Droplet Breakup Research Articles

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.

Study of the mechanism of shock-induced and detonation-induced droplet breakup based on hybrid solvers

Study of the mechanism of shock-induced and detonation-induced droplet breakup based on hybrid solvers

Aerodynamic thermal breakup droplet ionization combined with a β-irradiation source for mass-spectrometric analysis of samples in a nonpolar solvent

A method is proposed for increasing the number of ions during mass-spectrometric analysis of samples in a nonpolar solvent (benzene). For this purpose, aerodynamic thermal breakup droplet ionization (ATBDI) with the impact of β-radiation on the aerosol droplets used in ATBDI was evaluated. This modification of the method, which we named β-ATBDI, allows to shift a nonvolatile analyte (trinitrotoluene in the negative ionization region and cocaine in the positive ionization region, as an example) into a gas phase as an aerosol at room temperature (in contrast to atmospheric pressure chemical ionization). In addition, β-ATBDI enables a researcher to distinguish mass spectrometric peaks of the compounds located in an aerosol droplet from compounds located outside the droplet, i.e., to identify background peaks. Also briefly discussed the ionization of two antibiotics—azithromycin in methylene chloride and sulfadiazine in salt water with β-ATBDI, ATBDI and electrospray ionization source.

Taylor droplet breakup in T-type microchannels: A detailed flow analysis

The study of droplet break-up in microchannels can be used to improve designs for both biological or chemical microreactors and microfluidic devices e.g., heat exchangers and microchips. In this study, 3D computer simulations were carried out to investigate the detailed behavior of droplet breakup in a T-junction microchannel using the Volume of Fluid (VOF) and Level Set (LS) coupling methods (S-CLSVOF) implemented in OpenFoam. The evolution of droplet morphology and the rupture regimens were analyzed considering pressure results, velocity neck thickness, and tunnel width. The results show four rupture regimes for a range of capillary numbers and dimensionless initial droplet length of 0.005≤Ca≤0.055 and 1≤l0/w≤5. The stages of each rupture were also analyzed and compared with experimental and numerical results from literature. A detailed description of each step in the droplet breakup is also given, including analyses of the influence that the local pressure field and Laplace pressure have on the rupture processes. The 3D results reveal that the Laplace pressure decreases in the main channel due to continuous flux at the top and bottom of the 3D channel. This in turn, leads to a slight change in the rear curvature of the bubble. Laplace pressure is an important parameter that can be used to predict the opening tunnel point, large changes in curvature, and whether the droplet will breakup or not. Just before breakup, the 3D simulations captured a backflow near the neck, a region, characterized by pressure drops, and thus smaller breakup times.

Numerical Simulation on Metallic Droplet Deformation and Breakup Concerning Particle Morphology and Hollow Particle Formation During Gas Atomization

Numerical Simulation on Metallic Droplet Deformation and Breakup Concerning Particle Morphology and Hollow Particle Formation During Gas Atomization

Optimization of the gas system for gas–water combined atomization technique in FeSiBC amorphous powder production

The gas–water combined atomization is an advanced technology for Fe-based amorphous powder preparation, and its gas/water system parameters have significant impacts on powder properties. In this study, numerical simulations and industrial trials were combined to optimize the gas atomization parameters. The results showed that increasing the atomization pressure promotes the transition of the flow field to the closed wake. Moreover, the median particle size was significantly refined and the cooling rate was improved. Extending the extrusion length facilitated the decrease in suction pressure, while excessively long extrusion lengths led to instability in the atomization process. The decrease in delivery tube diameter enhances droplet breakup and cooling, but increases the risk of clogging. Industrial trials at different atomization pressures showed that low atomization pressure led to the formation of needle-shaped powder, and the FeSiBC amorphous powder prepared at 3.0 MPa exhibited optimal comprehensive properties, with saturation magnetization of 166.1 emu·g−1 and coercivity of 4.5 Oe.

Numerical study and experimental validation: A portioned calculation method for a large atomization field

Atomization and sprays are widely used in industry and agriculture. An appropriate atomization simulation method is essential in analyzing the liquid film-breaking process and atomization performance, especially in large-scale atomization field calculations. This study innovatively proposes a portioned method that combines existing fundamental atomization calculation models to balance computational accuracy and speed, finally achieving a full-scale numerical study of large atomization fields. This study employs the volume of fluid (VOF) model to measure the two-phase flow in the inner flow field and applies the discrete particle model (DPM) to analyze droplet behavior in the far-atomization field. In the near-atomization field, the VOF-to-DPM method connects the nozzle with the jet space, providing an effective numerical simulation of the liquid film formation and droplet breakup processes. Additionally, experiments on atomization using a pressure-swirl nozzle at different flow rates were conducted. Experimental data, such as atomization cone angle, flow distribution, and droplet particle size distribution, were obtained, and numerical calculations were performed using the large atomization field partitioned calculation model. The simulation results are utilized to explain the mechanisms of liquid film disintegration, while the experimental results are employed to validate the accuracy of the numerical model. The comparison revealed that the calculated results of the partitioned simulation approach align well with the experimental data. The maximum error in flow characteristics is 9.53%, in atomization cone angle is 6.16%, and in flow distribution is 3.67%, and there is a good agreement in particle size distribution with a maximum error of 17.58% in Sauter mean diameter, validating the accuracy of the portioned calculation method for large atomization fields.

Analytical solutions for airborne droplet trajectory: Implications for disease transmission

Airborne droplets containing viruses from infected persons can present long range disease transmission risks. In this study, we examine the trajectory of an airborne droplet based on a point force model. The interplay between gravity, drag, inertia and deformation are factored in, for drop sizes ranging from micrometer to millimeter size. In particular, we propose an expression for the drag force which enables analytical solutions for cases of practical interest, such as the transient velocity behavior and spreading distances. This allows us to obtain physical insights which are not obvious from direct numerical simulations. Effects such as droplet deformation, breakup and evaporation are also considered. Our analytical solutions compare favorably with numerical and experimental data. The evaporation rate was determined experimentally with a levitating device and some experimental drop velocity versus time data were obtained with falling millimeter sized droplets.

Numerical study on the effect of actuation parameters on the formation characteristics of metal droplets in magnetohydrodynamic drop-on-demand (DOD) jetting

Numerical simulations have investigated the effects of electromagnetic drive waveform amplitude and pulse width on the state of metal droplet generation during drop-on-demand printing of molten metal aluminum by electromagnetic drive. It indicates that positive and negative pulses of magnetohydrodynamic(MHD) actuation applied to the molten metal in the crucible can form a so-called “push-pull” effect, which is the main mechanism affecting droplet generation and breakup. The positive pressure pulse causes the liquid metal to be extruded outward, and the negative pressure pulse promotes the fluid retraction at the nozzle to promoting the thinning of the liquid bridge and the generation of liquid droplets. With the increase in the amplitude of the applied current, the ejected molten aluminum from the crucible experiences three states: no droplet, single droplet, and droplet with satellite droplets. In the single drop state, the droplet formation time is shortened as the current peak increases and the droplet volume increases with the increase of pulse width. The energy conversion and force changes during droplet formation and falling are analyzed. When the peak kinetic energy of the ejected molten metal exceeds its surface energy, the jet breaks up and generates a single droplet. For the actuation with constant amplitude and duration for positive pulses, extending the duration of negative pulses weakens the "pull" effect, leading to a transition from the generation of a single droplet to multiple droplets. The numerical simulation results provide information on the current amplitude, pulse width, and Weber number range necessary for stable single droplet generation.

Breakup of Single Droplet Induced by High Weber Number Flow behind Shock Wave

Breakup of Single Droplet Induced by High Weber Number Flow behind Shock Wave

Dynamic breakup of Janus droplet in a bifurcating microchannel

Dynamic breakup of Janus droplet in a bifurcating microchannel

Modelling the effect of soluble surfactants on droplet deformation and breakup in simple shear flow

A hybrid lattice Boltzmann and finite difference method is applied to study the influence of soluble surfactants on droplet deformation and breakup in simple shear flow. First, the influence of bulk surfactant parameters on droplet deformation in two-dimensional shear flow is investigated, and the surfactant solubility is found to influence droplet deformation by changing average interface surfactant concentration and non-uniform effects induced by non-uniform interfacial tension and Marangoni forces. In addition, the droplet deformation first increases and then decreases with Biot number, increases significantly with adsorption number $k$ and decreases with Péclet number or adsorption depth; and among the parameters, $k$ is the most influential one. Then, we consider three-dimensional shear flow and investigate the roles of surfactants on droplet deformation and breakup for different capillary numbers and viscosity ratios. Results show that in the soluble case with $k=0.429$ , the droplet exhibits nearly the same deformation as in the insoluble case due to the balance between surfactant adsorption and desorption; upon increasing $k$ from 0.429 to 1, the average interface surfactant concentration is greatly enhanced, leading to significant increase in droplet deformation. The critical capillary number of droplet breakup $C{{a}_{cr}}$ is identified for varying viscosity ratios in clean, insoluble and soluble ( $k=0.429$ and 1) systems. As the viscosity ratio increases, $C{{a}_{cr}}$ first decreases and then increases rapidly in all systems. The addition of surfactants always favours droplet breakup, and increasing solubility or $k$ could further reduce $C{{a}_{cr}}$ by increasing average interface surfactant concentration and local surfactant concentration near the neck during the necking stage.

Influence of triangular obstacles on droplet breakup dynamics in microfluidic systems

Microfluidic devices with complex geometries and obstacles have attracted considerable interest in biomedical engineering and chemical analysis. Understanding droplet breakup behavior within these systems is crucial for optimizing their design and performance. This study investigates the influence of triangular obstacles on droplet breakup processes in microchannels. Two distinct types of triangular obstructions, positioned at the bifurcation (case I) and aligned with the flow (case II), are analyzed to evaluate their impact on droplet behavior. The investigation considers various parameters, including the Capillary number (Ca), non-dimensional droplet length (L*), non-dimensional height (A*), and non-dimensional base length (B*) of the triangle. Utilizing numerical simulations with COMSOL software, the study reveals that the presence of triangular obstacles significantly alters droplet breakup dynamics. Importantly, the shape and location of the obstacle emerge as key factors governing breakup characteristics. Results indicate faster breakup of the initial droplet when the obstacle is positioned in the center of the microchannel for case I. For case II, the study aims to identify conditions under which droplets either break up into unequal-sized entities or remain intact, depending on various flow conditions. The findings identify five distinct regimes: no breakup, breakup without a tunnel, breakup with a tunnel, droplet fragmentation into unequal-sized parts, and sorting. These regimes depend on the presence or absence of triangular obstacles and the specific flow conditions. This investigation enhances our understanding of droplet behavior within intricate microfluidic systems and provides valuable insights for optimizing the design and functionality of droplet manipulation and separation devices. Notably, the results emphasize the significant role played by triangular obstacles in droplet breakup dynamics, with the obstacle’s shape and position being critical determinants of breakup characteristics.

A dynamic droplet breakup model for Eulerian-Lagrangian simulation of liquid-fueled detonation

This study proposes a dynamic model to reflect the physical image of the droplet breakup process in two-phase detonation flows. This breakup model is implemented in a two-phase detonation solver developed based on an open-source computational fluid dynamic platform, OpenFOAM, and compared with three prevalent models (TAB, PilchErdman, and ReitzKH-RT model) under different droplet diameters (30–70 μm) in one- and two-dimensional detonation problems. The simulating results show that the present breakup model well predicts experimentally determined detonation parameters such as detonation velocities and post-wave temperature. In addition, the present model has the advantage of being free of the KH breakup time parameter, which the ReitzKH-RT model needs to fit the experimental data. The one-dimensional detonation simulations indicate that different breakup models slightly impact the detonation wave velocity because the droplet breakup process does not significantly affect the total heat release as long as it is sufficiently fast to sustain the detonation. However, the two-dimensional detonation simulations show that the breakup model and the droplet initial diameter significantly affect the detonation cell size due to the different droplet distributions predicted by various models. The breakup length, which is the distance from the shock wave to the location at which sufficiently small child droplets appear, affects the chemical reaction zone thickness and then the detonation cell size. A longer breakup length will result in a larger detonation cell size.

Spray combustion characteristics and soot formation potential of Nano-Al2O3 diesel at low ambient temperatures

Nanoparticle additives have the potential to enhance combustion efficiency and reduce emissions in liquid combustion processes. This study thoroughly investigates the spray evaporation, ignition, and combustion characteristics of nanoparticle/fuel blends, especially under challenging environmental conditions, to improve combustion efficiency and reduce emissions. Employing diffused background illumination, shadowgraph, Mie scattering, high-speed natural luminosity, and two-color method, the spray combustion dynamics of thermally stable nano-Al2O3/diesel (NAD) blends were examined within a constant volume combustion chamber. Diesel, a prevalent primary or auxiliary fuel, represents other highly volatile renewable fuels, while Al2O3, a significant component in combustion fly ash, is noted for its specific catalytic properties and ability to re-burn, eliminating organic toxic elements. The results indicated that NAD blends exhibit a faster evolution rate for both liquid and vapor sprays compared to pure diesel, attributed to the increased kinetic energy of breakup droplets. Under most experimental conditions, incorporation of nano-Al2O3 slightly reduced the ignition delay time due to enhanced heat transfer. Notably, nano-Al2O3 altered the ignition mechanism of the diesel spray, transitioning it from partially premixed to diffusion combustion and relocating the ignition point nearer to the injector. The combustion flame of NAD blends demonstrated increased luminosity, a shorter flame lift-off length, and variations in start-of-injection and total-injection-nozzle-lift-off. However, due to a high equivalence ratio mixture between the liquid phase spray and combustion region, the soot concentration in NAD blends was higher than that in diesel. Additionally, the elevated flame temperature in NAD blends significantly increased the soot concentration. Overall, nano-Al2O3 markedly enhances diesel combustion, especially in low ambient temperatures, suggesting its impressive potential for the high-efficiency and low-emission utilization of renewable fuels or the combustion of recycled waste solvents.

Puffing/micro-explosion of two-liquid droplets: Effect of fuel shell composition

Theoretical research into the heat and mass transfer, hydrodynamic and physicochemical processes in combustion chambers of gas turbine engines usually implies that multi-component jet fuels are modeled using single-component liquids (saturated or cyclic hydrocarbons) and their substitutes. Due to an insoluble dispersed phase (e.g., water) in their composition, droplets consist of a noncombustible core and a liquid fuel shell. During heating, water droplets coalesce in fuel droplets to produce explosion-triggering volumes of liquid superheated to the boiling point. When heated, these heterogeneous droplets breakup in the micro-explosion and puffing modes. This study reports the numerical simulation results providing the temporal characteristics of heating and evaporation of heterogeneous droplets until puffing/micro-explosive breakup, when varying the composition of the fuel shell in the homologous series of saturated and cyclic (as illustrated by monocycloparaffins) hydrocarbons from C7 to C16. The conducted research has revealed that the variations in the breakup delay times in the homologous series of saturated and cyclic hydrocarbons are nonlinear. The breakup delay rates were found to increase substantially in the boundary points of the investigated series. Mechanisms to control droplet fragmentation delay time were identified for different initial and boundary conditions. A dimensionless complex reflecting the correlation between the critical conditions of composite liquid droplet breakup and the physicochemical properties of the fuel shell components was proposed.

Component Effects in Binary Droplet Impact Behaviors on the Heated Plate: Comparison Study of Ethanol/Propanol and Ethanol/Water Droplets and Observation of Novel Bubble Shrinkage Phenomenon

This work aims to investigate the effect of liquid physical properties on the behavior of binary droplets impact on the heated smooth aluminum alloy plate with a high-speed imaging system. Two groups of mixed solutions with similar boiling point differences are selected as the working liquid, in which the low-boiling-point components are both ethanol and the high-boiling point components are propanol and water, respectively. Compared to the ethanol/propanol binary droplets, the experimental results show that the ethanol/water binary droplets have diverse impact phenomena and significantly broad transition boiling regimes, as well as the reduced droplet residence time and increased Leidenfrost temperature point. With the decreasing ethanol content in ethanol/water binary droplets, these effects become more prominent. For secondary atomization, the ethanol/water binary droplet undergoes parent droplet breakup into fragment droplets with larger diameters (Ds > 0.3 mm). Both binary droplets produce satellite droplets with small diameters (Ds < 0.3 mm) by puffing and ejection. In terms of the ethanol/propanol binary droplet impact, the probability of puffing is higher and the satellite droplet diameters are small. In the ethanol/water binary droplet impact, the probability of ejection is higher and the satellite droplet diameter distribution is wider. When an ethanol/water binary droplet of 25 vol.% ethanol content impacts the heated wall at Ts = 120 °C, a novel large bubble shrinkage phenomenon occurs at the late stage of droplet evaporation. This phenomenon is proposed to be relevant to the increasing surface tension and saturation temperature with decreasing ethanol content, as well as the decreasing ambient temperature above the top surface of the bubble.

Aerodynamic breakup of gel suspension droplets loaded with aluminum particles

The persistent and frequent space explorations are affected by the performance of propellants, among which the closely related gel propellants have been widely used due to their unique characteristics. Currently, research efforts primarily concentrate on the metallization and atomized combustion of gel propellants, emphasizing the combustion performance of metallized gel propellants. However, little attention has been given to the secondary breakup process that occurs just before combustion. By utilizing high-speed flow visualization technology, this study examines the key aerodynamic breakup process of gel suspension droplets loaded with micro aluminum particles, experimentally and theoretically investigating the droplet dynamics induced by particle volume fraction. The experimental confirmation of the three breakup behaviors exhibited by gel suspension droplets, namely oscillation mode, bag-stamen breakup, and hemline breakup, is presented along with the construction of a phase diagram. The impact of heterogeneous effects resulting from an increase in particle volume fraction on the critical breakup Weber number is revealed, with a concentration of 9 % as the turning point. Within the range of moderate Weber numbers, the tail flick deformation of suspension droplets in airflow is observed and reported for the first time. At different particle concentrations, there keeps a linear correlation between the dimensionless tail length and Weber number. Spherical suspension droplets will deform into thin disks in the airflow, with a nearly constant deformation rate regardless of changes in particle concentration. Additionally, the relationships between characteristic times and droplet parameters are discussed through statistical analysis of the results. As Ohnesorge number and particle volume fraction increase, the characteristic times both show a monotonically increasing trend. Moreover, a model of unstable growth rate associated with the particle volume fraction is established through linear instability analysis, accurately predicting the variation in critical Weber number for gel suspension droplet breakup. The congruence between our theoretical framework and experimental findings not only enhances comprehension of the impact of micro aluminum particles on the secondary breakup of gel suspension droplets but also provides valuable guidance for the aerospace applications of metallized gel propellants.

Pseudo-Eulerian model formulation for the determination of breakage frequency and daughter size distribution: Single droplet breakup experiments in turbulent flow

The study of breakage of single oil droplet in a turbulent flow is experimentally performed by means of direct visualization using high-speed imaging. A comparison of two different approaches for estimation of the breakage frequency is presented using the same set of experimental data. The first approach, estimates the breakage frequency based on the estimation of the averaged breakage time and breakage probability calculation based on single oil breakage experiments. The second approach consists in combining and superposing the different measurements into one population of droplets, to investigate the evolution of the size distribution of droplets emulating a pseudo-Eulerian representation. Using this approach, the breakage frequency can be estimated by following the evolution of the number of droplets of a particular size. We found that the pseudo-Eulerian approach is more accurate for construction of closure model for the implementation of population balance equation.

Modelling the passive breakup of a surfactant-contaminated droplet in a T-junction microchannel

A lattice Boltzmann method is used to explore the effect of surfactants on the unequal volume breakup of a droplet in a T-junction microchannel, and the asymmetry due to fabrication defects in real-life microchannels is modelled as the pressure difference between the two branch outlets ( $\Delta {P^\ast }$ ). We first study the effect of the surfactants on the droplet dynamics at different dimensionless initial droplet lengths ( $l_0^\ast $ ) and capillary numbers (Ca) under symmetric boundary conditions ( $\Delta {P^\ast } = 0$ ). The results indicate that the presence of surfactants promotes droplet deformation and breakup at small and moderate $l_0^\ast $ values, while the surfactant effect is weakened at large $l_0^\ast $ values. When the branch channels are completely blocked by the droplet, a linear relationship is observed between the dimensionless droplet length ( $l_d^\ast $ ) and dimensionless time ( ${t^\ast }$ ), and two formulas are proposed for predicting the evolution of $l_d^\ast $ with ${t^\ast }$ for the two systems. We then investigate the effect of the surfactants on the droplet breakup at different values of $\Delta {P^\ast }$ and bulk surfactant concentrations ( ${\psi _b}$ ) under asymmetric boundary conditions ( $\Delta {P^\ast } \ne 0$ ). It is observed that, as $\Delta {P^\ast }$ increases, the volume ratio of the generated droplets ( ${V_1}/{V_2}$ ) decreases to 0 in both systems, while the rate of decrease is higher in the clean system, i.e. the presence of surfactants could cause a decreased pressure difference between the droplet tips. As ${\psi _b}$ increases, ${V_1}/{V_2}$ first increases rapidly, then remains almost constant and finally decreases slightly. We thus establish a phase diagram that describes the ${V_1}/{V_2}$ variation with $\Delta {P^\ast }$ and ${\psi _b}$ .