Breakup Regimes Research Articles

Aerodynamic breakup of emulsion droplets in airflow

AbstractAerodynamic breakup refers to the process where large droplets are fragmented into small droplets by the aerodynamic force in airflow, which plays a vital role in fluid atomization and spray applications. Previous research has primarily concentrated on the aerodynamic breakup of single‐component droplets, but investigations into the breakup of emulsion droplets are limited. This study experimentally investigated the aerodynamic breakup of water‐in‐oil emulsions in airflow, utilizing high‐speed photography to observe the breakup process and digital in‐line holography to measure fragment sizes. Comparative analyses between emulsion droplets and single‐component droplets are conducted to examine the breakup morphology, breakup regime, deformation characteristics, and fragment size distributions. The emulsion droplets exhibit higher apparent viscosity and shorter stretching lengths of the bag film and peripheral rim due to the presence of a dispersed phase. The breakup regime transitions of emulsions are modeled by integrating the viscosity model of emulsions and the transition model of the pure fluid. The fragment sizes of emulsion droplets are larger due to the shorter lengths of the bag film and peripheral rim.

Breakup regimes and heat transfer of an isolated bubble and Taylor bubble flow in the T-type microchannel

Current research on Taylor bubble flow heat transfer mainly focuses on straight channels, with less emphasis on the transport characteristics and heat transfer regimes of Taylor bubble flow in branching channels. Understanding the flow and heat transfer regimes of Taylor bubble flow in branching channels is crucial for enhancing microfluidic and microchemical applications. Numerical simulations can provide detailed flow information that experiments might miss, deepening our understanding of Taylor bubble flow for enhanced heat transfer. The research results indicate the presence of three bubble breakup regimes at the T-junction: Non-breakup (NB), Tunnel breakup (TB), and Obstructed breakup (OB). The phase diagrams were established based on Capillary number and bubble length to predict the transitions between these three breakup regimes. Pressure within the channel increases with bubble deformation but decreases after breakup. Bubble breakup converts potential energy to kinetic energy, enhancing the heat transfer coefficient in the branching microchannels. The stagnation of bubbles at the T-junction temporarily reduces the heat transfer coefficient but increases after the breakup. Up to 115 % of the best performance improvement was achieved for Taylor bubble flow when the channel width ratio was 1.2. When the channel width ratio was greater than or equal to 1, the TB regime influenced bubble breakup within the channel, resulting in asymmetric breakups and uneven heat transfer. When employing tree-like branched microchannels and Taylor flow for enhanced heat transfer, a channel width ratio of 0.8 is recommended.

Universal Correlation for Droplet Fragmentation in a Microfluidic T-Junction.

Despite extensive research on droplet dynamics at microfluidic T-junction, for different droplet lengths and Capillary numbers, there remains limited understanding of their dynamics at different viscosity ratio. In this study, we adopt a modeling framework in a three-dimensional (3D) configuration to numerically investigate the droplet dynamics as it passes through a symmetric T-junction with varying Capillary numbers, droplet lengths, and viscosity ratios. We present a 3D regime map for the first time to demarcate the droplet breakup and no breakup regimes. Herein, we propose a simple surface equation accounting for the critical Capillary number for breakup, in terms of viscosity ratio and dimensionless droplet length. The proposed universal relationship aligns well with experimental and computational findings from the existing literature. Furthermore, we reveal a new droplet breakup characteristic at high viscosity ratio and high Capillary number where the droplet spreads almost twice its initial value before splitting. Overall, this research provides comprehensive understanding of droplet dynamics at the T-junction and has significant implications for several related applications, including the large-scale synthesis of microdroplets using microchannel networks.

Numerical simulations of secondary atomization characteristics of shear-thinning kerosene gels

Owing to their unique non-Newtonian characteristics, gel propellants have become a new type of fuel with promising applications in the aerospace industry. However, the complex rheological properties and high viscosity of gels pose significant challenges for flow control in gel engines, particularly in terms of fuel atomization and combustion efficiency. Focusing primarily on kerosene gel, this study investigates the secondary atomization characteristics of shear-thinning droplets, where the volume of fluid method is employed to capture interface dynamics on adaptive grids, and the viscosity of the gel is described by a power-law model. The accuracy of the numerical model is validated by comparing it with the experimental observations of the breakup process of 1% silica kerosene gel droplets. Numerical simulations are conducted to analyze the droplet breakup processes and formation mechanisms under bag, multimode, and shear breakup. Compared to kerosene, the apparent viscosity distribution inside the kerosene gel droplets is non-uniform, and the inhibition of viscous force delays the breakup time and reduces the droplet deformation, thereby affecting the transition of the breakup mode. By varying the Weber number, We, the rheological parameters, and the density ratio, the quantitative effects of these factors on droplet deformation, centroid velocity, average viscosity, the breakup time, etc. are revealed. The results indicate that the breakup becomes more severe with increasing We number, resulting in larger deformation and a greater number of smaller droplets; the rheological parameters significantly affect the droplet breakup by altering the fluid viscosity; moreover, under the same flow condition, the higher the density ratio, the more difficult it is for the droplet to break up; however, on dimensionless scales, the effect of density ratio appears weak. Additionally, a breakup regime diagram is constructed in the We–Oh (Ohnesorge) number space for low Weber numbers. This study validates the numerical method for simulating the secondary atomization of kerosene gel propellants, providing a thorough analysis of three breakup mechanisms and the impact of key parameters on kerosene gel droplets. These insights offer a theoretical foundation for the optimization of gel propellants, aiding in the development of efficient and stable propulsion systems.

Time-Resolved Imaging Of Wavy Interface In The Primary Atomisation Region Of An Air Assist Atomiser Using An Event-Based Camera

The current work discusses the demonstration of a low-cost event-based camera for time-resolved imaging (10000 frames/sec) of a primary atomization zone in canonical air-assist atomizers. Experiments have been conducted simultaneously with traditional high-speed and event-based cameras, enabling us to quantitatively assess the potential of event-based cameras in spray imaging applications. Three atomization breakup regimes are considered: columnar, bag, and multimode. Dynamic Mode Decomposition (DMD) is implemented over the instantaneous data sets acquired from both cameras to assess their performance in extracting turbulence statistics. The obtained DMD modes from both cameras are similar, highlighting the potential of low-cost event-based cameras in extracting coherent structures and their spectral contents. Finally, the limitations (e.g., event saturation) of event-based cameras in the context of primary atomization are also discussed.

Effect of elevated crossflow temperature on jet primary atomization

Liquid jet in hot gas crossflow is widely employed in industrial combustion devices, especially in the propulsion systems. In this work, the effect of elevated crossflow temperature on the primary atomization of a liquid jet is experimentally investigated, including breakup regime, surface wavelength, column breakup height, and near-field trajectory. The experiments are conducted at crossflow temperatures of 300 K and 500 K, with gas Weber number ranging from 8.82 to 67.55 and momentum flux ratio from 10 to 50. The gas Weber number and momentum flux ratio are kept constant via increasing the crossflow velocity when crossflow temperature increases. The results show that the elevated crossflow temperature weakens surface breakup, particularly at high momentum flux ratios, which makes the transition of breakup regime from column breakup to surface breakup require higher gas Weber number or momentum flux ratio. Besides, the elevated crossflow temperature leads to a slight increase in surface wavelength, column breakup height and near-field trajectory, with the increase in trajectory being more pronounced at lower gas Weber numbers. Finally, an empirical correlation for the near-field trajectory is obtained, including the effects of crossflow temperature, gas Weber number, and momentum flux ratio.

Multi-Scale methodology of breakup and atomization for liquid jets in crossflow

Liquid jets in crossflow represent a highly complex phenomenon, characterized by intricate interactions across multiple temporal and spatial scales spanning orders of magnitude. To better understand the atomization mechanisms and characteristics of liquid jets in crossflow, a hybrid multi-scale methodology, integrating volume of fluid and discrete phase models with an enhanced coupling algorithm based on Large Eddy Simulation, has been developed and implemented as a multiphase solver in OpenFOAM. This methodology enables the comprehensive reproduction of the atomization process, including primary breakup and secondary atomization, obviating the requirement for complex tunable atomization models. For further reducing computational costs, a parallel acceleration technology is proposed by integrating the adaptive mesh refinement with dynamic load balancing into the solver. The multi-scale methodology is validated against experimental data regarding the trajectory of the liquid jet, as well as the distribution of droplet sizes and velocities. The validated methodology is then used to predict the behavior of Jet-A in crossflow. Simulation results demonstrate that, under the multi-mode breakup regime, large droplets are formed through the deformation of ligaments, small droplets are stripped from threads, and even smaller droplets are generated after secondary breakup. In contrast, in the shear breakup mode, large droplets are directly squeezed from the sides of the liquid column, while smaller droplets are stripped from ligaments and threads. Additionally, compared to the multi-mode breakup, droplet diameters are smaller in the downstream field for the shear breakup mode.

Non-circular pressure swirl nozzles injecting into stagnant air

As far as we are aware, all the previous studies have focused on circular injectors. Here, the effect of non-circular orifice shape on pressure swirl injector was investigated experimentally. For this purpose, three injectors; circular, square and rectangular were manufactured. The nozzles had the same cross-sectional area. Water was used as the working fluid and tests were conducted at room temperature and pressure. The Weber number varied from 484 to 1936. Flow parameters of the swirl jets were obtained by an in-house image processing code. The measured parameters were: discharge coefficient, spray angle, breakup length, lateral spread, SMD of drops and wavelength of surface waves formed on the cone. The influence of nozzle shape on all these parameters are discussed.A new breakup regime morphology for non-circular pressure swirl nozzles was defined. At lower Weber numbers, the air core was trapped inside the liquid sheet and relatively large ligaments were formed and separated from the edges (fiber breakup regime). As the Weber number was increased Rayleigh-Taylor instability produced cell-like structures near the nozzle exit. These cell-like structures eventually broke up at the thickened edges of the cells and formed Christmas tree like structures (Christmas tree regime). We found that all the measured flow parameters can best be correlated as a function of Reynolds number and Weber number based on liquid sheet thickness at nozzle exit. All these correlations were reported. Breakup length and lateral spread were heavily influenced by the breakup regime. In fiber breakup regime the breakup length and lateral spread increased with increasing Weber number. However, in Christmas tree (cellular) breakup regime, breakup length and lateral spread decreased with increasing Weber number.

Surface wettability-induced modulations of droplet breakup in a bifurcated microchannel

We explore the dynamics of droplet propagation and subsequent disintegration in a symmetric bifurcating Y-microchannel by varying the wettability characteristics of one of the daughter channels while maintaining the wettability of the other constant. The temporal evolution of the droplet is numerically investigated using the phase-field method. Based on the neck-width evolution, the droplet bifurcation phenomenon has been divided into three separate stages, namely, squeezing, transition, and pinch-off. During the squeezing stage, the rate of change of neck width increases as the wettability angle decreases, while an opposite trend is observed at the pinch-off stage, leading to almost identical breakup time for the droplet regardless of the wettability angle. We identify pertinent regimes of droplet breakup, such as symmetric breakup, asymmetric breakup, no-breakup upper channel, no-breakup lower channel, and spreading regime, over wide ranges of capillary numbers (Ca) and viscosity ratio (μr). Our study indicates that an increase in the relative influence of viscous force (high Ca) reduces the droplet's wettability effect. The same pattern is obtained when the viscosity of the droplet is increased in relation to the viscosity of the carrier fluid. In contrast, for low Ca flows, the relatively strong interfacial tension favors the wettability characteristics of the surface, resulting in a dominance of non-breakup regimes. The regime plots proposed in this paper depict the roles of Ca and μr on various breakup regimes in detail. Such regime diagrams may emerge as fundamental design basis of microfluidic devices in diverse applications, such as biopharmaceuticals, microreactors, and food processing.

Experimental investigation on liquid breakup regimes and spray characteristics in slinger atomizers with various injection orifices

Slinger atomizers, known as one type of rotary atomizers, have been widely applied in various small gas turbine engines. The fuel can be well atomized by taking advantage of the high rotational speed of the turbine shaft. The geometric characteristics of the injection orifice play an important role in determining the atomization performance of the slingers. The breakup regimes and the droplet size of the slinger atomizers with slot-shaped orifices have rarely reported in the past. Herein, three types of slinger atomizers with different orifice shapes and orifice diameters are tested at rotational speeds of 8000–20 000 rpm and liquid feed rates of 4 up to 20 g/s. High-speed shadowgraph imaging, high-speed digital imaging, and planar Mie technologies are applied to provide the spray breakup process, liquid film injection features, and droplet distribution, respectively. Spray visualizations show that the orifice diameters strongly affect the breakup modes, whereas the orifice shapes have a slight effect. The variation regarding droplet sizing under different heights from the slinger plane is analyzed. The uniformity of the droplet distribution in slot-shaped slinger atomizers is better than that in round-shaped slinger atomizers. Moreover, the smaller orifice diameter results in a small Sauter mean diameter (SMD) for the slinger atomizers with slot-shaped orifices. Finally, a mathematical expression is obtained to predict non-dimensional droplet size (SMD/d) for different slinger atomizers. The present results appear to be the first systematic investigation of the spray characteristics in slinger atomizers with slot-shaped orifices.

Bubble deformation and breakup in a non-uniform electric field

In this paper, the deformation and breakup characteristics of bubbles during their growth in a mixture solution were studied with a needle-ring electrode configuration in the presence of an electric field by using high-speed photography. Four distinct bubble breakup behaviors of the dripping, cone, kink and branch regimes were achieved successfully by manipulating the electric field. By considering the applied voltage and gas flow rate, the cone structure, interface stability, departure frequency and bubble size distribution in different regimes were quantitatively discussed. It is found that the cone height, interface curvature, departure frequency and bubble size distribution are evidently associated with bubble breakup regimes. As the electric field strength increased, the bubbles stretched in the direction of the electric field and the instability of the gas–liquid interface became stronger, which eventually resulted in the bubbles fragmenting into a cluster of microbubbles. This work provides a fundamental understanding of bubble deformation and breakup mechanisms under the electric field.

Experimental analysis of the near-nozzle direct injection spray for gasoline and ethanol-gasoline blended fuels

The current work analyses the spray during the early stage of development in the near-nozzle region using a single-hole gasoline direct injection system for gasoline (G100) and ethanol-gasoline (E85) blended fuels. The present work combined a Galilean-type beam expander with an achromatic lens, using an LED light and a high-speed camera to visualize a magnified view of the near-nozzle microscopic aspect of the emerged jet. Three different shapes, spheroids, cylindrical, and steeple, were visualized experimentally during the evolution of the fuel jet under different fuel injection and chamber pressures. A transition in shape from the steeple-to-spheroid shape was detected. It was observed that the velocity of the emerged jet is affected by its evolved shapes, attributing to differences in the initial breakup characteristics and shot-to-shot variation in the spray at a given condition. The velocity of the cylindrical-shaped jet was 14% and 42% higher than the steeple and spheroid-shaped jets, respectively. The Rayleigh-Plesset equation describes the reason for an increase in the spheroidal-shaped jet velocity by 84% when the chamber pressure is reduced from 0.6 to 0.1 MPa. The Ohnesorge and Reynolds number of the emerged liquid jets were estimated to describe the breakup regime of the jet during the initial stage. It was observed that the cylindrical and the steeple-shaped jets pertain to a higher tendency to atomize during the initial stage of evolution compared to the spheroidal jet. The average jet velocities and liquid penetrations in the near-nozzle region for E85 and G100 were estimated.

HIGH FIDELITY SIMULATION ON THE BREAKUP AND DEFORMATION CHARACTERISTICS OF LIQUID JET IN CROSSFLOW WITH RECTANGULAR NOZZLES

High fidelity simulation was carried out by ANYS software to investigate the primary breakup characteristics of liquid jet in crossflow. The influence of nozzles shapes on the breakup features and deformation characteristics of liquid column was analyzed. Air velocity ranged from 40 m/s to 80 m/s and the shape of nozzles included circular, Rectangle-A, and Rectangle-B. The results indicated that, at the higher velocity of crossflow, the surface breakup of liquid column was mainly affected by the air velocity passing through the sides, rather the sharp corner of the rectangular nozzles. As compared with the circular nozzle, the boundary of breakup regime was expanded for Rectangle-A, while it was shrunk for Rectangle-B. In contrast to circular nozzles, the deflection of edges of liquid column was avoided for rectangular nozzles, thus preventing the shrinkage of liquid column during injection. The results of the analysis of the flow structures of crossflow showed that the air velocity in the flow direction of liquid column might be the reason for the difference of breakup regime of nozzles. In addition, the difference of windward trajectory between rectangular nozzles with the different widths was decreased as the air velocity increased.

Interactions between liquid sprays and shock waves in underexpanded flows

The interactions between droplets and shock waves have many applications, but few studies have investigated how the distributions of droplet diameters and droplet velocities in a spray are modified after passing through a shock. This study examines the droplet statistics upstream and downstream of shock waves in an underexpanded jet by performing phase Doppler interferometry in combination with Schlieren imaging. A mixture of water and propylene glycol is employed as the liquid. It is observed that, when the spray passes through an oblique shock, the droplet diameters decrease and then increase but the droplet velocities remain steady. When the droplets pass through a weak normal shock, on the other hand, the most probable droplet diameter decreases, some very large droplets appear, and the velocity distribution splits into three distinct regions. However, a strong normal shock causes the mean droplet diameters and the mean droplet velocities to decrease consistently. Reasons for these different behaviors are given, and secondary breakup regimes are estimated in terms of the Weber number. Additionally, droplet probability distribution fits are compared to the measured values for each of the different cases. The droplet statistics presented here can be used to improve computational modeling of spray-shock interactions.

Direct Numerical Simulation of Atomization Characteristics of ECN Spray-G Injector: In-Nozzle Fluid Flow and Breakup Processes

AbstractIn order to sustain applications dealing with various liquid fuels in internal combustion engine (ICE), it is essential to make available prediction methodologies that allow an early evaluation of the potential usefulness of such fuels in terms of favourable mixture preparation process already in realistic configurations. Since the air-mixture formation and subsequent processes are predominantly governed by the fuel injection, a DNS based numerical investigation coupled with VOF as an interface tracking method is carried out in this paper to gain better insight on the fuel injection from an industrial injector "Spray G" configuration. Chosen from Engine Combustion Network (ECN), this is a gasoline direct injector (GDI) featuring 8-holes orifices and operating with high injection pressure (200 bar). Under consideration of the required computational cost associated with DNS, only the 1/8 of the nozzle geometry including one orifice is used. The numerical simulation is accomplished for the quasi-steady injection condition with nozzle needle fully opened. The obtained results are first validated with available experimental data for nozzle mass flow rate and spray spread angle showing a good agreement. Then, a detailed numerical analysis is provided for the in/near nozzle flow evolution especially for flow turbulence, primary and secondary atomization. Furthermore, droplet statistics in terms of droplet shape, and droplet size-velocity distribution together with a breakup regime map are reported. Finally, a 2-D data curation technique is proposed to extract the droplet statistics along selected planes and evaluated by direct comparison with three-dimensional droplet data, which may allow handling of the DNS data in more feasible and economical way especially for time series data with higher frequency. The comprehensive DNS data generated by this DNS-VOF approach enable not only to carry out detailed numerical analysis of in- and near-nozzle physical phenomena for which experimental data are still scarce, but also to provide a hint of more reliable injector boundary conditions useful for lower order spray injection method based on Lagrangian particle tracking.

Experimental characterization of early, quasi-steady and post-injection evolution of single-hole direct injection sprays for spark-ignited engines

The atomization characteristics of the ethanol-gasoline (E85) blended fuel using a single-hole GDI atomizer were studied. The spray at early development, quasi-steady, and post-injection stages were studied in the near-nozzle region. A Galilean-type beam expander and an achromatic lens were used to achieve the desired magnification. In the early development stage, the jet velocity was measured to estimate the breakup regime based on the Ohnesorge (Oh) and Reynolds (Re) numbers. Spray during the initial stage at 2 MPa injection pressure shows second wind-induced breakup under 0.6 MPa chamber pressure (Pch). A transition between second-wind and atomization breakup for jets under 0.03 and 0.05 MPa Pch. The jet experiences atomization breakup under 0.1 MPa Pch. The effect of the injection pressure and chamber temperature were also studied. The post-injection liquid sheet formation near the nozzle exit was observed. Results show that the size of the liquid sheet depends on the liquid-to-gas density ratio (ρlg). The size of the sheet for E85 fuel at ρlg of 2363 was found to be 6 mm, which was reduced by 75 % with a reduction in ρlg by 95 %. The chamber temperature increased from 303 K to 363 K, ρlg increased from 702 to 842, and a reduction in the liquid sheet was observed from 2.5 mm to 1.8 mm, respectively. The necking phenomenon of the liquid sheet was observed at the nozzle tip before the sheet detached and disintegrated into droplets. The necking length for the E85 was compared with gasoline fuel.

Breakup regimes of the long-time dynamics of a finite-size air filament in a dense fluid

The long-time dynamics of a quiescent finite-size three-dimensional air filament in a static liquid is studied using three-dimensional numerical simulations. The two ends of the air filament retract under the effect of surface tension and form bulges. A long filament with the aspect ratio Γ = 30 is considered to trigger the end-pinching regime of the filament rupture. The study focuses on the effect of the Ohnesorge number, which relates viscous forces to inertia and surface tension forces. Depending on the value of the Ohnesorge number, two or three successive ruptures of the filament are observed. Wavy structures form at the interface of the air filament after the first rupture and lead to a subsequent breakup in the middle of the filament or in several places depending on the corresponding Ohnesorge number. The size distribution of the bubbles generated is provided and shows an average diameter twice as large as the initial diameter of the air filament. Microbubbles are generated, and their number is shown to increase when the Ohnesorge number decreases.

Multiscale numerical modeling of a complete spray evolution including breakup of liquid jet injection in gaseous cross flow

A direct coupling of the Volume of Fluid method (VOF) and the Lagrangian Particle Tracking (LPT) technique within a Large Eddy Simulation (LES) framework is revisited and assessed by numerically investigating a liquid jet injection into a cross-flowing air (JICF). To close the subgrid scale (SGS) terms appropriate SGS models are considered. In particular, an interfacial SGS term is included into the VOF scalar transport equation in order to achieve a suitable LES and to describe properly the liquid column and the break-up processes (primary and secondary). The dilute spray region including the droplets resulting from the secondary breakup is solved using the LPT approach. Since the latter allows the cell size to be multiple times larger than the droplet diameter, this reduces the computational expenses significantly making it possible to handle the simulation of the whole spray evolution including the complete liquid atomization without complex tunable atomization models. For this purpose, two operating conditions of a JICF existing in the literature are considered. The achieved results are first presented in terms of mesh sensitivity, required computational costs and impact of the interfacial SGS term. Next, comparisons between pure VOF/LES and VOF/LPT/LES for both qualitative features and quantitative properties are provided and discussed. In overall, a good agreement with experiments for the quantitative properties is reported. It is especially found that both the Rosin–Rammler and Gaussian distributions offer nearly a more accurate fit to the particle size distribution except for the range from 0 to 10 μm. Together with the detailed characteristics of vortices evolving and an extended breakup regime map, the spray characteristics including the spray structure are well captured. Finally, even though the interfacial SGS term has an evident influence on the jet and atomization dynamics, its impact on quantitative properties is not so prominent under these operating conditions.

Fine characterization of the capillary instability of free viscoelastic jets

The experimental approach presented in this article investigates low-velocity small-diameter cylindrical viscoelastic jets. It consists in producing and analyzing sets of jet portion images taken at different positions along the jet axis, i.e., from the nozzle producing the jet down to the spray region. A very significant part of the work was dedicated to develop analyzing tools: a proper orthogonal decomposition to identify the onset of the capillary instability, a procedure to determine the mean jet breakup length and a multi-scale tool to describe the jet local shape and its evolution along the jet axis. Used in a similar previous study, the multi-scale analysis has been extended in the present work. These tools are shown to be well suited to the experimental study conducted here, and in particular to meet the need for statistical analysis that free jets impose. They are applied to study the capillary instability of free jets of dilute polyethylene oxide solutions produced from a micrometer nozzle. The parameters of the study are the molar mass of the polymer and the velocity of the jet. Results on the detection of the capillary instability onset, the jet evolution dynamics towards the Bead On A String (BOAS) pattern, the local extensional relaxation time and the terminal elongational viscosity of the solutions, the bead coalescence mechanism and its influence on the size of the spray drops are obtained. Analysis of these results reveals that the polymers undergo two episodes of mechanical degradation during the process: the first one at injection, the second one during the elasto-capillary regime of the jet breakup. Moreover, the scenario in the literature suggesting a sufficiently relaxed axial tension of the flow for the capillary instability to occur is found to be not always valid. It is also shown that the average size of the drops correlates with the intensity of the bead coalescence mechanism that takes place on the BOAS. These results demonstrate the richness of the adopted approach and in particular of the multi-scale analysis to study the development of a fluid interfacial instability.

Detailed numerical investigation of the drop aerobreakup in the bag breakup regime

Aerobreakup of drops is a fundamental two-phase flow problem that is essential to many spray applications. A parametric numerical study was performed by varying the gas stream velocity, focusing on the regime of moderate Weber numbers, in which the drop deforms to a forward bag. When the bag is unstable, it inflates and disintegrates into small droplets. Detailed numerical simulations were conducted using the volume-of-fluid method on an adaptive octree mesh to investigate the aerobreakup dynamics. Grid-refinement studies show that converged three-dimensional simulation results for drop deformation and bag formation are achieved by the refinement level equivalent to 512 cells across the initial drop diameter. To resolve the thin liquid sheet when the bag inflates, the mesh is refined further to 2048 cells across the initial drop diameter. The simulation results for the drop length and radius were validated against previous experiments, and good agreement was achieved. The high-resolution results of drop morphological evolution were used to identify the different phases in the aerobreakup process, and to characterize the distinct flow features and dominant mechanisms in each phase. In the early time, the drop deformation and velocity are independent of the Weber number, and a new internal-flow deformation model, which respects this asymptotic limit, has been developed. The pressure and velocity fields around the drop were shown to better understand the internal flow and interfacial instability that dictate the drop deformation. Finally, the impact of drop deformation on the drop dynamics was discussed.