Droplet Collision Research Articles

The variability of volatile organic compounds during a persistent fog-haze episode

A persistent fog-haze process associated with high pollution occurred in the northern suburbs of Nanjing from November to December 2013. Based on the comprehensive chemical and microphysical observations during the intense observation period, the composition characteristics, and variation rules of volatile organic compounds (VOCs) in the atmosphere under four weather conditions (slight haze, haze, fog, and dense fog) were compared and analyzed, the influencing factors for VOCs during extremely dense fog were discussed in more detail. The average concentrations of VOCs displayed as alkanes > aromatics > alkenes > alkynes, and their concentrations were ranked as dense fog > fog > haze > slight haze, the main factor contributing to the difference in concentrations of VOCs under different weather conditions is the boundary layer characteristics and photochemical reaction rate. Microphysical parameters such as liquid water content (LWC) were negatively correlated with VOCs concentration in dense fog (LWC>0.008 g m−3). Also, the concentration of VOCs showed an oscillating decrease in extremely dense fog (LWC>0.12 g m−3), and the total VOCs removal rate was close to 30%, which may be attributed to an indirect/direct removal effect, in which the enhanced collision and deposition of fog droplets promote the redistribution of VOCs gas-aqueous/particle partitioning, and remove them from the atmosphere by fog water.

Waltz of tiny droplets and the flow they live in

This article describes the dynamics of small inertial particles centrifuging out of a single vortex. It shows the importance of caustics formation in the vicinity of a single vortex: both for particle collisions and void formation. From these single-vortex studies we provide estimates of the role of caustics in high Reynolds number turbulence, and in the case of clouds, estimate how they may help in rain initiation by bridging the droplet-growth bottleneck. We briefly describe how the Basset- Boussinesq history force may be calculated by a method which does not involve huge memory costs, and provide arguments for its possible importance for droplets in turbulence. We discuss how phase change could render cloud turbulence fundamentally different from turbulence in other situations.

Interaction between Droplets and Particles as Oil–Water Slurry Components

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

Weber number and the outcome of binary collisions between quantum droplets

A theoretical analysis of binary collisions of quantum droplets under feasible experimental conditions is reported. Droplets formed from degenerate dilute Bose gases made up from binary mixtures of ultracold atoms are considered. Reliable expressions for the surface tension of the droplets are introduced based on a study of low energy excitations of their ground state within the random phase approximation. Their relevance is evaluated considering an estimation of the expected excitation energy having in mind the Thouless variational theorem. The surface tension expressions allow calculating the Weber number of the droplets involved in the collisions. Several regimes on the outcomes of the binary frontal collisions that range from the coalescence of the quantum droplets to their disintegration into smaller droplets are identified. Atoms losses of the droplets derived from self-evaporation and three-body scattering are quantified for both homo- and hetero-nuclear mixtures. Their control is mandatory for the observation of some interesting effects arising from droplets collisions.

PDMS 표면에서 충돌 액적의 점도에 따른 거동의 주기성

Periodicity behavior was observed after droplet collision in viscous solutions with controlled glycerol concentrations onto a PDMS surface. No periodic movement was observed in the droplets at glycerol concentrations of 50% or more. In contrast, the vertical diameter of the droplets increased and decreased periodically at glycerol solution concentrations of 40% or less. Moreover, there was little change in the periodicity of the impacting droplet movement, and the vibration frequency was measured at approximately 80 to 98 Hz in the entire range. The maximum droplet spreading factor after collision decreased significantly with increasing glycerol concentrations of 40% or more (Ohnesorge number 1.4 × 10SUP-2/SUP or more). The results suggest that the effect of viscosity became greater than that of the solution surface tension with increasing glycerol concentrations.

Dynamic characteristics of moving droplets impacting sessile droplets with different Reynolds numbers

The collision of moving droplets with sessile droplets is a common occurrence in fields of industry, including power generation, chemical engineering, and aerospace, among others. In this paper, the collision of propylene glycol, glycerol, and de-ionized water droplets is studied for given collision speeds and different volume ratios of moving and sessile droplets using high-speed photography. It is found that droplet collision at a speed of about 0.25 m/s leads to compression deformation, whereas collision at about 1.10 m/s typically produces features, such as a non-splashing liquid crown and a central liquid jet. In this paper, the main characteristics of the above phenomena are quantified, the influence of the Reynolds number at different volume ratios is studied, and the main phenomena are explained from the perspective of an energy analysis. The findings reported here are significant for the solution of practical engineering problems and improving the stability of equipment operation.

Surveying the effects of concave obstacles with different edge walls on hollow glycerin droplet impacting using the volume of fluid approach

The numerical analysis of the impact of a hollow droplet on a solid concave obstacle at various heights of the edge wall and impact velocity is presented in this study. The Volume Of Fluid (VOF) approach was used to simulate hollow droplet impact using OpenFoam software. The Newtonian, incompressible, and laminar properties of the glycerin hollow droplet are assumed. Hollow droplet collision hydrodynamic behavior and jet characteristics were explored. The height of the counter jet rose as the impact velocity and height of the edge wall increased. Maximum height and length have happened to case 8 in t = 2 ms and case 3 in t = 1 ms, so the amount of it is 9.75 and 19.75 mm, minimum height and length in the last time computed is 1.4 and 2.73 mm for cases 1 and 7, respectively.

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

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

A simulation approach to predict electrostatic coalescence performance of water-in-oil emulsion using population balance model

ABSTRACT Electrostatic coalescence of the water droplets in water-in-oil (W/O) emulsion plays a significant role in demulsification process. As the droplets collide and aggregate with each other, the dispersed droplets may coalesce and separate from continuous phase. Several mechanisms simultaneously contribute to the complexity associated with droplets coalescence. Thus, an appropriate simulation approach is needed to predict electrostatic coalescence performance in W/O emulsion. In this work, the population balance model (PBM) was used to predict coalescence performance of W/O emulsion under electric field. The proposed approach is novel as it integrates various mathematical models to study dispersed droplets collision frequency considering Boltzmann motion, differential settlement, and electric field. Moreover, a new collision efficiency model, which is best proper for the droplet size between 1 μm and 100 μm, is also proposed to make the prediction model more accurate. The simulation results were verified by comparing with existing experimental datasets. After that, a sensitivity analysis was carried out to study parameters that affect electrostatic coalescence performance, such as electric field strength, dispersion permittivity, continuous viscosity, and collision efficiency coefficient. The results show that the higher voltage and droplet permittivity both lead to the greater electrostatic coalescence performance, but the increments are gradually smaller under the condition of neglecting the droplet fragmentation. With the increasing of continuous viscosity, the droplet size median significantly decreases at first and then flattens. Meanwhile, the performance of electrostatic coalescence is greater at bigger collision efficiency coefficient. But the variation of volume size median with collision efficiency coefficient is modest. The simulation approach proposed in this paper provides an in-depth understanding of the dispersed droplets collision in electric field, which provides useful insights for prediction of electrostatic coalescence performance of W/O emulsion in petroleum industries.

Research on Value-Seeking Calculation Method of Icing Environmental Parameters Based on Four Rotating Cylinders Array

This paper calculates and analyzes the collision and freezing characteristics of water droplets on the cylindrical conductors with different diameters and deduces the variation law of icing weight change rate on different diameters rotating cylindrical conductors with wind speed, ambient temperature, liquid water content, and median volume diameter. Then, a prediction and value-seeking method of icing environment parameters based on rotating circular conductors is determined. The laboratory test results show that the measured values of ambient temperature T, wind speed U, liquid water content w and median volume diameter MVD are in good conformity with the reference values measured by other instruments, and the average errors of each parameter are 3.41%, 5.44%, 6.08% and 7.20%, respectively. The field natural experiment was carried out at Hunan Xuefengshan Energy Equipment Safety National Observation and Research Station. The obtained icing environmental parameters were substituted into the icing simulation calculation model of the LDQ-100 glass insulator. Compared with the field experiment results, the simulated ice shape and the field ice shape were in good agreement. The error between the numerical calculation results of the icing weight and the field icing weight of the experimental insulator was less than 6%.

Synergetic roles of dynamic and cloud microphysical processes in extreme short‐term rainfall: A case‐study

AbstractBoth dynamic and cloud microphysical processes play significant roles in the intensity of severe rainfall within a convective storm. In this study, a quantitative analysis has been performed to investigate dynamic and cloud microphysical contributions to extreme hourly rainfall (EHR) with the peak value of 201.9 mm in Zhengzhou City, China, on 20 July 2021. It is found that the EHR is generated by the overlay of rainwater provided by both dynamic delivery and cloud microphysical production within a meso‐γ‐scale convective storm over Zhengzhou. Specifically, part of the rainwater is directly produced by cloud microphysical processes over the EHR region. More importantly, considerable rainwater, which is produced in the front of the storm associated with strong updraughts, is delivered into the EHR region. The dynamically delivered rainwater overlays the rainwater produced by cloud microphysical processes, forming a deep layer with a large amount of rainwater over the EHR region. As the massive rainwater pours down within a short time, EHR is formed. It should be highlighted that the dynamic delivery plays a decisive role in EHR formation, although sometimes EHR can be generated mainly through cloud microphysical production in the case of weak dynamic delivery. Concerning the cloud microphysical processes, the collision of cloud droplets by raindrops produces the largest amount of rainwater, followed by graupel melting. Linking the EHR with dynamic and cloud microphysical processes within a convective storm, a new light on further understanding and forecasting of short‐duration extreme rainfall would be shed.

Large‐eddy simulations of drizzling shallow cumuli using a turbulence‐aware autoconversion parametrization

AbstractTurbulence in clouds enhances collision between cloud droplets, which is critical to drizzle formation in shallow cumulus clouds. In this study, we develop an autoconversion parametrization that considers the turbulence‐induced collision enhancement (TICE) obtained from a particle trajectory model. We implement the developed parametrization to a large‐eddy simulation model and simulate the Rain In Cumulus over the Ocean (RICO) case to investigate the effects of TICE on shallow cumulus clouds and their sensitivities to the cloud droplet number concentration and horizontal grid resolution. TICE significantly increases the autoconversion rate, leading to large increases in the accretion rate, rainwater path and surface rain rate. The simulations of shallow cumulus clouds are highly sensitive to the cloud droplet number concentration and horizontal grid resolution, and the inclusion of TICE overall reduces the sensitivities. For example, the large rainwater path and surface rain rate that appear when cloud droplet number concentration is relatively low are reduced by TICE, whereas the very small rainwater path and surface rain rate that appear when cloud droplet number concentration is relatively high are increased by TICE. Comparisons with two other TICE‐aware parametrizations based on direct numerical simulation results of droplet collisions in turbulence show that there are substantial differences in TICE effects for different parametrizations, which implies a large uncertainty in simulating turbulent clouds. Among the three parametrizations, the parametrization developed in this study shows intermediate TICE‐induced increases in rainwater path and surface rain rate.

Effects of impacted dynamic hollows and extending dense drops on the concave surface with practical obstacles

In this study, the center of a concave surface was analytically studied using the volume of fluid approach to simulate hollow and dense droplets on a variety of solid obstacles. OpenFoam software was used to carry out the numerical simulations. The hollow droplet’s fluid phase, Glycerine, has an outer diameter of 5.25 mm and its gas phase, air, has a diameter of 4 mm. We looked at the laminar flow of an incompressible Newtonian fluid phase. Jet characteristics and droplet collision hydrodynamic behavior were investigated. Due to the interaction between droplets and shells on the obstacle’s surface and the concave surface, which causes a pressure difference and improves fluid movement, the largest jet size is consequently produced in rectangular obstacles. The sharp obstacle, on the other hand, molds the jet’s shortest length and height.

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

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

Efficient fabrication of tilt micro/nanopillars on polypropylene surface with robust superhydrophobicity for directional water droplet rebound

SummaryThe directional rebound and transport of water droplets plays an important role in microfluidic devices, anti-fogging, and water harvesting. Herein, an extrusion compression molding and directional stretch demolding method was used to prepare a polypropylene (PP) surface with tilt micro/nanopillars with a contact angle of 157 ± 3°. The rolling angle is the highest (9 ± 4°) when the direction of rotation is opposite the tilt direction of the micro/nanopillars, showing excellent water repellency and anisotropy of the surface. Compared with the position of the first collision of the water droplet, the position of the second collision shifted ∼1.5 mm along the tilt direction of the micro/nanopillars, driven by the surface tension component during the collision. The directional rebound behavior is controlled by the droplet energy and the tilt angle. The micro/nanopillars demonstrate excellent self-cleaning property and mechanical durability, which shows the possibility of their practical engineering applications.

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

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

Simulation of two-phase flows at large density ratios and high Reynolds numbers using a discrete unified gas kinetic scheme

In order to treat immiscible two-phase flows at large density ratios and high Reynolds numbers, a three-dimensional code based on the discrete unified gas kinetic scheme (DUGKS) is developed, incorporating two major improvements. First, the particle distribution functions at cell interfaces are reconstructed using a weighted essentially non-oscillatory scheme. Second, the conservative lower-order Allen–Cahn equation is chosen instead of the higher-order Cahn–Hilliard equation to evolve the free-energy-based phase field governing the dynamics of two-phase interfaces. Five benchmark problems are simulated to demonstrate the capability of the approach in treating two-phase flows at large density ratios and high Reynolds numbers, including three two-dimensional problems (a stationary droplet, Rayleigh–Taylor instability, and a droplet splashing on a thin liquid film) and two three-dimensional problems (binary droplets collision and Rayleigh–Taylor instability). All results agree well with the previous numerical and experimental results. In these simulations, the density ratio and the Reynolds number can reach a large value of O(1000). Our improved approach sets the stage for the DUGKS scheme to handle realistic two-phase flow problems.

A review of vaporization models as design criterion for bipropellant thrust chambers

In the beginning of liquid propellant rocket engine development, the thrust chamber sizes were obtained, mainly, empirically. With the technological advancements over the years, several approaches have been developed in order to optimize its sizes and predict more accurately the performance. Besides the clear contribution in predicting efficiencies, the use of accurate vaporization models to optimize combustion chambers decreases losses and the number of required tests. To increase efficiencies, the chamber must be optimized. In case the chamber is too small, incomplete combustion is achieved and combustion instability may occur. In case the chamber is too large, losses due to weight and heat transfer increase and the vehicle becomes larger (leading to more drag losses). Additionally, the number of tests is reduced since models were experimentally validated and less experimental iterations are required in order to obtain the optimized design. Although there are many models, all of them reach similar conclusions, such as an increase in chamber pressure, a decrease in injected droplet size and velocity, and others, lead to a decrease in the required chamber size. Nowadays, with the advancements in computing budget, more complex and accurate models have be developed. Some of these models account for chemical reactions, turbulence effects, droplet collisions and interactions, two- and three-dimensional modeling, and others. Also, the use of CFD codes provides relevant contributions to the analytical and numerical models, especially in validating them, and, additionally, decreases the amount of required experimental tests. The main propulsive parameter that rules this phenomenon is the characteristic length, which accounts the required chamber size for the propellants to be injected, atomized, vaporized, mixed and combusted. Most of the available models neglect the atomization, mixing and combustion of the propellant, since those phenomena occur much faster compared with the vaporization. This work provides a review of those vaporization models, focusing on the main used models worldwide. Such review is of great importance in order to supply enough information and comparison between models, making possible for the researcher/engineer to choose the model that better fit its necessities, requirements and limitations.

Modeling droplet collision dynamic for Lagrangian simulation of impinging spray under high ambient pressures using an improved approach

Spray-to-spray impingement under ambient pressures of 10–60 atm was investigated numerically and experimentally, with particular interest in illustrating the influence of droplet collision dynamic on the spray characteristic. Specially, an improved approach for collision probability prediction was proposed by taking into account the initial size and distance of colliding droplets. The numerical simulations show our approach accounts for the collisions among droplets located in the different computational cells, hence producing substantial independence of computational cell size and total parcel number. The improved approach was subsequently implemented with a recently proposed pressure-dependent collision outcome model to simulate the impinging spray characteristics, the results were compared to the widely-used O’Rourke’s and Estrade et al.’s models. By implementing the improved approach with the pressure-dependent collision outcome model, our numerical simulations successfully reproduce the tendency that droplet bouncing is promoted with increasing ambient pressure, which has been fully recognized in the previous experimental and theoretical studies, however, rarely reflected in previous numerical works. Spray microcosmic characteristic was further discussed based on the numerical simulations.

Enhancing the Emulsification and Photostability Properties of Pectin from Different Sources Using Genipin Crosslinking Technique.

Pectin is a potential polysaccharide-based emulsifier, but the stabilized emulsions suffer from insufficient emulsion stability. Therefore, modification is needed to enhance its emulsification performance to cater to practical applications. The genipin-crosslinking strategy was used in this work to modify pectin with different sources and extraction conditions. Chemical composition analysis, molecular weight (Mw), and radius of gyration (Rg) measurement revealed that sugar beet pectin (SBP) has a more compact and flexible conformation than commercial citrus pectin (CP) and apple pectin (AP), indicated by the significantly (p < 0.05) larger Mw/Rg of SBP (18.1–11.3 kg/mol/nm) than CP (8.3 kg/mol/nm) and AP (8.0 kg/mol/nm). Crosslinking modification significantly increased the Mw, radius of gyration, and viscosity. This significantly (p < 0.05) improved the emulsifying stability (a smaller increase in droplets size) by the contribution of both thicker adsorbed hydrated layers at the oil-water interface with a stronger steric-hindrance effect and larger viscosity effect to slow down droplet collision. The formation of a blue-black substance from crosslinking reaction was able to block the UV radiation, which significantly improved the photostability of β-carotene-loaded emulsions. Altogether, genipin-crosslinking is feasible to modify pectin of different sources to enhance the emulsion stability and for use as a vehicle for delivering bioactive compounds.