Breakup Mode Research Articles

Primary breakup dynamics and spray characteristics of a rotary atomizer with radial-axial discharge channels

An experimental investigation of primary breakup dynamics and spray characteristics of a rotary atomizer with high aspect ratio radial-axial discharge channel is described. A high-resolution shadow imaging technique with pulsed backlight illumination was used for spray visualization. For the rotary atomizer with high aspect ratio discharge channel and radial-axial orientation, visualization showed the occurrence of Centripetal–Coriolis-induced stream-mode injection for all operating conditions. In this mode of injection, a crescent liquid film forms in the channel exit and issues from the orifice as a liquid column or a thin liquid sheet depending on atomizer operating conditions. It was found that the liquid Weber number value determines the state of the injected liquid stream (column or sheet) during the breakup process. The breakup process was studied with the variation of rotational speed and liquid volume flow rate. A morphological classification of the different modes of breakup is obtained from the flow visualizations. Four breakup modes have been observed for the present range of test parameters. These include Rayleigh, bag, stretched ligament, and shear breakup modes. These modes are separated by transitional regimes. The results suggest qualitative similarities between the primary breakup of the present rotary atomizer with a non-turbulent liquid jet and a fan like liquid sheet under a gas cross flow. The maximum liquid stream penetration height was measured for some operating conditions. It is concluded that this parameter is depend on the momentum flux ratio and liquid Weber number. To further evaluate features of the spray, the droplet size measurements have been conducted using the PDIA technique. The results indicate that there is a relatively strong correlation between the SMD values and liquid Weber number, while a properly non-dimensionalized droplet size is well correlated with the momentum flux ratio and Rossby number.

How boiling happens in nanofuel droplets

We report detailed analyses of evaporation and atomisation characteristics of nanofuel droplets in a contactless environment (acoustic levitation) under external radiative heating. Two base fuels, ethanol and n-dodecane with a significant difference in their respective vapour-pressures, are considered. Nanoparticles (NPs) of cerium oxides (CeO2) are utilised as nano-additives at a dilute particle loading rate (PLR) of 0%-0.5% by weight. Pure ethanol droplets vaporise at a faster rate than pure dodecane droplets and do not exhibit any secondary atomisation. However, pure dodecane droplets exhibit two modes of secondary breakup; Kelvin-Helmholtz instability induced stripping and catastrophic breakup beyond a certain threshold value of the initial droplet size. Nanofuel droplets of ethanol neither exhibit any significant change in the vaporisation rate nor exhibit secondary atomization. Contrarily, dodecane-based nanofuels show enhanced vaporisation due to heat absorption by nanoparticles and consequently different modes of secondary breakup. Interestingly, dodecane-based nanofuel droplets exhibit internal boiling induced atomization. A time scale analysis considering orthokinetic NP aggregation, evaporation lifetime, and bubble growth rate is presented to elucidate the mechanism of such internal boiling. The theoretical non-dimensional time scale (τ*) so coined is extended to estimate the minimum value of the droplet size necessary for exhibiting boiling. The analysis shows excellent agreement with the experimental observations. Furthermore, we propose a unique three-dimensional regime map to correlate the breakup modes with droplet sizes, PLR, and heating rates.

Inertia-driven jetting regimes in microfluidic coflows

Inertial breakup modes are identified at microscale for co-flowing jets using a high-pressure mixer.

Experimental investigation of spray characteristics of a liquid jet in a turbulent subsonic gaseous crossflow

Two perforated plates with different solidity ratios, S=50% and 67%, were used to investigate the effect of the velocity fluctuations of a subsonic gaseous crossflow on the spray characteristics of a liquid jet including droplet size and velocity distributions. The experiments were conducted over a range of jet-to-crossflow momentum flux ratio of q=16.5-172, and two gas Weber numbers of Weg=2.7 and 5.9, corresponding to the enhanced capillary breakup and bag breakup regimes, respectively. The experimental results of this study revealed that the distribution of droplets size associated with a turbulent and a uniform crossflow for each specific breakup regime were approximately identical. The bimodal and single peak distributions of droplets size, respectively, associated with enhanced capillary and bag breakup regimes were generally consistent with the literature reports. However, the transition of the liquid primary breakup regime from enhanced capillary to bag breakup mode was delayed in a turbulent crossflow compared to its uniform counterpart. The general behavior of droplets size-velocity profiles were also consistent with the literature reports. Nonetheless, complex variations in the distribution of droplets velocity when changing the crossflow turbulence intensity were observed and linked with the presence of instabilities on the liquid jet's surface. Finally, the present experiments allowed shedding more light on the reason why the breakup mechanisms of a liquid jet in a conventional uniform crossflow should not be generalized to predict the distinct breakup process of a liquid jet in a turbulent crossflow.

Droplets generator: Formation and control of main and satellite droplets

The continuous-ink-jet printing technology is of great importance in many fields of the industrial and scientific applications due to its low-cost, simplicity, and flexible to a wide variety of material, which entail important technological advantages over existing technology. The perturbation imposed on the surface of the jet-flow is one of the primary parameters to control droplets formation. Therefore, a simple method for producing steady disturbance is presented in this article for further development the technology of continuous-ink-jet printing. Using high-speed photography technology along with image processing, the formation of droplets are investigated at different nozzle diameter and disturbance frequency. The results showed that the droplets formation experienced different breakup modes at different nozzle diameters. The satellite droplet was produced by breakup of jet-tip at the nozzle diameter of 0.15 mm. When the nozzle diameter was increased to 0.33 mm, the satellite droplet was produced by breakup of the trailing of primary droplet. The vibration region of uniform droplets could be controlled by the change of the gas pressure. The wavelength of surface wave decreased with the rose of the disturbance frequency, and a model was established to illuminate the relationship between the disturbance frequency and the wavelength of surface wave. The diameter of droplets decreased with the rose of the disturbance frequency.

Prediction of Microdroplet Breakup Regime in Asymmetric T-Junction Microchannels.

Splitting droplets is becoming a major functional component in increasing number of droplet microfluidic applications, and there is an increasing interest in splitting droplets into two daughter droplets with different volumes. However, designing an asymmetric droplet splitter and predicting how a droplet splits in such designs is not trivial. In this study, numerical simulations were conducted to study droplet breakup in asymmetric T-junctions of square cross-sections having different pressure gradient ratios (i.e. T-junctions with outlet branches of different lengths). The goal of the simulation is to identify the conditions where a parent droplet breaks or does not break into two smaller droplets of different sizes (so called critical condition) and to identify the important fluid and microchannel parameters in this process. Four modes of droplet breakup (primary-, transition-, bubble-, and non-breakups) are identified and an empirical correlation is introduced that can predict the breakup/non-breakup of the droplet based on the parent droplet size and the capillary number. The simulation results are then compared with experimental data to verify its accuracy and the effect of fluids properties on the proposed correlation are studied. Two major asymmetric breakup mechanisms are determined, namely "breakup with permanent obstruction" and "unstable breakup". The numerical results show that the splitting ratio for the asymmetric breakup mechanisms depends on flow conditions and dwell time of the droplet at the junction prior to splitting. Finally, the results from two-dimensional and three-dimensional simulations were compared. It is shown that two-dimensional simulation may not accurately predict the breakup behavior for asymmetric droplet breakup and viscosity ration has a greater effect on the prediction critical condition.

Breakup modes of a planar liquid jet in a static electric field

Breakup process of a two-dimensional planar jet is analytically investigated when the jet is placed between two parallel sheath walls on which an external electric field is imposed. In the analysis, we consider the influence of finite relaxation time and convection of surface charges and tangential stress on the jet surfaces due to the parallel electric field. According to the leaky dielectric model and the slender jet approximation, nonlinear evolution equations for the jet are derived when the aerodynamic effect is neglected. The equations are solved numerically under the initial-boundary condition that the jet is emanating from a slit nozzle. It is found that there exist two types of breakup modes for symmetric deformations: (i) the surface tension dominant mode where a tip of the jet grows larger as the jet flows downward and (ii) the electric force dominant mode where the tip of the jet is accelerated and becomes thinner and thinner to the downstream. The existing regions of these two modes are mainly determined by two parameters (=charge relaxation time/time of fluid motion) and Λ (=electric pressure/fluid inertial). We find critical curves in the parameter space, across which the mode is transferred from (i) to (ii) with the increase of Λ and/or the decrease of . It is also found that antisymmetric deformations of the jet grow to the downward as long as the jet is thick, but the deformations are suppressed and stabilized as the jet becomes sufficiently thin.

Crown formation and atomization in burning multi-component fuel droplets

We report the observation of a crown-like sheet expansion and its subsequent atomization in burning multi-component fuel droplets. The droplets consisting of constituents with significant volatility differential are chosen in this study. Through homogeneous nucleation, the breakup of high-pressure vapor bubble results in the formation of radially expanding crown-like hemispherical sheet. The expansion of liquid sheet and the development of radial ligaments on its destabilized rim results in the creation of several secondary droplets. This pattern of breakup bears a close resemblance to the universally recognized Worthington-Edgerton crown configuration, which forms when a liquid droplet impact on a deep pool/thin film of same liquid or another liquid. Two distinct modes of liquid sheet fragmentation are identified during the experiments: (i) unstable sheet fragmentation and (ii) stable sheet fragmentation. In particular, the breakup characteristics of stable sheet mode is comparable to the commonly observed crown breakup by drop impingements. The modes of sheet breakup are primarily influenced by the proportion of volatile component in the mixture, which in turn dictates the onset of fragmentation and the intensity of breakup.

Numerical investigation of dynamic behavior of a compound drop in shear flow

We present a numerical investigation of the deformation and breakup of a compound drop in shear flow. The numerical method used in this study is a two-dimensional front-tracking/finite difference technique for representing the interface separating two fluids by connected elements. The compound drop with the initially circular and concentric inner and outer fronts is placed at the center of a domain whose top and bottom boundaries move in the opposite direction. Because of the shear rate, the compound drop deforms and can break up into drops, depending on the flow conditions based on the Reynolds number Re, the Capillary number Ca and the interfacial tension ratio σ21 of the outer to inner interfaces. We vary Re in the range of 0.1-3.16, Ca in the range of 0.05-0.6 and σ21 in the range of 0.8-3.2 to reveal the transition from the non-breakup to breakup regimes. Numerical results indicate that the compound drop breaks up into drops when there's an increase in Re or Ca or a decrease in σ21 beyond the corresponding critical values. We also propose a phase diagram of Ca versus Re that shows the region in which the compound drop changes from the deformation mode to the breakup mode.

Dual-mode nonlinear instability analysis of a confined planar liquid sheet sandwiched between two gas streams of unequal velocities and prediction of droplet size and velocity distribution using maximum entropy formulation

Twin fluid atomizers utilize the kinetic energy of high speed gases to disintegrate a liquid sheet into fine uniform droplets. Quite often, the gas streams are injected at unequal velocities to enhance the aerodynamic interaction between the liquid sheet and surrounding atmosphere. In order to improve the mixing characteristics, practical atomizers confine the gas flows within ducts. Though the liquid sheet coming out of an injector is usually annular in shape, it can be considered to be planar as the mean radius of curvature is much larger than the sheet thickness. There are numerous studies on breakup of the planar liquid sheet, but none of them considered the simultaneous effects of confinement and unequal gas velocities on the spray characteristics. The present study performs a nonlinear temporal analysis of instabilities in the planar liquid sheet, produced by two co-flowing gas streams moving with unequal velocities within two solid walls. The results show that the para-sinuous mode dominates the breakup process at all flow conditions over the para-varicose mode of breakup. The sheet pattern is strongly influenced by gas velocities, particularly for the para-varicose mode. Spray characteristics are influenced by both gas velocity and proximity to the confining wall, but the former has a much more pronounced effect on droplet size. An increase in the difference between gas velocities at two interfaces drastically shifts the droplet size distribution toward finer droplets. Moreover, asymmetry in gas phase velocities affects the droplet velocity distribution more, only at low liquid Weber numbers for the input conditions chosen in the present study.

Liquid jet breakup unsteadiness in a coaxial air-blast atomizer

The aim of this paper is to experimentally characterize the liquid jet breakup unsteadiness in a coaxial air-blast atomizer. The current research focuses on the measurement of the fluctuations of the jet breakup length and the flapping instability of the liquid jet, which contribute to the downstream fluctuations of the spray characteristics. The optical connectivity technique was used to measure the instantaneous breakup length of the water jet. Also, time resolved shadowgraph images of the primary jet breakup process were captured by high-speed imaging to characterize the jet instabilities at different axial locations from the atomizer exit. Experiments were performed for a wide range of air-to-liquid momentum flux ratio ( M) and aerodynamic Weber number ( Weg) corresponding to membrane- and/or fiber breakup mode of the jet disintegration process. The mean jet breakup length was found to vary inversely with M through a power law relation in agreement with the literature, while the breakup length fluctuations were found to first decrease and then increase with M. In order to capture the unsteady dynamics of the jet breakup process, the proper orthogonal decomposition analysis of the optical connectivity images was performed. The jet flapping and the fluctuations of the jet breakup length were identified as the second and the third spatial proper orthogonal decomposition modes, respectively, for all operating conditions of the atomizer. The amplitude and the frequency of the instabilities were measured by temporal tracking of the liquid–air interface on the shadowgraph images. The disturbance close to the injector exit corresponds to the Kelvin–Helmholtz instability, while close to the jet breakup point the jet exhibits the flapping instability, which is characterized by lateral oscillation of the jet about the atomizer axis. The influence of the liquid jet Reynolds number and momentum flux ratio on the KH and the flapping instabilities are examined.

Experimental investigation of the deformation and breakup of a droplet in high-speed flow

Experiment on the droplet deformation and breakup is performed in high-speed flow passaged by shock wave. Vibration deformation, shear stripping breakup and catastrophic breakup are obtained in the droplet breakup experiment. The deformation and breakup of droplet are recorded by high-speed photography. The morphologies of droplet in different breakup modes are analyzed quantitatively including the evolution of length and width respectively identified in streamwise and spanwise direction. At early time the development curves of droplets are almost overlapped indicating that droplets in three conditions are compressed by the flow in the same manner. As a result of the shear and stripping of flow, droplet breaks into fragments when the spanwise width increases to approximately three times of initial diameter. There is some similarity between shear stripping breakup and catastrophic breakup in the temporary evolution which indicates that catastrophic breakup mode has the same mechanism with shear stripping mode somehow. The analysis of experiment results supports the viewpoint that catastrophic breakup is not an independent mode. The results agree well with the types of breakup which are put forward by Theofanous based on breakup mechanisms. Thanks to the planar laser-induced fluorescence (PLIF) measurement, the distribution of atomized particles is visualized which shows that the increase of flow speed enhances the strength of vortex pair in the wake. The vorticity makes the area more unstable and accelerates the mixing between ambient air and atomized liquid. By the sufficient contact with hot air produced by shock wave, liquid in droplet is vaporized rapidly and becomes a bulk of droplet particles which are more easily to be blew off. Flow shear and stripping play an essential role in the breakup and atomization.

Breakup modes and criterion of droplet with surfactant under direct current electric field

The breakup behaviors of aqueous droplets with surfactant in oil under direct current (DC) electric field are investigated in this paper. Four different droplet breakup modes were observed, namely conical shape-conical jetting, ellipsoidal shape-conical jetting, ellipsoidal shape-filamentous breakup and cylindrical shape-filamentous breakup. The conversion of breakup modes was discussed with the electric capillary number (Ca)-surfactant concentration (CSDBS) phase diagram and the droplet elongation process. The mechanism validated here is that the lobes appear in all breakup modes, and the liquid volume at the both ends of droplet differentiate the breakup modes under different conditions. Different from previous studies, the criterion of droplet breakup modes was proposed based on the interfacial dynamics instead of the bulk phase properties. By analogy with stress–strain relationship, it is found that the electrostatic pressure shows a linear relationship with droplet specific surface area during the droplet elongation process, and the slope K could be employed to identify different breakup modes. These findings are of great significance to the promotion and application of electric-induced droplet breakup.

Using Planar Laser Induced Fluorescence to explore the mechanism of the explosive disintegration of water emulsion droplets exposed to intense heating

In this paper, we study the boiling of heated water emulsion droplets in the air flow at a temperature of 20–800°С. The relative volume concentration of the flammable components in the emulsion varies from 10% to 70%. We explore the unsteady temperature fields of droplets using a contactless optical diagnostic technique, Planar Laser Induced Fluorescence, with a cross-correlation system featuring a camera, a laser, a synchronizer, and the ActualFlow software. Rhodamine B acts as a fluorophore. We also use a high-speed video camera (up to 105 fps) and continuous automatic tracking algorithms (Tema Automotive software) to record the rates of heating and evaporation, as well as transformation of droplet surfaces. We demonstrate the unsteady temperature fields of droplets and three modes of their boiling and breakup. These differ in the number and dimensions of the emerging droplets as well as the durations of the main stages. The temperature differentials at the water – flammable component interface are determined corresponding to three boiling and breakup modes. We show that the third mode provides the greatest number of fine droplets (no less than 200) if the heating temperature exceeds 400°С and the concentration of the flammable component is over 66%. The temperature at the phase interface reaches 100 °C–125 °C before disintegration, and the droplet heating times before explosive breakup may vary from 0.1 s to 10 s. Finally, we analyze how the temperature, additive concentration and droplet size affect the conditions and characteristics of these modes.

Influence of rotary disk configurations on droplets characteristics in molten slag granulation for waste heat recovery

The influence of rotary disk configurations on droplets characteristics in molten slag ligament granulation was investigated by using the high-speed camera visualized system. Rosin/Paraffin mixture was adopted as the analogue of the molten slag according to the similarity theory. Four groups of disks with different configurations were studied. The droplets characteristics, such as droplets size, droplets size distributions, filament mass fraction and particle uniformity were analyzed. Generally, the droplets mean diameters (dm) decreases by increasing the rotary speed (ω) or disk diameter (D), and increases slightly by increasing the liquid flow rate (Q). Based on the experimental data, a simple correlation of dm for four types of disks is presented. The majority of the droplets size is populated in the range of 0.2–1.0 mm for all ω and Q, which accounts for over 70% mass fraction. The ω has significant influence on droplets size distribution and particle uniformity. The flat/arc-edge disk configurations can contribute to produce the narrower size range of droplets and obtain the superior particle uniformity compared to the other two types of disks. In the ligament breakup mode, the filament mass fraction reduces by increasing Q or decreasing ω. Meanwhile, the liquid film breakup mode also has significant impact on the formation of filament.

Transition Weber number between surfactant-laden drop bag breakup and shear breakup of secondary atomization

Fuel spray and atomization characteristics play an important role in the performance of internal combustion engines and many applications of industry. In drop aerobreakup, there are two main breakup modes: general bag breakup and shear breakup, which are induced by Rayleigh-Taylor instability and Kelvin-Helmholtz instability, respectively. Influence of surfactant on drop deformation and breakup is remarkable. So the transition condition of surfactant-laden drop between general bag breakup and shear breakup will be different from pure liquid. In this work, we investigate the influence of surfactant on drop breakup morphology in a continuous air jet stream, and find the physical mechanism of surfactant-laden drop atomization. Finally the predicted expression of transition Weber number is obtained.

Effects of unconventional breakup modes on incomplete fusion of weakly bound nuclei

The incomplete fusion dynamics of $^6$Li + $^{209}$Bi collisions at energies above the Coulomb barrier is investigated. The classical dynamical model implemented in the {\sc platypus} code is used to understand and quantify the impact of both $^6$Li resonance states and transfer-triggered breakup modes (involving short-lived projectile-like nuclei such as $^8$Be and $^5$Li) on the formation of incomplete fusion products. Model calculations explain the experimental incomplete-fusion excitation function fairly well, indicating that (i) delayed direct breakup of $^6$Li reduces the incomplete fusion cross-sections, and (ii) the neutron-stripping channel practically determines those cross-sections.

Three-body dissociation of OCS3+: Separating sequential and concerted pathways.

Events from the sequential and concerted modes of the fragmentation of OCS3+ that result in coincident detection of fragments C+, O+, and S+ have been separated using a newly proposed representation. An ion beam of 1.8 MeV Xe9+ is used to make the triply charged molecular ion, with the fragments being detected by a recoil ion momentum spectrometer. By separating events belonging exclusively to the sequential mode of breakup, the electronic states of the intermediate molecular ion (CO2+ or CS2+) involved are determined, and from the kinetic energy release spectra, it is shown that the low lying excited states of the parent OCS3+ are responsible for this mechanism. An estimate of branching ratios of events coming from sequential versus concerted mode is presented.

Colloidal stability dictates drop breakup under electric fields.

Electric fields can deform drops of fluid from their equilibrium shape, and induce breakup at sufficiently large field strengths. In this work, the electric field induced breakup of a squalane drop containing a colloidal suspension of carbon black particles with polyisobutylene succinimide (OLOA 11000) surfactant is studied. The drop is suspended in silicone oil. The breakup mode of a drop containing carbon black depends strongly on the suspension stability. It is observed that a drop of a stable suspension of carbon black has the same breakup mode as a drop with surfactant alone, i.e., without added carbon black. At lower electric fields, these drops break by the formation of lobes at the two ends of the drop; and at higher fields the homogeneous lobes break in a non-axisymmetric manner. However, a drop of an unstable suspension shows a drastically different breakup mode, and undergoes breakup much faster compared to a drop with surfactant alone. These drops elongate and form asymmetric lobes that develop into fingers and eventually disintegrate in an inhomogeneous, three-dimensional fashion. As a basis for comparison, the breakup of a pure squalane drop, and a squalane drop with equivalent surfactant concentrations but no carbon black particles is examined. Axisymmetric boundary integral computations are used to elucidate the mechanism of breakup. Our work demonstrates the impact of colloidal stability on the breakup of drops under an electric field. Colloidal stability on the time scale of drop deformation leads to rich and unexplored breakup phenomena.

Liquid jet in crossflow – Effect of liquid entry conditions

The focus of the present article is to study the effect of liquid jet injection entry conditions on the structure of liquid jet in crossflow (JICF). The experiments are conducted over a range of liquid-to-air momentum ratios (Q ∼ 3–100) and aerodynamic Weber numbers (We = 17–89). Control over the exit conditions of the injected liquid is achieved through the usage of different L/D ratios of the nozzle of the plain-orifice atomizer. The geometrical parameter L/D is varied between 10 and 100 in order to obtain fully-developed laminar flow, transition and turbulent flow. High-speed imaging and Shadowgraphy are used to study the trajectory, drop-sizing and transient behaviour of the resultant spray. It is observed that the dependence of trajectory of the spray is not just limited to the momentum ratio, Q, but also requires correction factors with respect to the injection entry conditions, which are in turn related to L/D. The trajectory of the turbulent jet is found to be lower at all times when compared to that of a laminar jet for the corresponding conditions. This behaviour may be attributed to the inherent instabilities present in a turbulent jet as opposed to a perfectly laminar jet. Turbulent jets also observed atomize better, producing smaller droplets when compared to their laminar counterparts. Further, we also investigate the transient phenomena of the liquid jet breakup at different conditions with the aid of Proper Orthogonal Decomposition (POD) analysis. Distinct modes of breakup are captured for the laminar and turbulent cases.