Single Droplet Research Articles

Experimental and Analytical Studies on Ignition of a Single Droplet and Spray

Experimental and Analytical Studies on Ignition of a Single Droplet and Spray

Interaction and motion of two neighboring Leidenfrost droplets on oil surface

Evaporation of droplets on a hot oil surface is a natural phenomenon. However, most of existing studies focus on the evaporation of a single droplet, and the evaporation of multiple droplets is insufficiently understood. Here, we explore the Leidenfrost evaporation of two identical FC-72 droplets on the surface of a hot oil bath. The oil temperature ranges from 73.6 to 126.6 ℃, and the evaporation of droplets each with an initial diameter of 1.5 mm is recorded by an infrared thermographer and a high-speed camera. The shallow oil depth keeps the oil temperature uniform relatively in the slot compared with that in the deep liquid pool due to the larger ratio of the surface area for copper-oil contact to the slot volume. We find that the neighboring droplets evaporate in three stages: non-coalescing, bouncing, and separating. The radius of neighboring Leidenfrost droplets follows the power law <i>R</i>(<i>t</i>)~(1−<i>t</i>/<i>τ</i>)<sup><i>n</i></sup>, where <i>τ</i> is the characteristic droplet lifetime and <i>n</i> is an exponent factor. Moreover, the diffusion-mediated interaction between the neighboring droplets slows down the evaporation process compared with the action of isolated Leidenfrost droplet and leads to an asymmetric temperature field on the droplet surface, thereby breaking the balance of the forces acting on the droplets. A simple dual-droplet evaporation model is developed which considers four forces acting horizontally on the droplet, namely, the Marangoni force resulting from the non-uniform droplet temperature, the gravity component, the lubrication-propulsion force, and the viscous drag force. Scale analysis shows that the Marangoni force and gravity component dominate dual-droplet evaporation dynamics. In the non-coalescence stage, the gravity component induces the droplets to attract each other, while the vapor film trapped between droplets prevents them from directly contacting. When the droplets turn smaller, the gravity component is insufficient to overcome the Marangoni force. Hence, the droplets separate in the final evaporation stage. Finally, we conclude that the competition between Marangoni force and gravitational force is the origin of the bounce evaporation by comparing the theoretical and experimental transition times at distinct stages. This study contributes to explaining the complex Leidenfrost droplet dynamics and evaporation mechanism.

Effect of pressure on heat and mass transfer performance in spray drying tower with low inlet temperature

• Pressure effect on spray-drying tower used in wastewater treatment is studied. • A three-stage single droplet model is developed and used in spray drying simulation. • Wastewater treatment using spray-drying tower under low pressure is tested. • Guidance on selection of optimal operating pressure for spray drying is provided. • The optimum pressure is due to increased mass transfer and reduced residence time. The spray drying tower is an important device of the air circulation evaporation-separation electroplating wastewater treatment system based on Brayton refrigeration cycle, which can acquire the optimum system performance by controlling the operational pressure. In order to explore the optimum pressure in the tower, the heat and mass transfer performances of the spray drying tower under different pressures are investigated. The experimental results show that the evaporation rate increases with the decrease of the pressure. The effect of pressure on tower height and inlet air temperature required for droplet to be completely dried is simulated to use the model verified by experimental data. A three-stage heat and mass transfer model for single-droplet is developed in the simulation. The simulation results show that there is an optimal pressure corresponding to the lowest required tower height. The optimal pressure is increased with the increase of inlet air temperature. When the tower height is fixed, the inlet air temperature required for drying droplet firstly decreases and then increases with the increase of pressure. In addition, through analyzing heat and mass transfer, it is found that the decrease of pressure results in the increase of not only the mass transfer driving force but also the air velocity, thus the droplet residence time is decreased, which leads to the existence of optimal pressure. It is feasible to achieve complete drying in spray drying process by reducing the pressure in the tower with low inlet temperature.

直接噴射式火花点火機関用ノズルにおける非蒸発噴霧の運動量交換過程

Direct fuel injection spray is used to improve thermal efficiency and environmental performance in SI engines. A mixture is formed through momentum exchange between fuel droplets and ambient gas, along with mixing accompanied by droplets evaporation and diffusion. Both the droplets development derived from the liquid fuel atomization and the entrainment ambient air predominate the mixture formation process. The purpose of this study is to elucidate the momentum exchange characteristics of the spray formed by a single-hole injector for direct injection gasoline engines. To clarify the momentum exchange characteristics of spray droplets, the spray was visualized by optical experiments and the spray development process was analyzed. The parameters are injection pressure and ambient density. A comparison of experimental results and a model analysis of a single droplet movement was conducted, and different of deceleration characteristics were evaluated. In the case of the fuel spray, the induced ambient air by the spray tip droplets decreases the relative velocity between the spray droplets and the spray internal flow. A model for predicting the temporal variation of the spray tip penetration from the different of momentum exchange characteristics between the spray and the droplet was proposed.

MORPHOLOGICAL EVOLUTION OF Al2O3 AND CNT NANOFLUID DROPLETS DURING SOLIDIFICATION

Nanofluid is an emerging heat transfer fluid with good heat transfer and thermal conductivity properties. It is important to investigate the phase change properties and morphological evolution during the freezing of nanofluid droplets to understand their practical applications. The effect of dynamic wettability on the deformation of a single droplet of aluminum trioxide (Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O) and graphene (CNT-H<sub>2</sub>O) nanofluids at different mass concentrations and substrate temperatures was investigated by visualizing the droplet freezing. The formation of solid-like and freezing front motions inside the droplet during the freezing process of these droplets was investigated. The solidification process was strongly influenced by the temperature gradient perpendicular to the cold surface and the change in the solid–liquid interface wettability during the phase change, resulting in volume redistribution at the top of the droplet. The freezing shape of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O nanodroplets resembled a "moon crater," and the influence of wettability decreased with increasing concentration, leading to a relative increase in the aperture of the top platform. The fully frozen state of the nanofluid droplet had an increasingly pointed tip, with a strong relationship between the substrate temperature and solidification time when the CNT-H<sub>2</sub>O concentration was 5 times higher and showed no change in the freezing droplet deformation rate under the experimental conditions. The contact angle of the two nanofluid droplets did not fluctuate significantly with increasing concentration, while that of the 1% nanofluid droplets remained at an average value of 85° during freezing. Under different freezing conditions, the freezing shape of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O droplets tended to increase in diameter as the subcooling temperature decreased, with the final deformation rate of 1% Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O being twice that at 5% concentration, while the contact angle of the same mass concentration of Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O decreased by 1° as the subcooling temperature dropped. The CNT-H<sub>2</sub>O droplet became sharper at the tip as the subcooling temperature increased, and its contact angle did not change with temperature.

Study on Dynamic Characteristics of Single Droplet Impingement on Heated Liquid Film

The wall combustion is one of the soot and unburned hydrocarbon formation sources in the engine cylinders, which is affected by both spray and wall parameters. The impingement dynamics of the droplets on liquid films have been widely studied. However, there is less focus on the droplet impingements on hot liquid film, which is more representative as a pre-wall combustion condition. This work investigates the impingement dynamics of ethanol on heated glycerol liquid film. The Weber number (10 ≤ We ≤ 275) and liquid film temperature (70°C ≤ T ≤ 175°C) are two main parameters that lead to a comprehensive understanding of impinging phenomena. In the experiment, a high-speed camera was used to visualize the droplets impingement behaviors, which could be classified to six categories: deposition-spreading, rebounding-sputtering, rebounding-floating, stable crown-spreading, stable crown-sputtering, and splash crown-sputtering. The critical temperature for sputtering is about 125°C, independent of the We. The dynamic phenomena were quantified by the diameter and height of the crown, sputtering time and droplet size distribution. The dimensionless diameter, dimensionless height, and maintenance time of the crown all increase with the increasing We or temperature. When the We is greater than 172, the dimensionless diameter increases less. The relationship between the maximum dimensionless height of the crown and the We is H*max = 0.0026 We. The change of the crown diameter with time is independent from the temperature. Additionally, the sputtering time decreases with the increasing We and temperature. For the diameter distribution of sputtering droplets, the fractions of the larger sputtering droplets increased at low We, while the smaller droplets increased their contributions at high We. With the increase of temperature, the proportion of small diameter sputtering droplets increases.

Thermal Analysis of Suspended Single Droplet Evaporation Measurements with a Coupled Lumped Parameter Model

Thermal Analysis of Suspended Single Droplet Evaporation Measurements with a Coupled Lumped Parameter Model

Numerical simulation of droplet evaporation based on the smoothed particle hydrodynamics method

Based on the smoothed particle hydrodynamics method, a numerical model of smoothed particle hydrodynamics in multi-phase flow evaporation accompanied with heat and mass transfer has been established, and phase transition of droplet evaporation is simulated. In this paper, an smoothed particle hydrodynamics mass transfer equation of gas phase in evaporation are proposed based on Fick?s law. In order to solve the problem of large mass difference of particles at the phase interface during evaporation, the particle splitting and merging techniques are introduced, and the tensile instability of particles during the splitting and merging is weakened by artificial stress. Besides, by adding the surface tension model, the mutual penetration of particles at the gas-liquid interface is effectively prevented. On this basis, the evaporation of single droplet and interacting droplets without gravity is simulated. The results show that the evaporation time of single droplet in this study conforms to D2 law and is within the theoretical range. There is a great influence between interacting droplets in the process of evaporation. Only when dimensionless number C (droplet spacing/droplet diameter) is larger than eight, the influence between droplets can be approximately ignored.

Incorporation of film theory in single droplet combustion model for prediction of precursor release in flame spray pyrolysis

The precursor release rate during flame synthesis has been shown to influence the uniformity of synthesized particles. However, its quantification through single droplet combustion modeling was based on its immediate release from the droplet without considering the effect of the mass boundary layer surrounding the droplet. Here, the film theory is applied with the single droplet combustion model to understand the precursor release in the droplet. The resulting mass boundary layer thickness is coupled with droplet temperature to qualitatively investigate precursor release in flame spray pyrolysis. It is shown that small droplets can enhance the precursor release rate due to their small mass boundary layer thickness and higher heating rate. Increasing the EHA content in the EHA/toluene solvent mixture reduces the mass boundary layer thickness and increases the droplet temperature due to EHA's low specific heat capacity. Using droplet sizes estimated by the phase Doppler interferometry, the model shows that the temporal droplet temperature profile remains relatively constant for six synthesis conditions. Concurrently, the mass boundary layer thickness is increased when the liquid and oxygen flow rates are reduced, and atomizing pressure drop is enhanced, resulting in the overall suppression of precursor release from the droplet and consequently increased formation of smaller-size primary particles. Insight into the relative tendencies of the pure gas-to-particle formation route during the synthesis was also derived as a function of the synthesis conditions. This new methodology for the characterization of precursor release is essential for a more accurate understanding and design of homogeneous nanomaterial using flame spray pyrolysis.

Formation and sedimentation of oil-mineral aggregates in the presence of chemical dispersant.

The formation and sedimentation of oil-mineral aggregates (OMAs) is the major method to transport spilled oil to the seafloor. In this study, the formation and sedimentation experiments of OMA using montmorillonite and four crude oils were performed in a wave tank in the presence of chemical dispersant. Most of the formed OMAs were droplet OMAs, and single droplet OMA would aggregate into multiple ones under the action of the dispersant. The size of the oil droplets trapped in the OMA increased with time and was larger for the oil with higher viscosity. The sinking velocities of OMAs formed in this study were between 100-1200 μm s-1 and they were positively correlated with their diameter. The density of OMA was of the same order as that of the crude oil that formed them. An increase in the dispersant dosage could promote the formation of OMAs. The oil content in OMAs was higher for the denser oil in the presence of a dispersant. The maximum oil trapping efficiency of OMAs was 48.05%. This study provides fundamental data on the formation kinetics of OMAs.

Neural network modeling of the dynamic inactivation of probiotics during single droplet drying for improved cell viability

Dairy products containing probiotics are often dried to improve shelf life and facilitate transportation. A reliable dynamic inactivation model has long been pursued to optimize the production by maximizing probiotics' survival during drying. How to take care of the dynamic drying process experienced by the cells for precise prediction of their survival remains a challenging task. In this work, a multi-task convolutional self-attention network (CSAN) has been developed for dynamic modeling of probiotics inactivation during single droplet drying (SDD). The convolution self-attention approach together with a unique double-branch architecture allows the neural network (NN) to learn effectively from historical data and predict inactivation dynamics throughout the whole drying process. In terms of prediction accuracy, our model ( R 2 > 0.96) outperforms many other existing models ( R 2 < 0.6 in most circumstances). By resorting to this model, two optimal SDD conditions have been identified with the resultant terminal solid contents higher than 90 wt% and cell survival ratios higher than 0.65. • A dynamic inactivation model for probiotics survival prediction during drying • Effective leaning of historical data by a convolutional self-attention neural network • A double-branch architecture allows precise prediction of inactivation dynamics • Optimized conditions identified for improved cell viability and sufficient solvent removal

Collective transport of droplets through porous media

The flow of deformable particles, such as droplets, dragged by a fluid, through a network of narrow pores inside rocks or other porous media is key in a range of applications, from enhanced oil recovery and water filtration to lab on a chip sorting of cells. The collective dynamics and its impact on the flow are poorly understood. Here, using droplets as a prototype, we show that collective transport can occur for conditions under which a single particle would get trapped at a pore channel. When a series of droplets gets trapped, the fluid flow is affected significantly, leading to an increase of the pressure difference across the pore channels, which, in turn, squeezes the particles through the channels. We analyze the conditions for a single droplet to flow through one pore and derive the corresponding Bond number. We also obtain a rule for the collective flow of droplets in porous media.

Similarity Analysis of Droplet Evaporation Trajectory in High-Temperature Gas Flow

In this study, a similarity analysis of droplet evaporation trajectory in high-temperature air crossflow based on numerical calculations was conducted. The procedure used for modeling the evaporation of a water droplet involved the use of a low boiling point liquid droplet. The trajectories of water and acetone droplets over an extensive temperature range of 695–1194 K were investigated, and the relationships for the corresponding parameters of the two different droplets, as well as the scale effect, were discussed. The results identify the existence of a comparative trajectory for different kinds of droplets, as well as the main dimensionless numbers governing the trajectory, by determining the Reynolds, Stokes, and Nusselt numbers. For each given gas flow Reynolds number, the droplet evaporation trajectory has a unique correspondence with the airflow temperature. Given this, a method of calculation to determine the modeling parameters of the droplet and crossflow was proposed. The applicability of the similarity modeling method of a single droplet to the spray droplets was examined, from which the spray structure in high-temperature crossflow was obtained.

Numerical Modeling of Laser Heating and Evaporation of a Single Droplet

Laser technology is being widely studied for controlled energy deposition for a range of applications, including flow control, ignition, combustion, and diagnostics. The absorption and scattering of laser radiation by liquid droplets in aerosols affects propagation of the laser beam in the atmosphere, while the ignition and combustion characteristics in combustion chambers are influenced by the evaporation rate of the sprayed fuel. In this work, we present a mathematical model built on OpenFOAM for laser heating and evaporation of a single droplet in the diffusion-dominated regime taking into account absorption of the laser radiation, evaporation process, and vapor flow dynamics. The developed solver is validated against available experimental and numerical data for heating and evaporation of ethanol and water droplets. The two main regimes—continuous and pulsed laser heating—are explored. For continuous laser heating, the peak temperature is higher for larger droplets. For pulsed laser heating, when the peak irradiance is close to transition to the boiling regime, the temporal dynamics of the droplet temperature does not depend on the droplet size. With the empirical normalization of time, the dynamics of the droplet shrinkage and cooling are found to be independent of droplet sizes and peak laser intensities.

Experimental and Three-Dimensional Numerical Study of the Single/Multiple Sessile Droplets Evaporation

In this work, a three-dimensional numerical simulation, accompanied by an experimental study, of the evaporation of one or more nonaligned liquid droplets deposited on a solid substrate and surrounded by air was produced. In addition to the central droplet, the behavior of other neighboring droplets was tracked. Evaporation of a single droplet S was compared with that of multiple droplets (a droplet C surrounded by four nonaligned droplets). The shielding effect exerted by the four droplets on the droplet C increases the life of the latter by 83% as compared to a single droplet. Evaporated flow of the droplet C is reduced by 58% compared to the case of the isolated droplet S. The droplets at the extremities evaporate faster (compared to the droplet C, the evaporated volume can increase up to 57%), since they have less interaction with nearby droplets. A geometric coefficient F characterizing the ratio of the distance separating two neighboring droplets, to the diameter of a droplet was introduced. The shielding effect decreases with the increase of F. For F greater than 5, the behavior of the extremity droplets tends toward that of a single droplet.

Cooling performance enhancement study of single droplet impingement on heated hole-patterned surfaces using improved GQPSO algorithm

A droplet impingement on solid surfaces plays a critical role in various spray and heat transfer applications. The main objective of this study is to investigate how to enhance the cooling performance of the single droplet impingement process on the heated hole-patterned surfaces by finding desired conditions using optimization techniques. The cooling effectiveness and total contact area at the maximum droplet spread were chosen for objective functions to be maximized. To estimate the optimal values of the unknown variables efficiently, the improved Gaussian-behaved particle swarm optimization (GQPSO) algorithm with a modified local attractor using Gaussian distributed random numbers was newly proposed and applied to the single droplet impingement problem. To verify the improved search performance of the proposed GQPSO approach, the optimization results were compared with the standard particle swarm optimization (PSO) and original GQPSO methods with the same conditions. By comparing the estimated maximum values of given objective functions, calculation times and numbers of penalty particles, we verified that the proposed GQPSO approach could search for more optimal solutions quickly and efficiently due to its improved local fine-tuning capability with the modified local attractor.

Online measurement of droplet size by actively exciting droplet oscillation in GMAW and GMAW-AM

Metal transfer plays a significant role in determining the process stability and deposition quality in GMAW and GMAW-AM. Predetermined parameters for controlling metal transfer may fail because of changes in applications or external disturbances. To obtain droplets of target sizes as expected and personalize control parameters for every single droplet, a low-cost method is proposed for the online measurement of the size of the droplet before being detached. The natural frequencies of droplets of different sizes are experimentally measured. The free oscillation of the droplet is actively excited first, during which the voltage falls as the droplet moves upward and rises as the droplet moves downward, with almost the same frequency as that of the droplet. By performing a frequency domain analysis of the voltage, the droplet size is able to be determined online from the oscillation frequency. This technique contributes to the feedback control of the transfer of every single droplet. The fluctuation of the voltage during the droplet oscillation is believed related to the variation of arc structure, the underlying mechanism still needs further study.

Exploiting ferrofluidic wetting for miniature soft machines

Miniature magnetic soft machines could significantly impact minimally invasive robotics and biomedical applications. However, most soft machines are limited to solid magnetic materials, whereas further progress also relies on fluidic constructs obtained by reconfiguring liquid magnetic materials, such as ferrofluid. Here we show how harnessing the wettability of ferrofluids allows for controlled reconfigurability and the ability to create versatile soft machines. The ferrofluid droplet exhibits multimodal motions, and a single droplet can be controlled to split into multiple sub-droplets and then re-fuse back on demand. The soft droplet machine can negotiate changing terrains in unstructured environments. In addition, the ferrofluid droplets can be configured as a liquid capsule, enabling cargo delivery; a wireless omnidirectional liquid cilia matrix capable of pumping biofluids; and a wireless liquid skin, allowing multiple types of miniature soft machine construction. This work improves small magnetic soft machines’ achievable complexity and boosts their future biomedical applications capabilities.

Influence of Nozzle Parameters on Spray Pattern and Droplet Characteristics for a Two‐Fluid Nozzle

AbstractIn this study, a two‐fluid nozzle, as, e.g., used in fluidized‐bed or spray drying applications, is comprehensively characterized regarding the spray pattern and droplet size. To analyze the spray cone, the spray cone angle and the radial mass distribution of the nozzle were measured at varied liquid flow rate, spray air pressure, liquid insert bore diameter, and air cap position. Additionally, droplet size distributions were recorded at different spray settings. In general, the overall spray cone and single droplets are significantly influenced by the spray parameters, especially the spray air pressure, as well as the nozzle geometry.

The internal circulations on internal mass transfer rate of a single drop in nonlinear uniaxial extensional flow

The internal flow of a droplet in the nonlinear extensional flow field will exhibit more than two internal circulations with the variation of nonlinear intensity (E). In this paper, the effect of positions and sizes of internal circulations on internal mass transfer rate of a single spherical droplet in a nonlinear extensional flow field is studied and compared with that in a linear extensional flow field. The simulation results show that when E ≥ 0, there are two symmetrical internal circulations in the droplet, which is the same with that in a linear extensional flow. The limit value of mass transfer rate Sh is 15, which is equal to that in a linear extensional flow, no matter how large E is. When E ≤ –3/7, the number of internal flow circulation of a droplet increase to four and the transfer rate Sh increases. When E = –1, the maximum internal transfer rate Sh equals 30 which is twice of that in a linear extensional flow. The generation of new flow circulations in droplets and the circulation positions will enhance mass transfer when E ≤ –3/7, which provides a new idea for enhancing the internal mass transfer rate of droplets.