Journal articles on the topic 'Droplet Collision'
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1
Li, Xiang-Yu, Axel Brandenburg, Gunilla Svensson, Nils E. L. Haugen, Bernhard Mehlig, and Igor Rogachevskii. "Effect of Turbulence on Collisional Growth of Cloud Droplets." Journal of the Atmospheric Sciences 75, no. 10 (October 2018): 3469–87. http://dx.doi.org/10.1175/jas-d-18-0081.1.
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We investigate the effect of turbulence on the collisional growth of micrometer-sized droplets through high-resolution numerical simulations with well-resolved Kolmogorov scales, assuming a collision and coalescence efficiency of unity. The droplet dynamics and collisions are approximated using a superparticle approach. In the absence of gravity, we show that the time evolution of the shape of the droplet-size distribution due to turbulence-induced collisions depends strongly on the turbulent energy-dissipation rate [Formula: see text], but only weakly on the Reynolds number. This can be explained through the [Formula: see text] dependence of the mean collision rate described by the Saffman–Turner collision model. Consistent with the Saffman–Turner collision model and its extensions, the collision rate increases as [Formula: see text] even when coalescence is invoked. The size distribution exhibits power-law behavior with a slope of −3.7 from a maximum at approximately 10 up to about 40 μm. When gravity is invoked, turbulence is found to dominate the time evolution of an initially monodisperse droplet distribution at early times. At later times, however, gravity takes over and dominates the collisional growth. We find that the formation of large droplets is very sensitive to the turbulent energy dissipation rate. This is because turbulence enhances the collisional growth between similar-sized droplets at the early stage of raindrop formation. The mean collision rate grows exponentially, which is consistent with the theoretical prediction of the continuous collisional growth even when turbulence-generated collisions are invoked. This consistency only reflects the mean effect of turbulence on collisional growth.
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Qian, Lijuan, Jingqi Liu, Hongchuan Cong, Fang Zhou, and Fubing Bao. "A Numerical Investigation on the Collision Behavior of Unequal-Sized Micro-Nano Droplets." Nanomaterials 10, no. 9 (September 3, 2020): 1746. http://dx.doi.org/10.3390/nano10091746.
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Micro-nano droplet collisions are fundamental phenomena in the applications of nanocoating, nano spray, and microfluidics. Detailed investigations of the process of the droplet collisions under higher Weber are still lacking when compared with previous research studies under a low Weber number below 120. Collision dynamics of unequal-sized micro-nano droplets are simulated by a coupled level-set and volume of fluid (CLSVOF) method with adaptive mesh refinement (AMR). The effects of the size ratio (from 0.25 to 0.75) and different initial collision velocities on the head-on collision process of two unequal-sized droplets at We = 210 are studied. Complex droplets will form the filament structure and break up with satellite droplets under higher Weber. The filament structure is easier to disengage from the complex droplet as the size ratio increases. The surface energy converting from kinetic energy increases with the size ratio, which promotes a better spreading effect. When two droplets keep the constant relative velocity, the motion tendency of the droplets after the collision is mainly dominated by the large droplet. On one hand, compared with binary equal-sized droplet collisions, a hole-like structure can be observed more clearly since the initial velocity of a large droplet decreases in the deformation process of binary unequal-sized droplets. On the other hand, the rim spreads outward as the initial velocity of the larger droplet increases, which leads to its thickening.
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Chen, Sisi, Man-Kong Yau, Peter Bartello, and Lulin Xue. "Bridging the condensation–collision size gap: a direct numerical simulation of continuous droplet growth in turbulent clouds." Atmospheric Chemistry and Physics 18, no. 10 (May 25, 2018): 7251–62. http://dx.doi.org/10.5194/acp-18-7251-2018.
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Abstract. In most previous direct numerical simulation (DNS) studies on droplet growth in turbulence, condensational growth and collisional growth were treated separately. Studies in recent decades have postulated that small-scale turbulence may accelerate droplet collisions when droplets are still small when condensational growth is effective. This implies that both processes should be considered simultaneously to unveil the full history of droplet growth and rain formation. This paper introduces the first direct numerical simulation approach to explicitly study the continuous droplet growth by condensation and collisions inside an adiabatic ascending cloud parcel. Results from the condensation-only, collision-only, and condensation–collision experiments are compared to examine the contribution to the broadening of droplet size distribution (DSD) by the individual process and by the combined processes. Simulations of different turbulent intensities are conducted to investigate the impact of turbulence on each process and on the condensation-induced collisions. The results show that the condensational process promotes the collisions in a turbulent environment and reduces the collisions when in still air, indicating a positive impact of condensation on turbulent collisions. This work suggests the necessity of including both processes simultaneously when studying droplet–turbulence interaction to quantify the turbulence effect on the evolution of cloud droplet spectrum and rain formation.
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Xing, Lei, Jinyu Li, Minghu Jiang, and Lixin Zhao. "Dynamic behavior of compound droplets with millimeter-sized particles impacting substrates with different wettabilities." Physics of Fluids 35, no. 2 (February 2023): 022108. http://dx.doi.org/10.1063/5.0137505.
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The dynamic behavior of compound droplets, which are made up of a millimeter-sized particle and distilled water, impacting substrates of different wettabilities is investigated via high-speed photography. The effects of the size of the particle within the compound droplet, substrate contact angle, and impact height on the deformation of the droplets and the characteristics of the impact are analyzed. It is found that the collisions of compound droplets with substrates can be classified into four categories based on the observed experimental phenomena that occur during the impact. These categories are referred to as adhesion collision, rebound collision, daughter-droplet collision (or partial rebound collision), and breakup collision. We consider both the impact of water droplets and compound droplets (with one of two different-sized particles) on substrates of different wettabilities. The effects of inertia, surface tension, and adhesion between the substrate and the liquid droplet, and adhesion between the particle and the liquid droplet are considered to explain the different collision phenomena of compound droplets and reveal the evolution mechanism of the droplet morphologies in the experiments. Furthermore, the effects of the height from which the droplet is released and the contact angle of the substrate (i.e., its wettability) on the maximum spreading diameter and maximum jet height of the droplet are presented quantitatively. The effect of the size of the particle within the compound droplet and the substrate contact angle on the dynamic behavior of the compound droplet subject to impact with the substrate is also described.
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Wang, Yiting, Lijuan Qian, Zhongli Chen, and Fang Zhou. "Coalescence of Binary Droplets in the Transformer Oil Based on Small Amounts of Polymer: Effects of Initial Droplet Diameter and Collision Parameter." Polymers 12, no. 9 (September 9, 2020): 2054. http://dx.doi.org/10.3390/polym12092054.
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In engineering applications, the coalescence of droplets in the oil phase dominates the efficiency of water-oil separation. To improve the efficiency of water-oil separation, many studies have been devoted to exploring the process of water droplets colliding in the oil phase. In this paper, the volume of fluid (VOF) method is employed to simulate the coalescence of water droplets in the transformer oil based on small amounts of polymer. The influences of the initial diameter and collision parameter of two equal droplets on droplet deformation and coalescence time are investigated. The time evolution curves of the dimensionless maximum deformation diameter of the droplets indicate that the larger the droplet diameter, the more obvious the deformation from central collisions. As the collision parameter increases, the contact area of the two droplets, as well as the kinetic energy that is converted into surface energy, decreases, resulting in an increase in droplet deformation. Furthermore, the effects of the initial droplet diameter and collision parameter on coalescence time are also investigated and discussed. The results reveal that as the initial droplet diameter and collision parameter increase, the droplet coalescence time increases.
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Pinsky, M., A. Khain, and H. Krugliak. "Collisions of Cloud Droplets in a Turbulent Flow. Part V: Application of Detailed Tables of Turbulent Collision Rate Enhancement to Simulation of Droplet Spectra Evolution." Journal of the Atmospheric Sciences 65, no. 2 (February 1, 2008): 357–74. http://dx.doi.org/10.1175/2007jas2358.1.
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Abstract The present study is a continuation of the series of studies dedicated to the investigation of cloud droplet collisions in turbulent flow with characteristics that are typical of real clouds. Detailed tables of collision kernels and collision efficiencies calculated in the presence of hydrodynamic interaction of droplets are presented. These tables were calculated for a wide range of turbulent parameters. To illustrate the sensitivity of droplet size distribution (DSD) evolution to the turbulence-induced increase in the collision rate, simulations of DSD evolution are preformed by solving the stochastic kinetic equation for collisions. The results can be applied to cloud modeling. The tables of collision efficiencies and collision kernels are available upon request. Some unsolved problems related to collisions of droplets and ice hydrometeors in turbulent clouds are discussed in the conclusion.
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Chen, Sisi, M. K. Yau, and Peter Bartello. "Turbulence Effects of Collision Efficiency and Broadening of Droplet Size Distribution in Cumulus Clouds." Journal of the Atmospheric Sciences 75, no. 1 (January 2018): 203–17. http://dx.doi.org/10.1175/jas-d-17-0123.1.
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This paper aims to investigate and quantify the turbulence effect on droplet collision efficiency and explore the broadening mechanism of the droplet size distribution (DSD) in cumulus clouds. The sophisticated model employed in this study individually traces droplet motions affected by gravity, droplet disturbance flows, and turbulence in a Lagrangian frame. Direct numerical simulation (DNS) techniques are implemented to resolve the small-scale turbulence. Collision statistics for cloud droplets of radii between 5 and 25 μm at five different turbulence dissipation rates (20–500 cm2 s−3) are computed and compared with pure-gravity cases. The results show that the turbulence enhancement of collision efficiency highly depends on the r ratio (defined as the radius ratio of collected and collector droplets r/ R) but is less sensitive to the size of the collector droplet investigated in this study. Particularly, the enhancement is strongest among comparable-sized collisions, indicating that turbulence can significantly broaden the narrow DSD resulting from condensational growth. Finally, DNS experiments of droplet growth by collision–coalescence in turbulence are performed for the first time in the literature to further illustrate this hypothesis and to monitor the appearance of drizzle in the early rain-formation stage. By comparing the resulting DSDs at different turbulence intensities, it is found that broadening is most pronounced when turbulence is strongest and similar-sized collisions account for 21%–24% of total collisions in turbulent cases compared with only 9% in the gravitational case.
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Saroka, Mary D., and Nasser Ashgriz. "Separation Criteria for Off-Axis Binary Drop Collisions." Journal of Fluids 2015 (May 25, 2015): 1–15. http://dx.doi.org/10.1155/2015/405696.
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Off-axis collisions of two equal size droplets are investigated numerically. Various governing processes in such collisions are discussed. Several commonly used theoretical models that predict the onset of separation after collision are evaluated based on the processes observed numerically. A separation criterion based on droplet deformation is found. The numerical results are used to assess the validity of some commonly used phenomenological models for drop separation after collision. Also, a critical Weber number for the droplet separation after grazing collision is reported. The effect of Reynolds number is investigated and regions of permanent coalescence and separation are plotted in a Weber-Reynolds number plane for high impact parameter collisions.
9
Wang, C. H., K. L. Pan, S. Y. Fu, W. C. Huang, and J. Y. Yang. "An Experimental Investigation on the Coalescent Behaviors of Colliding Droplets." Journal of Mechanics 23, no. 4 (December 2007): 415–22. http://dx.doi.org/10.1017/s1727719100001465.
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AbstractThe coalescent behaviors in collisions between two droplets respectively made of different alkanes, water and alkane, methanol and alkane, and ethanol and hexadecane were experimentally studied. The coalescent results between two droplets of different alkanes are qualitatively the same as that with the same material, which simply form a spherical droplet. However, it took time to have the concentration within the droplet to become uniformly distributed. The collision results of water and alkane droplets collision become slightly more complex, in most cases, the water droplet was either inserted into or adhesive to the hexadecane droplet while only insertion was observed if the target droplet was dodecane or heptane. The inserted water droplet tends to partially expose to the environment as the volume fraction of water is sufficiently high, say, ∼0.62 for hexadecane, > 0.70 for dodecane, and > 0.78 for heptane; and the limit is lowered with the decreasing of water or merged droplet size. For the cases of methanol and alkanes, and ethanol and hexadecane, the two colliding droplets were adhesive to each other in all the studies. Furthermore, in most conditions, air bubbles were observed immediately after the collisions, while only few or even none of them might be trapped within the final merged droplet.
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Demidovich, A. V., S. S. Kralinova, P. P. Tkachenko, N. E. Shlegel, and R. S. Volkov. "Interaction of Liquid Droplets in Gas and Vapor Flows." Energies 12, no. 22 (November 8, 2019): 4256. http://dx.doi.org/10.3390/en12224256.
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We investigated the conditions, characteristics, and outcomes of liquid droplet interaction in the gas medium using video frame processing. The frequency of different droplet collision outcomes and their characteristics were determined. Four interaction regimes were identified: bounce, separation, coalescence, and disruption. Collision regime maps were drawn up using the Weber, Reynolds, Ohnesorge, Laplace, and capillary numbers, as well as dimensionless linear and angular parameters of interaction. Significant differences were established between interaction maps under ideal conditions (two droplets colliding without a possible impact of the neighboring ones) and collision of droplets as aerosol elements. It was shown that the Weber number could not be the only criterion for changing the collision mode, and sizes and concentration of droplets in aerosols influence collision modes. It was established that collisions of droplets in a gaseous medium could lead to an increase in the liquid surface area by 1.5–5 times. Such a large-scale change in the surface area of the liquid significantly intensifies heat transfer and phase transformations in energy systems.
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Kropotova, Svetlana, and Pavel Strizhak. "Collisions of Liquid Droplets in a Gaseous Medium under Conditions of Intense Phase Transformations: Review." Energies 14, no. 19 (September 27, 2021): 6150. http://dx.doi.org/10.3390/en14196150.
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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.
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Khain, A., V. Arkhipov, M. Pinsky, Y. Feldman, and Ya Ryabov. "Rain Enhancement and Fog Elimination by Seeding with Charged Droplets. Part I: Theory and Numerical Simulations." Journal of Applied Meteorology 43, no. 10 (October 1, 2004): 1513–29. http://dx.doi.org/10.1175/jam2131.1.
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Abstract A new method of droplet collision acceleration, with the purpose of rain enhancement and fog elimination, is proposed. According to the method, some fraction of the droplets is taken from clouds (or fog) themselves, charged, and then injected back into clouds (or fog). To verify the efficiency of the method, a novel model has been developed, allowing simulation of droplet spectrum evolution by collision in case a certain fraction of the droplets in a droplet spectrum is charged. Simulations of droplet spectra evolution include several steps: (a) The forces arising between charged and neutral droplets, as well as between charged droplets, are calculated as the function of the value of the charges, droplet size, and distance between droplets. It is shown that because of the induction effect, significant attraction forces arise between charged and neutral droplets. (b) The results obtained have been used to calculate the collision efficiencies between charged and neutral, as well between charged droplets. As a result, a “four dimensional” table of the collision efficiencies (the collision efficiency is the function of the droplet size and charge) was calculated. The collision efficiencies between charged and neutral droplets turn out to be significantly higher than the pure gravity-induced values. (c) To accomplish these simulations, a novel numerical method of solving the stochastic collision equation has been developed. Cloud droplets are described by a two-dimensional size distribution function in which droplets are characterized by both their mass and charge. (d) This model, with the implemented table of the collision efficiencies, has been used to simulate droplet spectra evolution in clouds and fog in case some fraction of these droplets was charged. Simulations of the effects of seeding by charged droplets have been performed. Evolution of initially narrow droplet size spectra (typical of extremely continental clouds in highly smoky air), in the case of seeding and under natural conditions, has been simulated. It was shown that although a natural droplet spectrum does not develop and no raindrops are formed, the injection of just a small fraction of charged particles rapidly triggered the collision process and lead to raindrop formation a few minutes after the injection. Significant acceleration of raindrop formation has been found in the case of a maritime wide-droplet spectrum. Simulations of fog seeding were conducted using droplet spectra distributions of typical fog. Seeding by charged fog droplets of one or both polarities was simulated. In both cases a significant increase in fog visibility was found. The advantages of the seeding method proposed are discussed.
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QIAN, J., and C. K. LAW. "Regimes of coalescence and separation in droplet collision." Journal of Fluid Mechanics 331 (January 25, 1997): 59–80. http://dx.doi.org/10.1017/s0022112096003722.
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An experimental investigation of the binary droplet collision dynamics was conducted, with emphasis on the transition between different collision outcomes. A series of time-resolved photographic images which map all the collision regimes in terms of the collision Weber number and the impact parameter were used to identify the controlling factors for different outcomes. The effects of liquid and gas properties were studied by conducting experiments with both water and hydrocarbon droplets in environments of different gases (air, nitrogen, helium and ethylene) and pressures, the latter ranging from 0.6 to 12 atm. It is shown that, by varying the density of the gas through its pressure and molecular weight, water and hydrocarbon droplets both exhibit five distinct regimes of collision outcomes, namely (I) coalescence after minor deformation, (II) bouncing, (III) coalescence after substantial deformation, (IV) coalescence followed by separation for near head-on collisions, and (V) coalescence followed by separation for off-centre collisions. The present result therefore extends and unifies previous experimental observations, obtained at one atmosphere air, that regimes II and II do not exist for water droplets. Furthermore, it was found that coalescence of the hydrocarbon droplets is promoted in the presence of gaseous hydrocarbons in the environment, suggesting that coalescence is facilitated when the environment contains vapour of the liquid mass. Collision at high-impact inertia was also studied, and the mechanisms for separation of the coalescence are discussed based on time-resolved collision images. A coalescence/separation criterion defining the transition between regimes III and IV for the head-on collisions was derived and found to agree well with the experimental data.
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Pinsky, M. B., A. P. Khain, and M. Shapiro. "Collisions of Cloud Droplets in a Turbulent Flow. Part IV: Droplet Hydrodynamic Interaction." Journal of the Atmospheric Sciences 64, no. 7 (July 1, 2007): 2462–82. http://dx.doi.org/10.1175/jas3952.1.
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Abstract The paper presents a computationally accurate and efficient method for calculation of cloud droplets’ collision efficiency in a turbulent flow with the properties typical of atmospheric clouds. According to Part III, the statistical properties of a turbulent flow are represented by a set of noncorrelated samples of turbulent velocity gradients and Lagrangian accelerations. Long series of these samples were generated for turbulent parameters typical of different atmospheric clouds. Each sample can be assigned to a certain point of the turbulent flow. Each such point can be surrounded by a small elementary volume with the linear length scale of the Kolmogorov length scale, in which the Lagrangian acceleration and the velocity gradient tensor can be considered uniform in space and invariable in time. For each sample (or an elementary volume), fluxes of droplets of one size onto droplets of another size are calculated both in the presence and absence of hydrodynamical droplet interaction (HDI). In each elementary volume, the collision efficiency is calculated as the ratio of these fluxes. Using a set of the collision efficiency and kernels, the probability distribution functions (PDFs) and the mean values of collision efficiency and collision kernels are calculated under different dissipation rates and Reynolds numbers. It is shown that turbulence significantly increases the collision efficiency, especially for droplets of close sizes and droplet pairs containing a few-microns-radius droplet. The results suggest that the main mechanism by means of which turbulence increases the rate of cloud droplets’ collisions is its influence on HDI.
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Li, Xiang-Yu, Axel Brandenburg, Gunilla Svensson, Nils E. L. Haugen, Bernhard Mehlig, and Igor Rogachevskii. "Condensational and Collisional Growth of Cloud Droplets in a Turbulent Environment." Journal of the Atmospheric Sciences 77, no. 1 (December 26, 2019): 337–53. http://dx.doi.org/10.1175/jas-d-19-0107.1.
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Abstract We investigate the effect of turbulence on the combined condensational and collisional growth of cloud droplets by means of high-resolution direct numerical simulations of turbulence and a superparticle approximation for droplet dynamics and collisions. The droplets are subject to turbulence as well as gravity, and their collision and coalescence efficiencies are taken to be unity. We solve the thermodynamic equations governing temperature, water vapor mixing ratio, and the resulting supersaturation fields together with the Navier–Stokes equation. We find that the droplet size distribution broadens with increasing Reynolds number and/or mean energy dissipation rate. Turbulence affects the condensational growth directly through supersaturation fluctuations, and it influences collisional growth indirectly through condensation. Our simulations show for the first time that, in the absence of the mean updraft cooling, supersaturation-fluctuation-induced broadening of droplet size distributions enhances the collisional growth. This is contrary to classical (nonturbulent) condensational growth, which leads to a growing mean droplet size, but a narrower droplet size distribution. Our findings, instead, show that condensational growth facilitates collisional growth by broadening the size distribution in the tails at an early stage of rain formation. With increasing Reynolds numbers, evaporation becomes stronger. This counteracts the broadening effect due to condensation at late stages of rain formation. Our conclusions are consistent with results of laboratory experiments and field observations, and show that supersaturation fluctuations are important for precipitation.
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Nguyen, Khanh P., and Truong V. Vu. "Collision Modes of Two Eccentric Compound Droplets." Processes 8, no. 5 (May 18, 2020): 602. http://dx.doi.org/10.3390/pr8050602.
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A compound droplet with its single inner droplet appears in a broad range of applications and has received much attention in recent years. However, the role of the inner droplet location on the dynamical behaviors of the compound droplet is still not completely understood. Accordingly, the present study numerically deals with the eccentricity of the compound droplet affecting its colliding behaviors with the other droplet in a simple shear flow. The solving method is a front-tracking technique that treats the droplet interface as connected elements moving on a rectangular fixed grid. Initially, two compound droplets assumed circular are placed at a distance symmetrically to the domain center and they come into contact, because of the shear flow, when time progresses. During the collision process, the inner droplet that is initially located at a distance to its outer droplet center circulates around this center. It is found that this rotation also contributes to the formation of the collision modes including the reversing, passing-over and merging ones. Starting from a passing-over mode, a transition to a reversing mode or a merging mode can appear when the inner droplets, in terms of their centroids, are closer than their outer droplets. However, the location of the inner droplet within the outer droplet only has an effect when the value of the Capillary number Ca is varied from 0.01 to 0.08. For Ca < 0.01 corresponding to the merging mode and Ca ≥ 0.16 corresponding to the passing-over mode, the inner droplet position has almost no impact on the collision behaviors of two compound droplets.
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Islamova, Anastasia, Pavel Tkachenko, Nikita Shlegel, and Genii Kuznetsov. "Secondary Atomization of Fuel Oil and Fuel Oil/Water Emulsion through Droplet-Droplet Collisions and Impingement on a Solid Wall." Energies 16, no. 2 (January 16, 2023): 1008. http://dx.doi.org/10.3390/en16021008.
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This paper presents findings from an experimental study investigating the secondary atomization of liquid fuel droplets widely used in the heat and power industry exemplified by fuel oil and environmentally promising fuel oil/water emulsion. The scientific novelty comes from the comparative analysis of the critical conditions and integral characteristics of the secondary atomization of the liquid and composite fuels with the greatest potential for power plants. Here, we used two fuel atomization schemes: droplet–droplet collisions in a gas and droplets impinging on a heated solid wall. The temperature of the liquids under study was 80 °C. The velocities before collision ranged from 0.1 m/s to 7 m/s, while the initial droplet sizes varied from 0.3 mm to 2.7 mm. A copper substrate served as a solid wall; its temperature was varied from 20 °C to 300 °C. The main characteristics of droplet interaction were recorded by a high-speed camera. Regime maps were constructed using the experimental findings. It was established that the critical Weber number was several times lower when water and fuel oil droplets collided than during the collision of fuel oil droplets with 10 vol% of water. The secondary atomization of fuel oil/water emulsion droplets by their impingement on a heated solid wall was found to reduce the typical sizes of liquid fragments by a factor of 40–50. As shown in the paper, even highly viscous fuels can be effectively sprayed using primary and secondary droplet atomization schemes. It was established that the optimal temperature of the fuel oil to be supplied to the droplet collision zone is 80 °C, while the optimal substrate temperature for the atomization of fuel oil/water emulsion droplets approximates 300 °C.
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Sonawan, Hery, Abdurrachim Halim, and Nathanael P. Tandian. "The theoretical approach of how to predict the critical rotational speed of the rotating nozzle in flashing purification to increase the evaporation rate." IOP Conference Series: Earth and Environmental Science 1157, no. 1 (April 1, 2023): 012032. http://dx.doi.org/10.1088/1755-1315/1157/1/012032.
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Abstract We developed a new flashing method in this study to improve the evaporation rate in flashing purification. In this case, a stationary nozzle is replaced by a rotating nozzle during the flashing process. The use of a rotating nozzle may distribute water droplets in previously unreachable spaces; the more droplets, the greater the evaporation rate. The use of a rotating nozzle during flashing causes problems if the rotational speed causes the water droplets to collide. To avoid a collision, a rotating nozzle must rotate at an appropriate rate. The kinematics of drifting water droplets after being sprayed by a rotating nozzle can be used to calculate this speed. It could be calculated by periodically observing droplet movement to determine the critical rotational speed of a rotating nozzle. By studying the trajectories of water droplet movement after being sprayed by a rotating nozzle, we can obtain no droplet collision circumstances. Water droplet evaporation during drifting reduces droplet size, resulting in a lower nozzle rotational speed to avoid a collision. For a water particle diameter of 153.5 µm, the smallest nozzle rotational speed predicted is 85.4 rpm. A droplet with a diameter of 80 µm produces 82 rpm of nozzle rotational speed, while a droplet with a diameter of 40 µm produces 80 rpm of nozzle rotational speed. Evaporation may reduce the size of the water droplets during the flashing process. To avoid droplet collision and increase evaporation rate, finer water droplets require a lower nozzle rotational speed.
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Dziekan, Piotr, and Hanna Pawlowska. "Stochastic coalescence in Lagrangian cloud microphysics." Atmospheric Chemistry and Physics 17, no. 22 (November 14, 2017): 13509–20. http://dx.doi.org/10.5194/acp-17-13509-2017.
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Abstract. Stochasticity of the collisional growth of cloud droplets is studied using the super-droplet method (SDM) of Shima et al.(2009). Statistics are calculated from ensembles of simulations of collision–coalescence in a single well-mixed cell. The SDM is compared with direct numerical simulations and the master equation. It is argued that SDM simulations in which one computational droplet represents one real droplet are at the same level of precision as the master equation. Such simulations are used to study fluctuations in the autoconversion time, the sol–gel transition and the growth rate of lucky droplets, which is compared with a theoretical prediction. The size of the coalescence cell is found to strongly affect system behavior. In small cells, correlations in droplet sizes and droplet depletion slow down rain formation. In large cells, collisions between raindrops are more frequent and this can also slow down rain formation. The increase in the rate of collision between raindrops may be an artifact caused by assuming an overly large well-mixed volume. The highest ratio of rain water to cloud water is found in cells of intermediate sizes. Next, we use these precise simulations to determine the validity of more approximate methods: the Smoluchowski equation and the SDM with multiplicities greater than 1. In the latter, we determine how many computational droplets are necessary to correctly model the expected number and the standard deviation of the autoconversion time. The maximal size of a volume that is turbulently well mixed with respect to coalescence is estimated at Vmix = 1.5 × 10−2 cm3. The Smoluchowski equation is not valid in such small volumes. It is argued that larger volumes can be considered approximately well mixed, but such approximation needs to be supported by a comparison with fine-grid simulations that resolve droplet motion.
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Ahmed, Fatma, Nobuyuki Kawahara, and Eiji Tomita. "Binary collisions and coalescence of droplets in low-pressure fuel injector." Thermal Science, no. 00 (2020): 185. http://dx.doi.org/10.2298/tsci191120185a.
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The phenomena of binary collisions and coalescence of droplets was investigated from experimental studies but still are missing from real applications such as from fuel injector. The main purpose of the current study is to investigate the phenomena of binary collisions and coalescence of droplets from a practical port fuel injector (PFI). To accomplish this, direct microscopic images are taken from high-speed video camera coupled with a long-distance microscope and Barlow lens using the backlighting method. Experimental optimization of the spatial resolution and the depth -of -field of the long-distance microscope and Barlow lens are achieved. Experimental results from the direct microscopic images are compared with predictions from empirical equations for different collision regimes. Droplet sizes and velocities of experimental coalescence droplets from collisions are compared with the values predicted by the equations. The main results of this study are: The probability of collision and coalescence is very low in a PFI. The tangential velocity components of small droplets play an essential role in shape deformation during collisions and coalescence of the droplets. The previous published empirical equations to calculate dimensionless parameters, the Weber number (We), the droplet diameter ratio (?), and impact parameter (B) are applicable to the coalescence of droplets in a PFI.
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Wang, Jian, Jichuan Wu, Shouqi Yuan, and Wei-Cheng Yan. "CFD simulation of ultrasonic atomization pyrolysis reactor: the influence of droplet behaviors and solvent evaporation." International Journal of Chemical Reactor Engineering 19, no. 2 (February 1, 2021): 167–78. http://dx.doi.org/10.1515/ijcre-2020-0229.
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Abstract Previous work showed that particle behaviors in ultrasonic atomization pyrolysis (UAP) reactor have a great influence on the transport and collection of particles. In this study, the effects of droplet behaviors (i.e. droplet collision and breakage) and solvent evaporation on the droplet size, flow field and collection efficiency during the preparation of ZnO particles by UAP were investigated. The collision, breakage and solvent evaporation conditions which affect the droplet size distribution and flow pattern were considered in CFD simulation based on Eulerian-Lagrangian method. The results showed that droplet collision and breakage would increase the droplet size, broaden the droplet size distribution and hinder the transport of droplets. Solvent evaporation obviously changed the flow pattern of droplets. In addition, both droplet behaviors and solvent evaporation reduced the collection efficiency. This study could provide detail information for better understanding the effect of droplet behaviors and solvent evaporation on the particle production process via UAP reactor.
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Qian, Lijuan, Hongchuan Cong, and Chenlin Zhu. "A Numerical Investigation on the Collision Behavior of Polymer Droplets." Polymers 12, no. 2 (January 24, 2020): 263. http://dx.doi.org/10.3390/polym12020263.
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Binary droplet collisions are a key mechanism in powder coatings production, as well as in spray combustion, ink-jet printing, and other spray processes. The collision behavior of the droplets using Newtonian and polymer liquids is studied numerically by the coupled level-set and volume of fluid (CLSVOF) method and adaptive mesh refinement (AMR). The deformation process, the internal flow fields, and the energy evolution of the droplets are discussed in detail. For binary polymer droplet collisions, compared with the Newtonian liquid, the maximum deformation is promoted. Due to the increased viscous dissipation, the colliding droplets coalesce more slowly. The stagnant flow region in the velocity field increases and the flow re-direction phenomenon is suppressed, so the polymer droplets coalesce permanently. As the surface tension of the polymer droplets decreases, the kinetic and the dissipated energy increases. The maximum deformation is promoted, and the coalescence speed of the droplets slows down. During the collision process, the dominant pressure inside the polymer droplets varies from positive pressure to negative pressure and then to positive pressure. At low surface tension, due to the non-synchronization in the movement of the interface front, the pressure is not smooth and distributes asymmetrically near the center of the droplets.
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He, Guo, Xiao Chuan Wang, and Yan Fei Li. "Effects of Droplets' Collision on Heat and Mass Transfer between High-Temperature Gas and Micron Water Droplets." Key Engineering Materials 609-610 (April 2014): 1386–91. http://dx.doi.org/10.4028/www.scientific.net/kem.609-610.1386.
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The heat and mass transfer process between gas and micron water droplets is complicated. During the process, the collision of droplets can affect the evaporation rate of droplets, and the heat and mass transfer between two phases as well. Unsteady numerical simulation was done in this paper to investigate the effect of droplets collision on the heat and mass transfer. The results show that after the water droplets collide and combine together, the total heat transfer area decreases, thus the accelerated rate of droplet temperature drops. As a result, the unsteady time of droplet temperature rising with considering droplets collision in calculation is longer than that of without considering droplets collision. After droplets collide and combine together, the evaporation rate drops, thus the droplets survival time and time for evaporating completely are extended. The steady gas temperature with droplets collision is higher slightly than that without droplets collision, which indicates that the droplets collision affect the results by using numerical prediction. Against the experimental data, the predicted results with droplets collision are more believable than that without collision. Considering the droplets collision and combination, the change rate of the droplets diameter drops, thus the gas velocity picks up slowly.
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Li, Fang-Fang, Chen Huang, Xie En, Guang-Qiang Wang, and Jun Qiu. "Microscopic experimental study on acoustic agglomeration of the droplets on wall." Thermal Science, no. 00 (2020): 233. http://dx.doi.org/10.2298/tsci200309233l.
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Study on the effect of acoustic wave on droplet collision-coalescence process is interesting and helps to better understand acoustic agglomeration mechanism. This study designs and carries out a microscopic experiment to investigate the effect of acoustic wave on wall droplet collision-coalescence process. The derived microscopic images of droplets under the action of different sound waves at different moment are processed and analyzed by binaryzation with iterative threshold, cavity filling, morphological open arithmetic processing, and identification of connected regions, etc. Using a newly defined parameter, equivalent droplet size, the growth rates of the droplets in natural state and under the action of different acoustic parameters are compared and analyzed. The results show that the effect of sound wave greatly accelerates the collision-coalescence process of the droplet, and comparing with sound pressure level (SPL), the frequency of the sound wave is a more effective parameter in promoting the collision-coalescence process of wall droplets, and the lower the acoustic wave frequency results in larger collision-coalescence rate.
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Islamova, Anastasia, Pavel Tkachenko, Kristina Pavlova, and Pavel Strizhak. "Interaction between Droplets and Particles as Oil–Water Slurry Components." Energies 15, no. 21 (November 6, 2022): 8288. http://dx.doi.org/10.3390/en15218288.
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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.
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Huang, Zheng Yong, Jian Li, Fei Peng Wang, Huan Huan Xia, and Mao Chang Li. "The Collision Behavior of Droplets Splitted from a Droplet that Rebounded on Super-Hydrophobic Surface." Applied Mechanics and Materials 723 (January 2015): 968–71. http://dx.doi.org/10.4028/www.scientific.net/amm.723.968.
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Droplet rebounding on super-hydrophobic surfaces is critical to suppress pollution flashover (i.e. enhancement of pollution flashover-voltage) and to reduce ice accumulation on insulators. This paper presents a novel way to reduce water accumulation on surface via the elastic collision between droplets splitted from a droplet that has rebounded from super-hydrophobic surface. The water-mass that contacted with surface will be reduced resultantly. The influence of hydrophobicity of the surface on contact time and spreading time of water droplets are discussed. The collision behavior between the splitted droplets is indicated by the surface charge that was induced by the rebounding droplets on super-hydrophobic surface. Experimental results show that the super-hydrophobic surface endows water droplets with shorter contact time, spreading time than those values obtained on a bare glass. Specific Web and Reynolds numbers can lead to the elastic rebounding between water droplets, delaying the water contact with the super-hydrophobic surface. The contact electrification between the rebounded droplet and the super-hydrophobic surface renders the droplet charged, thus determines the collision behavior of the splitted droplets that born from the rebounded droplet.
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Islamova, Anastasia, Pavel Tkachenko, Nikita Shlegel, and Geniy Kuznetsov. "Effect of Liquid Properties on the Characteristics of Collisions between Droplets and Solid Particles." Applied Sciences 12, no. 21 (October 24, 2022): 10747. http://dx.doi.org/10.3390/app122110747.
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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.
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Hoffmann, Fabian, Yign Noh, and Siegfried Raasch. "The Route to Raindrop Formation in a Shallow Cumulus Cloud Simulated by a Lagrangian Cloud Model." Journal of the Atmospheric Sciences 74, no. 7 (June 13, 2017): 2125–42. http://dx.doi.org/10.1175/jas-d-16-0220.1.
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Abstract The mechanism of raindrop formation in a shallow cumulus cloud is investigated using a Lagrangian cloud model (LCM). The analysis is focused on how and under which conditions a cloud droplet grows to a raindrop by tracking the history of individual Lagrangian droplets. It is found that the rapid collisional growth, leading to raindrop formation, is triggered when single droplets with a radius of 20 μm appear in the region near the cloud top, characterized by large liquid water content, strong turbulence, large mean droplet size, broad drop size distribution (DSD), and high supersaturations. Raindrop formation easily occurs when turbulence-induced collision enhancement (TICE) is considered, with or without any extra broadening of the DSD by another mechanism (such as entrainment and mixing). In contrast, when TICE is not considered, raindrop formation is severely delayed if no other broadening mechanism is active. The reason for the difference is clarified by the additional analysis of idealized box simulations of the collisional growth process for different DSDs in varied turbulent environments. It is found that TICE does not accelerate the timing of the raindrop formation for individual droplets, but it enhances the collisional growth rate significantly afterward by providing a greater number of large droplets for collision. Higher droplet concentrations increase the time for raindrop formation and decrease precipitation but intensify the effect of TICE.
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Ramirez-Argaez, Marco A., Diego Abreú-López, Jesús Gracia-Fadrique, and Abhishek Dutta. "Numerical Study of Electrostatic Desalting Process Based on Droplet Collision Time." Processes 9, no. 7 (July 15, 2021): 1226. http://dx.doi.org/10.3390/pr9071226.
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The desalting process of an electrostatic desalting unit was studied using the collision time of two droplets in a water-in-oil (W/O) emulsion based on force balance. Initially, the model was solved numerically to perform a process analysis and to indicate the effect of the main process parameters, such as electric field strength, water content, temperature (through oil viscosity) and droplet size on the collision time or frequency of collision between a pair of droplets. In decreasing order of importance on the reduction of collision time and consequently on the efficiency of desalting separation, the following variables can be classified such as moisture content, electrostatic field strength, oil viscosity and droplet size. After this analysis, a computational fluid dynamics (CFD) model of a biphasic water–oil flow was developed in steady state using a Eulerian multiphase framework, in which collision frequency and probability of coalescence of droplets were assumed. This study provides some insights into the heterogeneity of a desalination plant which highlights aspects of design performance. This study further emphasizes the importance of two variables as moisture content and intensity of electrostatic field for dehydrated desalination by comparing the simulation with the electrostatic field against the same simulation without its presence. The overall objective of this study is therefore to show the necessity of including complex phenomena such as the frequency of collisions and coalescence in a CFD model for better understanding and optimization of the desalting process from both process safety and improvement.
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Woittiez, Eric J. P., Harm J. J. Jonker, and Luís M. Portela. "On the Combined Effects of Turbulence and Gravity on Droplet Collisions in Clouds: A Numerical Study." Journal of the Atmospheric Sciences 66, no. 7 (July 1, 2009): 1926–43. http://dx.doi.org/10.1175/2005jas2669.1.
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Abstract This paper examines the combined influences of turbulence and gravity on droplet collision statistics in turbulent clouds by means of direct numerical simulation (DNS). The essential microphysical mechanisms that determine the geometric collision kernel are explored by studying how gravity affects droplet relative velocities and preferential concentration of both monodisperse and bidisperse droplet distributions. To this end, collision statistics of large amounts of droplets with radii ranging from 10 to 90 μm, driven by a turbulent flow field and gravity, are calculated. The flow is homogeneous and isotropic and has a dissipation rate of ε = 4.25 × 10−2 m2 s−3. The results show that in the calculation of collision statistics, the interplay between gravity and turbulence is an essential element and not merely an addition of separate phenomena. For example, the presence of gravity leads to clustering of large droplets interacting with the larger scales of turbulence in the DNS. The collision statistics of a bidisperse droplet distribution, even with a very small radius difference, shows profoundly different behavior than the monodisperse case.
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Shayunusov, Doston, Dmitry Eskin, Boris V. Balakin, Svyatoslav Chugunov, Stein Tore Johansen, and Iskander Akhatov. "Modeling Water Droplet Freezing and Collision with a Solid Surface." Energies 14, no. 4 (February 16, 2021): 1020. http://dx.doi.org/10.3390/en14041020.
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Water droplets released from the sea surface represent one of the major causes of ice accretion on marine vessels. A one-dimensional model of the freezing of a spherical water droplet moving in cold air was developed. The crystallization model allows one to obtain an analytical solution if a uniform temperature distribution over the liquid’s core is assumed. The model was validated using STAR CCM+ Computational fluid dynamics (CFD) code. A collision of a partially frozen droplet with a solid wall assuming the plastic deformation of an ice crust was also considered. The ratio of the crust deformation to the crust thickness was evaluated. It was assumed that if this ratio were to exceed unity, the droplet would stick to the wall’s surface due to ice bridge formation caused by the water released from the droplet’s core.
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GLASNER, KARL, FELIX OTTO, TOBIAS RUMP, and DEJAN SLEPČEV. "Ostwald ripening of droplets: The role of migration." European Journal of Applied Mathematics 20, no. 1 (February 2009): 1–67. http://dx.doi.org/10.1017/s0956792508007559.
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A configuration of near-equilibrium liquid droplets sitting on a precursor film which wets the entire substrate can coarsen in time by two different mechanisms: collapse or collision of droplets. The collapse mechanism, i.e., a larger droplet grows at the expense of a smaller one by mass exchange through the precursor film, is also known as Ostwald ripening. As was shown by K. B. Glasner and T. P. Witelski (‘Collision versus collapse of droplets in coarsening of dewetting thin films’, Phys. D209 (1–4), 2005, 80–104) in case of a one-dimensional substrate, the migration of droplets may interfere with Ostwald ripening: The configuration can coarsen by collision rather than by collapse. We study the role of migration in a two-dimensional substrate for a whole range of mobilities. We characterize the velocity of a single droplet immersed into an environment with constant flux field far away. This allows us to describe the dynamics of a droplet configuration on a two-dimensional substrate by a system of ODEs. In particular, we find by heuristic arguments that collision can be a relevant coarsening mechanism.
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Li, Wen, Bo Miao, Chun-Ling Zhu, and Ning Zhao. "An experimental study of water droplets deformation and collision with airfoil." International Journal of Modern Physics B 34, no. 14n16 (May 30, 2020): 2040094. http://dx.doi.org/10.1142/s0217979220400949.
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When aircraft pass through the clouds that contain super cooled water droplets during aviation, the droplets collide with the wing surface and ice is formed, which induces a significant threat to aviation safety. Studies on droplet movement in gaseous medium are prerequisite for deicing/anti-icing researches. So, in this work, an experimental study is performed on water droplet deformation as the droplets approach the leading edge of an airfoil. This experiment is carried out in a vertical wind tunnel, with a NACA0012 airfoil model assembled 4.3 m downstream of the droplet generator. The influence of Weber number (ranging from 0.2 to 36) on the deformation of a 2 mm diameter droplet is thoroughly investigated. The results indicate that droplets maintain initial form with Weber number under 10; after that droplet deforms into remarkable bag deformation as Weber number reaches to 19, and bag-stamen deformation mode as Weber number is above 20. This observed correlation between Weber number and deformation mode is validated through comparing with published simulation results. Furthermore, using the high-speed camera, clear images of the droplet structure during the deformation process are taken and are shown in detail in this work.
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Nagare, Baban, Claudia Marcolli, André Welti, Olaf Stetzer, and Ulrike Lohmann. "Comparing contact and immersion freezing from continuous flow diffusion chambers." Atmospheric Chemistry and Physics 16, no. 14 (July 19, 2016): 8899–914. http://dx.doi.org/10.5194/acp-16-8899-2016.
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Abstract. Ice nucleating particles (INPs) in the atmosphere are responsible for glaciating cloud droplets between 237 and 273 K. Different mechanisms of heterogeneous ice nucleation can compete under mixed-phase cloud conditions. Contact freezing is considered relevant because higher ice nucleation temperatures than for immersion freezing for the same INPs were observed. It has limitations because its efficiency depends on the number of collisions between cloud droplets and INPs. To date, direct comparisons of contact and immersion freezing with the same INP, for similar residence times and concentrations, are lacking. This study compares immersion and contact freezing efficiencies of three different INPs. The contact freezing data were obtained with the ETH CoLlision Ice Nucleation CHamber (CLINCH) using 80 µm diameter droplets, which can interact with INPs for residence times of 2 and 4 s in the chamber. The contact freezing efficiency was calculated by estimating the number of collisions between droplets and particles. Theoretical formulations of collision efficiencies gave too high freezing efficiencies for all investigated INPs, namely AgI particles with 200 nm electrical mobility diameter, 400 and 800 nm diameter Arizona Test Dust (ATD) and kaolinite particles. Comparison of freezing efficiencies by contact and immersion freezing is therefore limited by the accuracy of collision efficiencies. The concentration of particles was 1000 cm−3 for ATD and kaolinite and 500, 1000, 2000 and 5000 cm−3 for AgI. For concentrations < 5000 cm−3, the droplets collect only one particle on average during their time in the chamber. For ATD and kaolinite particles, contact freezing efficiencies at 2 s residence time were smaller than at 4 s, which is in disagreement with a collisional contact freezing process but in accordance with immersion freezing or adhesion freezing. With “adhesion freezing”, we refer to a contact nucleation process that is enhanced compared to immersion freezing due to the position of the INP on the droplet, and we discriminate it from collisional contact freezing, which assumes an enhancement due to the collision of the particle with the droplet. For best comparison with contact freezing results, immersion freezing experiments of the same INPs were performed with the continuous flow diffusion chamber Immersion Mode Cooling chAmber–Zurich Ice Nucleation Chamber (IMCA–ZINC) for a 3 s residence time. In IMCA–ZINC, each INP is activated into a droplet in IMCA and provides its surface for ice nucleation in the ZINC chamber. The comparison of contact and immersion freezing results did not confirm a general enhancement of freezing efficiency for contact compared with immersion freezing experiments. For AgI particles the onset of heterogeneous freezing in CLINCH was even shifted to lower temperatures compared with IMCA–ZINC. For ATD, freezing efficiencies for contact and immersion freezing experiments were similar. For kaolinite particles, contact freezing became detectable at higher temperatures than immersion freezing. Using contact angle information between water and the INP, it is discussed how the position of the INP in or on the droplets may influence its ice nucleation activity.
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Islamova, A. G., S. A. Kerimbekova, N. E. Shlegel, and P. A. Strizhak. "Droplet-droplet, droplet-particle, and droplet-substrate collision behavior." Powder Technology 403 (May 2022): 117371. http://dx.doi.org/10.1016/j.powtec.2022.117371.
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Fang, Long, Guoding Chen, and Deng Liu. "A determination criterion for predicting the outcome of oblique collision between an oil droplet and solid surface." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 231, no. 11 (January 28, 2016): 2066–74. http://dx.doi.org/10.1177/0954406215627840.
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In a bearing chamber, oil droplets shed from the bearing impinge on the outer chamber housing with different angles. The outcome of oblique collision between an oil droplet and the outer chamber housing determines the flow characteristic of wall oil film which has an important meaning to realize the rigorous lubrication design in the bearing chamber. However, the study of predicting the outcome of oblique collision between an oil droplet and solid surface is relatively rare. In this paper, an experimental setup about oil droplet–solid surface oblique collision and a numeric calculation model using Volume of Fluid (VOF) method have been built. And a lot of experimental work and numerical calculations have been done in a wide range of conditions. Based on that, a determination criterion is ultimately established for predicting the outcome of oblique collision between an oil droplet and solid surface. With the experimental data from literatures and this paper, the determination criterion is verified. The research work in this paper is not only a foundation work for better understanding the conditions of lubrication in bearing chamber but also an important reference for the study of droplet–solid surface collision.
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Guo, Shian, and Huiwen Xue. "The enhancement of droplet collision by electric charges and atmospheric electric fields." Atmospheric Chemistry and Physics 21, no. 1 (January 5, 2021): 69–85. http://dx.doi.org/10.5194/acp-21-69-2021.
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Abstract. The effects of electric charges and fields on droplet collision–coalescence and the evolution of cloud droplet size distribution are studied numerically. Collision efficiencies for droplet pairs with radii from 2 to 1024 µm and charges from −32 r2 to +32 r2 (in units of elementary charge; droplet radius r in units of µm) in different strengths of downward electric fields (0, 200, and 400 V cm−1) are computed by solving the equations of motion for the droplets. It is seen that the collision efficiency is increased by electric charges and fields, especially for pairs of small droplets. These can be considered as being electrostatic effects. The evolution of the cloud droplet size distribution with the electrostatic effects is simulated using the stochastic collection equation. Results show that the electrostatic effect is not notable for clouds with the initial mean droplet radius of r¯=15 µm or larger. For clouds with the initial r¯=9 µm, the electric charge without a field could evidently accelerate raindrop formation compared to the uncharged condition, and the existence of electric fields further accelerates it. For clouds with the initial r¯=6.5 µm, it is difficult for gravitational collision to occur, and the electric field could significantly enhance the collision process. The results of this study indicate that electrostatic effects can accelerate raindrop formation in natural conditions, particularly for polluted clouds. It is seen that the aerosol effect on the suppression of raindrop formation is significant in polluted clouds, when comparing the three cases with r¯=15, 9, and 6.5 µm. However, the electrostatic effects can accelerate raindrop formation in polluted clouds and mitigate the aerosol effect to some extent.
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Trainor, Thomas A. "QGP droplet formation in small asymmetric collision systems." EPJ Web of Conferences 235 (2020): 02006. http://dx.doi.org/10.1051/epjconf/202023502006.
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The journal Nature recently published a letter titled "Creating small circular, elliptical, and triangular droplets of quark-gluon plasma" [1]. The basis for that claim is a combination of measured Fourier amplitudes v2 and v3 from collision systems p-Au, d-Au and h-Au (helion h is the nucleus of atom 3He), Glauber Monte Carlo estimates of initial-state transverse collision geometries for those systems and hydrodynamic Monte Carlo descriptions of the vn data. Apparent correspondence between hydrodynamic model vn trends and data trends is interpreted as confirmation of “collectivity” occurring in the small collision systems, further interpreted to indicate QGP formation. QGP formation in small systems runs counter to pre-RHIC theoretical assumptions that QGP formation should require large collision systems (e.g. central A-A collisions). There is currently available a broad context of experimental data from p-p, A-A and p-Pb collisions at the RHIC and LHC against which the validity of the Nature letter claims may be evaluated. This talk provides a summary of such results and their implications. [1] Nature Phys. 15, no. 3, 214 (2019).
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Franklin, Charmaine N., Paul A. Vaillancourt, M. K. Yau, and Peter Bartello. "Collision Rates of Cloud Droplets in Turbulent Flow." Journal of the Atmospheric Sciences 62, no. 7 (July 1, 2005): 2451–66. http://dx.doi.org/10.1175/jas3493.1.
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Abstract Direct numerical simulations of an evolving turbulent flow field have been performed to explore how turbulence affects the motion and collisions of cloud droplets. Large numbers of droplets are tracked through the flow field and their positions, velocities, and collision rates have been found to depend on the eddy dissipation rate of turbulent kinetic energy. The radial distribution function, which is a measure of the preferential concentration of droplets, increases with eddy dissipation rate. When droplets are clustered there is an increased probability of finding two droplets closely separated; thus, there is an increase in the collision kernel. For the flow fields explored in this study, the clustering effect accounts for an increase in the collision kernel of 8%–42%, as compared to the gravitational collision kernel. The spherical collision kernel is also a function of the radial relative velocities among droplets and these velocities increase from 1.008 to 1.488 times the corresponding gravitational value. For an eddy dissipation rate of about 100 cm2 s−3, the turbulent collision kernel is 1.06 times the magnitude of the gravitational value, while for an eddy dissipation rate of 1500 cm2 s−3, this increases to 2.08 times. Therefore, these results demonstrate that turbulence could play an important role in the broadening and evolution of the droplet size distribution and the onset of precipitation.
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Wu, Jiandong, Jiyun Xu, and Hao Wang. "Numerical simulation of micron and submicron droplets in jet impinging." Advances in Mechanical Engineering 10, no. 10 (October 2018): 168781401880531. http://dx.doi.org/10.1177/1687814018805319.
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Micron droplet deposition onto a wall in an impinging jet is important for various applications like spray cooling, coating, fuel injection, and erosion. The impinging process is featured by abrupt velocity changes and thus complicated behaviors of the droplets. Either modeling or experiment for the droplet behaviors is still challenging. This study conducted numerical modeling and compared with an existing experiment in which concentric dual-ring deposition patterns of micron droplets were observed on the impinging plate. The modeling fully took into account of the droplet motion in the turbulent flow, the collision between the droplets and the plate, as well as the collision, that is, agglomeration among droplets. Different turbulence models, that is, the v2− f model, standard k–ε model, and Reynolds stress model, were compared. The results show that the k–ε model failed to capture the turbulent flow structures and overpredicted the turbulent fluctuations near the wall. Reynolds stress model had a good performance in flow field simulation but still failed to reproduce the dual-ring deposition pattern. Only the v2− f model reproduced the dual-ring pattern when coupled with droplet collision models. The results echoed the excellent performance of the v2− f model in the heat transfer calculation for the impinging problems. The agglomeration among droplets has insignificant influence on the deposition.
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Qiu, Facheng, Xianming Zhang, Xinjie Chai, Yingying Dong, Xingjuan Xie, Zuohua Liu, Renlong Liu, and Wensheng Li. "Simulation of Mass and Heat Transfer of Droplets Collision in a Flash Evaporation Pattern." International Journal of Chemical Engineering 2023 (February 10, 2023): 1–13. http://dx.doi.org/10.1155/2023/3574285.
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The behavior of droplets collision in a flash evaporation ambient widely exists in various fields. In this work, the deformation analysis and thermal analysis models were established under the condition of flash via a computational fluid dynamics (CFD) method. First, the effects of initial temperature and collision velocity on heat and mass transfer during evaporation were considered. Then, the morphology change of the liquid phase, the mass change, and their influencing factors during the droplet evaporation process were analyzed. A very good agreement is observed between the results of this paper and the published literature. The results show that the interaction between the initial collision velocity and the initial temperature affects the heat and mass transfer performance. The initial collision velocity influences the heat and mass transfer process of the evaporating droplet by affecting the deformation characteristics of the droplet. The collision velocity and the liquid temperature have a competitive relationship with the evaporation process. Under a low-initial temperature, the collision velocity played a leading role in the evaporation of the liquid phase and the mass transfer of steam.
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He, Fuyou, Jiawei Li, Chuan Li, Pengyu Wang, Zutao Wang, Ming Zhang, Kexun Yu, and Yuan Pan. "Investigation on collision-coalescence of droplets under the synergistic effect of charge and sound waves: orthogonal design optimization." Journal of Physics D: Applied Physics 55, no. 7 (November 12, 2021): 075204. http://dx.doi.org/10.1088/1361-6463/ac34ac.
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Abstract 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.
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Sardina, Gaetano, Stéphane Poulain, Luca Brandt, and Rodrigo Caballero. "Broadening of Cloud Droplet Size Spectra by Stochastic Condensation: Effects of Mean Updraft Velocity and CCN Activation." Journal of the Atmospheric Sciences 75, no. 2 (January 24, 2018): 451–67. http://dx.doi.org/10.1175/jas-d-17-0241.1.
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Abstract The authors study the condensational growth of cloud droplets in homogeneous isotropic turbulence by means of a large-eddy simulation (LES) approach. The authors investigate the role of a mean updraft velocity and of the chemical composition of the cloud condensation nuclei (CCN) on droplet growth. The results show that a mean constant updraft velocity superimposed onto a turbulent field reduces the broadening of the droplet size spectra induced by the turbulent fluctuations alone. Extending the authors’ previous results regarding stochastic condensation, the authors introduce a new theoretical estimation of the droplet size spectrum broadening that accounts for this updraft velocity effect. A similar reduction of the spectra broadening is observed when the droplets reach their critical size, which depends on the chemical composition of CCN. The analysis of the square of the droplet radius distribution, proportional to the droplet surface, shows that for large particles the distribution is purely Gaussian, while it becomes strongly non-Gaussian for smaller particles, with the left tail characterized by a peak around the haze activation radius. This kind of distribution can significantly affect the later stages of the droplet growth involving turbulent collisions, since the collision probability kernel depends on the droplet size, implying the need for new specific closure models to capture this effect.
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Fletcher, Neville H. "Effect of Electric Charge on Collisions between Cloud Droplets." Journal of Applied Meteorology and Climatology 52, no. 2 (February 2012): 517–20. http://dx.doi.org/10.1175/jamc-d-12-093.1.
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AbstractA simple model is presented for the calculation of the effect of electric charges on the collision and coalescence of cloud droplets, a topic that is of importance for coalescence-induced natural rainfall and for the possible effectiveness of techniques for increasing rainfall by injection of ions into the atmosphere. Whereas electric charges of opposite sign enhance the collision efficiency of cloud droplets, the effect when all droplets bear charges of the same sign depends strongly upon the droplet sizes and separations and upon the ratio of charges on each of a droplet pair. The conditions under which coalescence is increased exist for only a very small fraction of actual cloud structures.
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de Lozar, Alberto, and Lukas Muessle. "Long-resident droplets at the stratocumulus top." Atmospheric Chemistry and Physics 16, no. 10 (May 30, 2016): 6563–76. http://dx.doi.org/10.5194/acp-16-6563-2016.
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Abstract. Turbulence models predict low droplet-collision rates in stratocumulus clouds, which should imply a narrow droplet size distribution and little rain. Contrary to this expectation, rain is often observed in stratocumuli. In this paper, we explore the hypothesis that some droplets can grow well above the average because small-scale turbulence allows them to reside at cloud top for a time longer than the convective-eddy time t*. Long-resident droplets can grow larger because condensation due to longwave radiative cooling, and collisions have more time to enhance droplet growth. We investigate the trajectories of 1 billion Lagrangian droplets in direct numerical simulations of a cloudy mixed-layer configuration that is based on observations from the flight 11 from the VERDI campaign. High resolution is employed to represent a well-developed turbulent state at cloud top. Only one-way coupling is considered. We observe that 70 % of the droplets spend less than 0.6t* at cloud top before leaving the cloud, while 15 % of the droplets remain at least 0.9t* at cloud top. In addition, 0.2 % of the droplets spend more than 2.5t* at cloud top and decouple from the large-scale convective eddies that brought them to the top, with the result that they become memoryless. Modeling collisions like a Poisson process leads to the conclusion that most rain droplets originate from those memoryless droplets. Furthermore, most long-resident droplets accumulate at the downdraft regions of the flow, which could be related to the closed-cell stratocumulus pattern. Finally, we see that condensation due to longwave radiative cooling considerably broadens the cloud-top droplet size distribution: 6.5 % of the droplets double their mass due to radiation in their time at cloud top. This simulated droplet size distribution matches the flight measurements, confirming that condensation due to longwave radiation can be an important mechanism for broadening the droplet size distribution in radiatively driven stratocumuli.
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Bhowmick, Taraprasad, and Michele Iovieno. "Direct Numerical Simulation of a Warm Cloud Top Model Interface: Impact of the Transient Mixing on Different Droplet Population." Fluids 4, no. 3 (August 1, 2019): 144. http://dx.doi.org/10.3390/fluids4030144.
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Turbulent mixing through atmospheric cloud and clear air interface plays an important role in the life of a cloud. Entrainment and detrainment of clear air and cloudy volume result in mixing across the interface, which broadens the cloud droplet spectrum. In this study, we simulate the transient evolution of a turbulent cloud top interface with three initial mono-disperse cloud droplet population, using a pseudo-spectral Direct Numerical Simulation (DNS) along with Lagrangian droplet equations, including collision and coalescence. Transient evolution of in-cloud turbulent kinetic energy (TKE), density of water vapour and temperature is carried out as an initial value problem exhibiting transient decay. Mixing in between the clear air and cloudy volume produced turbulent fluctuations in the density of water vapour and temperature, resulting in supersaturation fluctuations. Small scale turbulence, local supersaturation conditions and gravitational forces have different weights on the droplet population depending on their sizes. Larger droplet populations, with initial 25 and 18 μ m radii, show significant growth by droplet-droplet collision and a higher rate of gravitational sedimentation. However, the smaller droplets, with an initial 6 μ m radius, did not show any collision but a large size distribution broadening due to differential condensation/evaporation induced by the mixing, without being influenced by gravity significantly.
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Nagare, B., C. Marcolli, O. Stetzer, and U. Lohmann. "Comparison of measured and calculated collision efficiencies at low temperatures." Atmospheric Chemistry and Physics 15, no. 23 (December 15, 2015): 13759–76. http://dx.doi.org/10.5194/acp-15-13759-2015.
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Abstract. Interactions of atmospheric aerosols with clouds influence cloud properties and modify the aerosol life cycle. Aerosol particles act as cloud condensation nuclei and ice nucleating particles or become incorporated into cloud droplets by scavenging. For an accurate description of aerosol scavenging and ice nucleation in contact mode, collision efficiency between droplets and aerosol particles needs to be known. This study derives the collision rate from experimental contact freezing data obtained with the ETH CoLlision Ice Nucleation CHamber (CLINCH). Freely falling 80 μm diameter water droplets are exposed to an aerosol consisting of 200 and 400 nm diameter silver iodide particles of concentrations from 500 to 5000 and 500 to 2000 cm−3, respectively, which act as ice nucleating particles in contact mode. The experimental data used to derive collision efficiency are in a temperature range of 238–245 K, where each collision of silver iodide particles with droplets can be assumed to result in the freezing of the droplet. An upper and lower limit of collision efficiency is also estimated for 800 nm diameter kaolinite particles. The chamber is kept at ice saturation at a temperature range of 236 to 261 K, leading to the slow evaporation of water droplets giving rise to thermophoresis and diffusiophoresis. Droplets and particles bear charges inducing electrophoresis. The experimentally derived collision efficiency values of 0.13, 0.07 and 0.047–0.11 for 200, 400 and 800 nm particles are around 1 order of magnitude higher than theoretical formulations which include Brownian diffusion, impaction, interception, thermophoretic, diffusiophoretic and electric forces. This discrepancy is most probably due to uncertainties and inaccuracies in the description of thermophoretic and diffusiophoretic processes acting together. This is, to the authors' knowledge, the first data set of collision efficiencies acquired below 273 K. More such experiments with different droplet and particle diameters are needed to improve our understanding of collision processes acting together.
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Gavaises, M., A. Theodorakakos, G. Bergeles, and G. Brenn. "Evaluation of the Effect of Droplet Collisions on Spray Mixing." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 210, no. 5 (September 1996): 465–75. http://dx.doi.org/10.1243/pime_proc_1996_210_220_02.
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A spray model, implemented in a three-dimensional computational fluid dynamics (CFD) code has been used to evaluate the effect of droplet collisions on spray mixing resulting from the overlapping of liquid spray cones produced by two parallel hollow-cone nozzles under the influence of a cross-flow. The computations are compared with experimental results from phase Doppler anemometer (PDA) measurements in mixing steady sprays. The results show that the droplet collisions, which mainly occur in the mixing area of the two different sprays, have great influence on the droplet size and, as a consequence, on the predicted droplet velocities, especially at distances far from the spray nozzles. Information about the collision mechanisms as well as about droplet velocities and droplet dispersion due to collisions is also presented.
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Chen, Sisi, Peter Bartello, M. K. Yau, P. A. Vaillancourt, and Kevin Zwijsen. "Cloud Droplet Collisions in Turbulent Environment: Collision Statistics and Parameterization." Journal of the Atmospheric Sciences 73, no. 2 (February 1, 2016): 621–36. http://dx.doi.org/10.1175/jas-d-15-0203.1.
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Abstract The purpose of this paper is to quantify the influence of turbulence in collision statistics by separately studying the impacts of computational domain sizes, eddy dissipation rates (EDRs), and droplet sizes and eventually to develop an accurate parameterization of collision kernels. Direct numerical simulations (DNS) were performed with a relatively wide range of EDRs and Taylor microscale Reynolds numbers . EDR measures the turbulence intensity levels. DNS model studies have simulated homogeneous turbulence in a small domain in the cloud’s adiabatic core. Clouds clearly have much larger scales than current DNS can simulate. For this reason, it is emphasized that obtained from current DNS is fundamentally only a measure of the computational domain size for a given EDR and cannot completely describe the physical properties of cloud turbulence. Results show that the collision statistics are independent of the domain sizes and hence of the computational for droplet sizes no bigger than 25 μm as long as the droplet separation distance, which is on the order of the Kolmogorov scale in real clouds, is resolved. Instead, they are found to be highly correlated with EDRs and droplet sizes, and this correlation is used to formulate an improved parameterization scheme. The new scheme well represents the turbulent geometric collision kernel with a relative uncertainty of 14%. A comparison between different parameterizations is made, and the formulas proposed here are shown to improve the fit to the collision statistics.
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Chen, R. H., and C.-M. Lai. "Collision outcome of a water drop on the surface of a deep diesel fuel pool." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 225, no. 7 (May 11, 2011): 1638–48. http://dx.doi.org/10.1177/0954406211403066.
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This study investigated the collision of water drops with diesel fuel. The target liquid was selected not only because this interaction is commonly observed in many fires but also because diesel fuel exhibits similar viscosity to heavy oils on fire. Investigated collision phenomena include water drop disintegration, cavity development, droplet ejection from the underside of the cavity, droplet ejection from the liquid (diesel fuel) crown rim, and formation of water-in-diesel compound drops. Results suggest that the number of water droplets from the disintegrated water drop increases non-linearly with increased Weber number. At a Weber number of 700, the number of water droplets reached a maximum while their size was minimized.