Collision Of Water Droplets Research Articles

Physical and Numerical Experimentation of Water Droplet Collision on a Wall: A Comparison between PLIC and HRIC Schemes for the VOF Transport Equation with High-Speed Imaging

Liquid films are often found in engineering applications with thicknesses ranging from micrometer scales to large scales with a wide range of applications. To optimize such systems, researchers have dedicated themselves to the development of new techniques. To further contribute to this development, the objective of this work is to present the results of the collision of water droplets on a wall by means of experimentation and numerical simulations. For physical experimentation, an injector is used to generate a chain of water droplets that collide with the opposite wall, forming a liquid film. Images of the droplets were obtained using two high-speed recording cameras. The results for different droplet sizes and impact angles are presented and the relationship between the momentum parameter and non-dimensional pool size was established. Modeling such processes is a common challenge in engineering, with different techniques having their advantages and limitations. The simulations in this work were run using the volume of fluid method, which consists of solving a transport equation for the volume fraction of each considered fluid. A correlation was found between the surface tension to momentum transport ratio, Scd, and the non-dimensional pool size for different droplet sizes and impact angles. Regions where partial depositions were most likely to occur were found via physical experiments.

Droplet collision with hydrophobic and superhydrophobic surfaces: Experimental studies and numerical modeling

The characteristics of the collision of water droplets with textured hydrophobic and superhydrophobic surfaces obtained using two techniques were studied experimentally. The surface processing techniques are based on laser modification of the surface layer and changes in its chemical composition. Weber numbers in the experiments were varied in the range of 10–200. Regimes of water droplet-surface collision, the critical conditions for the transition between regimes, as well as integral characteristics of the formed liquid fragments were identified. The main characteristics of the process under study, such as the droplet spreading diameter, the height of its rebound from the surface, the number of secondary fragments during its break up were obtained experimentally. A model with a two-dimensional axisymmetric formulation was used to predict the characteristics of water droplet-textured surface collision based on the phase field method. The results of experimental studies and numerical modeling are in satisfactory agreement (differences of no more than 3–5 %). The conditions, when it is necessary to consider the values of static and dynamic contact angles during the modeling process, were determined.

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.

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.

Interaction of water droplets with pyrolyzing coal particles and tablets

The paper presents the experimental research findings for the patterns of collisions of water droplets with pressed tableted samples used as substrates and with small particles of a pyrolyzing solid fuel. Brown coal samples were used. Droplet-substrate interactions were studied when varying the droplet diameter in the range from 1 to 4 mm and velocity from 0.5 to 4 m/s. That corresponded to the Weber number range of 7–830. The coal tablet surface temperature was varied from 20 to 700 °C. In the interactions of water droplets (0.7–1.5 mm in diameter, pre-collision velocity from 1 to 3 m/s) with coal particles (with a size of 0.2–1 mm, pre-collision velocity 0.7–2 m/s), the temperature of the latter was varied in the range of 330–480 °C. The following regimes of the interaction of droplets with solid particles during chemical reactions and phase transformations were distinguished: spreading/agglomeration and break-up/separation. Differences in the characteristics of the interaction of water droplets with coal particles at varying temperatures were identified. Droplet-particle interaction regime maps for B(We), We(Oh) and We(Ca) were constructed. The collision regime boundaries were described using fitted curves that can be utilized to develop the existing mathematical models of droplet-particle collisions in gas. It was established that the gaseous volatile production in coal pyrolysis has a modest effect on the regimes and characteristics of the droplet destruction in the temperature range under consideration (20–700 °C).

Experimental study on the impact forces of a water droplet colliding with a surface carrying a deposited droplet

In this study, the process of water droplet collision with a horizontal surface carrying a deposited droplet and the impact forces caused by the Rayleigh jet in the retraction stage of collision were experimentally investigated. Such collisions are common in various industrial applications. The droplet impact force on the surface was measured using a piezoelectric sensor and the corresponding instantaneous droplet morphology was obtained using high-speed photography. For comparison, the characteristics of Rayleigh jets and their corresponding impact forces were studied. When droplets collide with a surface carrying a deposited droplet, the Rayleigh jet may be accompanied by a peak impact force that is much greater than that in the spreading stage of the collision. This impact force exhibits a decaying oscillatory pattern. The appearance of decaying oscillatory impact forces is closely related to the mass ratio of deposited droplets to falling droplets, as well as the Weber number and Froude number of falling droplets. The reaction force of the upward Rayleigh jet on the surface in the case where decaying oscillatory impact force does not appear leads to a relatively small peak impact force. A scaling relationship for the reaction force peak is derived.

Intense deformation and fragmentation of two droplet collision at high Weber numbers

Droplet collision is common in nature and industrial processes, while understanding of the phenomena at high Weber numbers is not as comprehensive as the one at low Weber numbers. We experimentally investigate the collision dynamics of two droplets at high We ranging from 200 to 2000 by adopting pure water and pure ethanol droplets. Four distinct droplet collision regimes in the order of increasing deformation and fragmentation are reflection separation, finger separation, stretching separation and splashing. Due to the larger inertia force and lower viscous force, water droplet collision is easier to form finger separation and splashing regimes than ethanol droplets under the same We. The reinforcement of viscous force and surface tension weakens droplet deformation and fragmentation, and further changes droplet collision regime. Due to the viscous dissipation ratio of droplet at the maximum spread moment as a function of Ohnesorge number (Oh), the model of the maximum spread factor of coalesced droplet is different under various Oh. For a low Oh, a new formula of the maximum spread factor is proposed by optimizing the maximum spread time in the viscous dissipation model based on energy conservation, which agrees well with the experiment. For a high Oh, the formula of the maximum spread factor is developed by using the viscous dissipation ratio, and it is consistent with the experiment as the viscous dissipation ratio is 0.8.

Collisions between liquid droplets during the intersection of aerosol flows in a heated gas

• A rise in the gas temperature led to an increase in the number of secondary fragments. • At T g = 300–400 °C droplets with r d = 0.075–0.1 mm prevail. • A temperature rise contributed to a 2–3 times increase in area ratios S 1 / S 0. • At T g = 100–400 °C, droplets increased in size due to expansion of liquid on heating. • The motion of droplets in a heated gas led to a 10–30% change in their size due to evaporation. The paper presents the experimental findings on the water droplet collision behavior during the intersection of two aerosol flows in a high-temperature gas environment. Aerosol flows were produced by two strip pattern nozzles angled at 45° towards each other in the same plane. The resulting flow had an opening angle of 60°. The droplet impact angle (α d ) was varied from 0 to 90°, droplet radii ( R d1 , R d2 ) from 0.1 to 1.2 mm, and droplet velocities ( U d1 , U d2 ) from 2 to 12 m/s. The gas temperature in the droplet collision zone was varied using an induction heater with an internal volume of about 0.13 m 3 with viewing windows to record the key parameters (number, size, velocities, and trajectories) of liquid fragments before and after collision. The air temperature ranged from 20 to 400 °C in the experiments. Using a high-speed video camera, we recorded four collision regimes: coalescence, separation, disruption, and bounce. We also identified the main differences in the number and size of secondary droplets formed in an aerosol flow after the collision of two primary ones. A temperature rise led to an increase in the number of secondary fragments. The size distributions of secondary fragments are presented for three ranges of size ratios of initial droplets: Δ < 0.3; 0.3 < Δ < 0.7; Δ > 0.7. The impact of droplet collisions on their size variation rates was estimated at the gas temperature T g = 20–400 °C. Finally, we compared the contribution of this factor and evaporation without droplet-droplet collisions.

The Effect of Impurities on Water Droplet Collision Regimes and Behavior

The Effect of Impurities on Water Droplet Collision Regimes and Behavior

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.

Collisions of water droplets in the high-temperature air

The paper presents experimental research findings on the integral characteristics of the interaction of water droplets in a gas heated to high temperatures. An induction heater with an internal volume of about 0.13 m3 was used. It was fitted with quartz glass observation windows to record the characteristics of droplet motion before and after collisions. Air was used as a gas medium in the inductor. The parameters were varied in the following ranges: the air medium temperature 20–700 °С, the initial radius of droplets 0.3–0.9 mm, their velocity 0.1–7 m/s and impact angle 0–90°. At high gas temperatures, droplets were two-phase objects, because vapor bubbles formed in their near-surface layer. Frames with four collision regimes (coalescence, separation, disruption and bounce) were recorded. There were significant differences in the transformation of the water droplet surface and in the interaction of droplets with each other at various gas temperatures in the heating chamber. It has been shown that the growth of the air temperature increases the droplet lifetime and causes them to deviate from the spherical shape. The effect of the gas temperature on the position of droplet collision boundaries was determined with due consideration of the relative linear interaction parameter and the Weber number. When the gas temperature increases, two areas may form, corresponding to the bounce of droplets on the collision regime map. Differences were established in the number and dimensions of secondary water droplets formed from the collision of two initial ones. The total surface areas of liquid before and after droplet collisions were calculated.

Numerical study on melt drop collision and hydraulic fragmentation during FCI of a nuclear reactor severe accident

Melt drop collision is an important phenomenon in the circumstance of pressure wave propagation during fuel–coolant interaction (FCI). The deformation and fragmentation of melt drops can increase the contact area with coolant, and as a result will affect the heat transfer and melt oxidation. In this study, a numerical model was established by considering surface tension and validated with the experimental results that were obtained from water droplet collision in gaseous environment. Then, the head-on collision of two UO2 melt drops in water pool was investigated at different Weber numbers, and the melt morphology, contact area, and the number and size distribution of children droplets were analyzed. The results show that interfacial waves and wrinkles could be clearly observed on the melt film, and the finger structures presented at the rim of melt film and finally separated into children melt droplets. The increase of fuel–coolant contact area could be divided into the inducements by deformation and fragmentation. The melt deformation only had influence to the area increase at intermediate process but no effect to that at steady state. By contrast, the final fuel–coolant contact area was determined by the intensity of melt fragmentation. The size distribution of melt children droplets was like an off-centered normal distribution, where the maximum number of children droplets existed in the size range of 0.03 < D/D0 < 0.045, but the droplets in this size range contributed little to the fuel–coolant contact area. With the above analysis, an empirical model to calculate the contact area was developed. The model had a discrepancy of ±10% in the range of Wec = 290–1815 that the model was established in, but the discrepancies were about 13.3% and 19.3% for the cases of Wec at 2196 and 2612, respectively.

Simultaneous Effect of Droplet Temperature and Surface Wettability on Single Drop Impact Dynamics

In this paper, the influence of the liquid droplet temperature on thermo–hydrodynamics of a single droplet impinging on surfaces having different hydrophobicities is experimentally investigated. Variation in the liquid temperature typically results in alteration of properties such as the density, the viscosity, the surface tension, and the enthalpy, consequently, the droplet dynamics gets to be modified. Employing high-speed imaging technique, the morphology and spreading pattern are investigated for water droplet collision on hydrophilic, hydrophobic and super-hydrophobic surfaces. The droplet deformation is monitored qualitatively and quantitatively for drops in the temperature range from 5 to 85°C and the Weber number between 14.5 and 160. It is observed that with an increase in the liquid temperature the spreading factor increases owing to the combined effect of reduction in the density, the surface tension, the viscosity and the contact angle of the solid surface. The differences in extension of droplets under the extreme temperatures for hydrophilic, hydrophobic and super-hydrophobic surfaces are noted to be 62.7, 27.76, and 20.52%, respectively. At the low temperature, the surface tension force dominates and the Cassie–Baxter state prevails on a textured super-hydrophobic surface and the droplets bounce off. In contrast at elevated temperatures, the liquid–solid interface ruptures and liquid penetrates into the cavities and results in the Wenzel state. Furthermore, the drop which exhibits multiple bounces in the low temperature regime is found sticking on a super-hydrophobic substrate at the high droplet temperature irrespective of the Weber number.

Collision of water droplets with different initial temperatures

The experimental research into water droplet collisions resulting in their secondary atomization has been performed. The temperature of the first and second droplets was 20°С and 90°С, respectively. The interaction regime maps using the dimensionless linear collision parameter and the Weber number have been plotted. We also determined the integral characteristics of child droplets: size and number. When droplets with a temperature of 90°С were used, the critical Weber numbers went down by 10–30% for disruption, 5–15% for separation, and 15–35% for coalescence. For bounce, they increased by 50–80%. The number of newly formed child droplets for droplets with a temperature of 90°С increased by 5–20% relative to the droplets with a temperature of 20°С. The integral characteristics of child droplets and interaction regime maps of droplets with different temperatures can be used to optimize heat and mass transfer technologies.

Influence of the Concentration of Water Droplets in an Aerosol Cloud on the Characteristics of their Collisional Interaction

The authors have presented results of experimental investigations into the process of collisions of water droplets in a gas medium with variation of the basic parameters in wide ranges corresponding to advanced gas-vapor-droplet technologies: sizes (radii) 0.1???1 mm, velocities of travel 0???10 m/s, angles of interaction (attack) 0???90o, relative volume concentration 0.1???10???3???8.5??10???3 m3 of water/m3 of the gas, velocity of motion of the flow 0???10 m/s, and temperature 20???1000oC; directions of flows: opposing, codirectional, and lateral. In processing the experimental results, the emphasis was on the influence of the relative volume concentrations of the droplets on the characteristics of their interaction. The relative probabilities of four typical regimes of interaction of the droplets have been calculated: recoil, coagulation, spread, and breakup (reduction in size). Parametric ranges have been shown for droplets for which the concentration growth of the latter in an aerosol leads to an increase in the probability of coagulation, recoil, and size reduction. Also, the authors have presented ranges of variation of the concentration of droplets in a gas medium, in which the probabilities of all the investigated interaction schemes are high. The experimental results have been processed with the existing approaches to generalization by calculating the angular factor of interaction and the Weber number.

Electrochemical detection of single attoliter aqueous droplets in electrolyte-free organic solvent via collision events

We report the electrochemical detection of single water droplets in an electrolyte-free organic solvent by collision events using a platinum ultramicroelectrode (Pt-UME). Water droplet emulsions were prepared by the ultrasonication of an organic solvent (e.g., dichloroethane) and an aqueous solution containing hydroquinone (H2Q) as the redox species and MgSO4 as an emulsion stabilizer as well as the supporting electrolyte. Under the appropriate potentials, we could observe single water droplet collision phenomena at the Pt-UME by amperometric current-time (I-t) measurements. Water droplets that established contact with the Pt-UME led to amperometric current spikes due to the instantaneous electrolysis of the constituent redox species. The concentration of H2Q in the emulsion was adjusted to confirm that the amperometric spike resulted from the electrochemical reaction in the water droplet. As the concentration of H2Q increased, the oxidation peak current increased. The size and contact area of the water droplets were also estimated. Based on these single droplet collision results, we could successfully establish a facile water droplet detection system in an organic solvent without the addition of a hydrophobic organic electrolyte.

Experimentally determining the effects of water droplets collision when mixing aerosol with gas flow at different heating temperatures

The article presents the results of experimental studies of the collisions characteristics for water droplets in an aerosol at its entry into the air counter flow. The temperature of the latter ranged from 20?C to 500?C. Experiments were also carried out with the flow of combustion products having a temperature of 800- 850?C. The initial dimensions (radii) of the droplets in the aerosol were 50-1000 ?m. Visualization of the droplet motion in the counter flow of air and combustion products required the use of a hollow cylinder made of quartz glass with a height of 1 m and an internal diameter of 0.15 m, a cross-correlation complex and optical methods (particle image velocimetry, particle tracking velocimetry, interferometric particle imaging). The characteristics of the droplet interaction (size, velocity, total surface area of the liquid before and after) were controlled using a high-speed video camera and tracking algorithms in the TEMA AUTOMOTIVE software package. The main modes of drops interaction have been identified: bounce, coagulation, scatter, and breakup. The statistical information database has been obtained to describe the interaction modes using diagrams, taking into account the ratio of the sizes of colliding drops, velocities of their motion, and an angle between trajectories of motion. The influence of gas temperature on the probabilistic criteria of droplet collisions, as well as the integral criterion characterizing the change in the liquid surface area due to the intensification of droplet collisions in the gas medium has been established.

Interaction of Water Droplets in Air Flow at Different Degrees of Flow Turbulence

Presented are results of experiments on high-speed video recording of collisions of water droplets in a gas medium as part of aerosol. Parameters of the gas (air) flow and aerosol cloud were monitored using cross-correlation complexes and optical methods of Particle Image Velocimetry, Particle Tracking Velocimetry, Interferometric Particle Imagine, and Shadow Photography. Conditions at different degrees of gas flow turbulence were considered. The characteristic Reynolds numbers ranged from 1100 to 2800. The relative probabilities of coagulation, scattering, and fragmentation of water droplets upon their collisions were calculated. Experimental dependences of probabilistic criteria on parameters of droplets and flow have been obtained for subsequent mathematical modeling. It has been shown that fragmentation and complete breakup of droplets can enable several-fold increase in the relative area of the liquid. The effect of the degree of gas flow turbulence on parameters of recorded processes of interaction of droplets has been established. The results of the experiments were subjected to criterion processing, theWeber and Reynolds numbers taken into account.

Experimental estimation of the probabilities of collision of water droplets in the environment of heated gases

Experimental estimation of the probabilities of collision of water droplets in the environment of heated gases

Mesoscopic assessment of water droplet collision with a single stainless steel fiber in oil

Mesoscopic assessment of water droplet collision with a single stainless steel fiber in oil