Liquid Breakup Process Research Articles

Instability and breakup of rayleigh-plateau jets with capillary wave induced by electrowetting-on-dielectric

This paper discusses the lateral disturbances of vertical liquid flow enhancement using electrowetting-on-dielectric (EWOD) technique, which effectively reducing the breakup length of the liquid flow and minimizing the formation of satellite droplets. The EWOD technology is characterized by its minimal space occupation and low energy consumption. The successful implementation of the EWOD phenomenon relies on the formation of a three-phase contact line (TPCL) between the electrodes and the liquid, and the novel EWOD tube structure design provides an extended TPCL, enhancing the amplitude of capillary waves. Through experimental and theoretical analysis, this paper identifies key parameters affecting the liquid breakup process, such as the gap depth, angle of the EWOD tube, EWOD voltage amplitude, and frequency of the AC electrical signal. The research results indicate that the proposed EWOD tube responds rapidly and can consistently and stably manage liquid breakup and reduce the number of satellite droplets.

Effects of structured prefilmer lip on spray characteristics in planar prefilming airblast atomization

Effective liquid fuel atomization is an ultimate aim for the prefilming airblast atomizer in aero-engines, and the liquid fuel breakup behavior is the key indicator of atomization quality. Whereas, the prefilmer lip structure is unnoticed but of great influence on this breakup process. In this paper, the rectangular block structured prefilmer lip and the triangular prism structured prefilmer lip are delicately designed and adopted in planar prefilming airblast atomization. Accordingly, the spray characteristics, including the liquid film morphology, droplet distributions, and atomization quality are comprehensively investigated under structured prefilmer lips, using a coupled LB-LES model. The numerical results reveal that when the rectangular block structured prefilmer lip is adopted, the multihole morphology appears on the liquid film under the cutting effect of the rectangular sawtooth, thus contributing to the liquid breakup process. The droplet equivalent diameter in the atomization process distributes exponentially, and 70%-80% of the droplet sizes are less than 100 μm. The droplet velocity shows a normal distribution with its average value located from 1 m/s to 2 m/s in the streamwise direction when the air velocity is 20 m/s. Ultimately, according to droplet size, droplet velocity, and atomization angle, along with the atomization efficiency coefficient, the atomization quality under the rectangular block structured prefilmer lip is better than that with triangular prism structured prefilmer lip and unstructured prefilmer lip, and the highest atomization quality is obtained with rectangular block width of 0.3 mm.

Primary breakup of liquid fan sheet in crossflow

This paper presents the primary breakup of liquid sheet under the influence of crossflow. The atomization of liquid sheets is important for many applications, e.g., agricultural sprays, fuel atomization, and spray coatings, among others. The aerodynamic effects on the liquid breakup process is investigated experimentally to identify the breakup regime transitions. A subsonic wind tunnel was used to simulate the crosswind effects on flat fan nozzle spray. Two flat fan nozzles sizes were tested in this study. High-speed digital imaging technique was used to measure the breakup regime transitions, the location of the end of the liquid core, the breakup time, the location, and size of droplets at onset of breakup and the spray trajectory. The results were correlated using phenomenological analyses. The aerodynamic effects were responsible for initiating bag breakup and reducing the droplet sizes which are susceptible to drift. This would be problematic for spraying herbicides in windy conditions.

Liquid fuel refill dynamics in a rotating detonation combustor using megahertz planar laser-induced fluorescence

The highly dynamic response of a continuously injected liquid fuel jet to rotating detonation waves is a critical parameter in the design, performance optimization, and modeling of a rotating detonation engine (RDE). In this work, this response is spatio-temporally resolved from the fuel injection point to the detonation channel using planar laser-induced fluorescence (PLIF) at imaging rates up to 1 MHz. A rotating detonation combustor (RDC) is operated on hydrogen and air to sustain stable detonation waves that interact in a one-way coupled manner with a single liquid fuel jet that propagates into the combustion chamber with cycle periods of ∼250 µs. Diesel is utilized as a realistic fuel surrogate with higher aromatic compounds to enable fluorescence excitation using the 355 nm third-harmonic output of a burst-mode Nd:YAG laser. By optimizing the technique to accommodate orders of magnitude variations in the fuel density throughout the injection process, the PLIF data enable measurements of (i) the overall refill dynamics after the arrival of the detonation wave, (ii) changes in the liquid spray trajectory with microsecond temporal resolution, and (iii) the time required to reestablish the quasi-steady axial refilling process as a function of peak chamber pressure relative to the air- and liquid-injector pressure drops. As the passage of the detonation wave imparts significant changes in the momentum flux ratio, the qualitative liquid break-up process and spatial distribution of the spray also vary significantly in time. Only as the injection system recovers late in the cycle does the fuel spray eventually return to a quasi-steady position and allow comparisons with theoretical jet trajectories. These data, enabled by ultra-high-speed PLIF imaging, represent some of the first detailed measurements for quantifying the dynamic response and recovery of liquid jets exposed to periodic detonations in an operating RDC.

Investigation of Water Films Shed From an Airfoil in a High-Speed Flow

Abstract The breakup of liquid films in high-speed flows is found in many applications. These include prefilming air blast atomization found in fuel injectors and shedding of droplet off of airfoils. In this work, a test rig has been developed and applied to study the breakup of water films from the trailing edge of an NACA 0012 airfoil placed in a high-speed air flow. The results provided reflect a new detailed dataset focused on air velocities above 75 m/s. Computational fluid dynamic (CFD) simulations were used to guide the development of the rig and the associated boundary and inlet conditions. In this work, air velocities up to 175 m/s were used with water films between 0.4 cm2/s and 3.6 cm2/s. The water film was introduced onto one surface through a series of 0.5 mm holes separated by 1 mm at a location 35 mm downstream of the leading edge of the vane. High speed video was used to document the sheet breakup behavior from the edge of the airfoil. Image analysis was used to identify the breakup length of ligaments formed. Laser diffraction was used 50 mm downstream of the trailing edge of the vane to determine the droplet size distribution and associated representative average diameters generated. A series of test conditions were run between 50 m/s and 175 m/s using multilevel factorial designed experiments. The results obtained were subjected to analysis of variance to generate correlations for breakup length and droplet representative diameters as a function of the conditions studied. The analysis was conducted on dimensionless versions of the variables studied to help connect the results to the physics of the situation. For example, rather than using air velocity to correlate the behavior, the Weber number (based on gas phase density and velocity) was used owing to its historical ability to describe liquid breakup processes. The results obtained are compared with other studies in the literature, and good agreement was found for the conditions at which a comparison could be made. It is observed that, for the conditions studied, liquid film thickness has little effect on the resulting droplet sizes—especially for the higher air velocity cases studied. The results also illustrate the sensitivity to choice of the characteristic length, which is generally consisted of trailing edge geometry or boundary layer thickness. In contrast, liquid film does influence the ligament breakup length, although the time to break up is less affected. Overall, the correlations developed provide engineering tools to help estimate ligament and drop size behavior for water films shed from airfoils in a high-speed flow.

Influence of the wettability on the liquid breakup in planar prefilming airblast atomization using a coupled lattice Boltzmann–large eddy simulation model

In this paper, an efficient coupled lattice Boltzmann–large eddy simulation model [X. An et al., “Coupled lattice Boltzmann-large eddy simulation model for three-dimensional multiphase flows at large density ratio and high Reynolds number,” Phys. Rev. E 104, 045305 (2021)] based on the Allen–Cahn phase-field theory is introduced for simulating the liquid breakup in planar prefilming airblast atomization. This is the first time that the lattice Boltzmann method is used in the three-dimensional numerical investigation of prefilming airblast atomization to the best of our knowledge. The present model utilizes two evolution equations: one is used to capture the fluid interface, and another is adopted to solve hydrodynamic properties. An advanced multiple-relaxation-time scheme is also applied for the collision operator to enhance the numerical stability. To investigate the influence of the wettability on the liquid breakup accurately, a simple and efficient wetting boundary scheme is delicately designed and strictly validated. Additionally, to evaluate the atomization quality intuitively, an atomization efficiency coefficient is proposed for characterizing the liquid breakup process. The numerical results reveal that the influence of the wettability lies in the liquid accumulation phenomenon at the edge of the prefilmer and the droplet movement in the vertical direction. The atomization quality adopting a non-wetting prefilmer is better than other cases, according to the atomization efficiency coefficients, the mean droplet equivalent diameters with their size distribution proportions, and the atomization angles in the vertical direction. In addition, it is also found that the droplet proportion above the prefilmer increases as the contact angle increases, and the proportions on both sides of the prefilmer account for 50% at the contact angle of 90°.

Effect of orifice size on liquid breakup dynamics and spray characteristics in slinger atomizers

The ability of the rotary slingers to achieve fuel atomization by employing the rotational speed of the turbine shaft obviates the use of high-pressure fuel pump and makes those a popular choice for small gas turbine engines. Such atomizers contain arrays of orifices on their outer periphery for injection of the liquid into the combustion chambers. In the present work, we experimentally investigate the influence of the choice of orifice size on primary liquid breakup dynamics and spray characteristics in slinger atomizers. Three slinger discs with the same disc size and number of orifices but with different orifice size, d o (= 1 mm, 1.5 mm, and 2 mm) are examined in a high-speed slinger test rig for a range of rotational speed (2000–10,000 rpm) and liquid feed rate (0.2–1 lpm). The operating conditions are relevant to part-load and engine relighting conditions for which the selection of geometric parameters of the injector may play a crucial role in achieving the atomizer performance. In the current work, the liquid breakup process in the vicinity of the slinger orifices was visualized using front light illumination technique, whereas the pulse-shadowgraph technique was employed for droplet sizing. It is found that smaller orifice size promotes column mode of liquid breakup which results in larger primary liquid breakup length and subsequently results in larger Sauter mean diameter. The results highlight that the orifice size is an important design parameter as the liquid atomization in slinger atomizers is sensitive to the selection of orifice size especially for lower range of rotational speeds.

Coriolis-induced liquid breakup and spray evolution in a rotary slinger atomizer: Experiments and analysis

Understanding the physics of primary liquid breakup process and its correlation with the evolution of spray characteristics in a rotary slinger atomizer is the goal of the present research. Experiments were conducted in a high-speed slinger test rig that houses a static liquid delivery manifold to uniformly supply the liquid to the rotating slinger disc that contains a single row of orifices carved on its peripheral surface for liquid injection.The atomizer was operated for a wide range of conditions by varying the rotational speed and liquid feed rate. The liquid breakup structure at the exit of the slinger orifices was visualized using front light illumination technique, while the droplet size was measured at different radial stations away from the slinger surface by application of the Interferometric Laser Imaging for Droplet Sizing (ILIDS) technique. The visualization images highlighted strong influence of Coriolis force as the liquid tends to accumulate on one side of the channel (that is opposite to the rotational direction) for all cases. It was observed that while the liquid thickness is smaller for higher rotational speed, it does not vary much with liquid feed rate at the same speed and, instead, in such case the span of the liquid is wider. A theoretical analysis was developed to describe the in-channel liquid behaviour that accounts for the effect of Coriolis and surface tension forces. Interestingly, the theory could explain the above observations. The differences in the predictions in comparison to the analysis by Dahm et al. (2006a) was attributed to the assumption of annular film flow in the latter. The liquid breakup mode (stream, sheet or transition mode) could be described by Coriolis Bond number (Bo) that refers to the ratio of Coriolis to surface tension forces and proportional to the spread parameter (span to thickness ratio), while the liquid breakup regimes were identified on a Bo−q plot, where q is the liquid to air momentum flux ratio. The variation of characteristic droplet sizes with both rotational speed and feed rate were examined, and again some interesting trends are identified. The correlation between liquid breakup mode/regime with the measured droplet size was established using the above non-dimensional numbers.

Investigation of Liquid Breakup Process Solid Rocket Motor Part B: Vertical C-D Nozzle

Abstract Aluminized propellants are frequently used in solid rocket motors (SRMs) to increase specific impulse. However, as the propellant combusts, the aluminum is oxidized into aluminum oxide (Al2O3), it agglomerates into molten droplets that attach to the outside wall of the rocket nozzle. This phenomenon negatively impacts ballistics performance because the droplets remain attached to the inner wall of propulsion chambers. This buildup of particles tends to erode the wall, decreasing the performance and sustainability of the rocket. This study presents both experimental and computational fluid dynamics (CFD) to investigate the relationship between gas velocity and molten particle size for the vertically arrayed combustion chamber. Also, the Weber number and the Froude number are monitored to explain the breakup phenomenon and the condition of alumina flow in the whole testing channel. This study focused mainly on the vertical arrangement of the propulsion chamber with the cold experimental and simulation investigating the role of the liquid water in addition to a comparison with the horizontal chamber case. Unlike the horizontal setup, a greater number of droplets with smaller average droplet diameter present in the vertical setup; however, Froude number follows the same trend as for the horizontal C-D nozzle setup.

Primary breakup of a non-Newtonian liquid using a high-speed rotary bell atomizer for spray-painting processes

The present contribution deals with numerical and experimental studies of the primary liquid breakup process using a high-speed rotary bell atomizer. The first part of the investigations focuses on the film formation on the distributor disk and the inner surface of the rotary bell. Numerical simulations using the volume-of-fluid approach were carried out. A non-Newtonian liquid that has shear-thinning behavior is used to investigate the effect of the viscosity on the initial wetting, the film formation process, and the film thickness distribution on the bell. A nonhomogeneous film structure is found on the inner surface of the rotary bell. This is also observed in experimental investigations using a high-speed camera. The second part focuses on the disintegration process of the paint liquid in the near bell region. As inlet conditions for the breakup simulations, the properties of the liquid film at the bell edge, i.e., film thickness, velocities, and apparent viscosity, resulting from the film formation simulations were applied. Two different liquid disintegrations in the near-field were found, which were also observed in experimental investigations using a high-speed camera. Furthermore, user-defined functions were compiled in ANSYS Fluent to uniquely identify and characterize droplets formed through the breakup process. In this way, droplet properties such as diameter, velocity, and position can be determined.

Spray Characterization of a Slinger Injector Using a High-Speed Imaging Technique

This paper presents the experimental-visualization results of liquid breakup in a slinger injector with high-aspect-ratio radial–axial discharge channels over a range of rotational speeds between 600 and 10,000 rpm to identify detailed breakup pattern and spray characteristics. High-speed images of the jet breakup are captured and processed by a shadowgraph method. The particle/droplet imaging analysis technique is used to obtain the droplet-size distribution. Centripetal/Coriolis-induced stream-mode injection is observed for high-aspect-ratio axially canted discharge channels. The results show that the liquid-breakup process with rotational-speed values up to 4000 rpm corresponds to a subcritical stream-mode breakup. Moreover, a new phenomenon of jet bifurcation is observed at higher rotational speeds (more than 5000 rpm). Liquid-film disintegration is observed at rotational speeds of 10,000 rpm, which constitutes a transitional regime from subcritical to supercritical stream-mode breakup. The results show that the Sauter mean diameter decreases with increasing rotational speed. Furthermore, based on experimental observations, it is concluded that the number of injection orifices and their partitioning in different orientations affect the Sauter-mean-diameter values. Finally, a correlation is obtained between Sauter mean diameter, rotational speed, and volume-flow rate.

Atomizing industrial gas-liquid flows – Development of an efficient hybrid VOF-LPT numerical framework

Atomizing gas-liquid flows are used in industrial applications where high interphase heat and mass transfer rates and good mixing are of primary importance. Today, there is no single mathematical framework available to predict the entire liquid breakup process at an acceptable computational cost for a typical problem of industrial size.In this work, we develop a volume-of-fluid (VOF) framework that is combined with Lagrangian particle tracking (LPT) to take advantage of the respective strengths of these two approaches. The two frameworks are coupled via a statistical model that enables a transition from the VOF to the LPT formulation using input data about the primary breakup process obtained from detailed VOF simulations in dedicated switching zones. LPT-to-VOF transitions are handled directly by analyzing the proximity of LPT parcels to larger VOF structures.The combined framework is specifically designed to accommodate situations where atomization occurs in several locations simultaneously and when separated and dispersed turbulent gas-liquid flows co-exist in the same industrial unit. The procedure in which the statistical model is derived is presented and discussed, its performance is verified and the computational efficiency of the combined VOF-LPT model is assessed. Finally, the application of the coupled framework to the simulation of an industrial gas-liquid mixer with four separate atomization regions is presented.

Study of the Liquid Breakup Process in Solid Rocket Motor

In a solid rocket motor, when the aluminum based propellant combusts, the fuel is oxidized into alumina (). It tends to agglomerate into molten droplets, impinge on the chamber walls, and then flow along the nozzle wall. Such agglomerates cause erosive damage. The focus of the current research is to characterize the agglomerate flow within the nozzle section by studying the breakup process of the liquid film that flows along the wall of a straight test channel while a relatively higher-speed gas moves over it. We have used an unsteady-flow Reynolds-averaged Navier–Stokes code to investigate the interaction of the liquid film flow with the gas flow. The rate of the wave breakup was characterized by introducing breakup length, Ohnesorge number, and Weber number for various flow conditions. Based on the volume fraction of the liquid, these three numbers are indicators of the level of liquid breakup. We summarize that a larger breakup length relates to a high breakup state of the liquid because the appearance of droplets contributes to a larger total boundary length during calculation.

Assessment and validation of liquid breakup models for high-pressure dense diesel sprays

Liquid breakup in fuel spray and atomization significantly affects the consequent mixture formation, combustion behavior, and emission formation processes in a direct injection diesel engine. In this paper, different models for liquid breakup processes in high-pressure dense diesel sprays and its impact on multi-dimensional diesel engine simulation have been evaluated against experimental observations, along with the influence of the liquid breakup models and the sensitivity of model parameters on diesel sprays and diesel engine simulations. It is found that the modified Kelvin-Helmholtz (KH)–Rayleigh-Taylor (RT) breakup model gives the most reasonable predicted results in both engine simulation and high-pressure diesel spray simulation. For the standard KH-RT model, the model constant Cbl for the breakup length has a significant effect on the predictability of the model, and a fixed value of the constant Cbl cannot provide a satisfactory result for different operation conditions. The Taylor-analogybreakup (TAB) based models and the RT model do not provide reasonable predictions for the characteristics of high-pressure sprays and simulated engine performance and emissions.

Energy considerations in spraying process of a spill-return pressure-swirl atomizer

The work focuses on energy conversion during the internal flow, discharge and formation of the spray from a pressure-swirl (PS) atomizer in the simplex as well as spill-return mode. Individual energy forms are described in general and assessed experimentally for a particular PS atomizer and light heating oil as a medium. The PS spray was observed at various loads to investigate the liquid breakup process and the spray characteristics. Spatially resolved diameters and droplet velocities, measured by means of phase-Doppler anemometry, served for estimation of the energy characteristics in the PS spray.The input energy given by the potential energy of the supplied liquid partially converts into the kinetic energy (KE) in the swirling ports with hydraulic loss in per cent scale. Most of the pressure drop is associated with rotational motion in the swirl chamber with total conversion efficiency at the exit orifice ∼58%. The rest of the input energy ends up as friction loss, leaving room for improvement.The overall value (ID32) of the Sauter mean diameter of droplets in the spray, D32, varies with pressure drop Δpl powered to −0.1. The radial profiles of D32 widen with the increase in spill/feed ratio (SFR), but the ID32 remain almost constant within the studied SFR range. The spray KE at closed spill line covers the droplet KE (21–26%) and that of entrained air (10–13%), both moderately varying with Δpl. The specific KEs of both the liquid and air markedly drop down with the spill line opening.Atomization efficiency is less than 0.3% for the studied range of operation regimes and depends on Δpl and SFR. Our results confirm low power demand of simplex PS atomizers, with extra energy consumption in spill mode. Several recommendations are given for PS atomizer innovations and development of new, more efficient, designs meeting more stringent environmental requirements.

Energy conversion during effervescent atomization

The paper qualitatively describes processes during the internal flow, discharge of two-phase mixture as well as spray formation with a particular focus on the energy transfer during effervescent atomization of light heating oil. The near nozzle spray visualization elucidates the liquid breakup process at different operation modes. Numerical results illustrate the forms of energy involved in the atomization process, their values and the influence of operational conditions on relations between the energy forms. The main part of the paper focuses on the atomization efficiency. A simple method for estimation of the atomization efficiency of pneumatic atomizers is proposed; surface tension energy of created droplets, obtained from phase-Doppler anemometry data, is compared with the energy required for atomization. The atomization efficiency of effervescent atomizers is found to be in fragments of per cents for atomizing pressures ranging between 0.1 and 0.5MPa, and gas-to-liquid ratios (GLRs) between 0.02 and 0.1, and it is inferior, by about one order of magnitude to the efficiency of simple pressure and pressure-swirl atomizers for a comparable droplet size. The efficiency declines with both the pressure and GLR with approximately logarithmic tendency.

Ballistic imaging of liquid breakup processes in dense sprays

Ballistic imaging is the name applied to a category of optical techniques that were originally developed for medical applications. Recently, ballistic imaging was adapted to acquire instantaneous images of the liquid core inside atomizing sprays; a region that has been heretofore inaccessible to spray researchers. An important difference between spray research and the medical imaging problem is the need for high fidelity single-shot (within 10 μs) imaging in a spray whereas stationary tissue images can be averaged. Transient ballistic imaging diagnostics have been used to reveal details of the primary breakup process in a LOX injector, a turbulent water jet, a water jet in cross-flow, a transient diesel fuel spray, a rocket fuel injector, and an aerated spray. This paper briefly discusses various methods for imaging the liquid core, it introduces ballistic imaging and provides specific examples, it describes detailed studies of photon transmission through dense media, and it then discusses incorporation of those results into a model for a ballistic imaging instrument that can evaluate and optimize various concepts.

111 ノズル内キャビテーションが液体分裂機構に及ぼす影響 : 第 3 報 : 2 次元ノズル内の圧力変動

111 ノズル内キャビテーションが液体分裂機構に及ぼす影響 : 第 3 報 : 2 次元ノズル内の圧力変動

BREAKUP MECHANISMS AND DRAG COEFFICIENTS OF HIGH-SPEED VAPORIZING LIQUID DROPS

An experimental and modeling study was performed to investigate drop drag and breakup mechanisms of liquid drops injected into a transverse high-velocity air jet at room and elevated temperature conditions. The range of conditions included three drop breakup regimes previously referred to as bag, shear or boundary-layer stripping, and "catastrophic" breakup regimes. In the experiments the injected dieselfuel drop's initial diameter was 189 μm, and its velocity was 16 m/s. The transverse cur jet velocity was varied from 70 to 200 m/s, and the air jet's temperature was varied from room temperature up to 450 K. The conditions tested correspond to drop Weber numbers (based on gas density and drop-gas relative velocity) from 56 to 463. Double-pulse, high-magnification photography was used to study the temporal progress of the breakup of the drops. The experiments gave information about the microscopic structure of the liquid breakup process, drop breakup regimes, drop trajectories and drag coefficients, and the acceleration of atomizing drops. The results show that the breakup mechanism consists of a series of processes in which dynamic pressure effects deform the drop into a thin liquid sheet. This drop flattening significantly affects the drop's drag coefficient. It was found that drop trajectories could be modeled adequately using a modified dynamic drag model that accounts for drop distortion. The flattened drop subsequently breaks up into small droplets. At high relative velocities, in the "catastrophic" breakup regime, drops are flattened and fragmented by relatively large-wavelength waves whose wavelengths and growth rates are consistent with estimates from Rayleigh-Taylor instability theory. The minute drops that are also produced at these high relative velocities appear to originate from short-wavelength Kelvin-Helmholtz waves growing on the larger liquid fragments. At high gas temperatures the decreased surface tension and increased vaporization causes the bag to disappear earlier in the bag breakup regime, and causes shortened ligament lengths in the other breakup regimes.

MECHANISMS OF AIR-ASSISTED LIQUID ATOMIZATION

A well-controlled experimental apparatus was used to investigate the air-assisted liquid drop atomization process. High-magnification, high-speed photography as well as conventional spray field photography was used to study the breakup of a monodisperse stream of drops injected into a transverse high-velocity air stream. The gas velocity profile, liquid drop velocity, and size were determined by laser Doppler velocimetry (LDV) and a phase Doppler particle analyzer (PDPA). The experiments gave information about the microscopic structure of the liquid breakup process, drop breakup regimes, drag coefficients of atomizing drops, breakup drop size distributions, and drop trajectories. At low gas-liquid relative velocities, the microscopic photographs confirmed the existence of bag and stripping breakup regimes. The photographs also revealed the nature of high-speed, "catastrophic," liquid atomization, which appears to be due to the development of instability waves on the liquid surface. A model based on aerodynamic liquid breakup theory was used to study the influences of the drop drag coefficient and the breakup time on the drop trajectory during the drop atomization process. Previously proposed expressions for the drop drag coefficient and the breakup time constant were tested by comparing the calculated trajectories with the measurements.