Turbulent Jet Mixing Research Articles

Opposed jet burner and Tsuji burner for representative laminar flamelet with differential molecular diffusion for non-premixed combustion modeling

Modeling differential molecular diffusion in turbulent non-premixed combustion remains challenging for flamelet models. Laminar flamelet is a key building block of flamelet models for turbulent combustion. One significant challenge that has not been adequately addressed is the representativity of laminar flamelet for the characteristics of differential molecular diffusion in turbulent combustion. Laminar flamelet is typically generated based on two conceptual burner configurations, the opposed jet burner and the Tsuji burner. The two burners are commonly viewed to be equivalent for the description of laminar flamelet structures. In this work, a major difference between them is revealed for the first time when they are used to represent differential molecular diffusion. The traditional opposed jet burner yields almost fixed equal diffusion locations defined as the points with equal values of mixture fractions based on different elements in the mixture fraction space. The Tsuji burner, with a slight extension, can produce a continuous variation of the equal diffusion locations in the mixture fraction space. This variation of the equal diffusion locations is shown to be an important feature of non-premixed combustion, as demonstrated in a laminar jet mixing layer, a turbulent jet mixing layer, and a turbulent jet non-premixed flame. Theoretical analysis is conducted based on an idealized opposed jet mixing layer to identify the major cause of the difference between the two burners. It is found that the inlet diffusion caused the major difference in differential molecular diffusion in the two burners. The curvature difference, another main difference between the two burners, however, is not found to cause any major difference in differential molecular diffusion in the two burners. Based on the finding, a modified opposed jet burner with disabled inlet diffusion is introduced and is shown to be able to reproduce the feature of differential molecular diffusion observed in the Tsuji burner. Both burners, after properly generalized with suitable boundary conditions and additional parameters (like the velocity ratio of fuel and oxidizer), are expected to be suitable and equivalent choices for the generation of representative laminar flamelets that can be used eventually in flamelet modeling of differential molecular diffusion in turbulent non-premixed combustion.Novelty and significance statement: (1) A significant difference between the opposed jet burner and the Tsuji burner is revealed for generating representative laminar flamelet with differential molecular diffusion (under specific conditions); (2) The flamelet generated from the Tsuji burner is shown to be representative of differential molecular diffusion found in practically relevant mixing and combustion problems; (3) The treatment of inlet diffusion is identified to be the major cause of the difference observed between the two burners; (4) Modifications are introduced to the opposed jet burner to reconcile the two different burners for producing representative flamelet that can be used in the modeling of differential molecular diffusion in turbulent non-premixed combustion.

A physics merged deep neural network-based prediction method for jet turbulent mixing noise

Turbulent mixing noise is a vital component of jet noise, and its rapid, accurate prediction has always been persistently pursued. Recent advancement in machine learning has been applied to jet noise prediction. However, these applications are pure curve fitting and lack physical constraints. In this study, a physics-merged deep neural network (PMNN)-based prediction method was developed for turbulent mixing jet noise by merging the physics of the jet noise. This deep neural network (DNN)-based method employed recent advancements in jet turbulent mixing noise containing large- and fine-scale turbulence structures. Two simple rational functions for large- and fine-scale turbulent noise similarity spectra were proposed to replace the original complex similarity spectra functions and incorporated into the DNN-based prediction method. For comparison, we present two data-driven DNN-based prediction methods (DDNN). The first DDNN method used the sound pressure level (SPL) as the output of neural networks, directly establishing the nonlinear relationship between the input features and SPL. In the second DDNN method, the dominant modes of the jet noise spectra extracted using the proper orthogonal decomposition method were merged with DNN. These DNN-based methods were then trained using a set of experimental data over a wide range of jet operating conditions. Their performance was evaluated and compared. It was evident that all these DNN-based methods were capable of predicting turbulent mixing noise reasonably well. In contrast to the DDNN methods, the PMNN method could provide insights into the jet turbulent mixing noise components. It demonstrates that the turbulent mixing jet noise spectra at the mid polar angle is generated by the large-scale noise component at low-frequency range and by the fine-scale noise component at high-frequency range.

Process Robustness in Lipid Nanoparticle Production: A Comparison of Microfluidic and Turbulent Jet Mixing.

The recent clinical and commercial success of lipid nanoparticles (LNPs) for nucleic acid delivery has incentivized the development of new technologies to manufacture LNPs. As new technologies emerge, researchers must determine which technologies to assess and how to perform comparative evaluations. In this article, we use a quality-by-design approach to systematically investigate how the mixer technology used to form LNPs influences LNPstructure. Specifically, a coaxial turbulent jet mixer and a staggered herringbone microfluidic mixer were systematically compared via matched formulation and process conditions. A full-factorial design-of-experiments study with three factors and three levels was executed for each mixer to compare process robustness in the production of antisense oligonucleotide (ASO) LNPs. ASO-LNPs generated with the coaxial turbulent jet mixer were consistently smaller, had a narrower particle size distribution, and had a higher ASO encapsulation as compared to the microfluidic mixer, but had a greater variation in internal structure with less ordered cores. A subset of the study was replicated for mRNA-LNPs with comparable trends in particle size and encapsulation, but more frequent bleb features for LNPs produced by the coaxial turbulent jet mixer. The study design used here provides a road map for how researchers may compare different mixer technologies (or process changes more broadly) and how such studies can inform process robustness and manufacturing control strategies.

Multi-scalar mixing in turbulent coaxial jets

Although natural and industrial flows often transport and mix multiple scalars, relatively few studies of turbulent multi-scalar mixing have been undertaken. In the present work, a novel three-wire thermal-anemometry-based probe – capable of simultaneously measuring velocity, helium concentration and temperature – is used to investigate the evolution of multiple scalars ($\phi _1$,$\phi _2$,$\phi _3$) and velocity in turbulent coaxial jets. The jets consist of (i) a centre jet containing a mixture of helium and air ($\phi _1$), (ii) an annular jet containing pure (unheated) air ($\phi _2$) and (iii) a coflow of (pure) heated air ($\phi _3$). Axial measurements are made at three different momentum flux ratios ($M=0.77, 2.1, 4.2$). Increasing$M$was observed to result in complex, competing effects. Larger momentum flux ratios cause the potential core of the centre jet to decrease in size, greater scalar fluctuations and more rapid correlation of$\phi _1$and$\phi _2$. However, at the same time, certain statistics, including those describing the velocity field, evolve more slowly. Moreover, the flow near the beginning of the fully merged region appears to be less mixed at higher values of$M$. The present work finally demonstrates that differences can be observed in the evolution and mixing of coaxial jets between those in which$M<1$and those in which$M>1$, thereby presenting an opportunity by which the mixing process in coaxial jets may be controlled.

Effect of Jet Velocity on the Formation of Moderate or Intense Low-Oxygen Dilution Combustion of Pulverized Coal.

Moderate or intense low-oxygen dilution (MILD) combustion of pulverized coal is regarded as a new combustion technology with great potential due to its advantages of reducing NO x emission and improving the uniformity of heat flux in the furnace. Increasing the jet velocity has been proved to be an important technical means to achieve MILD oxy-coal combustion, especially without a high level of preheat, but the transition mechanism of coal combustion modes with the increase of jet velocity is not clear enough. In this work, a high-velocity coal-laden jet combustion system on the basis of a flat-flame burner was designed to study the effect of jet velocity on the formation of MILD combustion of pulverized coal. The combustion mode transition of the pulverized coal jet is revealed by analyzing the flame structure, temperature, radiation, and reaction intensity through a variety of optical measurement methods. Theoretical criteria for combustion mode were applied to predict the formation of MILD combustion and were validated by the experimental data. In the environment with an ambient temperature of 1600 °C, the transition velocity of the coal jet into the MILD combustion regime is about 50 m/s and about 100 m/s at 5 and 15% O2 molar fraction, respectively. The dilution effect, jet entrainment, and turbulent mixing in the high-velocity jet, as the key factors to achieve an MILD combustion regime, were analyzed theoretically and experimentally. The dilution effect inherent in the jet significantly reduces the reactant concentration and ultimately reduces the reaction intensity and flame brightness while the entrainment of the jet promotes the radial dispersion of particles and the flame uniformity, which is dominant at lower jet velocities. Strong turbulent mixing promotes the ignition and volatile combustion, which is dominant at higher jet velocities.

Reduction of Chlorine Disinfection Dosage through Optimal Jet Design in the Hong Kong Harbor Area Treatment Scheme

Chlorine is commonly used in disinfection processes in wastewater treatment plants. The chlorine solution is typically dosed by turbulent mixing through multiport jet diffusers. The chlorine reacts with the inorganic and organic nitrogenous compounds at a fast rate (in the order of 1 s or less), and a significant portion of the dosed chlorine can be rapidly consumed by the reactions instead of destroying the pathogens. Despite extensive previous research, the chlorine consumption in the complex sewage flow remains an elusive problem. Field-scale experiments have been carried out to study the chlorine consumption in the turbulent mixing of dense chlorine jets with the primary treated sewage effluent of the Stonecutter’s Island Sewage Treatment Works (serving a population of 5.7 million). Based on jet theory and the field tests, it is proposed to discharge the chlorine jets at a location of highest velocity to minimize the contact time of the sewage with high concentration chlorine. Full-scale in-plant experiments demonstrated that significant savings in chlorine can be achieved with optimal jet diffuser design and placement; at nominal applied chlorine dosages of 12 and 20 mg/L in winter and summer, respectively, the chlorine demand can be reduced by up to 30% compared with the conventional dosing design. The relatively simple dosing reconfiguration results in significant savings in chlorine and energy, reduces harmful impact to the environment, and improves plant operation reliability. The chlorine demands from daily operations are well-correlated with the results of the field-scale models. Predictions of the chlorine demand as a function of applied dosage and temperature were obtained from the collective field data.

Measurement of turbulent supersonic steam jet flow characteristics using TDLAS

Ejectors have no moving parts and are preferable to mechanical compressors in many applications, but ejectors typically have a relatively low efficiency. To aid in the ejector design process, thorough understanding of the turbulent mixing of multi-phase compressible jets is beneficial.This paper reports experimental results for Tunable Diode Laser Absorption Spectroscopy (TDLAS) measurements derived from an axisymmetric supersonic steam jet apparatus.In this experimental work, a supersonic steam jet nozzle exit of a diameter 13.6 mm was surrounded by a low-speed flow of dry nitrogen. The TDLAS system was traversed through the flow at three different planes downstream from the ejector nozzle exit: 15, 20, and 30 mm distance. At each of the three planes, line-of-sight measurements were made with the laser passing through locations between 0 and 15 mm from the jet centreline.Through the analysis of the TDLAS data and application of the Abel inversion method, the radial distribution of the pressure, temperature, and the concentration of the water-vapour were obtained. The key findings are that it is possible to determine key physical parameters using experimental TDLAS measurements when combined with a suitable numerical optimization approach.

Circular turbulent wall jets in quiescent and coflowing surroundings

The dynamics and mixing of circular turbulent wall jets released into both a quiescent background and coflowing stream have been investigated experimentally. The statistics of the velocity field (measured by way of acoustic Doppler velocimetry) for the wall jets emitted into a quiescent background agree well with those of the other studies. The experiments involving coflowing wall jets were undertaken at three different jet-to-coflow velocity ratios. The coflowing wall jets were found to decay and spread at slower rates and have lower mean lateral velocities compared to wall jets in quiescent surroundings. Moreover, the decay and spreading rates of the coflowing wall jets increased with increasing jet-to-coflow velocity ratios. The wall jets issued into a coflow also developed more slowly and reached self-similarity at farther downstream distances relative to those emitted into a quiescent background. Given the decreased decay rate, spreading rate, and mean lateral velocities of wall jets in the presence of a coflow, it was inferred that the entrainment into, and mixing of, the wall jets was reduced, presumably due to the suppresion by the coflow of the vortical structures that characterize wall jets in quiescent backgrounds. Finally, the root-mean-square velocities of the wall jets increased when a coflow was present, and were found to be nearly self-similar in the range of measurements studied herein, in contrast with coflowing jets (that are not released in the vicinity of a wall).

Computational optimization of a hydrogen direct-injection compression-ignition engine for jet mixing dominated nonpremixed combustion

Hydrogen (H2) nonpremixed combustion has been showcased as a potentially viable and preferable strategy for direct-injection compression-ignition (DICI) engines for its ability to deliver high heat release rates and low heat transfer losses, in addition to potentially zero CO2 emissions. However, this concept requires a different optimization strategy compared to conventional diesel engines, prioritizing a combustion mode dominated by free turbulent jet mixing. In the present work, this optimization strategy is realized and studied computationally using the CONVERGE CFD solver. It involves adopting wide piston bowl designs with shapes adapted to the H2 jets, altered injector umbrella angle, and an increased number of nozzle orifices with either smaller orifice diameter or reduced injection pressure to maintain constant injector flow rate capacity. This work shows that these modifications are effective at maximizing free-jet mixing, thus enabling more favorable heat release profiles, reducing wall heat transfer by 35%, and improving indicated efficiency by 2.2 percentage points. However, they also caused elevated incomplete combustion losses at low excess air ratios, which may be eliminated by implementing a moderate swirl, small post-injections, and further optimized jet momentum and piston design. Noise emissions with the optimized DICI H2 combustion are shown to be comparable to those from conventional diesel engines. Finally, it is demonstrated that modern engine concepts, such as the double compression-expansion engine, may achieve around 56% brake thermal efficiency with the DICI H2 combustion, which is 1.1 percentage point higher than with diesel fuel. Thus, this work contributes to the knowledge base required for future improvements in H2 engine efficiency.

Effect of nozzle geometry on the dynamics and mixing of self-similar turbulent jets.

The effect of nozzle geometry on the dynamics and mixing of turbulent jets is experimentally investigated. The jets with a Reynolds number of 13,000 were issued from four different pipes with circular, elliptical, square and triangular cross sections. The velocity field was measured in the self-similar region of the jets using an acoustic Doppler velocimeter. Statistical parameters, such as the mean velocities, velocity variances, spreading rates, mass flow rates, and entrainment rates are presented. The results show that despite having approximately similar decay rates for the mean centerline velocities, the radial profiles of the axial mean velocity varied in jets with different nozzle cross sections and were widest for elliptical jets and narrowest for the triangular ones. On the other hand, velocity variances were greatest for the triangular jet when compared to the jets released from cross sections of other geometries. Furthermore, the spreading rate, mass flow rate, and entrainment rate were highest for the elliptical jet, and lowest for the triangular jet. From this it can be inferred that the elliptical jet has the highest mixing and dilution. The results of this study could help to improve the initial mixing of pollutants by optimizing the initial conditions.

Reduction of soot carbon in the exhaust gases of a tractor gas-diesel engine

The rate of oxidation of carbon (including its dispersed forms) has a value much higher than the rate of gasification. Therefore, in the initial part of the flame, when oxygen is still contained in the gas phase, the oxidation of soot will be the process on which the change in the size and concentration of dispersed carbon particles mainly depends. The intensity of oxidation and gasification of dispersed carbon in the flame largely depends on the development of the mixing process, determined by the aerodynamics of the fuel and air jets. The paper presents an analysis of the influence of mixing processes on the oxidation and gasification of dispersed carbon in a natural gas flame in the study of homogeneous flames and mixing of turbulent jets. The results of industrial studies of the mixing of fuel and air in a diffusion torch are taken into account. The results allow us to evaluate the influence of various aerodynamic factors on the processes occurring in the glowing flame of natural gas in the combustion chamber of gas diesel.

Effects of velocity shear layer on detonation propagation in a supersonic expanding combustor

This study investigates the mechanism of detonation propagation in a stoichiometric hydrogen–oxygen mixture with non-uniform flow velocity entering an expanding combustor. For simulation of the detonation propagation, the Navier–Stokes equations with a one-step two-species chemistry model are solved by employing the hybrid sixth-order weighted essentially non-oscillatory centered difference scheme. The self-sustaining mechanism of detonation propagation in an expanding combustor under the action of non-uniform supersonic flow with a velocity shear layer is revealed. The results show that under the influence of velocity shear layer, two different unburned jets are produced behind the detonation front. These jets are induced by the velocity shear layers and the Prandtl–Meyer expansion fan. The two jets interact and mix gradually. The interaction between the mixed unburned jets and highly unstable shear layers creates large-scale vortices that intensify the turbulent mixing of the unburned jets. Meanwhile, the baroclinic mechanism generates numerous vortices on the boundary of the unburned jet. These vortices promote the mixing of the burned and unburned gases, which eventually leads to the rapid consumption of the unburned pockets. The heat released due to the burning of the unreacted pockets behind the detonation wave supports a self-sustaining propagation of the detonation wave. When the velocity difference among the shear layers increases, the surface fluctuation of the detonation wave increases.

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

Abstract

Large-Eddy Simulations of Transverse Jet Mixing and Flame Stability in Supersonic Crossflow

Large-eddy simulations (LES) combined with the partially stirred reactor (PaSR) combustion model are employed to investigate the turbulent mixing, combustion mode, and flame stability of a sonic hydrogen jet injecting into high-enthalpy supersonic crossflow at three momentum flux ratios , in other words, 0.71, 2.11, and 4.00, respectively. The LES accuracy in terms of the turbulent kinetic energy, power spectra density, and subgrid Damköhler number is carefully addressed against various LES resolution criteria and the experimental mean pressure distribution on the upper wall. The ignition processes with autoignition and shock compression effects are identified and analyzed. At , the shock-induced ignition occurs behind the reflected shock wave, and the combustion heat release is dominated by the premixed combustion. While for the high jet-to-crossflow momentum flux ratios, for example, and 4.00, ignition happens around the jet orifice due to the strong bow shock and reflected shock effects, and the combustion heat releases are dominated by the nonpremixed combustion. Furthermore, the mechanisms of flame stabilization, local extinction, and reignition in the transverse jet combustion in supersonic crossflow are further analyzed with the chemical explosive mode analysis.

Computational characterization of hydrogen direct injection and nonpremixed combustion in a compression-ignition engine

With the revived interest in hydrogen (H2) as a direct combustion fuel for engine applications, a computational study is conducted to assess the characteristics of H2 direct-injection (DI) compression-ignition (CI) non-premixed combustion concept. Development of a CFD modeling using CONVERGE CFD solver focuses on hydrogen's unique characteristics by utilizing a suitable numerical method to reproduce the direct H2 injection phenomena. A grid sensitivity study is performed to ensure the fidelity of results with optimal cost, and the models are validated against constant-volume optical chamber and diesel engine experimental data. The present study aims to contribute to the future development of DICI H2 combustion engines, providing detailed characterization of the combustion cycle, and highlighting several distinct aspects of CI nonpremixed H2 versus diesel combustion. First, unlike the common description of diesel sprays, hydrogen jets do not exhibit significant flame lift-off and air entrainment near injector nozzle, and the fuel-air interface is drastically more stratified with no sign of premixing. It is also found that the DICI H2 combustion concept is governed first by a free turbulent jet mixing phase, then by an in-cylinder global mixing phase. The former is drastically more dominant with the DICI H2 engine compared to conventional diesel engines. The free-jet mixing is also found to be more effective that the global mixing, which indicates the need to completely rethink the optimization strategies for CI engines when using H2 as fuel.

Turbulent mixing and trajectories of jets in a supersonic cross-flow with different injectants

Abstract

High-speed turbulent gas jets: an LES investigation of Mach and Reynolds number effects on the velocity decay and spreading rate

The aim of this work is the investigation of Mach and Reynolds numbers effects on the behaviour of turbulent gas jets in order to gain new insights into the fluid dynamic process of turbulent jet mixing and spreading. An in-house solver (Flow-Large Eddy and Direct Simulation, FLEDS) of the Favre-filtered Navier Stokes equations has been used. Compressibility has been analyzed by considering gas jets with Mach number equal to 0.8, 1.4, 2.0 and 2.6, and Re equal to 10,000. As concerns the influence of Re on gas jets, four cases have been investigated, i.e. mathrm{Re} = 2500, 5000, 10,000 and 20,000, with Mach number equal to 1.4. The results show that, in accordance with previous experimental and numerical studies, the potential core length increases with Mach number. As regards the velocity decay and the spreading rate downstream of the potential core, compressibility effects are not relevant except for the jet with Mach number of 2.6. The normalized turbulent kinetic energy along the centerline as a function of the normalized streamwise distance shows a similar peak at the end of the potential core for all jets, except for the case with Mach number of 2.6. By increasing Re, the length of the potential core decreases up to the same value for all Re higher than 10,000. In the region downstream of the potential core, the velocity decay decreases as Re number increases from 10,000 to 20,000, whereas, for lower values of Re, the influence is almost negligible.

Mathematical Modeling of the Mixing and Heat Transfer in Turbulent Two-Phase Jets of Mutually Immiscible Liquids

The paper is devoted to the development and analysis of the mathematical model for mixingand heat transfer in the two-fluid turbulent heterogeneous jet of mutually immiscible liquids. Many natural andtechnical processes deal with the turbulent jets of mutually immiscible liquids, which represent an importantclass of the modern multiphase system dynamics. Differential equations for the axially symmetrical twodimensional stationary flow and the integral correlations in a cylindrical coordinate system are considered forthe jet of fluid flowing from a nozzle into a pool of another fluid immiscible with the first one. The results maybe of interest for researchers and engineers in the multiphase turbulent jets, mixing and heat transfer processes.

Theoretical analysis and numerical simulations of turbulent jets in a wave environment

A thorough understanding of the mixing and diffusion of turbulent jets released in a wave flow field is still lacking in the literature. This issue is undoubtedly of interest because, although stagnant ambient conditions are well known, they are almost never present in real coastal environmental problems, where the presence of waves or currents is common. As a result, jets cannot be analyzed without considering the surrounding environment, which is only rarely under stagnant conditions. The aim of the present research is to analyze from a theoretical point of view a pure jet vertically discharged in a wave motion field. Specifically, starting from the fundamental Navier–Stokes equations governing the problem joined to the continuity equation, the equations of motion and the integral equations of momentum, energy, and moment of momentum are derived. Therefore, the laws of variation of the jet length and velocity scales are deduced. Results from experiments and numerical simulations of a jet issuing in a wave environment demonstrate the validity of the proposed laws.

Hydrodynamics and Local Turbulent Mixing of Submerged, Parallel Liquid Jets: Experiments and CFD Simulations

The hydrodynamics and local turbulent mixing of parallel multiple liquid jets, submerged in liquid, were investigated by means of experiments and computational fluid dynamics (CFD). A renormalization group (RNG) k-ε turbulence model was used to simulate the flow field. The model was validated experimentally by particle image velocimetry (PIV) measurements. In the converging region adjacent to the nozzle exits, the recirculation region disappears, and there is only ambient fluid entrainment. Different jet arrays were compared to evaluate the effects of the jet spatial arrangement on the hydrodynamics and mixing performance. A shorter mixing length in the merging region suggests that mixing is more efficient in the triple-jet system than in other jet systems. Compared with the jet Reynolds number, the jet spacing plays a more significant role in determining the critical mixing regions, while the linear relationship between them is more sensitive than that for multiple parallel plane jets.