Primary Breakup Research Articles

Experimental Characterization of a Liquid Jet Emanating From An Effervescent Atomizer

Abstract The study focuses on experimental characterization of the primary atomization associated with an effervescent atomizer. Unlike the existing designs available in the literature that inject air perpendicular to the liquid flow direction, the present atomizer design utilizes coflowing air configuration. In doing so, the aerodynamic shear at the liquid–gas interface create instability and enhance the subsequent jet breakup. Both integrated and intrinsic properties of the liquid jet were extracted by utilizing high-speed flow visualization techniques. The integrated property consists of breakup length, while the intrinsic property involves primary and intermediate breakup frequencies. The primary instability is characterized by low-frequency sinusoidal mode, whereas the intermediate instability consists of high-frequency dilatational mode. Dimensionless plots of these parameters with Weber number ratio leads to a better collapse of data, thereby generating appropriate universal functions. The combined diagram of frequencies converge with increasing relative velocity. This may be due to the dominance of energy consuming sinusoidal wave as the aerodynamic shear increases.

Combustion Characteristics of a Peripheral Vortex Reverse Flow Combustor With Coaxial Fuel Injection

Abstract This paper deals with an experimental investigation of a novel and simple reverse flow combustor, operated stably with a liquid fuel (ethanol) for heat release intensities ranging from 16 to 25 MW/(m3·atm) with very low NOx and CO emissions. The liquid fuel is injected coaxially with the air jet along the centerline of the combustor. The high velocity air annulus helps in primary breakup of the liquid fuel jet. Air injection along the combustor centerline results in a strong peripheral vortex inside the combustor leading to enhanced product gas recirculation, internal preheating of the reactants, and stabilization of reaction zones. Single-digit NOx emissions were obtained for both coaxial fuel injection (non-premixed) and a premixed–prevaporized (PP) cases for all operating conditions. CO emissions for both the modes were less than 100 ppm (ϕ < 0.75). CH* chemiluminescence images revealed two distinct flame structures for coaxial fuel injection case. A single flame structure for PP case was observed extending from the injector exit to the bottom of the combustor. The instantaneous (spatially averaged) CH* intensity fluctuations were significantly lower for the PP case as compared to the coaxial fuel injection case.

Influence of Material Properties on the Spheroidisation Process in Gas Atomisation

For many contemporary powder metallurgical applications, spherical powders are preferred. Spherical particles have a lower oxygen content, a better flowability, and their behaviour is—compared to irregular particles—better predictable. The powder production process via melt atomisation can be divided into the steps a. primary breakup into ligaments, b. ligament breakup, and c. secondary breakup and/or spheroidisation, while simultaneously cooling and freezing take place. Apart from thermodynamic conditions during the process, melt properties such as viscosity, density, surface tension, heat capacity, and thermal conductivity will influence the processes around spheroidisation. As a first step, a 4-force model (viscosity, surface tension, external dynamic, and inertia forces) is applied on the melt droplet to predict the influence of the melt properties on spheroidisation separately. Secondly, the spheroidisation process is calculated for different materials such as Copper, Iron, or Titanium for existing atomisation systems. Finally, suggestions are presented which may help to produce more spherical particles.

Effects of co-current airflow on water atomization in a curved diffuser

To explore the effect of airflow on water atomization mechanism in mine exhaust heat recovery system, an experimental study of the dispersed phase fraction for droplets is conducted by Malvern laser particle size analyzer. This paper focuses on the influence of gas disturbance on water atomization in a curved diffuser in a ventilation system. With the theory of liquid breakup, it shows how liquid sheet occurs in primary breakup under different operating conditions. In case that the spray flows in the same direction as the airflow, the dominant factor affecting the breakup length of liquid film near the atomizing nozzle is the injection pressure. At a long distance from the nozzle, the droplet goes through vibrational shape oscillation and eventually further breakup in the complete atomization area. The main conclusions are as follows: (1) the dispersed phase fraction for droplets reaches maximum when the airflow is 3.2 m3/s; (2) droplets achieve the optimal atomization effect of the spray; (3) the heat and moisture should be fully exchanged.

Experimental study of liquid jets injected in crossflow

In this paper, we studied the behavior of liquid sheet and jet injection into a low-speed crossflow. Characteristics of primary breakup in a liquid sheet and jet are investigated for circular and rectangular orifices. Effect of dimensionless numbers on primary breakup regimes including jet trajectory, length, height, breakup, and spray characteristics are surveyed by the shadowgraphy technique at ambient pressure and temperature. Tests were performed in the range of 0.8–16.57 for the Weber number, 5–250 for the momentum flux ratio, and 380–1850 for the Reynolds number. This comprehensive study on rectangular jets in crossflow leads to introduce the transition regions of the primary atomization regime. Our observation demonstrated that the most substantial effects relating to the axis-switching in rectangular orifices. The axis-switching determines the region of mode transition, the height and the lenght of the jet, and the structure of bags formation. Besides, the inherent differences in axis-switching behavior are the substantial difference in the various configurations of rectangular orifices. In the orthogonal configuration, no axis-switching for moderately high Weber number causes to be more bent of column jet and consequently delay on bag regime threshold, regardless of our expectations. A similiar explanation could be applied for other observations in the primary atomization of rectangular nozzles.

Influence of the ambient pressure on the liquid accumulation and on the primary spray in prefilming airblast atomization

The influence of the ambient pressure on the breakup process is investigated by means of PIV and shadowgraphy in the configuration of a planar prefilming airblast atomizer. The ambient pressure is varied from 1 to 8 bar. Other investigated parameters are the gas velocity and the film loading. From single-phase PIV measurements, it is found that the gas velocity in the vicinity of the prefilmer partly matches the analytical profile from the near-wake theory. As done in previous publications, the characteristics of the liquid accumulation are extracted from the shadowgraphy images of the liquid phase directly downstream of the prefilmer. Two different characteristic lengths, as well as the ligament velocity and a breakup frequency are determined. In addition, the droplets generated directly downstream of the liquid accumulation are captured. Hence, the spray Sauter Mean Diameter (SMD) and the mean droplet velocity are given for each operating point. The novelty of this study is that a scaling law of these quantities with regard to ambient pressure is derived. A correlation is observed between the characteristic length of the accumulation and the SMD, thus reinforcing the idea that the liquid accumulation determines the primary spray characteristics. In this paper, a threshold to distinguish the zones between primary and secondary breakup is proposed based on an objective criterion. It is also shown that taking non-spherical droplets into account significantly modifies the shape of the dropsize distribution, thus stressing the need to use shadowgraphy when investigating primary breakup. Additionally, the ambient pressure and the velocity are varied accordingly to keep the aerodynamic stress ρgUg2 constant. This leads to almost identical liquid accumulation and spray characteristics. Hence, it is confirmed that the aerodynamic stress is a more appropriate parameter than the gas velocity or the ambient pressure to characterize prefilming airblast breakup. Finally, SMD correlations from the literature are compared to the present experiment. Most of the correlations calibrated with LDA/LDT measurement underestimate the SMD. This highlights the need to use shadrowgraphy for calibrating primary breakup models.

Experimental investigation on the influence of system pressure on resulting spray quality and jet breakup applying pressure adapted twin-fluid nozzles

The present work aims for the isolated investigation of the influence of system pressure on spray quality of twin-fluid atomizers. An approach of pressure adapted nozzles was applied, allowing for constant mass flows, gas-to-liquid ratio as well as fluid velocities at the nozzle orifice independent of system pressure. Two Newtonian liquids featuring viscosities of 1 and 100 mPa · s were used, varying the system pressure from 1 to 16 bar for gas velocities of 60, 80 and 100 m·s−1. A phase doppler analyzer was applied for measurement of resulting drop size and velocity. Primary breakup morphology was detected by a high-speed camera. Two regions with different dependencies of spray quality on system pressure were identified. Applying pressure adapted nozzles while increasing system pressure, first results in a decrease of droplet size followed by an increase. A maximum of the dynamic pressure of the gas phase was determined at minimum droplet size, which is explained based on the theory of a free jet. The observations are underlined by images of the high-speed camera. Here, a change in breakup morphology from fiber type to a mixture of fiber type and non-axisymmetric Rayleigh type breakup at high system pressure was observed.

Effect of ambient pressure oscillation on the primary breakup of cylindrical liquid jet spray

The present simulation study investigates the effects of ambient pressure oscillation on cylindrical liquid jet sprays, using the volume of fluid method. The research is motivated by combustion instability in combustion engines, where strong harmonic pressure oscillation can damage internal structures. Oscillating pressure modulates not only the fuel mass flow rate but also the ambient gas density and liquid surface tension, and in liquid sprays, the ambient fluid density and surface tension can have substantial effects on spray breakup. In order to investigate the multiple property changes with ambient pressure oscillation, therefore, a new solver in OpenFOAM is developed. In the solver, liquid mass flow rate, ambient gas density, and liquid surface tension change simultaneously as a result of pressure oscillation. Simulations were conducted at a Reynolds number of 2000 and Weber number over 2000, conditions that are conducive to primary breakup in laminar flows. The simulations show that oscillations in ambient pressure significantly strengthen the surface instability of the liquid ligament, which depends on the surface tension–pressure coefficient, the mean pressure, and the amplitude of oscillation.

Large scale instabilities in coaxial air-water jets with annular air swirl

The aim of this paper is to characterize large-scale instabilities during the primary breakup process in liquid centered coaxial air-water jets. The interest here is to investigate the role of annular air swirl on such instabilities. A coaxial airblast atomizer that incorporates an axial swirler is considered for this purpose. The atomizer was operated in a wide range of the Weber number, Weg(80–958), momentum flux ratio, M(1–26), and air swirl strength, S(0–1.6). High-speed shadowgraphic images of the primary jet breakup process were recorded. Proper orthogonal decomposition (POD) analysis of the time-resolved images was performed for each operating condition. The 2nd and 3rd POD modes depicted some universal spatial features which refer to large scale instabilities. Three different dominant large scale instabilities were identified, viz., jet flapping, wavy breakup, and explosive breakup, for the entire range of the injector operating condition either in the presence or absence of air swirl. It was found that jet flapping (referred to as the lateral oscillation of the tail end of the jet) is the dominant mode of jet instability for a lower range of M, while explosive jet breakup (referred to as the radial expansion of the jet) governs jet breakup unsteadiness for a higher range of M. The wavy or sinuous mode of breakup is a secondary mechanism relevant under low M conditions. The mechanisms of large scale instabilities and the role of air swirl in that context are explained based on the Fourier analysis of the temporal coefficients of the corresponding POD modes.

Non-intrusive uncertainty quantification in the simulation of turbulent spray combustion using Polynomial Chaos Expansion: A case study

A major source of input uncertainties in the simulation of turbulent spray combustion is introduced by the need to specify the state of the liquid spray after primary breakup, i.e. a spray boundary condition for the lagrangian transport equations. To further enhance the credibility and predictive capabilities of such simulations, output uncertainties should be reported in addition to the quantities of interest. Therefore, this paper presents results from a comprehensive quantification of uncertainties from the specification of a spray boundary condition and numerical approximation errors. A well characterized lab-scale spray flame is studied by means of an Euler-Lagrange simulation framework using detailed finite rate chemistry. As direct Monte Carlo sampling of the simulation model is prohibitive, non-intrusive Polynomial Chaos expansion (PCE) is used for forward propagation of the uncertainties. Uncertain input parameters are prioritized in a screening study, which allows for a reduction of the parameter space. The computation of probabilistic bounds reveals an extensive uncertainty region around the deterministic reference simulation. In an a posteriori sensitivity analysis, the majority of this variance is traced back to the uncertain spray cone angle of the atomizer. The explicit computation of input uncertainties finally allows for an evaluation of total predictive uncertainty in the case considered.

The effect of bubbles on primary breakup of diesel spray

The spray atomization and related nozzle internal flow dictate the subsequent combustion of air/fuel mixture and emissions. The dynamics of internal flow and spray breakup under various injection pressures and initial conditions were studied by employing the combination of high-speed microscope imaging technique and CFD numerical simulation with VOF and RNG K-ε models. Four main types of spray fragmentation were observed and classified. The energy stored in the compressed bubbles was released to accelerate and atomize the fuel film in the front, which is more effective under higher injection pressure. Besides, it is surprising to discover the ‘C’ shaped fuel in the nozzle which divided the bubble into three parts rather than puncturing it directly. The probability of this ‘C’ shaped fuel decreased with increasing injection pressure due to the strong pressure shock. This study is believed to be essential for further revealing the spray breakup mechanisms.

Unstable breakup of a power-law liquid fuel jet in the presence of a gas crossflow

A theoretical model is presented for the effect of a slight gas crossflow on the instability of the power-law fuel jet in this paper. Based on a linear approximation, the dispersion equation was deduced theoretically with axisymmetric disturbance. The influence of various dimensionless factors for jet instability is studied. Theoretical calculation results of temporal mode show that the transverse motion of gas phase could effectively accelerate the power-law fuel breakup. With the increase of transverse gaseous velocity, power-law fuel jets would become more susceptible to be broken up. Increasing gas-liquid density ratio could enhance the aerodynamic forces in the axial and vertical directions and promote fuel fragmentation. Simultaneously, a huge gas density could weaken the influence of transverse airflow. With the increase of the transvers airflow, Weber number has a weakening effect on the fuel breakup, but it has a significant positive effect on the primary breakup scale. However, with gas crossflow, the liquid viscous force also takes strong disadvantage of development of surface wave. Besides, the effects of crossflow and co-current airflow environments on the instability of power-law liquid jet are studied. For the generalized Reynolds number and power law exponent, the fuel jet under cross-flow conditions is more unstable, but the jet under the co-current flow condition is smaller on the fracture scale. These issues are beneficial to understand the disintegration of the power-law fuel jet with gas crossflow, and supply reference for the application of power-law liquids in real engineering technologies.

Assessment of varying primary/secondary breakup mechanism of diesel spray on performance characteristics of HSDI engine

Fuel spraying process and hydrodynamic interaction of fuel jet-air plays a crucial role in the mixture formation pattern. A baseline diesel engine is considered and based on the operational values ​​of the experimental engine, the geometric and spraying model is designed and meshed. The values ​​of the primary and secondary breakup coefficients are adjustable according to the tested engine. By fixing the geometrical features, different atomization models with related tuning constants are selected to determine the effect of these factors on the spray quality and engine output in terms of combustion and emissions, and to provide a general framework for model performance for the users. In the present study, both the primary breakup and atomization models, as well as the secondary breakup model are considered emphasizing the modification and identification of the functions of these parameters. In the second breakup of the WAVE model is adopted and the C2 coefficient or constant in the range of 10–60 in the 10 steps is launched by the software for each mode, then in the secondary breakup model, KH-RT with a coefficient of C2 = 12 is investigated. In the next step, the primary breakup model of “Core Injection” is evaluated with C1 coefficients between 10 and 50, and its C2 coefficient will be analyzed for the values 15, 25, and 35. It has been understood that there is a linear correlation between equivalence ratio and C2 of secondary breakup with the determination coefficient of R2 = 0.954. Increasing C2 of WAVE results in deceleration in spray droplet strip (dR/dt) such that C2 = 60 gives the biggest SMD, the lowest pressure, heat release rate, and soot amount.

Numerical investigations on stability of the spatially oscillating planar two-phase liquid jet in a quiescent atmosphere

The liquid jet when perturbed sinusoidally will lead to instability under certain conditions. Understanding the causes and consequences of such a behavior is still obscure. Hence, numerical investigations are reported in the present study for a two phase spatially oscillating planar jet in a quiescent air. Simulations are performed by solving the Navier-Stokes equations and using the volume of fluid method to track the air-water interface. It is demonstrated that an increase in amplitude of oscillation is caused due to the formation of a low pressure region created by the vortical structures in air near the leading edge of the jet when deflected. This two way coupling between air and water is analyzed with the help of enstrophy, divergence of the Lamb vector, and vortex forces. It is found through a parametric study that surface tension and viscosity stabilize the perturbations in an oscillating planar jet. On the other hand, an increase in Froude number (Fr) initially leads to an augmentation of perturbation amplitude and later causes its damping when surface tension forces become dominant. The numerical analysis for different inlet velocity profiles establishes that the jet is more stable when subjected to a parabolic inlet velocity profile as compared to a uniform profile due to lower relative velocity at the interface. The present work also reveals the role of capillary instability in addition to Kelvin-Helmholtz and Rayleigh-Taylor instabilities that induce primary breakup in the jet.

Liquid jet primary breakup in a turbulent cross-airflow at low Weber number

The influence of turbulence characteristics of a cross-airflow including its velocity fluctuations and integral length and time scales on the primary breakup regime, trajectory and breakup height and time of a transversely injected liquid jet was investigated experimentally. Turbulence intensity of the incoming airflow was varied from $u_{rms}/u_{g}=1.5\,\%$ to 5.5 % (where $u_{g}$ is cross-airflow streamwise mean velocity and $u_{rms}$ is the r.m.s. of the corresponding cross-airflow streamwise mean velocity fluctuation) by placing at the inlet of the test section a perforated plate/grid with a solidity ratio of $S=50\,\%$. Over the range of gas Weber number, $3.1<We_{g}<7.14$, the ensuing liquid jet exhibited more fluctuations and late breakup transitional behaviour under turbulent airflow conditions than in a uniform cross-airflow. Proper orthogonal decomposition of the liquid jet dynamics revealed that the use of grid caused a rise in the wavelength of travelling waves along the liquid jet, which hindered the transition of the liquid jet primary breakup regime from enhanced capillary breakup to the bag breakup mode. The quantitative results demonstrated that, at a constant airflow mean velocity, turbulent cross-airflow caused the liquid jet to bend earlier compared with its uniform counterpart. A power-law empirical correlation was proposed for the prediction of the liquid jet trajectory which takes into account the effect of turbulent Reynolds number. The liquid jet breakup height (in the $y$-axis direction) normalized by the jet diameter, and accordingly the liquid jet breakup time normalized by the airflow integral time scale, were found to decrease with increasing the airflow turbulence intensity. Two power-law empirical correlations were proposed to predict the liquid jet breakup height and time.

Characteristics of Water and Urea–Water Solution Sprays

In spray studies related to selective catalytic reduction (SCR) systems a common approach is to replace the urea–water solution (UWS) with pure water, even though there is very limited detailed information on the spray properties for these two liquids obtained under the same conditions using the same experimental equipment. Neither is it known how the possible differences in spray properties influence computational fluid dynamics (CFD) simulations. In this study, besides the flow characteristics, we compare both global and local spray parameters measured for UWS and pure water in the same conditions. To our knowledge, this is the first study which examines the influence on the injection process of replacing UWS with water over such a wide range. Moreover, the influence of different spray properties on CFD simulations is also examined. The experimental studies showed differences in almost all considered spray parameters. Moreover, different spray behaviour was noticed in terms of primary break-up. One important finding is that water and UWS sprays do have a similar Sauter mean diameter, but at the same time the droplet size distribution is considerably different. The simulation results indicated noticeable differences in terms of wall film formation; nevertheless, the overall mixing performance was not significantly affected.

On the correlation of the primary breakup length with fuel temperature in pressure swirl nozzle

This work is focused on the effect of viscosity on the estimation of liquid sheet thickness and breakup length of a pressure swirl nozzle that is undergoing fuel temperature variation. The pressure swirl nozzle tested in this study was developed for aviation gas turbine engines, where the ignition characteristics included an importantly low-temperature environment. Jet A-1 aviation fuel was used for various injection pressures at different fuel temperatures. With the aid of a laser sheet planar imaging, instantaneous spatial spray images were acquired. In lower fuel temperature cases, the fuel viscosity increased, resulting in a slowing of the liquid sheet breakup process. An increased mass flowrate and a decreased spray cone angle were observed at lower fuel temperatures and fixed injection pressures. With a decrease in fuel temperature, the primary breakup length and secondary breakup length increased dramatically. A newly proposed expression for predicting the primary breakup length of the liquid sheet and accounting for viscosity effects with the change of fuel temperature is presented.

Primary breakup of power-law biofuel sprays in pressurized gaseous crossflow

The spray characteristics of biofuels have attracted broad interests, due to its bright prospect as one of the alternative fuels. The rheology of some biofuels was found to be power-law behavior. The spray performance of these fuels still remains unclear. Besides, biofuel atomization in pressurized gaseous crossflow can be of interest for combustion engines. The influential factors, namely the momentum flux ratios, Weber number and liquid-gas density ratios, are investigated employing controlling variable approach in the present. Power-law behavior fuel has shown great advantages in bringing a better atomization compared to the Newtonian one with case-control method. Jet column surface disturbance and the deformation magnitude during the injection process is emphasized. The penetration core lengths and breakup points are identified to illustrate the column deformation and breakup times. Besides, the column furcation processes which form finger-like features, as well as surface wavelengths are detected and compared along the studies of the influential factors. A larger injection momentum brings a smaller column deflection while the deflection is independent of Weber number. Besides, the locations of the furcation points increase with the raise of the momentum flux ratios while reduce with the increase of Weber number. The significant effect for increasing the density ratios is on promoting jet atomization. Column breakup processes tend to take place when keep increasing the gas densities. The column surface fluctuations are enhanced while the quantity of the ligaments raises and the shapes become richer.

Spray characterization for engine combustion network Spray G injector using high-fidelity simulation with detailed injector geometry

This article presents a computational fluid dynamics study of the engine combustion network Spray G, focusing on the transient characteristics of the spray during the start of injection and the impacts of nozzle geometry details derived from the manufacturing process. The large-eddy-simulation method, coupled with the volume-of-fluid method, was used to model the high-speed turbulent two-phase flow. A moving-needle boundary condition was applied to capture the internal flow boundary condition accurately. The injector geometry was measured with micron-level resolution using X-ray tomographic imaging at the Advanced Photon Source at Argonne National Laboratory, providing detailed machining tolerance and defects from manufacturing and a realistic rough surface. For comparison, a nominal geometry and a modified geometry incorporating measured large-scale geometric features but no surface details were also used in the simulations. Spray characteristics such as mass flow rate, injection velocity, and Sauter mean diameter were analyzed. Significantly distinct spray characteristics in terms of injection velocity, spray morphology, and primary breakup mechanism were predicted using different nozzle geometries, which is mainly attributable to the realistic surface finish and manufacturing defects. The measured high-resolution geometry predicts a lower injection velocity, a wider-spreading spray, and an overall slower breakup rate with evident injector tip wetting compared to the ideally smooth nozzle boundary. This result implies that the manufacturing details of the injector, which are usually ignored in fuel injection studies, have a significant impact on the spray development process and should be taken into account for design optimization.

Numerical Simulation of Two-Phase Flow Within Aerated-Liquid Injectors

Aerated-liquid atomization involves the injection of a small amount of gas into a liquid stream to initiate primary breakup within the injector itself. The sprays that result have good penetration and lateral dispersion characteristics, and are suitable for fueling high-speed air-breathing propulsion systems. In this study, the flow within aerated-liquid injectors featuring both inside-out and outside-in aeration schemes is simulated using an implicit large-eddy simulation method equipped with a sharp phase-interface-capturing technique and a continuum model for surface tension. Results are compared with time-averaged and time-resolved X-ray measurements of liquid-phase density and imaging obtained at the 7-BM beamline at Argonne National Laboratory using beryllium injector models. A good agreement between computational predictions and experimental measurements is generally achieved, with each indicating that the end state of the aeration process for both injector designs is the formation of a core-annular two-phase flow in the nozzle portion of the injectors. The numerical model is less successful in predicting the structure of the separated two-phase flows observed before the expansion of the mixture through the nozzle. One-dimensionalized liquid-phase density, velocity, and momentum flux distributions obtained from the numerical model and the experiment display a close agreement for all injector configurations.