Breakup Mode Research Articles

Experimental study of a dental airotor cooling spray system

In this paper, we have characterized a spray system used in dental airotors. Experimental data with respect to droplet size and velocity are generated at different locations in the spray. The impact dynamics of the spray upon substrates are also analyzed. The breakup modes have been identified in the system, and appropriate physical insights into the dynamics are provided. The impact of the spray upon both the hard substrates results in highly rebounding daughter droplets, which can contribute to bio-aerosols. The risks of cross-contamination due to aerosol can thus be prevented with appropriate modifications of the spray nozzles.

Experimental Study on Breakup Characteristics of Hollow Cone Spray in Gas Crossflow

Hollow cone spray with a medium spray half-cone angle (in the range of 36°–45°) exposed to a gaseous crossflow is a situation that has been extensively investigated experimentally. In this experiment, flow visualization techniques are employed to examine the initial liquid sheet thickness at the nozzle orifice exit and the sheet breakup process. The dynamics of the primary breakup of the sheet is analyzed based on the observations of the sheet breakup in the near-nozzle region. The experimental results show that the breakup mechanism of the liquid sheet varies greatly depending on liquid centrifugal momentum and aerodynamic force. Four distinct breakup modes are observed, and the breakup regimes are mapped in the parametric space based on the liquid Weber number (84–2175) and the liquid-to-air momentum ratio (22–8455). It is observed that the leeward side of the sheet is dominated by shear oscillation breakup within the range of the liquid-to-air momentum ratio less than 600. The influences of the aerodynamic Weber number on breakup frequency and the number of surface waves observed on the leeward side sheet are discussed by analyzing the spray images. The breakup-regime transition criteria for hollow cone spray in crossflow and the trajectory of spray in crossflow are proposed by employing a multivariable regression method.

Impingement and breakup characteristics of free opposed impinging jets with unequal nozzle diameter

The free opposed impinging jets with unequal nozzle diameter is developed to improve mixing efficiency at unequal flowrates for liquid–liquid reactions. The liquid sheet breakup characteristics and droplet behavior are investigated using particle image velocimetry. Water, 40w% glycerol-water (40G), and 50% glycerol-water (50G) under different jet velocities and nozzle diameters are studied for comparison, the correlation equations are obtained and the errors are analyzed. When the jet velocity increases, the liquid sheet breakup mode exhibited a closed-rim mode, open-rim mode, rimless mode, wave or ligament mode, and fully developed mode for water and 40G under a nozzle diameter of 1.5–2 mm, and the first four modes were observed with the high viscosity liquid (50G) or under a large nozzle diameter (2–3 mm). As the jet velocity increases, the dimensionless breakup length increases sharply at the closed-rim and open-rim modes, and then decreases at the rimless mode. The sheet thickness, Sauter mean diameter, average droplet diameter, and droplet velocity decrease, and the droplet size becomes more uniform with increasing jet velocity. However, the opposite variation trend is observed with increasing viscosity and nozzle diameter, and the effect is more significant at u < 4.25 m/s. The droplet average diameter increases at u = 2.12 m/s and 3.18 m/s and decreases at u ≥ 4.25 m/s downwards along y-axis due to the coalescence and secondary breakup of droplets. The droplet velocity becomes lower as droplets move downwards along y-axis from −40 mm to −50 mm due to the dissipation of momentum. The results reveal that the increase of viscosity and nozzle diameter has a negative effect on liquid sheet and droplet behavior at low jet velocities. The rimless mode is the key mode affecting the transition of the sheet breakup mechanism, and after this mode, the negative effects of viscosity and nozzle diameter weaken due to the high momentum, which promotes the breakup of the liquid sheet and droplets. It will provide theoretical support for the mixing mechanism with unequal flowrates, and is helpful to expand the application of free opposed impinging jets.

Understanding the breakup behaviors of liquid jet in gas atomization for powder production

Gas atomization (GA) is the main method to produce metal powders for additive manufacturing (AM) because of its low cost and high efficiency. However, the liquid breakup behaviors in high-speed gas flow during GA remains unclear, especially the primary breakup in the near field and the secondary breakup in the far field. Great difficulty exists in in-situ observation of the interactions between high-speed gas and high-temperature molten metal because of the limitations of the enclosed environment. Here, we built a new GA simulation system with a close-coupled atomizer utilizing liquids with low melting temperature, such as water, glycerin, etc. We use a high-speed camera to capture the evolution of liquid behaviors at various parameters. We first reveal that four primary breakup modes and four secondary breakup modes exist in the GA process, and these breakup modes significantly influence the particle size distribution (PSD) and the defects of the powder. Besides, the breakup modes are clarified by the dimensionless analysis. This manuscript provides an effective experimental platform to understand the breakup behaviors of the liquid jet in GA and suggests the optimal breakup mode for powder production.

Conical breakup of droplets in dielectric liquid medium

The conical breakup behavior of droplets in immiscible dielectric liquids under non-uniform alternating current (AC) electric fields is experimentally investigated using high-speed photography. The optical-microscopic morphological characteristics of the dynamic behavioral process of charged droplets under different electric field conditions are precisely captured. Two distinct conical breakup configurations, called the trident vibration and lotus vibration modes, are proposed, and their detailed structural parameters are discussed. The extremely small stable conical meniscus (semi-angles range of 11°–18.6°) is found in the liquid–liquid system. The distribution of cone angles under various spatial conditions is determined. In addition, the multilevel asymptotic characteristics of conical fragmentation are found due to the variances in spatial and temporal scales. The diameters of the droplets produced under the cone-jet mode could be as small as a few micrometers. The differences between the droplet breakup mode of AC and DC electric fields are also discussed. Overall, several interesting features are reported for the axisymmetric multi-cone broken morphology when droplets undergo breakup.

Macaroni-type breakup in the primary breakup of discontinuous shear-thickening suspension

The primary breakup of discontinuous shear-thickening suspension is studied experimentally. A divergent nozzle was used as the annular passage of a coaxial two-fluid nozzle. The breakup morphology was observed with a DSLR camera and a high-speed camera. According to the experimental results, the discontinuous shear-thickening behavior and the geometry of the annular air nozzle of divergent section significantly affect the breakup morphology of the suspension jet. Under the action of the annular air, the suspension jet near the exit of the nozzle tends to expand radially and becomes hardened and incompressible in the axial direction. For discontinuous shear-thickening suspensions, the axial hardening does not affect the good fluidity in the radial direction because of their anisotropic microstructure. Therefore, a hollow structure appears, and the suspension jet exhibits the macaroni-type breakup. This breakup mode strengthens when the radial expansion increases. Based on the equation of continuity, the force analysis and mechanism of the macaroni-type breakup are discussed.

Experimental study on nucleation and micro-explosion characteristics of emulsified heavy fuel oil droplets at elevated temperatures during evaporation

The application of emulsified fuels can improve atomization quality which mainly benefits from the micro-explosion phenomenon. To explore bubble behaviors and droplet micro-explosion characteristics of emulsified heavy fuel oil (HFO) droplets during evaporation, the suspended droplet method was adopted. A water content of 0–30 % was used at atmospheric pressure and ambient temperatures of 573–873 K. Three trigger conditions for heterogeneous nucleation in the emulsified HFO droplet were proposed based on the properties of emulsified HFO. The dual-mode micro-explosion of emulsified HFO was defined as local and global micro-explosion. It is of great interest to mention that dual-mode micro-explosion accompanied by a honeycomb-like structure was clarified for the first time. Based on the experimental results, the working conditions of the droplet breakup modes and the appearance of a honeycomb-like structure were summarized. A regression formula was introduced to express the dependence of micro-explosion intensity on ambient temperature and water content. The temperature characteristic curve of the evaporation process presented an obvious plateau, which was a typical feature of emulsified HFO droplets in the evaporation process. Secondary droplets that entrapped bubble nuclei inside were also captured with an occurrence of droplet micro-explosion. This experimental investigation on bubble behaviors and micro-explosion characteristics during evaporation is expected to provide guidance for the application of emulsified HFO.

Understanding droplet breakup in a post-array device with sheath-flow configuration.

Microfluidic post-array devices have the potential to generate quasi-monodisperse emulsion droplets with high throughput and dispersed phase fractions by splitting droplets with regularly arranged posts. However, the lack of understanding of post-array devices makes it challenging to predict droplet size and quantitatively evaluate the influence of post geometry, hindering their widespread application. Therefore, we investigated the characteristics of droplet breakup through a post array using a series of devices with sheath-flow configurations, in which the dispersed and continuous flow rates could be flexibly tuned. Using a poly(dimethylsiloxane)-glass device fabricated via soft lithography, we found that the volume ratio of the dispersed phase to the continuous phase significantly affects the droplet size, even when the viscosity ratio is close to one. For the first time, we demonstrated that the effective capillary number calculated from the emulsion viscosity and effect of the dispersed phase fraction consistently describes various experimental results. Furthermore, our flow observations and droplet diameter measurement showed two breakup modes: the size-constant obstruction and shear-induced modes with a power-law correlation similar to droplet splitting in a T-junction. Thus, the power-law correlation in microfluidic droplet splitting successfully expresses the droplet generation characteristics in post-array devices. A combination of effective viscosity correction and curve fitting allowed us to evaluate the influence of the material and post-geometry on droplet generation characteristics. This study contributes to the understanding of droplet breakup in post-array devices and extends their unique droplet generation properties to include high-throughput, high-fraction, robust, and continuous emulsification processes.

Thermal nonequilibrium and mechanical forces induced breakup and droplet formation of superheated liquid jets under depressurized release

Superheated liquid jets disintegrate into numerous droplets when released into the ambient with lower saturated pressure, driven by thermal nonequilibrium induced flashing and the accompanying mechanical forces. Such a phenomenon facilitates fuel atomization in energy utilization while posing a serious threat during accidental releases. In this work, the breakup and droplet formation of superheated liquid jets under depressurized releases were investigated with an experimental 20 L tank. A high-speed camera was utilized to characterize breakup behaviors. The interaction between thermodynamic and mechanical effects during depressurization was discussed based on linear stability analysis and bubble dynamics. Furthermore, the quantitative relationship between the two driving effects under different conditions was established using dimensionless and multiple regression analyses. Results show that the thermodynamic effect increases with the decreased mechanical effect during depressurization because of the increased energy of bubble burst, regardless of the external or internal flashing regime. Non-flashing, partially flashing, and fully flashing breakup modes are identified. The dimensionless and multiple regression analyses show that in addition to thermodynamic (Ja, ρv/ρl, Rp, and ηp) and mechanical (Wev and Oh) effects, the inhibition induced by the cooling effect (Pr and Ec) should not be overlooked. The quantitative expression among them agrees well with experimental data with R2 = 0.976.

Electrohydrodynamic instability and disintegration of low viscous liquid jet

The simple-jet mode can be developed from dripping, dripping faucet (transition), and jetting when subjected to a sufficient strong electric field caused by a needle-plated electrode. In the present work, the instability and disintegration of an electrified jet of anhydrous ethanol in the electrohydrodynamic simple-jet mode are visualized by a high-speed camera. A systematic analysis and description on the evolution of the breakup morphology of the charged jet in the simple-jet mode was carried out. In the dripping mode at low Weber number, the unstable simple-jet occurs, while the stable simple-jet mode happens as Weber number higher than critical value. In a dripping faucet and jetting regime, the simple-jet mode can easily take place, and jet undergoes typical breakup modes including varicose, whipping, whipping assisted bifurcation, and ramified instabilities as electric potential increases. The growth rate of the maximum perturbation of the zero-, first-, and second-order surface waves determines the type of the instabilities. The operating window of the simple-jet mode is presented to indicate that the simple-jet mode only operates in the suitable range of Weber number and electric Bond number. The spray characteristics, including envelope angle, droplet size, and the stable length of the electrified jet, have been explored to demonstrate that the uniform drops could be generated in the simple-jet regime. The evolution from the cone-jet to the simple-jet mode is also observed when a special hemispherical nozzle is used. For an almost stable electric potential, the cone-jet can gradually transform into the simple-jet with an increase in the liquid flow rate.

Improved Quantum Molecular Dynamics Model and Its Application to Ternary Breakup Reactions

Collisions of very heavy nuclei 197Au+197Au at the energy range of 5–30 A MeV have been studied within the improved quantum molecular dynamics (ImQMD) model. A class of ternary events satisfying a nearly complete balance of mass numbers is selected and we find that the probability of ternary breakup depends on the incident energy and the impact parameter. It is also found that the largest probability of ternary breakup is located at the energy around 24 A MeV for the system 197Au+197Au. The experimental mass distributions and angular distributions for the system 197Au+197Au ternary breakup fragments can be reproduced well by the calculation with the ImQMD model at the energy of 15 A MeV. The modes and mechanisms of ternary and quaternary breakup are studied by time-dependent snapshots of ternary events. The direct prolate, direct oblate, and cascade ternary breakup modes, are manifested and their production probabilities are obtained. The characteristic features in ternary breakup events, three mass-comparable fragments, and the very fast, nearly collinear breakup, account for the two-preformed-neck shape of the composite system. The mean free path of nucleons in the reaction system is studied and the shorter mean free path is responsible for the ternary breakup with three mass comparable fragments, in which the two-body dissipation mechanism plays a dominant role.

Effects of shear stress on the behavior of volatile bubbles in the high‐density polyethylene melt

AbstractA novel visualization system, in which polymer melt is sheared by a sliding plate, was established to simulate different pressure shear zones of the screw extruder. In this work, the nucleation, growth, coalescence, and breakup of n‐hexane bubbles in the high‐density polyethylene (HDPE) melt under shear stress were observed through a high‐speed camera. It was found that the nucleation rate of bubbles in the HDPE melt increased with the shear rate. Moreover, the bubble diameter at high‐pressure shear zones decreased with time until bubbles disappeared but increased with time at low‐pressure shear zones. A modified prediction model of bubble coalescence time at high‐pressure shear zones was established, and the relative deviation was less than 1%. Three bubble breakup modes, that is, tensile breakup, erosive breakup, and breakup at the gas–liquid interface, were observed under shear. Finally, a new mechanism of foam devolatilization in the screw extruder was proposed.

The shattering of abrupt shear thickening suspension jet in an air-blast atomizer

The primary breakup process of shear thickening is observed via high-speed photography. It is indicated that discontinuous shear thickening (DST) suspension jets exhibit the unique shattering-type breakup mode when air velocity increases up to a high region. In the shattering-type breakup mode, solid-like cracks grow inside the suspension jets, which results in the shattering of the suspension jets. After the shattering, the suspension jets break into tiny fragments with granular and matte surfaces. Combined with the rheological measurement and force analysis, the shattering-type breakup of DST suspension is due to aerodynamic force overwhelming the maximum shear stress. The critical Weber number Wec and a dimensionless number N studied in our previous research (Wang et al., 2022) are used to divide different breakup modes.

Breakup of a laminar liquid jet by coaxial non-swirling and swirling air streams

This paper describes an experimental study on shear-based spray formation. A laminar liquid jet was ejected inside co-annular non-swirling and swirling air streams. The aerodynamic Weber numbers (WeA) and swirl numbers (S) of the flow cases ranged from 4 to 1426 and from 0 to 3.9, respectively. High-speed shadowgraphy was utilized to obtain data on the first droplet locations, breakup lengths of the liquid jets, and two-dimensional wave spatiotemporal spectra for the jets. In order to detect the large-scale instabilities of the central liquid jet, proper orthogonal decomposition (POD) was performed on the high-speed shadowgraphic images. Stereo particle image velocimetry was utilized to investigate the annular air flow fields with S in the range of 0–2.5. It was found that air swirl promotes the morphological development of the jets with S in the range of 1.2–2.5. Both the breakup length and axial distance between the first droplet separation and the nozzle exit reduce as WeA and S increase. Scaling of the first droplet locations and breakup lengths is also evaluated in this paper. In terms of the air flow fields, radial expansion of the annular swirling air jets was observed, and the annular swirling jets expand radially further as S goes up. Central reversal air flows appear near the nozzle exit when S≥1.2, and some small droplets are blown upward to the nozzle exit by these central reversal air flows. In terms of large-scale instabilities, flapping is the dominant instability across most of the flow cases (as revealed by the first POD mode). Wavy and explosive breakup appear as the secondary breakup modes when WeA is low (≤110). In the absence of the central reversal air flows, the temporal frequencies of the instabilities of the air–water interfaces increase as S goes up. It was found that the central reversal air flows tend to stabilize the air–water interfaces. The spatial frequencies of the instabilities of the air–water interfaces remain low (≤0.06 mm−1) across all the flow cases, which produce long-wave structures.

An experimental study on the influence of airflow temperature on the different silicone oil droplet breakup regimes

In industrial environments, droplet deformation and breakup in hot airflow are universal. The effect of airflow temperature (293–473 K) on the breakup of silicone oil droplets in continuous airflow was studied. The breakup morphology, deformation stage, and breakup regime map are discussed in detail by varying the airflow temperature and Weber number in this paper. The results show that the breakup regimes of silicone oil droplets are consistent with water droplets in the Weber number (5 &amp;lt; We &amp;lt; 30) considered in this paper. However, airflow temperature has an obvious effect on droplet breakup modes. The number of nodes generated at the toroidal rim of the silicone droplet is verified to be the same as that of the water droplet, following “the combined Rayleigh–Taylor /aerodynamic drag” mechanism. The rising airflow temperature will increase the droplet cross-stream diameter. The maximum cross-stream diameter relationship for droplet breakup at different temperature airflows is proposed. The liquid bag growth time can accurately describe the droplet breakup characteristics in a hot airflow environment. The correlation equation for the time characteristics is proposed. In addition, the droplet breakup angle is used to describe the space range of droplet breakup. Eventually, special attention is paid to sub-droplets. The airflow temperature has a considerable effect on the sub-droplets. The average number of sub-droplets and the average surface area ratio before and after breakup increased as the airflow temperature increased. However, the number of sub-droplets and surface area increase slowly after the airflow temperature increases to a certain level.

Mode of continental breakup of marginal seas

AbstractWe investigated the continent-ocean transition (COT) structure of three main marginal seas in the western Pacific Ocean (South China Sea, Coral Sea, and Woodlark Basin) to determine the tectono-magmatic processes acting during continental breakup. The COT formed from the activity of a low-angle normal fault system localizing deformation during final rifting. Extension was contemporaneous with magmatic activity, including volcanic edifices, dikes, and sills in the distalmost parts of these basins. The COT shows a sharp juxtaposition in space and time of continental crust against igneous oceanic crust, and its overall structure differs from that of magma-poor or magma-rich passive-margin archetypes. We propose that this mode of breakup is characteristic of marginal seas due to the high extension rates imposed by kinematic forces of nearby subduction zones. Revealed in the context of marginal seas, this mode of breakup and the resulting COT structures highlight the underestimated diversity of continental breakup mechanisms.

Impingement Atomization of Carbopol Gels

Experiments were performed on the atomization of Carbopol–water gels of compositions in the range 0.10–0.30 gellant wt.% on a like-on-like jet impingement configuration. Analysis indicates two primary modes of sheet breakup: perforation driven and wave driven. While perforation-driven breakup is not seen for the lowest gellant concentration case, 0.10 wt.%, the wave-driven breakup is present in all cases within the range of investigated jet velocities. Based on the observations, a mechanism involving feedback between the impingement point and the sheet vertex, and a separation front initiated in between the impingement point and the sheet vertex are proposed for the wave-driven breakup.

Spray trajectory and 3D droplets distribution of liquid jet in crossflow with digital inline holography

Liquid jet in crossflow (LJCF) has a wide application in the actual engine engineering. This article presents an experimental investigation of a water jet transversely injected into a subsonic crossflow through digital inline holographic imaging, and presents the relationship between the column trajectory and the downstream droplet distribution. A phenomenological analysis based on high-speed digital inline holography (DIH) with the frequency of 25 kHz is presented to interpret the source of droplets with different sizes in the bag breakup mode and the shear breakup mode. High-resolution DIH with a spatial resolution of 5.5μ m is applied to measure breakup point, droplet size, and location under 30 mm downstream of the orifice. The experiment is carried out under normal temperature and pressure. Gas Weber number Weg varies from 11 to 67, and the liquid to gas momentum ratio q changes from 10 to 28, which are mainly under bag breakup and multi-mode breakup regime. Crossflow velocity profiles are also measured. Liquid penetration were obtained and fitted through spray pattern under x/dj<10 and through downstream droplet statistics under 12<x/dj<70 separately. Under the cases studied, spray penetration evaluated by droplet statistics is larger than that evaluated through spray pattern. In addition, droplet size is relative large in the core region of the spray under higher Weg, possibly due to the presence of vortices in the core region and stronger aerodynamic effects at the periphery of spray. This research presents the quantification of the primary breakup process and corresponding three-dimensional (3D) downstream SMD distribution simultaneously, which helps improve the understanding of spray evolution in crossflow.

Effect of conductivity on the mechanism of charge ejection in Rayleigh breakup of a charged drop

A critically charged drop is known to undergo Rayleigh fission by forming oppositely directed symmetric jets, which further disintegrate into progeny droplets. In this work, we present numerical simulations based on the axisymmetric boundary integral method and show that the electrical conductivity of the droplet not only affects the characteristics of the progeny droplets but also modifies the mode of drop breakup. It is observed that, at higher conductivity, the droplet undergoes fission by jet detachment from the mother drop, losing higher charge, while at lower conductivity, the jet relaxes back after ejecting a few progeny droplets that carry a lower charge.

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.