted drain. According to this principle of operation, various activated extractors require air exchange in large volumes with significant energy consumption and at the same time with insufficiently high sanitary and hygienic indicators. In order to increase the efficiency of the local ventilation system, especially for tanks with large open surfaces, fundamentally new aerodynamic schemes are required, the feature of which is the formation of dynamic screens with inflow and rotation flows. On the basis of analytical research the basic possible variants of the device of the two-level air-jet screen of an industrial bath by drawing up balance equations for achievement of the maximum efficiency factor η are considered. The analysis of theories of interactions of inflow jets, suction flares and convective flows from liquid surfaces is carried out. The aerodynamic problem of a flat turbulent flow is considered, in which inflow jets interact with a 180 ° expansion, forming a two-level closed rotating system with concentrated flow, which is affected by convective flows with harmful substances from an industrial bath. The assumptions and calculations confirmed the fundamental possibility of using a system of two-level air-jet screens for large baths.