Disintegration Of Droplets Research Articles

Study on the impact of active intake air fluctuation on the performance and mass transfer of PEM Fuel Cell

Enhancing mass transfer is imperative for mitigating concentration polarization and thereby optimizing fuel cell performance. This study presents numerical investigation to explore the performance of a proton exchange membrane fuel cell, which involves implementation of actively controlled fluctuating mass fluxes at cathode inlet in order to achieve desirable mass transfer enhancement. The trajectories of liquid droplets within channel were predicted by employing volume of fluid method. The results revealed that the current density exhibits frequency-dependent fluctuations, aligning with input oscillations. A marginal increase in mean current density was observed with frequency amplified and the maximum power density with fluctuated flow increased by 3.9 % compared to that with steady flow inlet. A noteworthy and linear augmentation was evident with amplitude escalation. The mean current density surpassed 34.7 % of the steady current at the amplitude of 10. Beyond a frequency threshold of 1000 Hz, the fluctuating current density surpassed that of steady flow throughout the entire period, attributed to the time constant of gas transport within the gas diffusion layer. Moreover, fluctuation phenomenon of the pressure field within the fuel cell channel, which contributed to reinforced mass transfer and accelerated product alleviation in the region facing bipolar rib was identified during gas transport in the gas diffusion layer. Numerical simulations adopting the volume of fluid method revealed a heightened tendency for the disintegration and expulsion of liquid droplets in the channel attributable to fluctuating flow.

Statistical Characteristics of Droplets Formed due to the “Bag Breakup” Fragmentation Event at the Interface between Water and High-Speed Air Flow

Abstract Recent studies indicate that the dominant mechanism for generating sprays in hurricane winds is a “bag breakup” fragmentation. This fragmentation process is typically characterized by inflation and consequent bursting of short-lived objects, referred to as “bags” (sail-like pieces of water film surrounded by a rim). Both the number of spray droplets and their size distribution substantially affect the air–sea heat and momentum exchange. Due to a lack of experimental data, the early spray generation function (SGF) for the bag breakup mechanism was based on the assumed similarity with resembling processes. Here we present experimental results for the case with a single isolated bag breakup fragmentation event. These experiments revealed several differences from similar fragmentation events that control the droplet sizes, such as secondary disintegration of droplets in gaseous flows and bursting of bubbles. In contrast to the bubble bursting, the film thickness of the bag canopy is not constant but is random with lognormal distribution. Additionally, its average value does not depend on the canopy radius but is determined by the wind speed. The lognormal size distribution of the canopy droplets is observed in conjunction with the established mechanism of liquid film fragmentation. The rim fragmentation results in two types of droplets, and their size distribution has been found to be lognormal distribution. The constructed SGF is verified by comparing it with experimental data from the literature. The perspectives of transferring the results from laboratory to field environment have also been discussed. Significance Statement The “bag breakup” fragmentation is the dominant mechanism for generating spray in hurricane winds. The number and the sizes of the spray droplets substantially affect the heat transport from the ocean to the atmosphere and, thereby, the development of hurricanes. This paper presents experimental data and analysis that demonstrate how droplet formation occurs during bag breakup fragmentation. It also shows analysis of the quantity and size of droplets formed during a single fragmentation event. This work demonstrates how obtained experimental results can be applied to real field conditions in the context of hurricane prediction models.

Shear flow-driven droplet motion with smoothed dissipative particle dynamics

Droplet motion in shear flow is a typical phenomenon in nature that also plays an important role in the spread of COVID-19 when viruses like SARS-CoV-2 exist in exhalation droplets. In this paper, we adopted smoothed dissipative particle dynamics (SDPD) to simulate the Couette flow and Poiseuille flow. Then, the self-diffusion coefficient and the Schmidt number of the quiescent flow in SDPD are confirmed by comparison with those in DPD. In addition, the SDPD method with pacific coefficients is used to simulate the disintegration of droplets and the collisional aggregation and separation of droplets in shear flow. The applicability of the SDPD method is demonstrated through these simulations, which can be useful in the investigation of many complex fluid flows during the expansion of COVID-19.

Interactions between a propagating detonation wave and circular water cloud in hydrogen/air mixture

Interactions between a propagating hydrogen/air detonation wave and circular water cloud are studied. Eulerian-Lagrangian method involving two-way gas-droplet coupling is applied. Different droplet (diameter, concentration) and cloud (diameter) properties are considered. Results show that droplet size, concentration and cloud radius have significant effects on peak pressure trajectory of the detonation wave. Three propagation modes are identified: perturbed propagation, leeward re-detonation, and detonation extinction. Leeward re-detonation is analyzed from unsteady evolutions of gas and liquid droplet quantities. The detonation is re-initiated by a local hot spot from shock focusing of upper and lower diffracted detonations. Disintegration of water droplets proceeds when the detonation wave crosses the cloud. In addition, detonation extinction is featured by quickly fading peak pressure trajectories when the detonation wave passes the larger cloud, and no local autoignition occurs in the shock focusing area. Evolutions of thermochemical structures from the shocked area in an extinction process are also studied. The transfer rates of mass, energy and momentum of detonation success and failure are analyzed. Moreover, parametric studies demonstrate that the critical cloud size to quench a detonation decreases when the droplet concentration is increased. However, when the droplet concentration is beyond 0.84 kg/m3, the critical cloud size is negligibly influenced due to small droplets. Two-phase fluid interfacial instability is observed, and the mechanism of cloud evolution is studied with the distributions of droplet, vorticity, density / pressure gradient magnitudes, and gas velocity. Analysis confirms that velocity difference (due to two-phase momentum exchange) dominates the formation of large-scale vortices from the southern and northern poles, corresponding to Kelvin–Helmholtz instability. Moreover, the influences of effective Atwood number on the evolutions of cloud morphology, vorticity, and gas velocity are evaluated. Results show that higher droplet concentration results in wider droplet dispersion range due to larger vortices.

Conical Shape Fluctuations Determine the Rate of Ion Evaporation and the Emitted Cluster Size Distribution from Multicharged Droplets.

The ion evaporation mechanism (IEM) is perceived to be a major pathway for disintegration of multi-ion charged droplets found in atmospheric and sprayed aerosols. However, the precise mechanism of IEM and the effect of the nature of the ions in the emitted cluster size distribution have not yet been established despite its broad use in mass spectrometry and atmospheric chemistry over the past half century. Here, we present a systematic study of the emitted ion cluster distribution in relation to their spatial distribution in the parent droplet using atomistic modeling. It is found that in the parent droplet, multiple kosmotropic and weakly polarizable chaotropic ions (Cs+) are buried deeper within the droplet than polarizable chaotropic ions (Cl-, I-). This differentiation in the ion location is only captured by a polarizable model. It is demonstrated that the emitted cluster size distribution is determined by dynamic conical deformations and not by the equilibrium ion depth within the parent droplet as the IEM models assume. Critical factors that determine the cluster size distribution such as the charge sign asymmetry that have not been considered in models and in experiments are presented. We argue that the existing IEM analytical models do not establish a clear difference between IEM and Rayleigh fission. We propose a shift in the existing view for IEM from the equilibrium properties of the parent droplet to the chemistry in the conical shape fluctuations that serve as the centers for ion emission. Consequently, chemistry in the conical fluctuations may also be a key element to explain charge states of macromolecules in mass spectrometry and may have potential applications in catalysis due to the electric field in the conical region.

Disintegration of Free-falling Liquid Droplets, Jets, and Arrays in Air

Disintegration of Free-falling Liquid Droplets, Jets, and Arrays in Air

Study on characteristics of fragment size distribution generated via droplet breakup by high-speed gas flow

In a design of scramjet engine using liquid hydrocarbon fuels, predictions of the fragment size distribution generated by liquid droplet breakup in high-speed gas flow are useful. However, the characteristics of fragment size distribution may be unclear, especially in high-velocity flows. In this study, the diameter of fragments formed by the disintegration of water droplets in a high-speed gas flow behind the shock wave was measured. The fragment diameters of several μm to several tens of μm moving at high speeds were clearly captured via high resolution visualization using a microscope and pulse laser with a flash time of 20 ns as a backlight. The parameters used to measure the fragment diameter from the captured images were determined from calibration experiments using a device that can change the working distance; thus, highly reliable particle size measurements were conducted. From the experimental results, the time variation in the volume probability density distribution of fragments size, Sauter mean diameter (SMD), and mass median diameter (MMD) were calculated. As a result, it was clarified that the volume probability density was successfully described by a root-normal distribution for high Weber number, for which catastrophic breakup should occur according to conventional classification. It was also observed that SMD and MMD increase with time, and the ratio of MMD to SMD was found to be 1.2, except at the initial stage. In this study, the characteristics of size distribution of the fragments generated by liquid droplet breakup in high-velocity flows, which was unclear, has been clarified.

Atomization mechanism and powder morphology in laminar flow gas atomization

Metal powders prepared by laminar flow gas atomization have the advantages of small particle size and narrow particle size distribution. At present, the research on laminar flow gas atomization mainly focuses on the influence of process parameters on atomization and powder characteristics, but the atomization mechanism of laminar flow gas atomization is still not clear. In this work, the atomization gas flow, primary and secondary breakup mechanism, and particle morphology of the laminar flow gas atomization process are systematically investigated through numerical simulation and experimental analysis. The characteristics of single-phase atomization gas flow through the De Laval nozzle are studied using the standard <i>k-ε</i> turbulence model. The flow field structure shows a “necklace”-like structure with an expansion wave cluster of oblique shock. The primary and secondary atomization mechanism are investigated using the coupled level-set and volume-of-fluid model, which is validated by solidified fragments and powders after the atomization experiment, and results of the numerical simulation also provide some important advices for the application and specific process of laminar gas atomization technology. The studies indicate that the melts at the periphery of the liquid column are mainly peeled off by filaments or ligaments, which exhibits the small dimension and pressurized melt atomization characteristics. The secondary atomization is mainly based on the disintegration of spherical droplets in the mode of Rayleigh-Taylor instability deformation and sheet-thinning breakup. The simulation results also show that increasing the gas pressure and melt superheat can effectively reduce the probability of irregular powders to occur. The AlSi10Mg powders are obtained under a pressure of 2.0 MPa in the experiment on gas atomization, and the properties of the powders are analyzed. The results show that the powders have good sphericity and flowability, and the proportion of hollow powders is very low. In addition, the mean particle size of the AlSi10Mg powders is 54.3 μm, and the yield of fine powders reaches 48.7%, which is greatly improved compared with the traditional gas atomization processes. Moreover, about 90% of the powders have particle sizes in a range of 30–100 μm, which indicates that a narrow particle size distribution can be obtained by the laminar gas atomization technology.

Effect of electromagnet-based fuel-reforming system on high-viscous and low-viscous biofuel fueled in heavy-duty CI engine

In the present study, a high-viscous biofuel, namely wheat germ oil (WGO), and a low-viscous biofuel, namely pine oil (PO), are used in a twin-cylinder diesel engine. The fuel ionization filter is fitted with a permanent magnet, an electromagnet, and the combination of permanent magnet and electromagnet, and their effect on the engine performance, emission, and combustion is studied. A fuel ionization filter placed in the fuel line, before the injection pump, ionizes the fuel molecules and increases the rate of disintegration of droplets due to a decrease in viscosity and surface tension. The tests are performed at a constant engine speed of 1500 rpm with loads varying from no load to full load at intervals of 25%. As compared to diesel, the engine operation with ionization filter increased brake thermal efficiency and reduced the fuel consumption for both PO and WGO. The increase in brake thermal efficiency is in the order: permanent magnet, electromagnet, and combination of electromagnet and permanent magnet. The magnetic field strength of electromagnet is higher than permanent magnet which tends to increase the ionization of the fuel. When both the magnets are combined, the magnetic field strength further increases resulting in more ionization of the fuel. It is also perceived that magnetic effect reduces the viscosity of the fuel. Regulated emissions, namely unburned hydrocarbons (HC), carbon monoxide (CO), and smoke emissions, reduced, whereas NOx emissions increased with WGO and ionization filter. With pine oil and ionization filter, all the regulated emissions decreased as compared to neat pine oil. The reduction in HC, CO, and smoke emissions was highest for combination of electromagnet and permanent magnet followed by electromagnet and permanent magnet. The study shows that combination of permanent magnet and electromagnet resulted in the best engine performance and emission characteristics.

Explosive disintegration of two-component droplets in a gas flow at its turbulization

The experimental results shown that the mode of droplet disintegration dominates in the laminar flow, and the intensive fragmentation is prevalent in the turbulent flow during almost the entire time of heating. Typical dependences of the time of droplet heatup before disintegration or fragmentation on the temperature, flow rate, structure and regime (laminar and turbulent) are established. The studies are conducted with heated air and flue gases to ensure the application of the research results in the technology of thermal and flame cleaning of liquids from irregular impurities. It is shown that in the flow of combustion products the droplet disintegration occurs 15-20% faster than in the air-flow. In this case, the explosive puffing is more often realized. At high-temperatures (more than 400?C) the characteristics of the explosive droplet disintegration in the studied flows are almost identical (differences in disintegration times do not exceed 5% at different flow turbulization). At lower temperatures, the disintegration times differ 3-4 times for the range Re = 2200-3400. In this case, the more Reynolds number is, the more intense is the fragmentation of two-component droplets throughout the heating time. Due to explosive disintegration of intensely evaporating two-component droplets the growth of the relative area of evaporation was 10-25 times.

Conditions and Characteristics of High-Temperature Processes of Ebullition and Disintegration of Droplets of Water Emulsions

Conditions and Characteristics of High-Temperature Processes of Ebullition and Disintegration of Droplets of Water Emulsions

Experimental understanding on the dynamics of micro-explosion and puffing in ternary emulsion droplets

Dynamics of puffing and micro-explosion phenomena occurring in ternary fuel emulsion droplets under high temperature environment were explored using high speed backlight imaging technique. A single droplet composed of diesel-biodiesel-ethanol emulsion was placed at the tip of a 75 µm gauge thermocouple and introduced rapidly into a furnace maintained at 500 °C. Several interesting features such as oscillation of suspended droplets, physical transformations occurring within the droplet, vapour expulsion, puffing, micro-explosion, sheet formation, perforations, growth of perforations, sheet disintegration and rotation of secondary droplets were observed. High resolution image analysis revealed separation of emulsion components within the core of the suspended droplet, which appeared either as a single nucleus or multiple nuclei. Two distinct types of micro-explosion were identified. For droplets encountering a single nucleus at the core resulted in a stronger vapour expulsion followed by intense micro-explosion. For droplets having multiple nuclei at the core resulted in a weaker vapour expulsion and slower growth of droplet prior to micro-explosion. Both types of micro-explosion process resulted in a number of child droplets. For the case of strong vapour expulsion nearly 80% of its child droplets have their sizes distributed within 150 μm compared to 60% for weaker vapour expulsion. The child droplets that were generated from the primary events of both puffing and micro-explosion cascaded further into secondary and tertiary events of puffing and micro-explosion in freely suspended environment.

Conditions for Explosive Disintegration of Inhomogeneous Water Droplets on High-Temperature Heating

Experimental investigations of the characteristic stages of the processes of heating, evaporation, and explosive disintegration of inhomogeneous water droplets (with a commensurate graphite inclusion) in a high-temperature (600–1200 K) gaseous medium are carried out. Three methods of heating droplets differing in the dominating mechanism of heat transfer are used: heating in a muffle tube furnace (thermal radiation), in a stream of heated air (radiative-convective heat transfer), and in a stream of high-temperature combustion products of a typical liquid fuel (radiative-convective heat transfer). Characteristic values of each heat flux component ate determined for the conditions of experiments, as well as their dependences on temperature. It is shown that the highest values of the radiative heat flux (determining one from the viewpoint of the origination of the effect of explosive fragmentation of droplets) correspond to the schemes of heating in a stream of combustion products and in a tubular muffle furnace. The threshold values of the gaseous media temperatures at which a stable explosive disintegration of evaporating inhomogeneous droplets is realized (Tg > 850 K for conditions of heating in a stream of heated air, Tg > 800 K for the tubular muffle furnace, and Tg > 600 K for a stream of combustion products) have been obtained experimentally. With the use of thermocouple measurements the assumption on accumulation of the energy of thermal radiation near the liquid–solid particle interface and on the resulting formation of an additional source of liquid fi lm heating has been confirmed, which leads to the overheating of the liquid and to explosive disintegration of the droplet.

Explosive Decay of Emulsion Drops Based on Water and Oil Products under Conditions of High-Temperature Purification of Liquids

In the present paper, we established the conditions and the main characteristics of the stable boiling and subsequent explosive disintegration of emulsion droplets based on water and oil products under high-temperature (from 350 to 1100 K) heating using high-speed video recording (up to 105 frames per second). The studies were carried out under various conditions of energy supply to the droplet: on a massive substrate, in contact with a heated rod (local heating), and in a heated air flow. The times of heating up the emulsion droplets before the explosive decay are determined. The scale of the effect of the ambient temperature (up to 1100 K), the heat flux density (up to 104 kW/m2), and the oil product concentration (up to 70%) in the emulsion on these characteristics is established. Using high-speed video tracking, the main characteristics of the emulsion droplet decay are determined: the duration, surface transformation stages, and the number of liquid fragments formed (up to 300) and their total area (7–8 times higher than the initial one). The conditions for the emulsion drop decay both before and after its ignition were studied.

An explosive disintegration of heated fuel droplets with adding water

This work experimentally studies the processes of explosive disintegration of heated fuel droplets with variations of the added water volume. The studies are performed for typical liquid fuels burnt in power plants, as well as flammable liquids being the main fuel component: kerosene, diesel, gasoline, oil, turbine and automobile oil. The minimum (sufficient for the explosive breakup of droplets) heating temperature and the ratio of concentrations of water and flammable liquid are determined. The optical method of Planar Laser Induced Fluorescence (with the operation principle based on the laser illumination of a drop or a liquid film with the introduction of a fluorophore dye) is used to determine temperatures at the inter-component boundary that are sufficient for the micro-explosion of a two-liquid drop. The influence of laser action on the drop on the processes of its heating, evaporation, transformation and breakup is evaluated. The effect of the laser on the mode of explosive disintegration of droplets, the time of its heating and destruction, and the minimum (limiting) temperature of droplet disintegration is determined. Laser irradiation intensifies heating and breakup of fuel droplets as a result of three processes: an additional radiative heat flux; the perturbation (deformation) of the medium interface; and a considerable temperature gradient on the inter-component boundaries, leading to high velocity of convection flows and the formation of vortex structures in the vicinity of the medium interface. The technology of fuel droplets disintegration is proposed.

Using Planar Laser Induced Fluorescence to explore the mechanism of the explosive disintegration of water emulsion droplets exposed to intense heating

In this paper, we study the boiling of heated water emulsion droplets in the air flow at a temperature of 20–800°С. The relative volume concentration of the flammable components in the emulsion varies from 10% to 70%. We explore the unsteady temperature fields of droplets using a contactless optical diagnostic technique, Planar Laser Induced Fluorescence, with a cross-correlation system featuring a camera, a laser, a synchronizer, and the ActualFlow software. Rhodamine B acts as a fluorophore. We also use a high-speed video camera (up to 105 fps) and continuous automatic tracking algorithms (Tema Automotive software) to record the rates of heating and evaporation, as well as transformation of droplet surfaces. We demonstrate the unsteady temperature fields of droplets and three modes of their boiling and breakup. These differ in the number and dimensions of the emerging droplets as well as the durations of the main stages. The temperature differentials at the water – flammable component interface are determined corresponding to three boiling and breakup modes. We show that the third mode provides the greatest number of fine droplets (no less than 200) if the heating temperature exceeds 400°С and the concentration of the flammable component is over 66%. The temperature at the phase interface reaches 100 °C–125 °C before disintegration, and the droplet heating times before explosive breakup may vary from 0.1 s to 10 s. Finally, we analyze how the temperature, additive concentration and droplet size affect the conditions and characteristics of these modes.

Using Planar Laser Induced Fluorescence to explain the mechanism of heterogeneous water droplet boiling and explosive breakup

Over the recent years, the research community has taken an increasing interest in high-temperature gas-steam-droplet systems. This promotes emerging technologies in thermal or flame water cleaning from unspecified impurities, and firefighting by water slurry aerosols. Unfortunately, these technologies have yet to become mainstream, although they have been regarded as extremely important and promising for several years already. The fact is that there are too few experimental data on the physical processes intensifying the evaporation of water slurries in hot gaseous media. The results of such experiments with temperatures over 1000°C are virtually impossible to find. These data are so evasive due to fast-paced processes and difficulties in measuring the temperature in evaporating heterogeneous water droplets. The typical durations of high-temperature heating and evaporation do not usually exceed several seconds. In this work, we conduct experiments using a heterogeneous water droplet with a single nontransparent solid inclusion to determine the unsteady temperature field of the latter. We use an optical diagnostic technique, Planar Laser Induced Fluorescence, to study the conditions, mechanism, reasons and characteristics of water boiling leading to an explosive breakup (disintegration) of water slurry droplets. Rhodamine B acts as a fluorophore. The typical temperatures are determined in the depth of a droplet, near its free (outer) surface, and at the interface. The water temperature at the water – solid inclusion interface is shown to be higher than at the outer surface of a droplet. Furthermore, we compare the temperature fields of a homogeneous and heterogeneous water droplet under identical heating conditions.

The features of heterogeneous water droplet evaporation in high-temperature combustion products of typical flammable liquids

This paper presents the experimental results on heating and evaporation features of heterogeneous (with opaque solid particles ? the size of 0.05-0.5 mm, relative mass concentration 0-1%) water droplets (the initial size ? radius 1-3 mm) during their motion through high-temperature (500-1800 K) gases. A significant increase in the integral characteristics of evaporation by introducing opaque inclusions into droplets was observed. The influence of energy accumulation on the conditions of droplet evaporation at the internal solid/liquid interfaces was established. For proportioned inclusions, the conditions of intensive vaporization (leading to the explosive disintegration of droplets) at internal inclusion/liquid interfaces was set. To summarize research results, experiments were conducted with the combustion products of kerosene, gasoline, industrial alcohol, acetone, and oil. The particles of graphite, carbon, and aluminum as solid inclusions were used. The investigation compared integral characteristics of heterogeneous droplet evaporation under the conditions of non-stationary (gas temperature varied from 1800 K to 500 K over the length of channel) and nearly stationary (gas temperature was maintained at about 1100 K) heating.

CFD with population balance model to predict droplet size distribution in submerged turbulent multiphase jets

During deepwater oil spill events, oil is released into a relatively stagnant environment (ocean water) in an uncontrolled manner. The oil phase initially emerges as a jet and the gushing oil loses its momentum energy and results in entrainment of surrounding water. The shear interaction between the oil mass and the ambient fluid results in generation of droplets with wide size distribution. In this study, we present a numerical model for predicting the droplet size distribution resulting from the interaction of turbulent oil jets with the surrounding quiescent environment. We achieve this objective by integrating traditional multiphase CFD models with a population balance approach. The developed model has been validated against the experimental observations reported in Johansen et al.[12] The ‘Mixture model’ has been employed for evaluating flow fields in the system. We restrict our study to the atomization regime, where the droplet disintegration process has a greater significance over the competing coalescence mechanism. The population balance equation has been solved using the ‘Class method’ and the disintegration of droplets has been modelled by including the breakage kernel suggested by Lehr.27 The developed model has been used to analyze the effect of dispersed (oil) phase flow rates, the presence of dispersants, and the presence of air in the jet phase on the overall size distribution of oil droplets. We also present a case which compares the droplet size distributions obtained by using the flow field evaluated by a more rigorous Eulerian Two‐Fluid model over Mixture model.

New mechanisms of macroion-induced disintegration of charged droplets

Molecular modeling has revealed that the presence of charged macromolecules (macroions) in liquid droplets dramatically changes the pathways of droplet fission. These mechanisms are not captured by the traditional theories such as ion-evaporation and charge-residue models. We review the general mechanisms by which macroions emerge from droplets and the factors that determine the droplet fission. These mechanisms include counter-intuitive “star” droplet formations and extrusion of linear macroions from droplets. These findings may play a direct role in determining macromolecule charge states in electrospray mass spectrometry experiments.