Droplet Collision Research Articles

Investigation on collision-coalescence of droplets under the synergistic effect of charge and sound waves: orthogonal design optimization

As an efficient approach to improve visibility, defogging technology is essential for the operation of ports and airports. This paper proposes a new and hybrid defogging technology, i.e. an electric–acoustic defogging method. Specifically, the droplets are charged by corona discharge, which is beneficial to overcome the hydrodynamic interaction force to improve the droplet collision efficiency. Meanwhile, sound waves (especially acoustic turbulence) promote the relative movement of droplets to increase the collision probability. In this study, the effects of acoustic frequency (f), sound pressure level (SPL), and voltage (V) on the droplet growth ratio were studied by orthogonal design analysis. The results of difference analysis and multi-factor variance analysis show that frequency and SPL are the dominant factors that affect the collision of droplets, and the effect of voltage is relatively weak. And f= 400 Hz, SPL = 132 dB, and V = −7.2 kV are the optimal parameters in our experiment. In addition, we further studied the impact of single factor on droplet growth ratio. The results show that there exists an experimental optimal frequency of 400 Hz. The droplet growth ratio increases with SPL and voltage level. The new technology proposed in this paper can provide a new approach for defogging in open space.

Collisions of Two-Phase Liquid Droplets in a Heated Gas Medium.

The paper presents the experimental research findings for the integral characteristics of processes developing when two-phase liquid droplets collide in a heated gas medium. The experiments were conducted in a closed heat exchange chamber space filled with air. The gas medium was heated to 400–500 °C by an induction system. In the experiments, the size of initial droplets, their velocities and impact angles were varied in the ranges typical of industrial applications. The main varied parameter was the percentage of vapor (volume of bubbles) in the droplet (up to 90% of the liquid volume). The droplet collision regimes (coalescence, bounce, breakup, disruption), size and number of secondary fragments, as well as the relative volume fraction of vapor bubbles in them were recorded. Differences in the collision regimes and in the distribution of secondary fragments by size were identified. The areas of liquid surface before and after the initial droplet breakup were determined. Conditions were outlined in which vapor bubbles had a significant and, on the contrary, fairly weak effect on the interaction regimes of two-phase droplets.

Spraying of Composite Liquid Fuels Based on Types of Coal Preparation Waste: Current Problems and Achievements: Review

This article discusses the atomization of composite liquid fuels. A large group of injectors is considered. A comparative analysis of the atomization characteristics (droplet sizes and velocities, jet opening angles) and the influence of the fuel characteristics (density, viscosity, component composition) and the process parameters (the ratio of the fuel–air mass flow rates, the features of the jet formation) has been carried out. Finally, the most effective types of injectors, which provide for the necessary characteristics of fuel atomization for its combustion, have been determined. The most favorable conditions for the applicability of each type of atomization have been formulated. Possible mechanisms of secondary fragmentation of droplets of composite fuels have been analyzed: those resulting from mutual collisions of droplets in the flux and from the interaction with a solid surface as well as those resulting from thermal overheating in the presence of a phase boundary or a large gradient of component volatility. A conclusion is made about the need of using a synergistic effect of primary and secondary atomization of fuel suspension droplets.

Droplet trajectories and collisions in gas forced vortexes: A molecular dynamics study

Molecular dynamics simulations of droplets in gas forced vortexes by applying a virtual rotational potential were presented. The result showed that the flow field scale of the simulation performed can reflect the macroscopic characteristics. The trajectories of water droplets are longer than that of oil droplets within a certain radius of flow field. A droplet trajectory model was proposed to predict droplet behavior. The study continued to investigate droplet collisions. It was observed that the large oil droplet and the downwind small water droplets produced near head-on, highly off-centre and scraping-adhering collisions, forming stable two-phase interfaces. The slight changes of the interfaces between water and oil droplets under the action of rotating flow field can be reflected by van der Waals interaction. The intermolecular hydrogen bonds form when two water droplets collide. Our study provides a new idea for further understanding of droplet collision in gas flow field.

Experimental research of liquid droplets colliding with solid particles in a gaseous medium

This article presents the research findings on binary collisions of water droplets and solid particles using three most widely spread ranks of coal. The experimental setup used in this research allowed the variation of the particle size from 0.25 mm to 0.75 mm and the velocity from 0.5 m/s to 4 m/s. The droplet velocity was varied from 0.3 m/s to 6 m/s. The impact angle of attack ranged 0–90°. These variation ranges of initial parameters correspond to promising gas–vapor–droplet technologies such as spray drying, fire extinguishing, water treatment, fuel mixing and combustion, heat and mass transfer power plants, etc. We analyzed the frames showing three collision regimes: coalescence, separation, and disruption. We determined how the temperature of the liquid medium affects the position of droplet collision regime boundaries on the corresponding maps in the B(We) coordinates. We also determined the impact of hydrophilic and hydrophobic behavior of solid particles on the collision characteristics. The size distributions were established for secondary fragments formed during the collision of water droplet and solid particle, as well as the collision of two droplets. The key patterns of the processes under study were singled out for various droplet shapes and size ratios. Finally, we compared the characteristics and conditions of interaction between water droplets and water-based slurries.

Electrosynthesis of palladium nanocatalysts using single droplet reactors and catalytic activity for formic acid oxidation

The synthesis of nanosized palladium (Pd) catalyst from a Pd2+ emulsion is performed using the emulsion nanodroplet-mediated electrodeposition method. In this method, aqueous emulsion droplets act as tiny reactors for the inner precursor (e.g., Pd2+) to reduce and generate the corresponding metal products while they are discretely colliding on the working ultramicroelectrode surface, which is biased at a suitable constant potential. The utilized volume of each reactor (with a given concentration of the Pd2+ precursor) and the monitored total number of deposition (i.e., droplet collision) events, being the critical factors, can facilitate the controllable (or quantifiable) synthesis of Pd nanocatalysts where the amount of Pd per deposition event and the generated product amount are determinable. In this research, individual Pd nanocatalysts nucleate from droplet collisions and become uniformly distributed on the substrate surface. The electrocatalytic activities of the synthesized Pd nanocatalysts for formic acid electrooxidation are higher than ca. 10 A mg(Pd)−1.

Collision dynamics of binary liquid metal droplets under horizontal magnetic field

We conduct experimental research on binary liquid metal droplet collisions under the influence of a horizontal magnetic field. For magnetic intensity smaller than 1.5 T, only reflexive separation is obviously facilitated, but other collision regimes show little difference with zero magnetic intensity. We infer that when magnetic intensity is larger than a critical value of 4 T, remarkable influences on all regimes would be observed.

Head-on Collision of Two Nanodroplets on a Solid Surface: A Molecular Dynamics Simulation Study.

Most researchers focus on the collision of a single droplet with a solid surface, while it is common for a droplet to collide with a sessile droplet on a solid surface in reality. This study performed the head-on collision of two nanodroplets on a solid surface using the molecular dynamics simulation method. The effects of impact velocity, interaction intensity between solid and liquid atoms, and the solid fraction of the surface on the collision process are studied with independent simulation cases. The maximum spreading factor and the dimensionless maximum spreading time are recorded and calculated to describe the collision process quantitatively. The simulation results indicate that the maximum spreading factor depends more on the solid fraction than the interaction intensity since it does not fundamentally change the wetting state of the droplet at its maximum spreading state. Because of two different effects, the maximum dimensionless spreading time decreases first and then increases with the interaction intensity, and both effects weaken with the increase of impact velocity. As the solid fraction increases, the maximum spreading factor increases significantly at high impact velocity, and the maximum dimensionless spreading time first decreases and then increases because the wetting state of the coalescent droplet at the maximum spreading moment gradually changes from the Wenzel state to the Cassie state. In general, the initial wetting state of the sessile droplet and the wetting state of the coalescent droplet at the maximum spreading moment have important effects on the maximum spreading factor and the maximum spreading time. We establish a theoretical prediction model for the maximum spreading factor on a smooth surface based on energy conservation with quite good accuracy. This research has improved our understanding of the head-on collision process of two nanodroplets on a solid surface.

Study of Stratus-Lowering Marine-Fog Events Observed During C-FOG

Two stratus-lowering marine fog events observed on 28 September and 4 October 2018 during the Coastal Fog (C-Fog) field campaign that took place offshore of eastern Canada from 1 September to 6 October 2018 are described. In situ, profiling, and remote sensing observations were made at selected land sites in eastern Newfoundland, Nova Scotia, and aboard the research vessel Hugh R. Sharp cruising in adjoining coastal waters. Synoptic-scale analysis shows that both fog episodes result from the interaction between synoptic-scale surface-level low-pressure systems and a contiguous high-pressure system. At the same time, back trajectories reveal that the bulk of the fog layer is formed due to differential advection. The diameter of the fog droplets at the surface gradually decreases from the centre of the fog layer to its leading/trailing edges. The bimodal fog-droplet diameter distribution with peaks at 5–10 µm and 20–25 µm provide clues on droplet collision and coalescence processes. The observed difference between microphysical variables and droplet distribution between the two fog events and within the same fog layer might be governed by the atmospheric-boundary-layer (e.g., humidity conditions and turbulence) that prevailed in the fog layer. Overall, it is concluded that the life cycle of observed stratus-lowering coastal-fog episodes depends on synoptic conditions and atmospheric-boundary-layer characteristics such as stability, cloud-top cooling, and entrainment.

Transitions of bouncing and coalescence in binary droplet collisions

In droplet impacts, transitions between coalescence and bouncing are determined by complex interplays of multiple mechanisms dominating at various length scales. Here we investigate the mechanisms and governing parameters comprehensively by experiments and scaling analyses, providing a unified framework for understanding and predicting the outcomes when using different fluids. Specifically, while bouncing had not been observed in head-on collisions of water drops under atmospheric conditions, it was found in our experiments to appear on increasing the droplet diameter sufficiently. Contrarily, while bouncing was always observed in head-on impacts of alkane drops, we found it to disappear on decreasing the diameter sufficiently. The variations are related to gas draining dynamics in the inter-droplet film and suggest an easier means for controlling bouncing as compared to alternating the ambient pressure usually sought. The scaling analysis further shows that for a given Weber number, enlarging droplet diameter or fluid viscosities, or lowering surface tension contributes to a larger characteristic minimum thickness of the gas film, thus enhancing bouncing. The key dimensionless group $(O{h_{g,l}},\;O{h_l},\;{A^\ast })$ is identified, referred to as the two-phase Ohnesorge number, the Ohnesorge number of liquid and the Hamaker constant, respectively. Our thickness-based model indicates that as ${h^{\prime}_{m,c}} > 21.1{h_{cr}}$ , where ${h^{\prime}_{m,c}}$ is the maximum value of the characteristic minimum film thickness $({h_{m,c}})$ and ${h_{cr}}$ is the critical thickness, bouncing occurs in both head-on and off-centre collisions. That is, when $1.2O{h_{g,l}}/(1 - 2O{h_l}) > \sqrt[3]{{{A^\ast }}}$ , a fully developed bouncing regime occurs, thereby yielding a lower coalescence efficiency. The transitional Weber number is found universally to be 4.

Interaction between Strong Sound Waves and Cloud Droplets: Theoretical Analysis

AbstractRecently, strong sound wave was proposed to enhance precipitation. The theoretical basis of this proposal has not been effectively studied either experimentally or theoretically. On the basis of the microscopic parameters of atmospheric cloud physics, this paper solved the complex nonlinear differential equation to show the movement characteristics of cloud droplets under the action of sound waves. The motion process of an individual cloud droplet in a cloud layer in the acoustic field is discussed as well as the relative motion between two cloud droplets. The effects of different particle sizes and sound field characteristics on particle motion and collision are studied to analyze the dynamic effects of thunder-level sound waves on cloud droplets. The amplitude of velocity variation has positive correlation with sound pressure level (SPL) and negative correlation with the frequency of the surrounding sound field. Under the action of low-frequency sound waves with sufficient intensity, individual cloud droplets could be forced to oscillate significantly. A droplet smaller than 40 μm can be easily driven by sound waves of 50 Hz and 123.4 dB. The calculation of the collision process of two droplets reveals that the disorder of motion for polydisperse droplets is intensified, resulting in the broadening of the collision time range and spatial range. When the acoustic frequency is less than 100 Hz (at 123.4 dB) or the SPL is greater than 117.4 dB (at 50 Hz), the sound wave can affect the collision of cloud droplets significantly. This study provides a theoretical perspective of the acoustic effect on the microphysics of atmospheric clouds.

Collisions of Liquid Droplets in a Gaseous Medium under Conditions of Intense Phase Transformations: Review

The article presents the results of theoretical and experimental studies of coalescence, disruption, and fragmentation of liquid droplets in multiphase and multicomponent gas-vapor-droplet media. Highly promising approaches are considered to studying the interaction of liquid droplets in gaseous media with different compositions and parameters. A comparative analysis of promising technologies is carried out for the primary and secondary atomization of liquid droplets using schemes of their collision with each other. The influence of a range of factors and parameters on the collision processes of drops is analyzed, in particular, viscosity, density, surface, and interfacial tension of a liquid, trajectories of droplets in a gaseous medium, droplet velocities and sizes. The processes involved in the interaction of dissimilar droplets with a variable component composition and temperature are described. Fundamental differences are shown in the number and size of droplets formed due to binary collisions and collisions between droplets and particles at different Weber numbers. The conditions are analyzed for the several-fold increase in the number of droplets in the air flow due to their collisions in the disruption mode. A technique is described for generalizing and presenting the research findings on the interaction of drops in the form of theoretical collision regime maps using various approaches.

Inertial stretching separation in binary droplet collisions

Binary droplet collisions exhibit a wide range of outcomes, including coalescence and stretching separation, with a transition between these two outcomes arising for high Weber numbers and impact parameters. Our experimental study elucidates the effect of viscosity on this transition, which we show exhibits inertial (viscosity-independent) behaviour over an order-of-magnitude-wide range of Ohnesorge numbers. That is, the transition is not always shifted towards higher impact parameters by increasing droplet viscosity, as it might be thought from the existing literature. Moreover, we provide compelling experimental evidence that stretching separation only arises if the length of the coalesced droplet exceeds a critical multiple of the original droplet diameters (3.35). Using this as a criterion, we provide a simple but robust model (without any arbitrarily chosen free parameters) to predict the coalescence/stretching-separation transition.

CFD simulation of agglomeration and coalescence in spray dryer

Particle size distribution is of utmost importance in spray dryers. Controlling the degree of agglomeration and coalescence is vital for manufacturing products that conform to the requirements. To predict the particle size accurately, this study presents a CFD model of particle agglomeration and droplet coalescence in a counter-current spray dryer. The stochastic agglomeration model was modified to incorporate more realistic droplet-droplet, droplet-particle and particle-particle interactions. Binary droplet collisions in the atomization zone of a spray dryer can result in either coalescence or separation. The boundaries between collision regimes were experimentally determined, and existing binary collision models were validated for water and milk concentrate to include the effect of viscous forces on collision outcomes. A high-speed camera was used to record the kinematic and physical properties of the droplets to identify various collision regimes. A suitable model was then incorporated in CFD simulations of spray drying. Using existing experimental data from a counter-current spray dryer with strong agglomeration, it is shown that the model can accurately predict product particle properties under different process conditions.

Experimental studies on binary water droplet collisions considering size ratio effects

Experimental studies on binary water droplet collisions considering size ratio effects

An improved algorithm to estimate droplet collision characteristics for Lagrangian simulation of impinging spray under different ambient pressures

An improved algorithm to estimate droplet collision characteristics for Lagrangian simulation of impinging spray under different ambient pressures

Suppressing Breakups in Binary Droplet Collisions by Surfactant

Suppressing Breakups in Binary Droplet Collisions by Surfactant

Euler/Lagrange Computations of Sprays Produced by Different Liquids using a Generalised Stochastic Droplet Collision Model

Euler/Lagrange Computations of Sprays Produced by Different Liquids using a Generalised Stochastic Droplet Collision Model

Size effects on dynamics of nanodroplets in binary head-on collisions

Head-on collision dynamics of 10, 50 and 100 nm droplets are investigated in vacuum by molecular dynamics, involving 35,858, 4,506,410 and 36,051,466 molecules, respectively. A variety of droplet collision dynamics are observed, such as coalescence, hole formation and shattering, as a function of the Weber number. It is found for the first time that the collision and reflexive separation can occur in the nanodroplet regime when the droplet diameter reaches 100 nm but not for 10 or 50 nm droplets. The size effect in droplet collisions is studied based on the analysis of stretching factors, energy dissipation and collision outcomes for droplets of different diameters. The kinetic energy dissipation due to the atomic interactions at nanoscales is identified to significantly influence the occurrence or otherwise of reflexive separation. Through quantitative analysis of the evolution of the internal structure of the 100 nm nanodroplets collision at the Weber number of 277, it is revealed for the first time that molecules from both parent nanodroplets have penetrated the full length of the merged nanodroplet in the direction of collision, due to a combination of molecular mixing and internal currents. Consequently, all three child nanodroplets have molecules from both parent nanodroplets, contrary to the perception gained from common imaging techniques. The results show that the dynamics, outcomes and mechanisms of nanodroplet collisions have both similarities and differences compared with their micro- and macro-counterparts.

Characterization of clay-stabilized, oil-in-water Pickering emulsion for potential conformance control in high-salinity, high-temperature reservoirs

In this study, a Na-montmorillonite clay-stabilized, oil-in-water (O/W) Pickering emulsion system was prepared and tested under high-salinity and high-temperature conditions. The emulsifying ability of the clay particles, emulsion stability against creaming and coalescence, and emulsion rheological behavior at different NaCl concentrations and temperatures were investigated. The results showed that decane could be effectively emulsified by clay particles into small emulsion droplets at salinities up to 25 wt% NaCl. The emulsion had a relatively narrow drop size distribution. However, the emulsion stability against creaming decreased with increases in NaCl concentration. At low NaCl concentrations, clay particles connected with each other by edge-edge and edge-face interactions, forming a three-dimensional network that stabilized the emulsion against creaming. A higher concentration of NaCl caused some clay particles to connect through face-face interactions, forming clay clusters. At high NaCl concentrations, the temperature had only a slight effect on the emulsion stability against creaming. However, emulsion stability against coalescence decreased with increasing temperature due to increased droplet collision caused by Brownian motion. A higher concentration of NaCl moderated this temperature effect of coalescence. The viscosity of emulsion decreased with increased NaCl concentration. Temperature increases caused emulsion viscosity to decrease at low NaCl concentration conditions. When the NaCl concentration was higher than 10 wt%, the temperature had little effect on the viscosity. In summary, this study identifies an O/W Pickering system with the potential for conformance control in high-salinity and high-temperature reservoirs.