Atomized Droplets Research Articles

Phenomenology of disruptive breakup mechanism of a levitated evaporating emulsion droplet

Atomization of emulsion droplets is ubiquitous across a variety of application domains ranging from NextGen combustors to fabrication of biomedical implants. An understanding of the atomization mechanism in emulsions can result in a paradigm shift in customized designs of efficient systems, be it in energy or biotechnology sectors. In this paper, we specifically study the breakup mechanism of an evaporating contact-free (acoustic levitation) emulsion droplet (water-oil) under external heating. Three distinct regimes are observed during the lifespan of the evaporating droplet. Initially, the droplet diameter regresses linearly with time, followed by vapor bubble nucleation due to a significant difference in the boiling temperature among the components of the emulsion. The collapse of this bubble results in a high-intensity breakup of the droplet leading to the propulsion of residual liquid in the form of a crown-like sheet. The area of the expanding crown varies linearly with the square of the time. It is hypothesized that the expansion of the liquid sheet centrifuges the larger water sub-droplets towards the edge, resulting in unique spatial segregation. Subsequently, we report the first observation of complex patches (representing water sub-droplets) and the rupture of the thin sheet adjacent to patches into holes (with hole growth rate ranging from 1.2 to 1.4 m/s) in the context of an evaporating isolated emulsion droplet. The hole formation results in the creation of ligaments which undergo breakup into secondary droplets with Sauter mean diameter (SMD) ∼ 50 µm.

Intensification of Vaporization and Secondary Atomization of Droplets of Fire-Extinguishing Liquid Composition

In this paper, we present the results of experimental studies of vaporization and fragmentation of droplets of fire-extinguishing compounds using suspensions and emulsions as examples. It was shown (using rapeseed oil as an example) that, when a combustible liquid of plant origin is added to their composition, micro-explosion or puffing, which are modes of secondary atomization of droplets, can take place. The ranges of changes in the lifetimes of droplets at different heating temperatures and component concentrations are established.

Theoretical Uniformity Analysis and Improvement of Spray Deposition by Mixing Nozzles with Heating Conditions

Spray coating is widely used in the manufacture of deposited layers of electronic devices due to its unique advantages of high-speed deposition over a large area. To improve the spray deposition process for further low-cost and uniform production, the uniformity of the spray deposition should be systematically investigated. The current study, however, mainly focuses on the experimental trials with few numerical directions especially for the mixing nozzle sprayers with heating conditions. In the paper, we conduct a theoretical study on the uniformity of the internal and external mixing nozzles. The influencing factors include the initial angle, the total ink flow rate, the transporting gas velocity and the distance from the nozzle to the substrate. Then, the orthogonal test method is adopted to obtain the optimal combination of the parameters. Finally, the effects of different heating modes on the uniformity have been further studied. The results show that these factors influence the uniformity with the two types of nozzles to a different degree. The evaporation of the atomized droplets can effectively improve the uniformity in a certain temperature range. The heating temperature with the highest uniformity is various depending on the heating modes, which should be carefully addressed during the actual production.

Phenomenon study on heat induced atomization of acoustic levitated methanol droplet

<sec>Atomization of droplets is ubiquitous in many natural and industrial processes, such as falling rain drops, inkjet printing, fuel injection in automotive and gas-turbine engines. Acoustic irradiation provides a very effective method of atomizing fluid. However, the acoustic atomization of acoustically levitated droplet is seldom studied. To assess the possibility of achieving ultrafine atomization, we, in this paper, systematically study the atomization of an acoustically levitated droplet placed in a hot gas of a flame. High speed camera is utilized to investigate the atomization characteristics of various droplets with diameters ranging from 0.5 mm to 3.5 mm. </sec><sec>The experimental results show that the sound pressure of the resonance acoustic field has the ability to atomize the droplet when it is suddenly bathed in hot gas. Here the heating acts as a switch to convert the droplet surface from an acoustic isolator to conductor by heating the surface to strong evaporation. The presence of a high concentration of vapor molecules surrounding the droplet caused the acoustic field to change, thus, a much larger pressure gradient is established along the droplet surface, resulting in the atomization of droplet from the equator. Furthermore, Faraday wave stimulation and discretization on the film cause the droplet to further disintegrate when the droplet diameter is large enough. The atomization consists of three different styles, i.e. rim spray (RS), film disintegration (FD) and normal sputtering (NS). When exposed to hot gas, the droplets with equivalent diameter <i>D</i><sub>0</sub> < 2.8 mm are depleted with RS until the whole mass is atomization. A thin rim is extruded at the equator and then splashed in the equator plane, the spray speed is around 9.5 m/s. Larger droplets end with the sudden FD of liquid film of the residual mass after the the RS has been consumed up. When the thickness of the rim and buckled film decrease to half of wave length, Faraday wave emerges, resulting in the vertical droplet ejection and the disintegration of the thin films. And the droplets with <i>D</i><sub>0</sub> > 3.2 mm undergo further film buckling, forming a closed bubble due to the Helmholtz resonator effect and NS at the bottom. This sound driven atomization of droplets enriches the understanding of fluid mechanism in multi-physical fields, and may provide new ideas for relative application research. </sec>

Numerical Simulation Research on Agglomeration between Coal-fired Fly Ash Fine Particulate and Atomized Droplets

In this paper, the interaction process between atomized droplets and particles after spraying chemical agglomeration solution was numerically simulated, the force model of particles in agglomeration solution was established, and the effects of density, viscosity, flow rate of agglomeration solution, and diameter of atomized droplets and other factors on the velocity, quality, diameter and number of particles after agglomeration were analyzed. The simulation results showed that the speed, mass, diameter and number of particles after agglomeration were greatly affected by the physical and chemical parameters of the chemical agglomeration solution. Increasing the density and viscosity of the solution could increase the variation trend of each characteristic quantity of particles. As the flow rate of agglomeration solution and diameter of atomized droplets became larger, the collision probability between the solution and particles got higher as a result. Since the particles gradually agglomerated, their mass and diameter increased. As their speed gradually decreased, the probability of collision and agglomeration with chemical agglomeration solution decreased as well.

CFD Simulation Research on Agglomeration between Coal-fired Ash Fine Particulate and Atomized Droplets

In this paper, a collision model between atomized droplets of agglomeration solution and particles is established. On this basis, the effects of flue gas temperature, atomized droplet diameter and other factors on the particle agglomeration process are studied. In addition, the evaporation model of agglomeration solution in the flue of a power plant is established for the coal-fired unit of power plant. Through CFD software, the variation of flow field velocity, temperature and pressure in the flue is simulated to determine whether the chemical agglomeration technology has negative impact on the actual operating conditions of the power plant. The simulation results show that the velocity and pressure of the flow field in the flue have no obvious change after the agglomerating agent is injected. Besides, the temperature drop of about 7°C. The droplets evaporate completely at a distance of 7-8 m after spraying. The evaporation time of droplets is within 1.6 s.

Degradation of 2,4,6-trichlorphenol by producing hydrogen using ultrasonic mist generated from photocatalysts suspension

Abstract Photocatalytic hydrogen production synergized with the oxidation of pollutants is an environmentally friendly and economical approach to generate clean energy and remove the pollution from environment. In this study, photocatalytic hydrogen production cooperating with 2,4,6-trichlorophenol (2,4,6-TCP) degradation have been reinforced by introducing an ultrasonic atomization. The degradation of 2,4,6-TCP in a mist of three photocatalysts (g-C3N4, TiO2, and Bi2O3) generated by ultrasonic atomization was performed under 254 nm ultraviolet (UV254) light irradiation. The results showed that, under UV254 irradiation, three different photocatalysts (g-C3N4, TiO2, and Bi2O3) all accelerated both hydrogen production and 2,4,6-TCP degradation. Additionally, 2,4,6-TCP degradation and photocatalytic hydrogen production exhibited an obvious synergistic effect, since 2,4,6-TCP has a strong tendency to react with photo-generated holes and their second radicals so as to inhibit the recombination of carriers, and thus improved the efficiency of hydrogen production simultaneously. Moreover, by introducing ultrasonic atomization, the atomized droplets acted as micro-photocatalytic units to replace the macro-photocatalytic reaction reactor. Therefore, the mass transfer distance for free radicals was restricted and the utilization of light energy by photocatalysts was increased. Further, the reaction efficiency was improved. The results reveal environmentally friendly and economical potential of hydrogen production by photocatalytic degradation of 2,4,6-TCP in atomized droplets.

Интенсификация парообразования и вторичного измельчения капель огнетушащих составов

The results of experimental studies of evaporation and fragmentation of droplets of fire extinguishing compositions by the example of suspensions and emulsions are presented. It is shown that when a flammable liquid of vegetable origin is added to their composition by the example of rapeseed oil, micro-explosion and puffing can be realized, which are the regimes of secondary atomization of droplets. The ranges of changes in the lifetimes of droplets at different heating temperatures and component concentrations are established.

Experimental study on flash-boiling spray structure of multi-hole gasoline direct injection injector in a constant volume chamber

The macroscopic and microscopic characteristics of flash-boiling spray were experimentally investigated with various optical measurement techniques. The effects of ambient pressure and fuel temperature on flash-boiling characteristics in multi-hole gasoline direct injection injector were analyzed. The analysis was focused on the spray structure and atomization droplet size distributions. In order to increase the understanding of the flash-boiling spray targeting, three injectors with different spray patterns were investigated under strong flash-boiling condition. The results show that ambient pressure and fuel temperature have significant influence on flash boiling. Both lower ambient pressure and higher fuel temperature could accelerate the flash-boiling process. For the macroscopic characteristics, similar influences could be found with the ambient pressure decreased by 0.4 bar and the fuel temperature increased by 10°C. Further, significant difference could be found within cold-jet spray and strong flash-boiling spray, such as the spatial structure. The spray structure always turns from hollow cone into solid when flash boiling occurs. With a higher fuel superheat degree, the spray droplet distribution moves toward smaller sizes and let the larger droplets reduce due to the promotion of atomization. For the strong flash-boiling spray, the Sauter mean diameter has decreased by 50% compared with cold-jet spray. There is a corresponding relationship between collapsed flash-boiling spray target and weighted geometric center of the injector. Spray collapse could be avoided by increasing the plume distance.

Interpreting atomization of agricultural spray image patterns using latent Dirichlet allocation techniques

Breakup patterns of agricultural formulations are explored using unsupervised learning techniques to elucidate the mechanics of atomization for oil-in-water formulations. Previous researchers have shown these formulations succumb to a different breakup mechanism than conventional formulations, beginning with inhomogeneities within the liquid sheet that nucleate holes within the material being sprayed, beginning the mechanism responsible for breaking the sheet into droplets. The Latent Dirichlet Allocation (LDA), a Bayesian hierarchical model, is used to explore unsupervised learning relationships between image analysis metrics on spray video data and the resulting atomization droplet size. Latent factors discovered by LDA were used for classification of video segments and achieved 99.9% accuracy (3-fold cross validation). Seventy-five videos were used for regression where each video had a unique measured droplet size distribution (D10, D50, and D90 values) for atomization. Experiments using the features learnt by Latent Dirichlet Allocation used with regression have extremely good results (R2 ~ 0.995 in 3-fold cross validation, R2 ~ 0.963 on never-seen videos), which serves as evidence for the potential use of this model in image analysis of agricultural spray patterns. LDA has huge potential in both learning and predicting atomization patterns [e.g., driftable fines (drops <150 μm)] when used with images based on the breakup phenomena in agricultural spray. These small drop sizes that occur during atomization have the greatest propensity for off-target movement through wind induced drift. LDA proved useful in characterizing current and future formulation designs using only images as witnessed by observations and excellent predictions summarized in this paper. In fact, these methods offer potential for use under field conditions to address spray performance based upon images of spray patterns at the nozzle without the need for expensive light scattering equipment often used to measure this phenomenon.

Reduction of fuel consumption of a small-scale gas turbine engine with fine bubble fuel

Thermal efficiency of the small-scale gas turbines (SSGTs) is much less than that of large-scale gas turbine. Therefore, improvement of thermal efficiency of SSGTs is very important. In this paper, fine bubble fuel (FBF) which is fuel mixed with air fine bubbles (AFBs) was introduced to SSGT to improve its thermal efficiency. Control fuel (CF) and FBF were tested on a SSGT. Specific fuel consumption (SFC) of the engine can be reduced about 2.6%–5.7% at a maximum and about 1.7%–2.7% as an average by using FBF compared to CF. To validate the effect AFBs on the reduction of fuel consumption (FC), droplet size distribution of CF and FBF injected by a nozzle was measured. It was found that atomization of droplets can be promoted by introducing AFBs to fuel, and D32 can be reduced about 4.4%. However, from the results of a simulation of combustion in the combustion chamber, considerable effect of atomization was not observed. Therefore, the effect of atomization of fuel on the reduction of FC is relatively small, and there are other reasons, such as promotion of diffusive combustion due to air which exists in fuel as AFBs.

Plasma-Assisted Fuel Atomization and Multipoint Ignition for Scramjet Engines

Estimates of energy and power requirements for shortening the ignition delay time in hydrocarbon-fueled scramjet engines using nonequilibrium plasma generation of radicals show that the uniform volumetric plasma ignition would be associated with extremely high power budget and substantial losses of total pressure. A multipoint plasma ignition scheme based on electron beams is proposed that would emulate the volumetric ignition while reducing the power budget by several orders of magnitude. The paper also explores an approach where the fuel is injected into the core flow as a liquid jet, and low-power electron beams are used to electrostatically charge the droplets and thus to control droplet breakup, atomization, mixing, and ignition. With a subcritical microwave field applied to the injection region, local enhancement of electric field strength at the surface of droplets, together with seed electrons and ions produced by the electron beam, would create subcritical microwave discharges and thus initiate combustion in multiple spots. The multispot ignition would help in spreading the flame across the combustor. Additionally, the initiation of combustion at the droplet surface, where local equivalence ratio is high, could help ignite lean (in average) mixtures.

Dynamic behaviors of droplets impacting on ultrasonically vibrating surfaces

Ultrasonic vibration has a wide application prospect in the fields of droplet atomization and aircraft anti/de-icing. The present work experimentally investigates the dynamic behaviors and anti-icing characteristics of droplets, with a wide range of velocities, impacting on ultrasonically vibrating surfaces. Two kinds of splash mode are observed, including edge splash and surface splash, and a novel rebound mode, named sub-droplet rebound, is found in our experiments. The surface splash is mainly induced by the Faraday instability of surface capillary waves, and the appearance of a large number of bubbles proves that cavitation is also one of the influence factors. By analyzing the interaction of aerodynamic force and surface tension, the critical curve of the edge splash is obtained. The convergence of capillary waves at the apex contributes to the sub-droplet rebound. In addition, the influences of impact velocities and ultrasonic vibration amplitudes on the droplet spreading and the size distribution of secondary droplets are elucidated and discussed. A higher excitation amplitude results in a wider secondary droplet size distribution and a larger average size. We also investigate the dynamic process of supercooled water droplets impacting on ultrasonically vibrating surfaces with and without preformed frozen ice, respectively. The ultrasonic vibration could effectively promote the droplet splash and prevent the ice accumulation even in the state that the surface has been frozen. The anti-icing efficiency of the ultrasonic vibration increases with the increase of the ultrasonic vibration amplitude. The results of this work could provide insights for controlling droplet spreading, atomization, and anti-icing by using ultrasonic vibration.

Effect of Atomized Delivery of Nutrients on the Growth Characteristics and Microstructure Morphology of Bacterial Cellulose.

This work demonstrates a general strategy for introducing remarkable changes in matrix organization and, consequently, functional properties of bacterial cellulose (BC). BC-producing cells were induced, using a well-defined atomized droplet nutrient delivery (ADND) system, to form pellicles with a regular layered morphology that persists throughout the mat depth. In contrast, the morphology of mats formed by conventional static medium nutrient delivery (SMND) is irregular with no distinguishable pattern. ADND also resulted in larger meso-scale average pore sizes but did not alter the fibril diameter (∼70 nm) and crystallinity index (92-95%). The specific modulus and specific tensile strength of ADND mats are higher than those of SMND mats. This is due to the regularity of dense layers that are present in ADND mats that are able to sustain tensile loads, when applied parallel to these layers. The density of BC films prepared by ADND is 1.63-fold lower than that of the SMND BC film. Consequently, the water contents (g/g) of ADND- and SMND-prepared BC mats are 263 ± 8.85 and 99.6 ± 2.04, respectively. A model that rationalizes differences in mat morphology resulting from these nutrient delivery methods based on nutrient and oxygen concentration gradients is proposed. This work raises questions as to the extent that ADND can be used to fine-tune the matrix morphology and how the resulting lower density mats will alter the diffusion of actives from the films to wound sites and increase the ability of cells to infiltrate the matrix during tissue engineering.

Planar drop-sizing and liquid volume fraction measurements of airblast spray in cross-flow using SLIPI-based techniques

This paper presents planar measurements of Sauter mean diameter (SMD) and liquid volume fraction distributions of airblast spray injected into cross-flow. The experiments are conducted using a combination of structured laser illumination planar imaging with laser sheet drop-sizing (SLIPI-LSD) and particle/droplet imaging analysis (PDIA) techniques. Effect of gas to liquid mass ratio (GLR) and cross-flow velocity (Ucross) is studied. Planar SMD distribution at low GLR improved with increase in Ucross due to secondary atomization of large droplets. Uniform SMD distribution in a range of 10–20 µm is observed for GLR more than 3. The distribution of liquid volume fraction at low GLR condition shows poor dispersion with most of the liquid concentrated near the injector. The liquid volume fraction distribution improves with increase in GLR and better dispersion is observed for GLR more than 3 and two-phase momentum ratio (q2) greater than 13.06. Spatial bifurcation in liquid fraction is found for high GLR conditions. The SMD in the range of 10–20 µm and uniform distribution of liquid are observed for GLR more than 3 and q2 > 25.6.

Study on flow fields of regional control constrained spray deposition under different atomization gas pressures

A regional control constrained spray deposition model was established to study the flow fields and deposition of atomized droplets in a semienclosed space. The FLUENT software was used to simulate the changing rules of the pressure field, velocity field, and temperature field under different atomization gas pressures (AGPs), aiming to optimize the equipment and parameters of spray Conform. The results show that the flow fields are prone to be disturbed by the controller of the two rotating disks when the AGP is low, which makes it difficult to control the droplet deposition. Besides, excessive AGP will bring a robust reverse airflow, which is not conducive to control droplet deposition either. However, the outlet pressure of the diversion tube is about 2.5 × 104 Pa when the AGP is between 2.5 and 3.0 × 105 Pa. In this condition, the melt will exit the nozzle without blockage. Moreover, the pressure near the basement is about 5 × 104 Pa at this point, making the droplet deposition controllable.

Power ultrasound and its applications: A state-of-the-art review

Ultrasonic processing has attracted increasing attention by people because ultrasonic technology may represent a flexible 'green' alternative for energy efficient processes. The major challenges for the power ultrasound application in real situations are the design and development of specific power ultrasonic systems for large-scale operations. Thus, new families of power ultrasonic transducers have been developed in recent years to meet actual needs, and this contributes to the implementation of power ultrasound of application in many fields such as chemical industry, food industry and manufacturing. This paper presents the current state of ultrasonic transducers of magnetostrictiv type and piezoelectric type as well as applications of power ultrasound in various industrial fields including chemical reactions, drying/dehydration, welding, extraction, heat transfer enhancement, de-ice, enhanced oil recovery, droplet atomization, cleaning and fine particle removal. The review paper helps to understand the current development of power ultrasonic technology and its applications in various situations, and induce extended applications of power ultrasound to more and more fields.

Bubble nucleation, micro-explosion and residue formation in superheated jatropha oil droplet: The phenomena of vapor plume and vapor cloud

The evaporation and secondary atomization of droplets in cylinder has been confirmed to be one of major factors determining combustion efficiency. Experiments were carried out to elucidate the bubble nucleation and micro-explosion characteristics of superheated jatropha oil (JO) droplet during evaporation process at 873, 973, 1073 K and atmospheric pressure. A single droplet was placed at the intersection of two quartz fiber and introduced rapidly into a high temperature chamber. Several interesting features such as bubble nucleation, bubble growth and coalescence, puffing, vapor jetting, droplet jetting, surface pit, floating raft and micro-explosion were observed. Two different types of micro-explosion, namely global and local micro-explosion were identified. It was also found that chemical reactions occurred during the evaporation process of JO droplet. Bubbles in JO droplet were produced partly by superheating of low boiling point fatty acid and partly by pyrolysis of long-chain fatty acids. There were four modes of bubble nucleation in JO droplet: surface pit nucleation, particle-droplet interface nucleation, fiber-droplet interface nucleation, and floating raft interface nucleation. Surface pit nucleation and floating raft interface nucleation were the main mode. The evaporation residue formed at the end of evaporation, which was caused by the polymerization and aromatization reactions. Most importantly, vapor plume and vapor cloud induced by evaporation have been discovered for the first time. Vapor loud was very similar to the liquid state in the dilute spray region. These were mainly due to the non-isothermal condensation caused by Stefan outflow.

Porosity and microstructure of steel tubes spray-formed by close-coupled atomizer

Abstract Close-coupled atomizers (CCA) can be used to reduce the porosity of spray-formed deposits compared to free fall atomizers (FFA), possibly due to smaller size and higher velocity of the atomized droplets. Previous studies on spray-formed AISI 52100 (100Cr6) tubular deposits show that CCA leads to less porosity in the inner and outer surface regions of the tubular deposits. However, there are very few investigations on the thermal profiles of the spray-formed deposits and on the influences of process conditions during spray forming via CCA. To provide insight into the thermal profile of the deposits, in this study in-situ measurement of the deposit surface temperature is performed by a two color pyrometer and the substrate temperature is measured by means of thermocouples. The porosity and microstructure of the spray-formed deposits under various process conditions are investigated by means of optical microscopy and image analysis. This investigation shows that the deposit thickness has a higher influence on the deposit surface temperature than the gas to melt flow rate ratio (GMR). Higher deposit temperature results in lower porosity and larger grain size. Moreover, the deposition positions on the deposit length has an influence on the porosity. The porosity in the vicinity of the substrate is higher at the deposit end position than the deposit start position. Finally, the present study suggests an optimal deposit surface temperature range for spray forming of highly dense tubular deposit by CCA.

Comparing the integral characteristics of secondary droplet atomization under different situations

In this paper, we describe the results of experimental research into secondary droplet atomization for several heterogeneous, water-oily liquid compositions including some highly heterogeneous emulsions and slurries. The group of schemes is studied, in addition to relevant experiments reported in the literature: droplets colliding with each other, with a heated or not heated walls, and with an air flow, as well as exposed to conductive or convective heating followed by micro-explosive breakup. After the analysis of experimental data, we calculate the size and number distributions of the liquid fragments generated using each of the approaches. We determine the duration of initial droplets' fragmentation as a function of the We numbers and the density of the supplied heat flux. The comparative analysis gave us optimal conditions for several aerosol generation techniques providing minimal fragment size with relatively low electric power consumption. It is shown that each of used atomization techniques provides a comparable number of liquid fragments on average, considering the amount of power required. Several techniques for combined atomization are proposed, based on the investigated approaches.