Droplet Particle Collision Research Articles

Rate constants of particle charging by electrospray nanodroplets

An analysis is presented of the attachment of electrospray nanodroplets by larger particles in a grounded metallic tube where both are carried by a laminar air flow. Droplets and particles that get charged by attaching droplets migrate toward the tube wall under the action of the electric field induced by their charges, but they do so at different velocities owing to the disparity of their electrical mobilities. The electric currents they cause in the tube wall originate therefore in different regions of the tube, and can be separately measured by splitting the tube into two insulated segments. It is shown that, in two limiting conditions, the cross-section for droplet–particle collisions can be extracted from these measurements.

Influence of collision conditions between aerosol flows of liquid droplets and solid particles typical for wet vortex dust collectors

Various industrial devices and systems make use of collisions of droplets with solid particles in aerosol flows. The characteristics of these collisions are usually predicted adopting the same approaches as for binary collisions of a liquid droplet with a solid particle. However, the conditions of such binary collisions are different from those in the spray composition. There are very few experimental findings for sprays. The purpose of this research was to experimentally determine the characteristics of interaction of droplets with particles of slurry fuel components in sprays. High-speed three-dimensional video recording made it possible to identify conditions when liquid droplets break up, and droplets and particles agglomerate to form slurry droplets. Droplet-particle interaction regimes were identified. Collective effects in a cascade of droplet-particle interactions were recognized. The frequency of collision regime occurrence was determined, considering the recorded collective effects in sprays. The characteristics of droplet-particle collisions in sprays were compared with those of binary interactions. Conditions were determined when the findings on binary collisions can be adapted to different systems with multi-phase sprays. The findings obtained promote research and development in the field of secondary atomization of composite fuel droplets.

Numerical investigation on heat transfer and vapour layer geometry of a droplet impacting on a spherical particle in Leidenfrost regime

Understanding the heat transfer mechanism and dynamic behaviors of droplet-particle collision in Leidenfrost regime is essential for the pyrohydrolysis process of pickling liquor, which is sprayed into a fluidized bed and then decomposes to oxides and acid vapour. In this study, the droplet deformation, heat transfer characteristics and vapour layer geometry of Leidenfrost droplet impacting spherical particle are investigated by constructing a microscale phase change model in VOF framework. The applicability of the computational model in capturing droplet dynamics and thermodynamics is verified by comparing it with experimental outcomes. The results indicate that the vapour layer geometry is relatively uniform in spreading but shows instability in recoiling. The vapour layer thickness increases rapidly in both spreading and early recoiling, while decreases in the late recoiling phase. Furthermore, the heat transfer and droplet evaporation depend on the vapour layer thickness strongly. Specifically, the thinner the vapour layer, the lower the heat transfer resistance. The increasing surface temperature, We and Re can effectively enhance heat transfer and raise droplet temperature. In addition, the maximum spreading factor and liquid–solid contact time play an important role in heat transfer and they both show a clear power-law dependency on We and Re for viscous droplets. The effect of droplet size on contact time is consistent with first-order vibration theory but the contact time is independent of surface temperature and impact velocity.

A numerical study of flow over supersonic projectile under heavy rain

This paper presents a computational fluid dynamics study of morphology and structure dynamics of the flow over a supersonic secant-ogive cylinder boat tail projectile under heavy rain. The discrete phase model is employed to approximate the process of droplet–particle collision with the projectile wall and the formation of a liquid film. The simulation results indicate that at certain angles of attack, rain impact decreases lift coefficient of projectile by as much as 14.9%, as the wall pressure distribution is distinctly reformed. Moreover, the rain condition induces the formation of a liquid film on the front end of the projectile, as the angle of attack increases, the stability of this liquid film gradually improves, while its thickness and coverage rise to the peak values and then decrease. On the projectile’s trailing half, however, the liquid film developed a watery pattern. The collision of liquid droplets causes momentum loss to the projectile, while an unstable liquid film tends to exacerbate aerodynamic performance loss, a stable liquid film would mitigate the performance loss.

Breakup of colliding droplets and particles produced by heavy fuel oil pyrolysis

Heavy fuel oil (HFO) obtained by blending oil refinery residue and cutter stock is actively used as fuel in marine engines and industrial boilers. However, the use of HFO presents a number of unresolved problems. In the lack of oxidizer, pyrolysis of HFO droplets in the flame core produces solid particles, and the fuel combustion temperature drops, increasing the solid phase concentration in the combustion chamber. This study investigates the water/HFO fragmentation behavior as the emulsion droplets collide with solid particles produced by their pyrolysis. According to the experimental findings, droplet-particle collisions can occur in two regimes: spreading and separation. A two-fold increase in the droplet to particle size ratio shifts the critical Weber number towards 10–20% higher values. To ensure the consistent separation regime with 2.5-mm droplets, the velocities should be 15–30% lower than with droplets of about 1 mm in radius. Larger particle size leads to a 20–30% increase in the resultant water/HFO droplet velocity. The research findings indicate that the arrangement of atomizers at an angle of less than 10° to each other will reduce the cost of thermal and electric energy production by almost 10%.

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).

Collision regimes and dynamic behaviors of a viscous droplet impacting on a spherical particle at high temperatures

Understanding droplet–particle collision behaviors is essential for the pyrohydrolysis process of pickling liquor, where the pickling liquor is sprayed and dried into particles. This study systematically investigated the collision characteristics between a viscous droplet and a heated stainless steel particle whose temperature ranged from 100 to 700 °C. The results indicate that the vapor thrust promotes droplet rebound in smaller spreading diameters but induces disintegration in larger spreading diameters under the film boiling regime. The collision regime map is summarized, and the transition thresholds of collision patterns are significantly increased with increasing liquid viscosity. The spreading factor and contact line velocity are strongly affected by the particle temperature at high liquid viscosities. In addition, the hydrophobic nature of particle surface in film boiling regime is favorable for droplet spreading. The dynamic contact angle significantly depends on the particle temperature and droplet properties. The dimensionless contact time shows a power law dependency on the Weber number in the rebound pattern, but it is almost a constant in the disintegration pattern.

Effect of surface roughness of solid particles on the regimes and outcomes of their collisions with liquid droplets

The paper presents the experimental research findings for the characteristics of the collisions of liquid droplets with solid particles (as illustrated by distilled water and metal particles) with a variable average roughness. The average roughness range (0.05–39 µm) corresponded to a group of practical applications: heat exchange, treatment, evaporation, fuel systems, etc. The regimes of droplet-particle collisions, as well as the characteristics of secondary liquid fragments, i.e., child droplets, were determined. Interaction regime maps for droplets and particles were constructed. Mathematical equations to describe the boundaries of droplet-particle interaction regimes were obtained. The findings were compared with the characteristics of collisions of liquid droplets with a flat solid substrate made of the same material as the particles with equivalent average roughness. Guidelines were provided for applying the research findings to the development of the technologies of secondary atomization of liquid droplets in applications.

Effects of rotation on collection characteristics of fine particles by droplets

Removing particles dispersed in fluid through drops is widely presented in various fields, and the critical factor is particles captured by droplets. Drop rotation effects play a dominant role in the capture process. However, their influences on collection characteristics remain unclear. Thus, a particle collection model was developed to simultaneously consider rotation and translation effects on fine particles captured by an individual droplet. The finite volume method was used to solve for flow field and collection efficiency, and the proposed model was verified by comparison with experimental and published results. The Liutex method was used to identify the vortex structure, on which dimensionless droplet rotation rates ranged from 0 to 0.1. Velocity, drag coefficient, radial position, and captured particle velocity distribution and collection efficiency were also investigated in relation to the rotation effect. The results show that the established model is reasonable. Vortex strength increases with increased rotation speed where the increment can be up to 480, and fluid rotation strength depends on the competitive relation between the increase in the rotation rate and the vortex movement. Radial velocity increases in regions where the angle between the positive x axis and the normal vector of drop surface ranges from 115° to 180° but decreases in regions where the angle ranges from −180° to −120°, and corresponding regions produce a comparative relation for improving particle capture. Increasing the rotation rate can increase the drag force coefficient by about 0.025, hindering droplet–particle collision. Average radial velocity of particles with higher than 3.7 mm/s is necessary at high rotation rates, while collection efficiency decreases at increased droplet rotation rates.

Interaction between Droplets and Particles as Oil–Water Slurry Components

The characteristics of the collisions of droplets with the surfaces of particles and substrates of promising oil–water slurry components (oil, water and coal) were experimentally studied. Particles of coals of different ranks with significantly varying surface wettability were used. The following regimes of droplet–particle collisions were identified: agglomeration, stretching separation and stretching separation with child droplets. The main characteristics of resulting child droplets were calculated. Droplet–particle interaction regime maps in the B = f(We) coordinates were constructed. Equations to describe the boundaries of transitions between the droplet–particle interaction regimes (B = nWek) were obtained. The calculated approximation coefficients make it possible to predict threshold shifts in transition boundaries between the collision regimes for different fuel mixture components. Differences in the characteristics of secondary atomization of droplets interacting with particles were established. Guidelines were provided on applying the research findings to the development of technologies of composite liquid fuel droplet generation in combustion chambers with the separate injection of liquid and solid components, as well as technologies of secondary atomization of fuel droplets producing fine aerosol.

Effect of Liquid Properties on the Characteristics of Collisions between Droplets and Solid Particles

The characteristics of the collisions of droplets with solid particles (52,100 steel) were experimentally studied when varying the key liquid properties: viscosity (1–6.3 mPa·s), surface tension (72.69–36.1 mN/m) and interfacial (liquid-liquid) tension (3.41–42.57 mN/m). Distilled water, aqueous solutions of glycerol, surfactants and diesel emulsions were used. The experimental conditions corresponded to the following ranges: Weber number 5–450, Ohnesorge number 0.001–0.03, Reynolds number 0.1–1000, capillary number 0.01–0.3. Droplet-particle collision regimes (agglomeration, stretching separation) were identified and the characteristics of secondary liquid fragments (size, number) were determined. Droplet-particle interaction regime maps in the We(Oh) and Re(Ca) systems were constructed. Equations describing the transition boundaries between the droplet-particle interaction regimes were obtained. The equations take the form: We = a · Oh + c. For the conditions of the droplet-particle interaction, the relationship We = 2214 · Oh + 49.214 was obtained. For the interaction with a substrate: We = 1.0145 · Oh + 0.0049. The experimental results were compared with the characteristics of collisions of liquid droplets with each other. Differences in the characteristics of secondary atomization of droplets as a result of collisions were identified. Guidelines were provided for applying the research findings to the development of liquid droplet secondary atomization technologies in gas-vapor-droplet applications.

Maximum spreading of droplet-particle collision covering a low Weber number regime and data-driven prediction model

In the present study, the maximum spreading diameter of a droplet impacting with a spherical particle is numerically studied for a wide range of impact conditions: Weber number (We) 0–110, Ohnesorge number (Oh) 0.001 3–0.786 9, equilibrium contact angle (θeqi) 20°–160°, and droplet-to-particle size ratio (Ω) 1/10–1/2. A total of 2600 collision cases are simulated to enable a systematic analysis and prepare a large dataset for the training of a data-driven prediction model. The effects of four impact parameters (We, Oh, θeqi, and Ω) on the maximum spreading diameter (β*max) are comprehensively analyzed, and particular attention is paid to the difference of β*max between the low and high Weber number regimes. A universal model for the prediction of β*max, as a function of We, Oh, θeqi, and Ω, is also proposed based on a deep neural network. It is shown that our data-driven model can predict the maximum spreading diameter well, showing an excellent agreement with the existing experimental results as well as our simulation dataset within a deviation range of ±10%.

Experimental characterization of the effect of liquid viscosity on collisions between a multi-component droplet and a heated particle

Understanding droplet-particle collisions is critical for regulating spray coating processes. Existing studies mainly focused on collision between a single component droplet and a particle either at room temperature or considerably higher than the boiling point. In this study, we perform experiments of a droplet impacting a heated particle with its temperature in the range of 35 °C–150 °C, where the droplet is generated from multi-component solutions with its viscosity adjusted by adding Hydroxypropyl methyl cellulose (HPMC). The collisions with droplets from 30% sodium benzoate solution show four regimes: deposition, deposition-bubble, breakup-rebound, and rebound. Byincreasingviscosity with adding 1% or 2% HPMC, only two regimes are observed: deposition and deposition-bubble. The variation of film thickness with time can be divided into three phases. The film thickness in phase I and II can be fitted by simple functions. With increasing liquid viscosity, the film thickness in phase III at different temperatures becomes consistent.

Adaptive mesh axi-symmetric simulation of droplet impact with a spherical particle in mid-air

In this study, droplet-particle collisions in mid-air are numerically investigated for a wide range of collision parameters: Weber number, contact angle and droplet/particle diameter ratio (4 ≤ We ≤ 150, 20° ≤ θeqi ≤ 160°, and Ω = 1/2 and 1). To perform these simulations in an efficient manner, a simple dual grid based adaptive mesh refinement (SDG AMR) strategy is proposed and implemented in our existing multiphase flow solver. A total of 90 collision cases are systematically analyzed and are compared with results for the case of impact on a stationary particle to understand how mid-air collision characteristics are changed relative to those on a stationary target. The simulation results show that mid-air droplet-particle collision behavior is significantly different from that of droplet collision on a stationary particle, thus the impact phenomena cannot be interpreted by extrapolating collision results for a stationary target.

Droplet-droplet, droplet-particle, and droplet-substrate collision behavior

In this paper, we present the research findings on the collision outcomes of water droplets with solid particles and substrates made of metal with variable surface wettability. We constructed the maps of two collision regimes (agglomeration and stretching separation) showing the number and size of secondary droplets in the coordinate system of the Weber number and dimensionless impact parameter. The key parameters of droplets and particles were varied: sizes and velocities, impact angles, and wettability of the solid surface. Droplet-particle collision regimes were compared with the outcomes of droplet collision with other droplets and with a massive solid surface. Finally, we determined in which conditions these outcomes differed significantly and, on the contrary, were quite similar in terms of the number and size of secondary droplets. The results obtained are important for the development of secondary droplet atomization technologies.

Comparison of droplet-particle interaction on a stationary and a moving particle

Droplet-particle collision is commonly found in nature and industry. The collision of a moving droplet against a fixed stationary particle has been investigated comprehensively, and the collision of a moving droplet against a moving particle also has been studied. However, there is no open literature to conduct a detailed comparison between the two occasions. In this study, the collision of a droplet against a moving/fixed stationary particle are studied by the lattice Boltzmann method, and a comprehensive comparison is made. The effects of Weber number and particle-droplet size ratio are investigated. For the collision of a moving droplet against a fixed stationary particle, three regimes are observed: deposition, agglomeration and disintegration. For the collision of a moving droplet against a moving particle, only two regimes are observed: deposition and disintegration. Moreover, for the first time, it is observed that the deposition regime of a droplet colliding against a moving particle can be divided into two sub-regimes. The variation of force of the particle is analyzed. The numerical results show that the amplitude of the vertical force of the fixed stationary particle is larger than its moving counterpart, although the fluctuation of the former will disappear quickly. The present study reveals the differences between the two collision situations, which are helpful to understand the collision phenomena in industrial applications.

Collision Characterization of Mixed Hydrocarbon Liquid Droplets with Polyethylene Particles

In the gas–solid fluidized bed with condensed liquid spray applied to ethylene polymerization, the collision characteristics of condensed droplets and polyethylene particles play an important role in determining the heat transfer and product properties. High-speed photography, noninvasive laser heating, and a quasi-circle data processing approach are employed to investigate the collision interaction between the hydrocarbon liquid droplets with different mixed compositions and a single polyethylene particle, under different particle temperatures and droplet Weber numbers. The results have found that the hydrodynamic behavior and heat transfer characteristics of the liquid films formed during the droplet–particle collision are more dependent on the liquid components with a higher mass fraction. The behavior of the liquid films is divided into two types, named the directly slip and recoil, the latter of which is beneficial for enhancing heat and mass transfer efficiency. Especially under low Weber numbers, the long-time recoil only occurs during the evolution of 1-octene and n-hexane/1-octene mixed liquid films. Generally, as the Weber number rises, the temperature thresholds required for recoil appearance increase for all kinds of liquid films. It is indicated that the necessary conditions for recoil are low inertia force and high temperature.

Experimental characterization of liquid film behavior during droplets–polyethylene particle collision

AbstractNowadays, the droplet–particle collision characteristics in the gas‐phase ethylene polymerization process are still unclear. The high‐speed photography and a quasi‐circle imaging approach are employed to study the collision interaction characteristics between liquid droplets and polyethylene particles. The liquid film evolution is studied through variations of the film thickness on the particle north pole, the dynamic contact angle, center angle and film thickness at the maximum extension. Results have found that for n‐hexane the threshold temperature of the recoil happening increases with increasing initial Weber number, but for 1‐hexene it is stable. Over 70°C evaporation and splash occurs immediately. Under low Weber numbers, the water droplet stays for damping oscillations, the reference stable height of which is linearly related to temperatures. Moreover, three regimes of film thickness variation with time are identified and mathematically described, while Regime 3 characteristics are found strongly dependent on the liquid species, Weber number, and particle temperature.

Collisions of droplets on spherical particles

Head-on collisions between droplets and spherical particles are examined for water droplets in the diameter range between 170 μm and 280 μm and spherical particles in the diameter range between 500 μm and 2000 μm. The droplet velocities range between 6 m/s and 11 m/s, while the spherical particles are fixed in space. The Weber and Ohnesorge numbers and ratio of droplet to particle diameter were between 92 < We < 1015, 0.0070 < Oh < 0.0089, and 0.09 < Ω < 0.55, respectively. The droplet-particle collisions are first quantified in terms of the outcome. In addition to the conventional deposition and splashing regimes, a regime is observed in the intermediate region, where the droplet forms a stable crown, which does not breakup but propagates along the particle surface and passes around the particle. This regime is prevalent when the droplets collide on small particles. The characteristics of the collision at the onset of rim instability are also described in terms of the location of the film on the particle surface and the orientation and length of the ejected crown. Proper orthogonal decomposition identified that the first 2 modes are enough to capture the overall morphology of the crown at the splashing threshold.

Numerical investigation of heavy fuel droplet-particle collisions in the injection zone of a Fluid Catalytic Cracking reactor, part II: 3D simulations

This study investigates the collisions between heavy gasoil droplets and solid catalytic particles taking place at conditions realized in Fluid Catalytic Cracking reactors (FCC). The computational model utilizes the Navier-Stokes equations along with the energy conservation and transport of species equations. The VOF methodology is used in order to track the liquid-gas interface, coupled with a dynamic local grid refinement technique in order to minimize the computational cost. Phase-change phenomena, as well as catalytic cracking surface reactions (2-lump scheme) are taken into account. In this paper, the numerical model is extended to investigate the droplet-particle collision process in three dimensions. In order to save computational resources, only half of the droplet is investigated, by imposing symmetry conditions. Firstly, single droplet-catalyst collisions are simulated and compared against the corresponding ones provided by 2D axisymmetric simulations and afterwards, the model is applied for the characterization of the collision dynamics between a single droplet and a particle cluster, i.e. a realistic 3D particle configuration. As the droplet flows through the space between the catalytic particles, important phenomena are observed, such as a) drop levitation due to the formed vapour layer and b) a thin liquid sheet formation, both of which affect the rate of gasoline production, as well as predictions for liquid pore blocking mechanism; a phenomenon frequently observed industrially. Results indicate that gasoline production decreases when the collision target is a particle cluster, instead of same number (as many as in the cluster) single catalysts, as the corresponding contact area decreases.