Atomization Of Liquid Jet Research Articles

Numerical simulation of primary atomization process of liquid jet in subsonic crossflow

Studies of primary atomization of liquid transverse jets in subsonic crossflow consider surface waves as the main cause of jet fragmentation. The characteristics of surface waves during jet fragmentation have been initially analyzed, but there is a lack of understanding of the mechanism of surface wave generation and development. This paper investigates the primary atomization of liquid transverse jets in subsonic crossflow based on the numerical method of Large Eddy Simulation (LES) with the coupled level set and volume of fluid method (CLSVOF) interface tracking, combined with adaptive mesh encryption techniques, where the liquid phase medium is water. This paper focuses on the fine structure of the primary atomization of the jet column, including the evolution of the jet column and the process of shedding the surface of the jet column, to clarify the physical process of primary fragmentation of the jet. The near-field flow structure and gas–liquid interaction are also analyzed to investigate the mechanism of primary atomization of liquid transverse jets and the mechanism of surface wave generation and development in subsonic flows. The results show that the shear action of gas and liquid leads to the generation of Kelvin–Helmholtz (KH) unstable wave, and KH surface wave dominates the surface breaking of jet column. The extrusion of gas and liquid results in the unstable fluctuation of Rayleigh–Taylor (RT), and the RT surface wave dominates the jet column, and finally the cylindrical fracture occurs. The development trend of RT surface wave is to increase the wavelength along the jet direction. When the wavelength of the RT surface wave develops from 0.12 mm to 1.2 mm, the jet column will be columnar broken at the trough of the last RT wave.

Machine learning accelerated turbulence modeling of transient flashing jets

Modeling the sudden depressurization of superheated liquids through nozzles is a challenge because the pressure drop causes rapid flash boiling of the liquid. The resulting jet usually demonstrates a wide range of structures, including ligaments and droplets, due to both mechanical and thermodynamic effects. As the simulation comprises increasingly numerous phenomena, the computational cost begins to increase. One way to moderate the additional cost is to use machine learning surrogacy for specific elements of the calculation. This study presents a machine learning-assisted computational fluid dynamics approach for simulating the atomization of flashing liquids accounting for distinct stages, from primary atomization to secondary breakup to small droplets using the Σ−Y model coupled with the homogeneous relaxation model. Notably, the models for thermodynamic non-equilibrium (HRM) and Σ−Y are coupled, for the first time, with a deep neural network that simulates the turbulence quantities, which are then used in the prediction of superheated liquid jet atomization. The data-driven component of this method is used for turbulence modeling, avoiding the solution of the two-equation turbulence model typically used for Reynolds-averaged Navier–Stokes simulations for these problems. Both the accuracy and speed of the hybrid approach are evaluated, demonstrating adequate accuracy and at least 25% faster computational fluid dynamics simulations than the traditional approach. This acceleration suggests that perhaps additional components of the calculation could be replaced for even further benefit.

Exploring on a three-fluid Eulerian-Eulerian-Eulerian approach for the prediction of liquid jet atomization

Predicting the atomization of a liquid jet in its applications, such as in fuel combustion and nuclear safety systems and in many other critical industrial applications, remains a challenging task. Using a low-computer consumption coarse-grid, this work presents a 3-D numerical method with three phases, which respectively represent the gas, continuous liquid and dispersed liquid. A Eulerian description is used for each of these phases. In addition, an algebraic interfacial area density (AIAD) model is used to consider the continuous liquid and gas phases. Meanwhile, a discrete population balance model is applied in order to take the droplet breakup and coalescences into account. The present Eulerian approach is then tested for the different liquid jets used in atomization regimes and then this is validated against the experimental data. The comparison reveals reasonable agreement in aspects such as jet spreading, droplet coalescence and the distribution of droplet sizes. Based on the simulation, the disintegration rates are found to increase from 175 kg/m3/s for case 1 to 310 kg/m3/s for case 2, which is due to increases in ejection velocity and turbulence intensity. In both the simulation and the experiment, larger-sized droplets form as the jet evolves downstream. This indicates that the coalescence between droplets overwhelms the possible breakup, meaning therefore that the diameter of the droplets increases streamwise, as shown in the comparison between D10 and D32 at the axial positions of 200 mm and 400 mm.

Air swirl effect on spray characteristics and droplet dispersion in a twin-jet crossflow airblast injector

Spray characterization in a novel twin-jet airblast injector is reported in this paper with the focus on the study of the effect of injector air swirl on droplet characteristics and dispersion behavior. The operational principle of the injector is based on achieving atomization of two liquid jets, injected in a radially opposite direction from a central hub by high-speed annular swirling cross-stream flow of air. Liquid jet atomization within model atomizers and the resulting spray study have not gained much attention in spite of its practical importance, for example, in lean premixed prevaporized combustors. In the present work, droplet size and three-component velocity measurements are measured in the above injector using the phase Doppler particle analyzer technique. Air velocity without liquid injection is also obtained using the laser Doppler velocimetry technique. For given inlet air and liquid mass flow rates, experiments are conducted in the absence and presence of annular air swirl corresponding to swirl number, S = 0 and 0.74, respectively. The addition of air swirl is found to dramatically affect the spray topology and also the measured spray characteristics as the droplet size reduces significantly downstream of the injector exit, which is explained. Droplet dispersion is studied by evaluating droplet size velocity correlation and also droplet Stokes number. The results not only provide insight into the physics behind improved atomization due to air swirl, but also demonstrate the ability of the novel injector to achieve atomization quality and high spray dispersion over a wide operating range.

Numerical simulation of the atomization of liquid transverse jet in supersonic airflow

The present study provides a numerical method for liquid jet atomization in supersonic gas crossflow. Compressibility of the gas and incompressibility of the liquid are considered. High-order accurate weighted essentially non-oscillatory schemes and the Harten–Lax–van Leer contact approximate Riemann solver are used for gas flows. Liquid flow is simulated by the Chorin projection method. The motion of the sharp interface between the gas and liquid is simulated by the volume of fluid method. In order to verify the accuracy of the numerical method, numerical and experimental results for the droplet breakup in the supersonic gas flow are compared. The method is employed to simulate the liquid jet atomization in the supersonic gas crossflow. According to numerical results, the breakup process is analyzed for four different stages. The discussion for the effect of the Mach number for the gas crossflow on the liquid jet atomization is given.

Linear instability of an annular liquid jet with gas velocity oscillations

Pressure fluctuation produced in liquid rocket engines affects atomization of the annular liquid jet in the combustion chamber. A linear stability analysis of an annular liquid jet under acoustic oscillations was conducted in this work. Parametric instability was applied to study the problem, and the oscillations of the gas velocity were considered. The Floquet theory was used to solve the disturbance, from which the dispersion equation containing the coefficients of the infinite order matrix was derived; the order of the coefficient matrix affected the accuracy of the results. There were several unstable regions in the wavenumber range, obtained by solving the dispersion matrix. When the gas velocity oscillated, the amplitude of the surface wave displacement also oscillated. In various unstable regions, the growth of the surface wave of the annular liquid sheet was also different, but the crest never passed through the equilibrium position, which was more complicated than in the planar liquid sheet. Increasing oscillation frequency contributed to an increase in the wavenumber corresponding to the unstable area. Increasing the oscillation amplitude increased the maximum growth rate. The effect of physical parameters on the instability of the annular liquid jet was also discussed.

Effect of elevated pressure on air-assisted primary atomization of coaxial liquid jets: Basic research for entrained flow gasification

Highly resolved numerical simulations have been conducted for a generic, coaxial air-blast atomizer designed for fundamental research of entrained flow gasification processes. Objective of the work is to gain a detailed knowledge of the influence of elevated reactor pressure on the primary atomization behaviour of high-viscous liquid jets. In agreement with measured breakup morphology and breakup regimes proposed in literature, the simulations yield a pulsating mode instability of liquid jet, along with disintegrations of fibre-type liquid fragments for different pressures. From the mechanism point of view, the breakup process has been shown to be triggered by concentric, axisymmetric ring vortices, which disturb the liquid jet surface in a first stage and penetrate further into the intact core, leading to interfacial instabilities and pinch-off of liquid ligaments. The liquid jet breaks up faster at elevated pressure, leading to a shorter core length LC. The calculated exponent (b≈−0.5) of the power law for fitting the decrease of LC with p agrees well with measured correlations from literature in terms of varied momentum flux ratio M and Weber number WeG, although water jets, atmospheric pressure and different air-assisted, external mixing nozzles were used in these works. Therefore, the effect of elevated pressure is equivalent to that of increased M or WeG, which scale linearly with p or the gas density for the current setup. The specific kinetic energy of liquid kL has been found to be increased with p, which is particularly pronounced in the high frequency range. A first-order estimate has been proposed, which can be used for the evaluation of liquid kinetic energy or droplet velocity within the spray. The results have been validated by simulations with twice-refined resolution, yielding a grid-independence behaviour with respect to the primary breakup characteristics. However, the follow-up processes with secondary breakup and spray dispersion are reproduced better by using the finer grid.

Primary atomization of a turbulent liquid jet in crossflow: a comparison between VOF and FGVT methods

The primary atomization of a turbulent liquid jet in crossflow was investigated using a mathematical, numerical and computational model. A comparison between the standard volume of fluid (VOF) method and the fine grid volume tracking (FGVT) method was reported. The FGVT method advects the interface between two fluids using a finer grid than the employed by the standard VOF method, and according to the literature, it provides a better interface resolution. Simulations were performed using adaptive mesh refinement in a three-dimensional domain subjected to gravitational field using the in-house code MFSim. The flow was modeled using an Eulerian–Lagrangian approach to capture the interface. The interface was tracked initially with VOF or FGVT methods until the initial breakup. Broken off, small-scale nearly spherical drops were transferred into the Lagrangian point particle description. Column breakup and shear breakup modes were observed on the liquid jet. Drops were small as one-hundredth the size of the injector diameter. The model was validated against experimental correlations for the liquid jet column trajectory, and the droplet size distribution was compared to a previous numerical study from the literature. In addition, the breakup mechanisms predicted were qualitatively compared to those in previous reports. The results of the liquid column trajectory from the simulations performed presented low differences with the literature for both methods tested. According to the numerical results obtained from the computational simulations, the liquid column trajectory was well captured and the droplet size distribution was similar to the literature; however, the FGVT method provided higher accuracy compared to VOF method. The two main breakup modes were identified, namely the column breakup with Kelvin–Helmholtz instabilities and the surface breakup with the formation of multiple ligaments which later lead to droplet formation. The FGVT method provided a more detailed interface contour and improved the number of droplets converted from the Eulerian to the Lagrangian approach compared to the standard VOF method. On the other hand, the FGVT method presented relatively higher computational costs compared to VOF. Therefore, the FGVT method presented a higher interface quality and allowed a larger number of droplet conversion to the Lagrangian approach compared to the VOF method, even though the simulation run time using VOF was lower than with FGVT.

A compressible two-phase flow framework for Large Eddy Simulations of liquid-propellant rocket engines

Atomization of liquid jets is a key feature of many propulsion systems, such as jet engines, internal combustion engines or liquid-propellant rocket engines (LRE). As it controls the characteristics of the spray, atomization has a great influence on the complex interaction between phenomena such as evaporation, turbulence, acoustics and combustion. In this context, Computational Fluid Dynamics is a promising way to bring better understanding of dynamic phenomena involving atomization, such as e.g. high-frequency combustion instabilities in LRE. However the unsteady simulation of primary atomization in reactive compressible two-phase flows is very challenging, due to the variety of the spatial and temporal scales, as well as to the high density, velocity and temperature gradients which require robust and efficient numerical methods. To address this issue, a numerical strategy is proposed in this paper, which is able to describe the dynamics of the whole chain of mechanisms from the liquid injection to its atomization and combustion. Primary atomization is modeled by a coupling between a homogeneous diffuse interface model and a kinetic-based Eulerian model for the spray. This strategy is successfully applied to the unsteady simulation of an operating point of the Onera’s Mascotte test bench, representative of one coaxial injector of LRE operating under subcritical conditions. The dynamics of the liquid core is retrieved and the flame shape as well as Sauter mean diameters are in good agreement with experimental results. These results demonstrate the ability of the strategy to deal with the harsh conditions of cryogenic combustion, and provide a promising framework for future studies of combustion instabilities in LRE.

Numerical simulation of in-nozzle flow characteristics under flash boiling conditions

Flash boiling spray has been extensively investigated because it has the potential in improving spray atomization. However, some phenomena, such as flow rate reduction, exit velocity choke and drastic bubble generation near the exit, etc., cannot be explained by existing theory of subcooled liquid jet and spray atomization. The phase change process occurring inside the nozzle still needs further exploration and investigation. However, due to the small dimensions of the nozzle, it is challenging to measure the flow characteristics experimentally. In this research, a one-dimensional two-phase flow model incorporating modified Stiffened Gas Equation of State is used to simulate the in-nozzle flow characteristics under flash boiling conditions. The model was validated against previous experimental data. The simulation results of the phase change process inside the nozzle was compared qualitatively with experimental data taken from a 2D optical nozzle. Different from subcooled conditions, superheated working liquid inside the nozzle evaporates more drastically as the result of local pressure distribution change. The flow velocity and mass flow rate are affected accordingly as well. In addition, the effects of the injection pressure, ambient pressure, and liquid temperature on these phenomena are also discussed. It was found that the ambient pressure and liquid temperature can affect the bubble generation by altering the evaporation rate of the fuel, while the injection pressure only influence the flow velocity and change the evaporation time of the liquid.

Subgrid-scale Capillary Breakup Model for Liquid Jet Atomization

ABSTRACT A subgrid-scale capillary breakup model has been developed by coupling a Eulerian interface-capturing approach to a Lagrangian point particle method. At the fully resolved scale, the evolution of phase interface is captured by the Refined Level Set Grid method (RLSG), while the under-resolved, small-scale liquid structures are represented by Lagrangian point particles. To couple the level-set method with the Lagrangian approach, a numerical algorithm is developed to identify the under-resolved structures and compute their shape metric. The capillary instability theory is then applied to predict the sizes and number of post-breakup drops needed to replace them with Lagrangian drops. The secondary atomization of the Lagrangian drops is modeled by the stochastic breakup model. To validate the present model, numerical simulations are performed for the breakups of a single liquid filament and a round liquid coaxial jet. Grid-convergence studies for the drop-size distribution of the resulting spray are conducted and compared to the experimental data. The numerical results show good agreement with the experimental data. The present subgrid-scale capillary breakup model reduces the computational cost by relaxing the stringent grid resolution required for multi-scale atomization simulations.

MESH RESOLUTION EFFECTS ON PRIMARY ATOMIZATION SIMULATIONS

In this work, we use direct numerical simulation through a volume of fluid solver with adaptivemesh refinement to analyze the atomization of a pulsating round liquid jet with a narrow length-scale range configuration. We propose three grid sizes based on characteristic scales we estimate from the deformation and fragmentation processes of this problem. We compute drop statistics and general spray features for each case. We found that mesh resolution affects the atomization rate and the probability density function of droplet size and position, not only for the underresolved drops but for all liquid structures. The two simulations with higher grid refinement presented volume-weighted distribution with minor differences. Therefore, we propose assessing the accuracy of atomization simulations based on the volume fraction of underresolved structures.

A 3D Moment of Fluid method for simulating complex turbulent multiphase flows

This paper presents the moment of fluid method as a liquid/gas interface reconstruction method coupled with a mass momentum conservative approach within the context of numerical simulations of incompressible two-phase flows. This method tracks both liquid volume fraction and phase centroid for reconstructing the interface. The interface reconstruction is performed in a volume (and mass) conservative manner and accuracy of orientation of interface is ensured by minimizing the centroid distance between original and reconstructed interface. With two-phase flows, moment of fluid method is able to reconstruct interface without needing phase volume data from neighboring cells. The performance of this method is analyzed through various transport and deformation tests, and through simple two-phase flows tests that encounter changes in the interface topologies. Exhaustive mesh convergence study for the reconstruction error has been performed through various transport and deformation tests involving simple two-phase flows. It is then applied to simulate atomization of turbulent liquid diesel jet injected into a quiescent environment. The volume conservation error for the moment of fluid method remains small for this complex turbulent case.

Characterizing primary atomization of cryogenic LOX/Nitrogen and LOX/Helium sprays by visualizations coupled to Phase Doppler Interferometry

There is a need for experimental data in conditions representative injection in rocket engines to validate or initiate droplet formation models used in numerical simulations. A new cryogenic vessel was built upon the MASCOTTE test bench to study the atomization of a single oxygen liquid jet, under non-reactive conditions, with simultaneous optical diagnostics. A test plan was built to explore the fiber-type regime occurring in liquid rocket injection systems, with a fixed Reynolds number and a large range of Weber number and momentum flux ratio, compared to existing studies. High-speed images are used to describe qualitatively the fiber-type regime and to visualize were droplets are present, in order to prepare the drop-size measurements. Phase Doppler Interferometry is used to measure the size and velocity of droplets produced by atomization of a liquid oxygen jet by a co-flowing gas. Droplet size and velocity measurements were performed with a PDI close to the nozzle exit in order to provide data on droplets produced by the primary atomization process, which can be useful for numerical simulations initialisation. The radial evolutions of the axial velocity and of the drop size distribution show similar trends as observed in the literature. The axial velocity is investigated for different operating conditions with helium or nitrogen as atomizing gas, showing an increase on the side of the spray. The radial evolution of the droplet size shows a translation of the drop size distribution on the edge of the spray towards the smaller sizes, indicating that the biggest liquid elements stay close to the LOX jet.

A zonal grid method for incompressible two-phase flows

We present a zonal grid based numerical method applicable to two-phase flows. The method is aimed at reducing the computational costs for interfacial flows involving local high velocity gradients such as those encountered in atomization systems. The objective is to fully resolve the primary atomization region while at the same time limiting the number of grid points in secondary or dispersed zones where a very fine grid is not required, along with the ability to use a local zone based time step. Calculations of a rising bubble and liquid jet atomization configurations were performed on a coarse grid with a fine zone superimposed on small domains with the strongest gradient. The results show good agreement with simulations carried out with a complete refined mesh with only a fraction of the CPU time. The results indicate that the two-phase zonal grid model approach introduced here has the potential to provide an accurate and cost-effective approach for modeling two-phase flow problems that have multiple temporal and spatial scales.

Flow topologies in primary atomization of liquid jets: a direct numerical simulation analysis

The local flow topology analysis of the primary atomization of liquid jets has been conducted using the invariants of the velocity-gradient tensor. All possible small-scale flow structures are categorized into two focal and two nodal topologies for incompressible flows in both liquid and gaseous phases. The underlying direct numerical simulation database was generated by the one-fluid formulation of the two-phase flow governing equations including a high-fidelity volume-of-fluid method for accurate interface propagation. The ratio of liquid-to-gas fluid properties corresponds to a diesel jet exhausting into air. Variation of the inflow-based Reynolds number as well as Weber number showed that both these non-dimensional numbers play a pivotal role in determining the nature of the jet break-up, but the flow topology behaviour appears to be dominated by the Reynolds number. Furthermore, the flow dynamics in the gaseous phase is generally less homogeneous than in the liquid phase because some flow regions resemble a laminar-to-turbulent transition state rather than fully developed turbulence. Two theoretical models are proposed to estimate the topology volume fractions and to describe the size distribution of the flow structures, respectively. In the latter case, a simple power law seems to be a reasonable approximation of the measured topology spectrum. According to that observation, only the integral turbulent length scale would be required as an input for the a priori prediction of the topology size spectrum.

LES of primary breakup of pulsed liquid jet in supersonic crossflow

The injection of a pulsed liquid jet into supersonic air flow is a promising approach to improving the fuel atomization performance in a Scramjet engine. Therefore, the primary breakup of a pulsed liquid jet in supersonic crossflow is numerically investigated in the present paper. A two-phase flow Large Eddy Simulation (LES) algorithm is developed for simulations of liquid jet atomization in supersonic gas flow. A coupled Level Set and Volume of Fluid (VOF) method is used to track the interface deformation and disintegration. The supersonic gas flow is solved using a compressible flow solver while the liquid phase is solved by an incompressible flow solver. Appropriate boundary conditions are specified at the interface for both solvers to correctly capture the interaction between the gas and liquid phases. The primary atomization of a steady liquid jet with the same average mass flow rate as the pulsed jet is also simulated as a benchmark test case. The liquid velocity pulsation produces a very different primary atomization morphology in comparison with the steady liquid jet, which significantly enhances the primary breakup process. It is observed that Rayleigh-Taylor instability dominates the development of surface waves for the steady liquid jet. For the pulsed liquid jet, the liquid column deformation induced by the liquid velocity pulsation determines the wavelength of the surface waves and thus the liquid jet breakup location. In comparison with the steady liquid jet, the penetration of the pulsed liquid jet increases by 20%, and the width of the wake zone expands by 25%, resulting in improved atomization and mixing performance.

Ultra-high speed visualization of a flash-boiling jet in a low-pressure environment

We visualize the flash-boiling atomization of liquid jets released into a low-pressure environment at frame rates of up to five million frames per second using a long distance microscope. Such temporal resolution allowed us to capture the details of the bubble expansion mechanism, responsible for the jet atomization, for the first time. We document an abrupt transition from a laminar to a fully external flashing jet by systematically reducing the ambient pressure. We perform experiments with different volatile liquids, ejected through micro-nozzles with different inner diameters. Surprisingly, minimum pixel intensity projections revealed spray angles close to θs ∼ 360° and speeds of bubble expansion up to 140 m/s. Particle tracking shows that ejected droplets achieve speeds much larger than the jet velocity and drop sizes order of magnitude smaller than the diameter of the nozzle. Furthermore, hole growth speeds measured on the bubble's film in combination with Taylor–Culick predictions suggest that the smallest droplet sizes are on the hundreds of nanometer or submicron range, which contravenes the general belief that flash-boiling atomization results in uniform drop sizes.

Effects of fuel viscosity on the primary breakup dynamics of a high-speed liquid jet with comparison to X-ray radiography

One of the major concerns in combustion engines is the sensitivity of engine performance to fuel properties. Recent works have shown that even slight differences in fuel properties can cause significant changes in performance and emission of an engine. In order to design the combustion engines with multi-fuel flexibilities, the precise assessment of fuel sensitivity on liquid jet atomization process is a prerequisite since the resulting fuel/air mixture is critical to the subsequent combustion process. The present study is focusing on the effect of physical fuel properties, mostly viscosity difference, on the breakup process of the liquid jet injected into still air. Two different jet fuels, CAT-A2 and CAT-C3, are considered here as surrogates for a fossil-based fuel and a bio-derived high-viscosity alternative fuel. The simulations are performed using the volume-of-fluid (VoF) interface tracking method coupled to Lagrangian particle method in order to capture the breakup instabilities of jets and the resulting droplets. The investigations take the actual geometry of the injector into account to resolve the unsteady flow phenomena inside the nozzle that impact the turbulence transition and atomization. The simulation results are compared to the experimental measurement using X-ray radiography. Both simulation and X-ray measurements consistently describe the effects of different fuels on the fundamental properties of atomization including the breakup length, transverse liquid volume fraction and the droplet sauter-mean-diameter. The application of a Detailed Numerical Simulation approach complemented by unique X-ray diagnostics is novel and providing new understanding and research directions in engine spray dynamics.

Discussion on instabilities in breaking waves: Vortices, air-entrainment and droplet generation

When waves are breaking, several instabilities are observed to be responsible for several features, like vortices, air-entrainment and droplets generation. Being able to ascertain if the number and sizes of droplets during breaking events can be controlled by instabilities and in which order these perturbations lead to droplets production, is evidently of great interest. Thanks to some numerical simulations and new experimental visualizations, a discussion is proposed to analyze the successive steps of atomization of a plunging liquid jet when a wave break. The complexity of the phenomenon will be highlighted, while some possible instability mechanisms will be described. Following the plunging jet impact, vortex filaments are produced, inducing air entrainment and complex structures which can be similar to the so-called obliquely descending eddies.