Droplet Collision Research Articles

Atomization characteristics of twin nozzles for outdoor snow-makers application

Swirl nozzles are the core component of outdoor snow-makers for forming snow crystal artificially. A general experiment setup for simulating the atomization process at the working conditions of outdoor snow-makers was developed. Phantom high-speed camera and PW180-B laser particle size analyzer (LPSA) were used to capture the dynamic atomization process and droplet diameter distributions respectively. The atomization characteristics of a single swirl nozzle and that of twin nozzles were tested and analyzed. For single nozzle, Sauter Mean Diameter (SMD) drops rapidly and then goes up gently due to the action of droplets breakup and coalescence. With the increase of the injection pressure, the SMD of the droplets decreases, and the mass flow rate of the nozzles is improved. Moreover, the spray cone angle increases rapidly and keeps a stable value. For twin nozzles, the cooperation of the swirl nozzles obviously reduces the SMDs of the atomization field because it enhances the probability of droplets collision and separation. The SMDs are determined by the cooperative distances and injection pressure.

Atomization and droplet dynamics of a gas-liquid two-phase jet under different mass loading ratios

Atomization and droplet dynamics of a gas-liquid two-phase jet were investigated experimentally, with the particular interest in the influence of mass loading ratio (the ratio of droplet mass flow rate to the carrier-phase mass flow rate, ranging from 0.60 to 3.23) on the jet atomization. Droplet size and velocity at selected positions were obtained by employing Phase Doppler Particle Analyzer (PDPA), combined with backlit illumination for spray visualization. Droplet transport characteristics are substantially influenced by the mass load ratio. Characterization of the local gas flow velocity by using droplets smaller than 5μm reveals a highly turbulent jet with Reynolds number exceeding 5×104, implying the possibility of droplet turbulent breakup. Critical equilibrium location xcrit between droplet breakup and coalescence is close to the nozzle exit and a positive correlation is found between mass loading ratio and xcrit. Exaltation of mass loading ratio increases the droplet size and decreases the velocity. Quantification of droplet collision outcome indicates a relatively high probability of coalescence, which explicates the downstream increasing of measured droplet Sauter mean diameter along the centerline of far-field jet.

Binary droplet coalescence in shear gas flow: A molecular dynamics study

Molecular dynamics (MD) simulations of droplet collision and coalescence in shear gas flow between gold plates were presented. The shear velocity, normal symmetry velocity and splitting phenomenon of gas were studied through the statistical analysis of gas molecular velocities. The normal symmetric velocity is the average velocity of the gas molecules moving symmetrically away from the plates in the normal direction. The change of adsorptive layers of gas molecules on solid plates with plate velocity was investigated by analyzing the density distribution of gas molecules. Capillary number and initial offset are the main parameters affecting droplet coalescence. By changing these two parameters, three main droplet collision results were obtained: coalescence, temporary bridge formation and sliding. The non-bonded interactions between droplets in various collision results were investigated and it was found that they can well correspond to the interface change processes. The regime diagrams were established to study the relationship between different results and parameters. Our study provides a new idea for further understanding of droplet coalescence in gas shear flow.

Lattice Boltzmann simulation of binary three-dimensional droplet coalescence in a confined shear flow

Small-scale microscopic phenomena determine the behavior of large-scale droplets, which brings great challenges to accurately simulate the droplet coalescence process. In this paper, the mesoscopic lattice Boltzmann method based on the phase field theory is used to simulate the collision and coalescence of binary three-dimensional droplets in a confined shear flow. The numerical prediction of droplet coalescence behavior was first compared with the experimental result, and good agreement was reported. Then, we investigated the influences of a comprehensive range of capillary numbers (0.01≤Ca≤0.5) and Reynolds numbers (0.01≤Re≤10) on the shearing dynamics of binary droplets and also provided a quantitative description of droplet behavior in terms of the droplet deformation parameter and relative trajectory. A shearing regime diagram is further constructed based on the coupling effect of Ca and Re, which reveals three distinct types of droplet behaviors, including coalescence, breakup after the coalescence, and non-coalescence. Concretely, three different patterns of droplets can be completely captured with the variation of Ca at low Re; only two types of coalescence and non-coalescence can be observed for a medium Re, and two droplets just slide over each other without the occurrence of the coalescence when Re is sufficiently large. Also, we identified two critical capillary numbers in the lower Re region and one critical capillary number in the middle Re region, respectively, characterizing flow type transitions from the coalescence to breakup, from the breakup to the non-coalescence, and from the coalescence to the non-coalescence. It is found that all the capillary numbers decrease with Re.

Effect of micro-explosion of biodiesel and ethanol droplets on evaporation: A three-stage mixed fuel droplet evaporation model

The ejection and micro-explosion of a mixed fuel droplet group affected the evaporation and combustion rate of fuel. In order to explore the evaporation, micro-explosion, and interaction rules of the biodiesel and ethanol droplet group with different component proportions and volumes during heating, a set of multiparameter tubular furnace experimental equipment was designed. By setting the experimental parameters of the tubular furnace, the heating sequence pictures of the biodiesel and ethanol droplet group were obtained by high-speed photography, and the droplet parameters were analyzed and processed by Image-Proplus. In the experiment, not only the phenomena of ejection, micro-explosion, and white fog but also the interactions of collision, interweaving, and fusion of droplets were observed, which affected the micro-explosion and evaporation of individual droplets in the droplet group. When the ratio of biodiesel to ethanol was 40:60 (40% biodiesel and 60% ethanol), the micro-explosion intensity was the highest. For the same type of droplet, the higher the micro-explosion intensity, the higher the evaporation rate, and the super micro-explosion made the droplet evaporate almost instantaneously. The modified equation for calculating the micro-explosion intensity was established, and the micro-explosion intensity of the mixed droplet was calculated. By integrating the droplet micro-explosion intensity equation and evaporation equation, a three-stage evaporation model of the mixed droplet was established, and the average evaporation rate of the second-stage BD30E70 (30% biodiesel and 70% ethanol) droplet was nearly 28 times the first stage, which was nearly 32 times the third stage.

Interferometry and Simulation of the Thin Liquid Film between a Free-Rising Bubble and a Glass Substrate.

Because of their practical importance and complex underlying physics, the thin liquid films formed between colliding bubbles or droplets have long been the subject of experimental investigations and theoretical modeling. Here, we examine the possibility of accurately predicting the dynamics of the thin liquid film drainage using numerical simulations when compared to an experimental investigation of millimetric bubbles free-rising in pure water and colliding with a flat glass interface. A high-speed camera is used to track the bubble bounce trajectory, and a second high-speed camera together with a pulsed laser is used for interferometric determination of the shape and evolution of the thin liquid film profile during the bounce. The numerical simulations are conducted with the open source Gerris flow solver. The simulation reliability was first confirmed by comparison with the experimental bubble bounce trajectory and bubble shape evolution during the bounce. We further demonstrate that the simulation predicted time evolution for the shape of the thin liquid film profiles is in excellent agreement with the high-speed interferometry measured profiles for the entire experimentally accessible film size range. Finally, we discuss the implications of using numerical simulation together with theoretical modeling for resolving the complex processes of high velocity bubble and droplet collisions.

The transition boundaries between interaction regimes of liquid droplets colliding in a gas

In this research, we use two different approaches to distinguishing the interaction regimes of liquid droplets in a gas environment. The first one distinguishes the following regimes: coalescence (parent droplets merge), bounce (parent droplets approach each other and then move away from each other), separation (impact produces two new droplets of a similar size as the initial ones), and disruption (two initial droplets break up into more than two fragments). The second approach distinguishes the following regimes: coalescence (merging of initial fragments), bounce (merging and consecutive splitting into two fragments of similar size as the initial ones), stretching separation (fragmentation due to the stretching of the merged droplet), and reflexive separation (fragmentation due to the tendency of each fragment to take spherical shape). Experimental results for different slurries, emulsions, solutions, and single-component compositions allowed us to distinguish transition boundaries of collision regimes using the Weber and Ohnesorge number, as well as angular and linear interaction parameters. Mathematical expressions are obtained for the boundaries, describing the variation ranges of droplet interaction parameters as well as their properties required for the consistent transition from one collision regime to another. The expressions are second order polynomials with droplet size ratio, Weber number, Ohnesorge number, and interaction parameters as variables. A database of empirical coefficients is obtained for general mathematical expressions that can be used to predict the transition boundaries of droplet collision regimes. Any of the two approaches to distinguishing the regimes is equally viable for this matter.

Spin-affected reflexive and stretching separation of off-center droplet collision

A volume-of-fluid (VOF) based numerical study on the spin-affected droplet separation of off-center collisions is presented. The roles of orbital angular momentum ${L}_{o}$ and spin angular momentum ${L}_{s}$ in affecting the droplet separation are illustrated by changing the chirality of droplet spin, showing that an augmented total angular momentum can promote the stretching separation and suppress the reflexive separation. A unified theoretical model based on conservation laws is proposed to qualitatively describe the boundaries of the coalescence-separation transition influenced by droplet self-spin motion.

An adaptive mesh refinement‐multiphase lattice Boltzmann flux solver for three‐dimensional simulation of droplet collision

AbstractA three‐dimensional adaptive mesh refinement‐multiphase lattice Boltzmann flux solver (3D‐AMR‐MLBFS) is developed for effectively simulating complex multiphase flows with large density ratio and high Reynolds number. In the method, the multiphase lattice Boltzmann flux solver (MLBFS) is used to solve the flow field and the level set method is adopted to capture the interface. In addition, a free energy‐based continuum surface tension force (FE‐CSF) model, which has a clear physical basis and does not need to introduce a regularization function, is applied to calculate the surface tension force. In order to improve the computational efficiency and at the same time ensure accuracy, the adaptive mesh refinement technology is introduced. This method is used to simulate three‐dimensional collision of two equal‐sized droplets for a wide range of parametric conditions. We not only verify the developed method by comparing with available literature data but also successfully reproduce four typical outcomes of droplet collision, namely coalescence, reflexive separation, stretching separation, and liquid membrane shattering, for which the evolutions of droplet morphologies and various energies are revealed.

Dynamics and outcomes of binary collisions of equi-diameter picolitre droplets with identical viscosities.

The dynamics of binary collisions of equi-diameter picolitre droplets with identical viscosities, varying impact speeds and impact angles have been investigated experimentally and compared to collision outcome prediction models. Collisions between pairs of pure water droplets with a viscosity of 0.89 mPa s and pairs of aqueous-sucrose (40% w/w) droplets with a viscosity of 5.17 mPa s were examined. The colliding droplets were ∼38 μm in diameter, which is around ten times smaller than those previously investigated when examining the effect of viscosity on the outcome of binary droplet collisions. Varying the impact speed and angle resulted in different collision outcomes, including coalescence, reflexive separation and stretching separation. The collision outcomes were plotted on two viscosity dependent regime maps. The regime boundaries are generally in agreement with earlier literature for both high and low viscosity cases. The agreement between experiment and theory, for both fluids, gives more confidence in the models tested here to predict collision outcomes for droplets of this size and these viscosities.

Hypergolic Ignition Induced by Head on Collision of Bi-Propellant Droplets: Monoethanolamine-Based Fuel and Hydrogen Peroxide

Hypergolic Ignition Induced by Head on Collision of Bi-Propellant Droplets: Monoethanolamine-Based Fuel and Hydrogen Peroxide

Extended Model of Bouncing Boundary for Droplet Collisions Considering Numerous Different Liquids

Extended Model of Bouncing Boundary for Droplet Collisions Considering Numerous Different Liquids

EFFECT OF GAS PRESSURE AND TEMPERATURE ON THE REGIMES OF LIQUID DROPLET COLLISIONS

EFFECT OF GAS PRESSURE AND TEMPERATURE ON THE REGIMES OF LIQUID DROPLET COLLISIONS

Improved well-balanced free-energy lattice Boltzmann model for two-phase flow with high Reynolds number and large viscosity ratio

Spurious velocities and inaccurate density properties arising from the imbalance of discretized forces at discrete level are frequently observed in numerical simulation of multiphase flows based on lattice Boltzmann equation (LBE) models. In this paper, an improved well-balanced free-energy LBE model is proposed for two phase flows with high Reynolds numbers and large viscosity differences based on the well-balanced LBE [Guo et al., Phys. Fluids 33, 031709 (2021)]. To this end, a free parameter associated with the shear rate is introduced into the equilibrium distribution function. This results in a fluid viscosity that is dependent not only on the relaxation time but also on the introduced parameter. The relaxation time can be chosen to improve the numerical stability and accuracy, while the viscosity is mainly determined by the new parameter. To test the capability of the present model in capturing discrete equilibrium states, both one-dimensional flat interface and two-dimensional stationary droplet are simulated. Numerical results show that the present model is capable of eliminating spurious velocities and maintaining a constant chemical potential when the system reaches an equilibrium state. To further validate the performance of the present LBE for dynamic problems, both binary droplet collision and single bubble rising are performed, which demonstrates that the present model has the capability to deal with two phase flows with high Reynolds number and large viscosity ratio.

Symmetric boundary condition for the MPS method with surface tension model

In order to supplement the investigation of boundary conditions for the MPS method, a general computational algorithm of symmetric boundary condition for MPS is presented to simulate the symmetric problems. By the MPS method with the symmetric boundary condition (the SBC-MPS method), only half or quarter of the full computational domain needs to be set up, thus, the computational cost is able to be slashed dramatically. In the SBC-MPS method, mirror particles are introduced, which will be arranged symmetrically according to the positions of real liquid particles near to the symmetric plane, but not involved in the solution of physical properties. The function of these mirror particles is to compensate the values for the calculation of real liquid particles. As numerical tests, various of problems including two-dimensional Poiseuille Flow, three-dimensional dam-break, and some surface tension dominant problems, such as droplet wetting / impinging to a solid surface and binary droplets collision have been systematically studied by the SBC-MPS method. As a result, the algorithm shows good robustness, stability and accuracy, which demonstrates a great potential to improve the computational efficiency for dealing with symmetric problems.

Peculiar encounter between self-propelled droplet and static droplet: swallow, rerouting, and recoil

Understanding the outcome and mechanism of droplet–droplet collision is important in digital microfluidics, especially for the encounter between the self-propelled and static droplets. Observation of the collision process between a self-propelled droplet and another static liquid droplet on a poly(methyl methacrylate) substrate. The former is a mixture of dimethylformamide and 10 wt% Silwet L-77, while the latter can be either miscible or immiscible with the former. Four types of interesting outcomes after the encounter are identified: (i) swallow-and-run (liquid paraffin), (ii) swallow-run-pinning (olive oil), (iii) rerouting-and-run (water), and (iv) recoil-and-run (glycerol). The swallow behavior is observed as the two droplets are immiscible, but the rebound behavior is acquired as they are miscible. The former can be explained by the two spreading coefficients, while the latter may be attributed to the buildup of the gradient of the interfacial tension upon contact.

The collision of immiscible droplets in three-phase liquid systems: A numerical study using phase-field lattice Boltzmann method

A ternary phase-field LB model was employed in this paper to study the collision between two immiscible liquid droplets in a continuous liquid medium. Five different cases were considered based on droplets’ density, viscosity, and interfacial tension ratio. The results were presented in the form of We-B. (Weber number, and collision parameter) nomograms. The hydrodynamics of the interaction was analyzed by visualizing droplets’ deformation and behavior during the impact, especially in the contact area. Three collision outcomes were identified for both partial and total spreading states: engulfment (I), bounce (II), and engulfment with encapsulation of a tiny bulk liquid drop (III). The highest probability of the latter was seen at We > 50 and B < 0.5 for the cases in which droplets had equal density and viscosity. It was found that the satellite droplet encapsulation in regime (III) occurs for the deformed droplets as a result of a concave interface at the moment of contact. This in turn leads to the formation of two contact points and liquid bridges. No satellite formation was observed for the case where the density ratio between the droplets was relatively high. Moreover, a drastic decrease (≈35%) in the boundary between regimes (II) and (I) was observed in this case, compared to the cases where the interacting droplets had equal density. Performed simulations for collisions near the transition point from partial to complete engulfment revealed that the hydrodynamics of interaction and the collision regimes are similar in both cases, despite the difference in the final morphology of the compound.

Simulating the impact of biomass burning aerosols on an intensive precipitation event in urban areas of the Pearl River Delta

The impact of biomass burning aerosols (BBAs) on heavy precipitation during an urban precipitation event on 21 February 2019 was simulated using WRF-Chem v 4.1.2 model. Two tests were conducted: one that considered BBAs and the other that did not. The results showed that, when BBAs were considered, the simulation performance was improved, with the positioning of the high-value precipitation area being more precise. The main precipitation area was dominated by convective precipitation surrounded by mixed precipitation. The stratiform precipitation accounted for the largest proportion (>40%). The intensity, spatial scope, and the persistence of the precipitation were suppressed due to BBAs. This suppression can be attributed to the enhancement in atmosphere stability induced by the BBAs, which weakened the vertical motion and the moisture transport. The convective (mixed) precipitation rate was enhanced by 0.5 mm/h (0.2 mm/h). The stratiform precipitation rate was weakened by 1.5 mm/h, which contributes the major suppression effect due to BBAs on precipitation. BBAs inhibited the condensation of cloud droplets and further inhibited the cloud formation under zero isotherm line. The weakened upward transport of moisture facilitated the formation of ice hydrometeors at lower levels. The major cloud microphysical processes related with the BBA-induced change were the condensation of cloud droplets, the collision and coalescence of cloud droplets by rain, and the melting of hail into rain.

Effects of liquid subcooling on droplet-wall collision heat transfer in film boiling

The effects of subcooling on single droplet-wall collision heat transfer characteristics during film boiling were experimentally investigated. Water droplet subcooling was varied from 0 to 60 K under atmospheric conditions, while the collision velocity and substrate temperature remained constant at 0.7 m/s and 500 °C. The instantaneous and localized heat transfer properties associated with dynamic collision of a water droplet were measured by integrated high-speed shadowgraph and infrared thermometry. Liquid subcooling greatly affected heat transfer. The residence time, effective heat transfer area, and local heat flux were measured. Only the local heat flux correlated strongly with liquid subcooling. When heat flux partitioning at the liquid–vapor interface was considered, i.e., conduction heat transfer into the subcooled droplet and the heat required for evaporation, heat transfer effectiveness was accurately predicted under various subcooling conditions.

Dynamics of equilibration and collisions in ultradilute quantum droplets

Employing time-dependent density-functional theory, we have studied dynamical equilibration and binary head-on collisions of quantum droplets taking as a case of study droplets made of a $^{39}\mathrm{K}\text{\ensuremath{-}}^{39}\mathrm{K}$ Bose mixture. The phase space of collision outcomes is extensively explored by performing fully three-dimensional calculations with effective single-component Quantum Monte Carlo-based and two-component LHY-corrected mean-field functionals. We exhaustively explored the important effect---not considered in previous studies---of the initial population ratio deviating from the optimal mean-field value ${N}_{2}/{N}_{1}=\sqrt{{a}_{11}/{a}_{22}}$. Both stationary and dynamical calculations indicate sensitivity to an initial nonoptimal concentration. When three-body losses (3BL) are present our two-component approach allows to theoretically address situations in which they mainly act on one of the components of the mixture. Our approach also allows to simultaneously explore the effect on the simulation of population imbalance and 3BL, which are coupled when they act.