Droplet Collision Research Articles

New insights into head-on bouncing of unequal-size droplets on a wetting surface

The impact of a liquid droplet with another droplet or onto a solid surface are important basic processes that occur in many applications such as agricultural sprays and inkjet printing, and in nature such as pathogens transport by raindrops. We investigated the head-on collision of unequal-size droplets of the same liquid on wetting surfaces using the direct numerical simulations technique at different size ratios. The unsteady Navier–Stokes equations are solved and the liquid–gas interface is tracked using the geometric volume-of-fluid method. The numerical model is validated by comparing simulation results of two extreme cases of droplets bouncing with the experimental data from previous studies and the agreement is quite accurate. The validated model is employed to simulate droplets bouncing at several size ratios at different Weber numbers and Ohnesorge number. Two distinct regimes are identified, namely, the inertial regime, where the restitution coefficient is a constant value close to 0.3, the viscous regime, where the restitution coefficient declines. To understand the bouncing behaviour, the velocity field is analysed and an energy budget calculation is performed. The distribution of the sessile droplet energy is found to be important and the sessile droplet surface energy is calculated by its deformation characteristics such as crater depth. Finally, a scaling analysis is performed to rationalize the insensitivity of the coefficient of restitution in the inertial regime, and its decline in the viscous regime, at large size ratios.

Droplet collision of hypergolic propellants

AbstractIn the present mini‐review, droplet impacting on a liquid pool, jet impingement, and binary droplet collision of nonreacting liquids are first summarized in terms of basic phenomena and the corresponding nondimensional parameters. Then, two representative hypergolic bipropellant systems, a hypergolic fuel of N,N,N′,N′‐tetramethylethylenediamine (TMEDA) and an oxidizer of white fuming nitric acid (WFNA) and a monoethanolamine‐based fuel (MEA‐NaBH4) and a high‐density hydrogen peroxide (H2O2), are discussed in detail to unveil the rich underlying physics such as liquid‐phase reaction, heat transfer, phase change, and gas‐phase reaction. This review focuses on quantifying and interpreting the parametric dependence of the gas‐phase ignition induced by droplet collision of liquid hypergolic propellants. The advances in droplet collision of hypergolic propellants are important for modeling the real hypergolic impinging‐jet (spray) combustion and for the design optimization of orbit‐maneuver rocket engines.

Numerical Simulation of Rotational Separation in Binary Droplet Collisions by the Improved Two-Phase Lattice Boltzmann Method

Numerical Simulation of Rotational Separation in Binary Droplet Collisions by the Improved Two-Phase Lattice Boltzmann Method

Optimization of the acoustic influence for precipitation enhancing inside natural clouds with ground-based technique

Problem setting. The paper belongs to precipitation enhancement methods. The new option is proposed inside natural clouds by using low-frequency acoustics, 40–300 Hz, emitted from air sirens near ground. The purpose. The purpose of the article is to determine the necessary acoustic parameters, frequency and power, for effective collision of water droplet with their merge in natural clouds. Results. In this study convenient analytical formulas are derived for utilization during field experiments, and example calculations are provided for different environment. Increasing for acoustic intensity directly in atmosphere by diffraction is proposed. Practical significance. The paper indicates the optimal options and regimes for acoustics in natural clouds and ways to achieve this effect.

Comparative study on the performance and properties of polyether and polyacrylate crude oil demulsifier

ABSTRACT Polyacrylate crude oil demulsifier is one of the hot topics in the research and development of demulsifier at present, there is a lack of systematic comparative study on performance and properties of polyether and polyacrylate crude oil demulsifier. In this paper, the demulsification performance and microscopic demulsification process of the two demulsifiers, namely polyacrylate P(MA-DBC) and polyether P199–59 are discussed. The demulsification performance of polyether P199–59 was superior to that of polyacrylate P(MA-DBC). The study of the microscopic process of demulsification established that for P199–59, the diffusion rate was faster, the saturated adsorption capacity was higher, the ability to destroy the oil-water interface film was stronger and the probability of water droplets collision coalescence was higher; therefore, it can achieve faster dehydration speed and higher dehydration rate than that of P(MA-DBC). The results of this comparative study are meaningful for improving the demulsification performance of polyacrylate demulsifier.

Evaporation characteristics of cis-1,1,1,4,4,4-hexafluoro-2-butene (R1336mzz(Z)) droplet in high pressure and temperature environments

Cis-1,1,1,4,4,4-hexafluoro-2-butene (R1336mzz(Z)) has emerged as an exceptionally promising low-global-warming-potential (GWP) refrigerant, ideal for spray cooling systems in the thermal management of electronic components. Research on the evaporation characteristics of an individual isolated cryogen droplet excludes uncertainties caused by droplet collisions and fusion, thereby laying the foundation for spray cooling. In this paper, a theoretical model for single R1336mzz (Z) droplet evaporation considering the effect of natural convection in a high pressure and temperature environment is proposed. The newly proposed model is validated by comparing the predicted results of the R1336mzz(Z) droplet evaporation with experimental data. Then, the effects of environmental temperature (323–523 K) and pressure (1–20 bar) on the R1336mzz(Z) droplet evaporation are investigated. The results reveal that the effect of increasing the ambient pressure on the droplet lifetime of R1336mzz(Z) undergoes a transition from deceleration to acceleration. Elevated temperature can promote droplet evaporation; however, the promoting effect of increasing the ambient temperature on droplet evaporation will be weakened in high-pressure cases. Increasing the ambient pressure and temperature both can enhance the heat transfer from the environment to the droplet through natural convection, while increasing the pressure greatly inhibits the molecular diffusion during droplet evaporation. Thus, the total evaporation rate depends on the competing effects of these two factors. In addition, the trend of the droplet temperature variation could differ based on droplet initial temperatures, ambient temperatures, and pressures. An increase in the ambient temperature or pressure corresponds to an increase in the droplet equilibrium temperature (Tequ). However, Tequ is almost independent of the droplet initial size and temperature.

Effects of fuel injection pressure and quantity on low octane gasoline sprays in a simulated high-pressure ambient environment of a gasoline compression ignition engine

Gasoline Compression Ignition technology using low-octane gasoline-like fuel can lead to very low NOx and Soot emissions and enhanced thermal efficiency than conventional diesel engines. The difference in test fuel properties is a significant reason for the difference in the fuel–air mixture formation in Gasoline Compression Ignition engines offering these advantages. In this study, microscopic and macroscopic spray characterisation of baseline diesel, 50 % v/v diesel blended with 50 % v/v gasoline (G50), and 20 % v/v diesel blended with 80 % v/v gasoline (G80) are conducted in a high-pressure ambient environment relevant to Gasoline Compression Ignition engines. The study was performed in a constant volume spray chamber in non-reacting conditions using Phase Doppler Interferometry and Diffused Backlit Illumination techniques. The test fuels were injected into a 30 bar ambient pressure environment at 400, 500, and 700-bar fuel injection pressures. The liquid spray penetration length of gasoline-diesel blends was slightly shorter than baseline diesel, but their spray cone angles and entrained air mass were significantly higher. The average axial droplet velocity was higher, and the Sauter Mean Diameter was lower for gasoline-diesel blends than baseline diesel 40 mm away from the injector nozzle exit, indicating superior mixture formation for the gasoline-diesel blends. The effect of droplet collision and injected fuel quantity on droplet size distribution was assessed in all test fuels. Droplet collisions increased the Sauter mean diameter of the spray at farther distances. Smaller fuel injection quantities decreased the Sauter mean diameter of the spray, increasing the probability of unburnt hydrocarbon emission producing ultra-lean regions in the engine combustion chamber.

NUMERICAL INVESTIGATION OF OFF-CENTER COLLISION BETWEEN TWO EQUAL-SIZED WATER DROPLETS

Droplet collision is a basic phenomenon in numerous natural and industrial processes, while the understanding of collision dynamics is still lacking. In this work, a numerical investigation of the offcenter collision of two equal-sized water droplets is performed with the Weber number of 14 to 196 and impact parameter of 0 to 0.8. The incompressible Navier-Stokes equations are solved by the finite volume method. The volume of fluid (VOF) method and adaptive mesh technique are used to capture the gas-liquid interface. First, by comparing with reliable published experimental data, the reliability of the numerical results is verified. Then, the shape evolution for coalescence, reflexive separation, and stretching separation is described in detail. The effect of the Weber number and impact parameter on the collision of two equal-sized water droplets is analyzed. Moreover, the analysis of the surface energy and kinetic energy is conducted for the collision process. Furthermore, the dimensions of ligament and bridge for high-impact parameter stretching separation are presented quantitatively. Finally, the collision outcome for the simulation cases in this work is depicted and discussed. This work is helpful for fundamentally understanding the mechanism of collision dynamics of droplets, as well as applying the droplet collision model to related processes.

Kinetic characterization of low velocity positive collision of double droplets

Droplet positive collision is a complex process involving heat transfer in gas-liquid-solid three-phase flow and droplet collide dynamics. In order to study the kinetic behavior of droplets under positive collision in low velocity (v < 2 m/s), this paper focuses on the spreading, vibration, and fracture characteristics of double droplets by numerical simulation. First, the accuracy of the model is verified by experimental comparison. The effects of droplet diameters, collision velocities and wall contact angles on the spreading process are analyzed, and the spreading factor curves are plotted. Then, the droplet rebound vibration after collision fusion is equated to a single-degree-of-freedom damped vibration system, and the peak vibration height variation curve of the fused droplet is obtained by non-linear fitting. Considering the droplet phase change, the influence law of different conditions on the vibration damping factor and vibration time of the fused droplet is studied. Finally, it is found that rebound fracture and spreading fracture occur after the fusion of double droplets under positive collision, and the critical values of collide velocity required for the occurrence of the aforementioned phenomenon are found. To provide a reliable theoretical basis for the study of heat and mass transfer processes after multiple droplets collide the wall.

COLLISION MORPHOLOGIES SUPERCOOLED WATER DROPLETS ON SMALL LOW-TEMPERATURE SUPERHYDROPHOBIC SPHERICAL TARGETS

The collision and freezing of supercooled water droplets exist in many fields and are usually uncon-ducive. The superhydrophobic surfaces used for anti-icing generally have microstructures or local protrusions which could be simplified as small spherical targets comparable to the droplet in size. The supercooled water droplets' collision and freezing on small low-temperature superhydrophobic spherical targets with the sphere-to-droplet diameter ratio D* ≤ 1 are studied numerically in this work. Coupling the solidification-melting model, the Volume of Fluid (VOF) method is used to implement numerical simulations. The supercooling degree, Weber number, and sphere-to-droplet diameter ratio effects on the collision and freezing behaviors and the area coverage ratio of the droplet on the low-temperature small sphere are investigated. Six typical morphologies are identified: full dripping, partial dripping, lower adhesion, wrapping adhesion, upper adhesion, and rebound. The water droplet is found to be more likely to drip down with the increasing Weber number, and the decreasing supercooling degree and the decreasing diameter ratio. A comprehensive morphology map is eventually established to illustrate the combined influence of the Weber number and diameter ratio on the occurrences of the rebound, adhesion, and dripping for different supercooling degrees. This work provides theoretical guidance for the engineering design and structural optimization of anti-icing surfaces.

Rupture of thin liquid trilayer films with soluble surfactants: fundamentals and applications to droplet coalescence.

Understanding the stability of thin liquid trilayer films is of direct relevance to applications such as multilayer coatings and polymer processing. The stability of trilayer films can also be used to provide insights into emulsion dynamics, such as the rupture of the thin film formed between two droplets during coalescence. Often, emulsions are laden with surfactants and other additives, which can be present in one or both phases as well as at the interfaces between the liquids. In experimental studies, complicating factors such as variations in droplet sizes, curvatures, and collision processes make it difficult to specifically isolate the influence of surfactant transport on droplet coalescence and film rupture. The present work addresses this issue by systematic consideration of a model problem involving a thin liquid trilayer film. Surfactant is soluble in either the outer layers or the inner layer, corresponding to surfactant soluble in the droplets or the continuous phase. Rupture of the inner layer is driven by van der Waals forces. Lubrication theory is applied to derive coupled nonlinear evolution equations describing the perturbations to the interface positions and the surfactant concentrations. Our findings reveal that surfactant better stabilizes the film when soluble in the inner layer, and the stabilizing effect is more pronounced when the outer layers are thicker. These findings are consistent with experimental observations involving emulsions, where emulsions tend to be more stable when surfactant is in the continuous phase rather than in the droplets, with the distinction being more pronounced when droplets are larger.

AN INTER-IMPINGEMENT SPRAY MODEL CONSIDERING BINARY DROPLET COLLISION BASED ON OPENFOAM

Collision and breakup are the two main processes of the inter-impingement spray within internal combustion engines. Previous models did not comprehensively account for five collision regimes that delineate collision dynamics under high-pressure conditions, leading to deviations in the prediction of Sauter mean diameter (SMD) and frequency of bouncing regime. Hence, this article integrates an improved composite collision model for high-pressure conditions, incorporating all collision regimes, as well as the generation of satellite droplets. Due to the significant dependence of collision regimes on the geometric relationships between collision pairs, this article advocates the combined utilization of the composite collision model with the absolutely deterministic trajectory (ADT) model. Furthermore, the calculation method of the breakup length Bl that separates the primary and secondary breakup is modified for high-speed inter-impingement sprays. Extensive validation of binary droplet collisions and inter-impingement sprays are conducted. The comparative analysis reveals that the conventional Bl overestimates the penetration, the simplified collision model overestimates the SMD, while the present model agrees well with experimental results. Results show that a slight increase in global SMD and a significant reduction in velocity after the impingement point. Downstream of the impingement point, SMD at the ends exceeds that in the middle when the impinging distance <i>S</i><sub>z</sub> > <i>B</i><sub>l</sub>, contrasting with the situation when <i>S</i><sub>z</sub> < <i>B</i><sub>l</sub> and closer to the impingement point. Moreover, the occurrence of the coalescence regime is more frequent during secondary breakup than in primary breakup before the impingement point.

Symmetry-Breaking-Induced Internal Mixing Enhancement of Droplet Collision

Binary droplet collision is a basic fluid phenomenon for many spray processes in nature and industry involving lots of discrete droplets. It exists an inherent mirror symmetry between two colliding droplets. For specific cases of the collision between two identical droplets, the head-on collision and the off-center collision, respectively, show the axisymmetric and rotational symmetry characteristics, which is useful for the simplification of droplet collision modeling. However, for more general cases of the collision between two droplets involving the disparities of size ratio, surface tension, viscosity, and self-spin motions, the axisymmetric and rotational symmetry droplet deformation and inner flow tend to be broken, leading to many distinct phenomena that cannot occur for the collision between two identical droplets owing to the mirror symmetry. This review focused on interpreting the asymmetric droplet deformation and the collision-induced internal mixing that was affected by those symmetry breaking factors, such as size ratio effects, Marangoni Effects, non-Newtonian effects, and droplet self-spin motion. It helps to understand the droplet internal mixing for hypergolic propellants in the rocket engineering and microscale droplet reactors in the biological engineering, and the modeling of droplet collision in real combustion spray processes.

Measurement of droplet coalescence in a mixing tank

AbstractIt is in urgent requirement to directly measure the droplet coalescence frequency for the application of the population balance model. In this study, a method was firstly developed to directly measure the droplet coalescence frequency in turbulent flow field by using a specially designed mixing tank and a high‐speed camera. The effects of the rotating speed and holdup fraction on the droplet coalescence frequency were quantitatively investigated. The increasing of rotation speed promotes first and then inhibits the coalescence, while the holdup fraction has little influence on the coalescence frequency function. The droplet collision frequency was also counted and the coalescence efficiency was calculated. The models in the literature were tested with our experimental data and were found failing to predict the coalescence frequency in the stirring tank. Empirical correlation was finally proposed and good agreement was found between the prediction results and the experimental data.

Changes in boiling regime and heat transfer effectiveness during water droplet collision with a superheated wall

We experimentally investigated changes in boiling regime and heat transfer effectiveness when single droplets collided with superheated substrates at various initial temperatures. Saturated water droplets colliding at constant speeds were studied over a range of initial substrate temperatures that spanned all boiling regimes from nucleate through transition to film boiling. Space- and time-resolved infrared thermometry and convectional shadowgraph imaging were used to precisely derive local heat transfer distributions and detect characteristic boiling regimes. The results confirmed the existence of the typical regions of a boiling curve: nucleate, transition, and film. The transition boiling regime could be subdivided into three sub-regimes—bubbly, oscillating, and fingering—during which the heat transfer effectiveness (THE) varied in a non-linear manner, exhibiting local minima and maxima. To facilitate interpretation of non-linear variations in HTE during transition boiling, the basic physical parameters of local heat flux, contact area, and residence time were quantitatively measured using obtained thermometry images. In the transition boiling regime, HTE first rapidly decreased during bubbly boiling principally because of a substantial decrease in the droplet residence time. And THE increased during oscillating boiling because of contact area expansion and increased heat flux. Finally, THE decreased during fingering boiling due to substantial reduction in the area fraction of liquid contact. When liquid-solid contact was completely prohibited, stable film boiling was stable. Thus, the dynamic Leidenfrost temperature point could be detected based on heat transfer characteristics, rather than collision dynamics.

Heat transfer process in the collision of high-temperature aluminum droplets with an inclined wall

The heat transfer process occurring when high-temperature aluminum droplets collide with a solid surface is investigated via numerical simulation and experimentation. The wall temperature at the impact point rapidly increases, with some oscillations, reaching a maximum value of 1054.5 K. The maximum wall heat flux density during the droplet–wall collision is 63,135 kW/m2. As the droplet spreads on the wall, the temperature at the impact point undergoes a two-stage decrease: a rapid decrease followed by a slow decrease. During the initial stage of the droplet–wall collision, the wall temperature is asymmetrically distributed, with the increase in temperature at the advancing edge spreading more slowly than that at the receding edge. Similarly, the corresponding heat flux density distribution at that time is also asymmetrically distributed, with higher heat flux density at the trailing edge of the spread than at the leading edge.

On the dynamics of head-on collision of sessile droplets on the traveling substrate

In this work, the dynamics of the head-on collision of two sessile water droplets, namely the moving droplet and pinning droplet, on a traveling substrate was experimentally investigated. Unlike the traditional scenario of sessile droplets collision, the traveling substrate propelled the moving droplet attached to it to move at a certain speed. The results revealed that there are only two distinct regimes of collision consequences: coalescence without separation and separation with separation. A series of time-resolved photographic images mapping all the collision regimes in the parameter space of Capillary number and size ratio were used to identify the controlling factors for different collision results. The breakup of the receding meniscus in separation regime was discussed in detail as it is caused by the change of the droplet morphology. The findings indicated that the length of the receding meniscus played a significant role in the droplet impact dynamics, and when it exceeded about 8.2 mm, the receding meniscus broke up. Furthermore, it was demonstrated that the sum of the volume of initial pinning droplet’s receding meniscus and the volume of the moving droplet before collision is equal to the volume of the receding meniscus at its maximum length after collision. The critical Capillary number/size ratio for the transition between these two regimes was derived based on the volume principle before and after the collision. The model-predicated results show good agreement with the experiments. The findings could contribute to a better understanding of the droplet collision dynamics and the optimization of the anti-icing, self-cleaning and inkjet printing devices.

Effects of secondary breakup, collision dynamics, gravity and evaporation on droplet size distribution in a pressure-swirl JET A-1 spray

Size and velocity of droplets in pressure-swirl sprays vary with distance downstream the nozzle. This study aims to explain the phenomena contributing to size-resolved spatial variation of the drop size distribution for Jet A-1 spray from a small pressure swirl atomizer injected into quiescent ambient air at atmospheric conditions. The effects of secondary breakup, droplet collisions, gravity, drag-driven spray dispersion, and evaporation are evaluated. Phase Doppler anemometer (PDA) was used to resolve the velocity and size of the droplets in radial profiles at several axial distances (Z) from the nozzle exit at injection pressures between 0.5 and 1.5 MPa. The droplet motion and collision dynamics were qualitatively characterised by high-speed imaging. The analysis focused on areas along 1) the spray axis and 2) the liquid sheet direction. The first area covers small droplets with marginal evolution, while the mean drop size in the second area significantly increases downstream. The local drop size distribution and its axial evolution results from a combined effect of ballistic filtering (drifting of small droplets from periphery to spray centre and large droplets vice versa), centrifugal and turbulent droplet dispersion, and droplet collisions (increasing drop size with distance). The relative droplet-gas velocity was found at investigated Z positions too small for drag-driven secondary droplet breakup to occur. Evaporation of Jet A-1 sprayed at room temperature and pressure into still atmosphere reduces the drop size marginally. Trajectory of all drop sizes is insignificantly altered by gravity. The smallest droplets are strongly drag-driven to the spray axis, while large ones disperse centrifugally and have sufficient momentum to neglect the gravity. Combining results from imaging and laser diagnostics, various collision outcomes were identified, with a conclusion that these depend on the size and velocity of colliding droplets. Coalescing collisions dominate and strongly increase the mean drop size downstream the spray at off-axis positions while insignificantly contribute along the spray axis. The drop size span factor reduces with Z as separative collisions between large droplets and mixed-type collisions between small and large droplets narrow the drop size distribution. The above effects can be most easily amplified for drop size reduction via modification of physical properties of sprayed liquid by its heating.

Migration Movements of Accidentally Spilled Oil in Environmental Waters: A Review

Accidentally spilled oil can cause great harm to the ecological balance of water once it enters the environmental waters. Clarifying its movement behavior and migration law in water has been the focus of environmental hydraulics research. This review starts from the mechanism of the oil spill migration process, and firstly reviews the kinematic characteristics of the smallest moving unit of the oil spill, the individual oil droplet, as well as focusing on several key aspects such as droplet shape, trajectory, terminal velocity and drag coefficient. Subsequently, considering the commonalities and differences between inland riverine and oceanic environments, different aspects of oil droplet collision, coalescence, breakage, particle size distribution, and vertical diffusion are discussed separately. Finally, the current status of research on the migration laws of accidental oil spills in environmental waters is summarized, and feasible future research directions are proposed to address the emerging research problems and research gaps.

Numerical simulation of binary droplet collisions using a Front Tracking interface technique

Droplet-droplet interactions of highly viscous liquids or suspensions are relevant in a broad range of industrial applications, such as spray drying, fuel injection or, coating and granulation. The efficiency of these processes is heavily determined by the surface area and volume of the droplets. To operate these processes efficiently, it is critical to accurately describe the physical mechanisms dominating the collision. Nonetheless, a complete description of the forces governing the collision is still missing, particularly when the liquids have a high viscosity. Front tracking techniques represent a promising alternative for studying droplet-droplet interactions as they ensure a sharper representation of the liquid-gas interface. In this work, the Local Front Reconstruction Method (LFRM) is the front tracking technique chosen as it allows merging and break-up. The validation of this method is extended for different Weber numbers, impact parameters, and liquid properties by determining the capabilities of LFRM to reproduce the collision of glycerol solution droplets. The simulation results are validated with dedicated experiments performed at the same Weber and impact parameter conditions. In the majority of the studied collisions, LFRM shows a good correspondence with experiments and literature, which proves the ability of LFRM to capture droplet collisions under the studied conditions.