Variation Of Droplet Size Research Articles

Vertical Variation of Turbulent Entrainment Mixing Processes in Marine Stratocumulus Clouds Using High‐Resolution Digital Holography

AbstractMarine stratocumulus clouds contribute significantly to the Earth's radiation budget due to their extensive coverage and high albedo. Yet, subgrid variability in cloud properties, such as aerosol concentration, droplet number, and precipitation rates, lead to considerable errors in global climate models. While these clouds usually have small vertical extent, turbulent entrainment‐mixing and precipitation can generate significant variations in droplet number, size, and relative dispersion with altitude. Here, we analyze turbulent entrainment‐mixing processes and the variability in cloud microphysical properties as a function of height within a warm marine stratocumulus cloud layer over the Eastern North Atlantic. We use high‐resolution airborne holographic measurements and compare them with local turbulence measurements. We find that entrainment‐mixing is primarily inhomogeneous near cloud top and homogeneous near cloud base. Further analysis of Damköhler number and transition scale number are able to explain the mixing mechanisms at different cloud heights using phase relaxation but not droplet evaporation as the microphysical time scale. A modified droplet evaporation time scale that considers local saturation deficit using a simple linear mixing model is developed, and it is able to reliably explain the observed mixing mechanisms. This study reinforces the importance of turbulent mixing and the use of appropriate microphysical time scales in determining cloud microphysical processes.

Key parameters for droplet evaporation and mixing at the cloud edge

AbstractThe distribution of liquid water in ice‐free clouds determines their radiative properties, a significant source of uncertainty in weather and climate models. Evaporation and turbulent mixing cause a cloud to display large variations in droplet number density, but quite small variations in droplet size (Beals et al., Science, 2015, vol. 350, pp. 87–90). However, direct numerical simulations of the joint effect of evaporation and mixing near the cloud edge predict quite different behaviours, and how to reconcile these results with the experimental findings remains an open question. To infer the history of mixing and evaporation from observational snapshots of droplets in clouds is challenging, because clouds are transient systems. We formulated a statistical model that provides a reliable description of the evaporation–mixing process as seen in direct numerical simulations and allows us to infer important aspects of the history of observed droplet populations, highlighting the key mechanisms at work and explaining the differences between observations and simulations.

Influence of the initial droplet distribution on the prediction of spray dynamics in Eulerian-Lagrangian simulations

Liquid fuel atomisation is largely affected by in and near nozzle phenomena. In this paper we establish a link between the initial droplet distribution at the nozzle exit, used as an input parameter in Lagrangian solvers, and the effect that this selection has on the subsequent predicted spray physics. Three initial PDFs with similar mean but different variance are tested in order to represent different aspects of the injection physics. Results are validated against an extensive range of experimental data from the Engine Combustion Network for the mildly cavitating Gasoline Spray G. Gasoline Direct injectors present high complexity both due to the importance of the near-nozzle phenomena for the fuel-air mixing, as well as due to the spray plume to plume interactions further downstream. Thus, they provide a well suited benchmark case for our study. We conclude that a correctly selected initial mean droplet size in combination to standard evaporation and break up models can provide reasonable predictions for a range of quantities including liquid and vapour penetration. Different droplet distribution models though exhibit differences in their prediction accuracy in terms of droplet dynamics depending on the initial droplet size variation around the mean. We demonstrate that these differences affect the coupling between the Lagrangian and the Eulerian dynamics through a Stokes numbers analysis close to the nozzle. We also analyse the effects that the stripping and catastrophic break up mechanisms have on the evolution of the droplet PDF at different axial locations depending on the selected initial droplet sizes. Overall, our results allow for a better understanding of the influence of the Lagrangian model input parameters on the global spray development on a numerical level, but can also help the optimisation of the design of new injectors by improving the understanding of the link between initial droplet distributions and the physics of the spray further downstream.

Theoretical investigation of global rainbow refractometry in spray combustion of polydisperse microdroplets

Global rainbow refractometry (GRR) is an important technique for temperature measurements of droplets and has been applied to a wide field of technical sprays. In this work, the application of GRR to sprays with droplets as small as m in diameter is investigated by means of Lorenz–Mie theory to evaluate the influence of varying droplet size distributions (DSD) and varying refractive indexes on the global rainbow. DSDs were found by fitting a Rosin–Rammler–Sperling–Bennett distribution to experimental data. Different DSDs, that are typical for flame spray pyrolysis applications were evaluated systematically with varying widths and mean diameters. A study of the angular rainbow maxima position with varying widths of DSDs shows a strong non-linearity. It is concluded that GRR experiments in polydisperse applications should be accompanied with an accurate sizing technique. Furthermore, possible droplet size-dependent distributions of refractive indexes within the samples may influence the results.

Study of an ESP's performance handling liquid-liquid flow and unstable O-W emulsions part II: Coupled CFD-PBM modelling

The internal flow behavior of an ESP handling dispersed oil-water and emulsion flows remains quite unexplored. Previous studies have focused either on the individual droplet dynamics or on a simplified and limited modelling of the ESP's global performance. Hence, this study seeks to implement and analyze the applicability of a more robust and far-reaching CFD-PBM model which maintains a reasonable computational expenditure. Good agreement with experimental data was attained by the Eulerian-PBM framework (<10% error) and acceptable mean flow predictions were achieved with a less robust and computationally demanding VOF model (15% error). The internal flow behavior was noted to be governed by droplet size and effective viscosity variations, which induce the formation adverse pressure gradients leading to the emergence of backflow currents and eddies, particularly at higher oil fractions and lower flowrates. At higher flowrates and latter stages, well adapted flow is always attained, regardless of the mixture handled.

Coriolis-induced liquid breakup and spray evolution in a rotary slinger atomizer: Experiments and analysis

Understanding the physics of primary liquid breakup process and its correlation with the evolution of spray characteristics in a rotary slinger atomizer is the goal of the present research. Experiments were conducted in a high-speed slinger test rig that houses a static liquid delivery manifold to uniformly supply the liquid to the rotating slinger disc that contains a single row of orifices carved on its peripheral surface for liquid injection.The atomizer was operated for a wide range of conditions by varying the rotational speed and liquid feed rate. The liquid breakup structure at the exit of the slinger orifices was visualized using front light illumination technique, while the droplet size was measured at different radial stations away from the slinger surface by application of the Interferometric Laser Imaging for Droplet Sizing (ILIDS) technique. The visualization images highlighted strong influence of Coriolis force as the liquid tends to accumulate on one side of the channel (that is opposite to the rotational direction) for all cases. It was observed that while the liquid thickness is smaller for higher rotational speed, it does not vary much with liquid feed rate at the same speed and, instead, in such case the span of the liquid is wider. A theoretical analysis was developed to describe the in-channel liquid behaviour that accounts for the effect of Coriolis and surface tension forces. Interestingly, the theory could explain the above observations. The differences in the predictions in comparison to the analysis by Dahm et al. (2006a) was attributed to the assumption of annular film flow in the latter. The liquid breakup mode (stream, sheet or transition mode) could be described by Coriolis Bond number (Bo) that refers to the ratio of Coriolis to surface tension forces and proportional to the spread parameter (span to thickness ratio), while the liquid breakup regimes were identified on a Bo−q plot, where q is the liquid to air momentum flux ratio. The variation of characteristic droplet sizes with both rotational speed and feed rate were examined, and again some interesting trends are identified. The correlation between liquid breakup mode/regime with the measured droplet size was established using the above non-dimensional numbers.

A Study on Aerosol Spray Characteristics of Different Size Atomizers

In this study, the variations of spray characteristics are tested with air-assisted siphon type different size liquid atomizers. The spray characteristics are analyzed considering three atomizers having the same inlet diameter, with different constricted areas and exit diameters for the flow rates of 42, 57, and 71l pm. Three different mixtures of Oleic Acid with Ethanol, and three different mixtures of Di-Octyl Phthalate (DOP) with Ethanol are employed as working fluids along with the six different suction heights. Experimental outcomes show that, for the same flow rates, solution, and suction height, the droplet size changes by varying the diameter of the constricted area and exit area. The change of any area in atomizer changes the level of turbulence in flow that results in the variation of droplet size and its distribution. The increasing flow rate also assists in increasing the droplet size. Since the droplet size distribution decreases with the increasing viscosity, the use of DOP and Ethanol mixture increases the droplet size as it is more viscous than the Oleic Acid and Ethanol mixture. It is also noticed that at a constant airflow rate, Number Median Aerodynamic Diameter (NMAD), and liquid consumption rate are inversely proportional, whereas the Geometrical Standard Deviation (GSD) and Mass Median Aerodynamic Diameter (MMAD) is directly proportional to the suction height.

Development of novel iron-regulated Pasteurella multocida B: 2 bacterin and refinement of vaccine quality in terms of minimum variation in particle size and distribution vis-a-vis critical level of iron in media

To enhance the qualitative bacterial biomass per unit of media and to overcome the limitations of the existing haemorrhagic septicaemia (HS) vaccines, a comprehensive study was undertaken encompassing the role of iron on the bacterial biomass of Pasteurella multocida B: 2 to vaccine development. Trypsin digested hydrochloric acid-treated sheep blood (THSB) as a novel iron rich supplement had been devised for the first time for augmenting the qualitative bacterial biomass per unit of media which was evident with growth kinetic study. The higher recovery of iron from THSB became evident via atomic absorbance spectrophotometry. The critical level of iron in the media as well as mode of iron supplementation showed a major impact on the outer membrane protein profile of P. multocida B:2 and variation in droplet size and particle-size distribution of formulated vaccine. Immune response study against iron-regulated bacterin adjuvanted with aluminum hydroxide gel in mouse model showed that 3% THSB supplementation of casein sucrose yeast (CSY) not only augmented the growth of P. multocida B:2 significantly but conferred highest pre-challenged ELISA IgG titer and protection against pasteurellosis. Thus, THSB supplementation of CSY can resolve existing up-scaling and immunogenic potential problems of HS vaccine production.

Self-assembly of Tween 80 micelles as nanocargos for oregano and trans-cinnamaldehyde plant-derived compounds

The self-assembly of Tween 80 (T80) micelles loaded with plant-based oregano essential oil (OR) and trans-cinnamaldehyde (TCA) was studied. The effect of different factors, including the surfactant to oil ratio, the presence of sodium chloride, thermal treatment, and dilution on their formation and physicochemical stability was evaluated. The creation of nano-cargos was confirmed by TEM. The self-associated structures had z-average droplet diameters of 92 to 337 nm without any energy input. Whereas addition of 10% (w/v) NaCl prevented the formation of oregano essential oil nano-assemblies of T80, swollen micelles containing TCA were successfully produced. Moreover, the OR or TCA loaded-micelles had only a slight droplet size variation upon thermal treatment. Ultimately, their antibacterial activity analysis against some food pathogens revealed that the encapsulation of OR and TCA within micelles crucially improved their antibacterial activity. These straightforward and cost-effective designed systems can be applicable in different products, including foods and agrochemicals.

Numerical investigation of rain droplet impact on offshore wind turbine blades under different rainfall conditions: A parametric study

The leading edge of a fiber composite wind turbine blade (WTB) is prone to erosion damages due to repeated rain droplet impact during its service life. Such damages are critical to the blade’s aerodynamic as well as structural performance, ultimately resulting in substantial repair costs. An effective design of a coating material for WTB is necessary and its analysis must include variables associated with erosive rain droplets such as (1) droplet diameter, (2) impact velocity, and (3) droplet impact angle. The present paper develops and validates a coupled fluid structure interaction (FSI) computational model for simulating rain droplet impact on WTBs, where the structure domain is modelled using conventional finite element method (FEM) and the fluid domain is modelled using smooth particle hydrodynamics (SPH). The 3D numerical model, developed in LS-DYNA, is validated with published experimental results. Further, a parametric study is considered to understand the effects of varying droplet size, impact angles and impact velocities on the impact responses of the leading edge coating system subjected to different rainfall conditions. The rainfall conditions considered for the analysis correspond to four different rainfall intensities (I) – light rainfall (2mm/hr), moderate rainfall (10mm/hr), heavy rainfall (25mm/hr), and very heavy rainfall (50mm/hr). The results show that the impact responses on the coating system increase with increasing droplet size and increasing droplet impact angle with maximum impulses, stresses and damages developed for normal impingement (90°). Also, the effects of droplet impact angles in the range of 50° to 90° are found critical for rainfall intensities representing very heavy rainfall conditions (I>25mm/hr). The results of the peak contact forces and impulses for the above combination of variables used in the numerical study are found in satisfactory agreement with analytical formulations developed through published experiments. Finally, repetitive rain droplet impact analyses are considered and number of impacts required for onset of erosion damages are found to increase by more than seven times upon reducing impact velocities from 140 m/s to 80 m/s for very heavy rainfall conditions (I>25mm/hr). The present study is expected to deliver a validated numerical model that can contribute towards enhancing the erosive capacity of a WTB.

The effect of adjuvants on spray droplet size from hydraulic nozzles.

BackgroundWhen spraying simple liquids through a hydraulic nozzle, the mechanisms that affect breakup into droplets have recently been described. However, it is unknown how the droplet size distribution changes when surfactant‐based adjuvants are added to the spray.ResultsWhen spraying different surfactant‐based solutions containing commercial adjuvants, the breakup of the different liquids behaves in the same way as for water‐only sprays, but the droplet sizes are smaller. By replacing the equilibrium surface tension with the dynamic surface tension at a surface age of ~20 ms, the volume mean droplet size variation with the Weber number, flow rate and nozzle geometry follows the predictions established for pure water sprays. When we rescale the droplet size distribution with the mean droplet size, all distributions collapse onto a single curve and can be described by a compound Gamma function.ConclusionAddition of a number of surfactant‐based adjuvants to an agricultural spray is observed to lead to a slight decrease in the volume mean droplet size. We show that the effects of these adjuvants on the drop size can be understood by taking into account the nozzle geometry, the flow rate, the liquid and air densities and the dynamic surface tension of the surfactant solutions at a surface age of approximately 20 ms. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Measurement of cloud properties using a self-designed cloud chamber

Present paper examines the dependency of ambient parameters such as humidity, and turbulence to determine the conditions on raindrop formation with the help of a self-designed cloud chamber. The research methods are experimental and observational in nature, where atmospheric phenomena are recreated through the usage of appropriate substitutes. Miniature droplets were created inside a box-like setup through the use of dry ice to cool the water vapor rising up, so as to create suspended water droplets, and to induce precipitation of heavier droplets. The experiment resulted in the creation of precipitated droplets, which were found at the base of the chamber at 99-100% relative humidity. The suspended droplets were used to study factors such as luminosity and variation of droplet sizes with turbulence. It was found that up to 14.4 m/s of turbulence, the droplet sizes increase with an increase in turbulence, with the luminosity decreasing with increase in turbulence. The gaussian profile of droplet size distribution has also been obtained, with a standard deviation of 2.83, 3.01 and 3.18 for low, medium and high turbulence speeds respectively. The experiment can be extended to incorporate a higher number of variables, so as to include a wider range of atmospheric phenomena.

Maillard conjugation of whey protein isolate with water-soluble fraction of almond gum or flaxseed mucilage by dry heat treatment.

The Maillard conjugation of whey protein isolate (WPI) by dry heat treatment (74% relative humidity at 60 °C) to either the water-soluble fraction of almond gum (SFAG) or flaxseed mucilage (SFM) was compared. Depending on the protein to polysaccharide ratio, carbohydrate type, and incubation time, different degrees of substitutions of the amino groups were obtained. The characterization of the conjugates by TNBS, SDS-PAGE, size exclusion chromatography, and circular dichroism analysis confirmed the formation of conjugates. SFAG was found to have less tendency for the formation of grafted WPI than SFM, which could be attributed to both the polysaccharide composition and/or a higher molecular weight. Ultimately, the emulsions stabilized by conjugates (pH 5.0 and 6.5) remained homogenous with no droplet size variation after heating, indicating that the conjugation of WPI to SFAG and SFM substantially improved its heat stability.

Theoretical Leidenfrost point (LFP) model for sessile droplet

In the present paper, a theoretical investigation is undertaken in pursuit of a new mechanistically based Leidenfrost point (LFP) model for a sessile droplet. The model consists of sub-models describing temporal variations of droplet size and shape, and thickness of the vapor layer separating the droplet from the heating surface during the evaporation process. Starting from the film boiling regime, it is shown that decreasing surface temperature causes monotonic thinning of the vapor layer. The primary hypothesis of the model is that as Leidenfrost temperature is reached, the vapor layer becomes sufficiently thin to enable surface roughness protrusions to breach the droplet underside. It is shown that, because of the stochastic nature of surface roughness, an appropriate statistical parameter of surface height must be determined for comparison with the vapor layer thickness. Using surface profiles measured by the authors along with those obtained from prior studies, it is shown how this statistical parameter may be related to other commonly available parameters. Overall, the model shows good accuracy in predicting temporal records of droplet size and shape, and vapor layer thickness for different liquids and surface temperatures. Combined with the statistical surface height parameter, the model shows very good accuracy in predicting the Leidenfrost temperature, evidenced by a mean absolute error of 7.77%.

Janus membrane emulsification for facile preparation of hollow microspheres

Hollow microspheres have aroused considerable interests in the materials community owing to their numerous potential applications; however, scalable, facile, low-cost production of hollow microspheres has not yet been demonstrated. Here, inspired by the idea that a hydrophilic surface allows the rapid detachment of gas bubbles to reduce the bubble size significantly, a novel and feasible approach to preparing gas-in-oil-in-water (G/O/W) emulsions on a large scale using Janus membrane emulsification is presented. A Janus membrane with asymmetric wettability, rather than a fully hydrophilic or a fully hydrophobic membrane, is employed in the emulsification system, in which the hydrophobic side allows gas/oil permeation and the hydrophilic side achieves fast release of oily bubbles. The wetting and chemical properties of each side of the membrane were investigated by contact angle and spectroscopy measurement. It was observed that the stability of the emulsions increased with increasing solution pH values. The variation of droplet size and distribution with the gas flow rate was also studied. The outer diameters of double emulsion drops are in the order of 100–300 μm. These droplets are further exposed to UV light to obtain hollow polymeric microspheres through solidification of the oil layers. Hollow structures of the microspheres were verified by HIM observations. This simple strategy opens a new avenue for generating double emulsions and hollow microspheres at scale with relatively narrow polydispersity.

The droplet size of emulsion adjuvants has significant impact on their potency, due to differences in immune cell-recruitment and -activation

Self-emulsification is routinely used for oral delivery of lipophilic drugs in vivo, with the emulsion forming in vivo. We modified this technique to prepare novel oil-in-water emulsions of varying droplet size and composition on bench to enable adjuvanted vaccine delivery. We used these formulations to show that smaller droplets (20 nm) were much less effective as adjuvants for an influenza vaccine in mice than the emulsion droplet size of commercial influenza vaccine adjuvants (~160 nm). This was unexpected, given the many claims in the literature of the advantages of smaller particulates. We also undertook cell-recruitment mechanistic studies at site of injection and draining lymph nodes to directly address the question of why the smaller droplets were less effective. We discovered that emulsion droplet size and composition have a considerable impact on the ability to recruit immune cells to the injection site. We believe that further work is warranted to more extensively explore the question of whether, the smaller is not ‘better’, is a more common observation for particulate adjuvants.

Experimental and mathematical investigations of spray angle and droplet sizes of a flash evaporation desalination system

This research experimentally studied the effect of operational parameters on spray characteristics of flash evaporation system which is the key component of the discharge thermal energy combined desalination (DTECD) system. For this purpose, initially the effect of superheat degree, inlet pressure, and inlet temperature on the spray angle of three different types of full cone spray nozzles was investigated using a high speed camera. Moreover, a force balanced mathematical model was developed to calculate droplet sizes and the results were compared with experimentally measured values using shadowgraph technique. The results show that the spray angle was largely affected by the superheat degree regardless of nozzle type. In addition, an empirical equation was suggested to correlate inlet pressure, saturation pressure, and spray angle. Further, a model was proposed to estimate the variation of droplet sizes along the spray centreline and concluded that the spray capacity significantly influenced the droplets' lifetime.

Droplet Misalignment Limit for Inkjet Printing into Cavities on Textured Surfaces.

The control of droplets deposited onto textured surfaces is of great importance for both engineering and medical applications. This research investigates the dynamics of a single droplet deposited into a confined space and its final equilibrium morphology, with emphasis given to droplet deposition under print head misalignment, the effect of nonuniform wettability, and deposition of droplets with varying sizes. A multiphase pseudopotential lattice Boltzmann methodology is used to simulate the process of deposition. The print quality is characterized in terms of a parameter referred to as the wetted fraction, which describes the proportion of the cavity that is wetted by the droplet. Our results show how single and multiple axis misalignment affect the final equilibrium morphology, and it was found for comparable configurations that multiaxis misalignment resulted in a higher wetted fraction. Investigations into wettabilities of the substrate and cavity wall revealed how larger ratios of the contact angles between the two enhance the ability for the droplet to self-align within the cavity. Additionally, a range of uniform wettabilities between the substrate and cavity were found, which mitigate against misalignment. Investigations into varying droplet sizes relative to the cavity revealed how misalignment can be compensated for with larger droplets, and limits for filling a cavity with a single drop are defined. Finally, we explore the deposition with misalignment into closely positioned cavities where it is found that the spacing between cavities is a key factor in determining the maximum permissible misalignment.

An investigation on the stability of the hazelnut oil-water emulsion

In this study, the stability of the emulsion of two immiscible liquids, including the effect of surfactants and agitation systems has been evaluated. The working fluid was hazelnut oil in water. Three different surfactants—two anionic sodium dodecyl sulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS) and a nonionic (Tween-20) — were tested. To assess the effect of surfactant type and agitation speed on droplet size distribution, focused beam reflectance measurement (FBRM) was employed. Emulsions prepared with both surfactant types, that is, anionic and nonionic, were better stabilized than emulsions with no surfactant. Droplet size decreased when the surfactant concentration was increased, until critical micelle concentration (CMC) of surfactants was reached. Once the CMC was reached, the variation in droplet size was significantly reduced or eliminated with any further increase in its concentration. The interfacial tension (IFT) was found to be a function of surfactant concentrations: with an increase in the surfactant concentration, the adsorption of surfactant molecules increased at the oil-water interface. The IFT remained unchanged when the micelles formed in the aqueous phase above CMC. Data analysis from FBRM revealed that higher agitation speed produced finer droplets. The destabilization time scale of emulsions in the presence of different surfactants was also assessed using electrical resistance tomography (ERT).

Ultrasonic/sonochemical synthesis and evaluation of nanostructured oil in water emulsions for topical delivery of protein drugs

Delivery of drugs and active agents to human skin by formulation containing nanosystems have shown remarkable advance in recent nanotechnology research. The aim of current investigation is to study protein drug, extracted from medicinal leech tissue and evaluate an isotropic and kinetically stable nanoemulsion formulation, with least surfactant and co-surfactant concentrations also with optimal solubility and stability, for topical delivery. It is a fact that physical properties of oil phase has an impact on nanoemulsion formation and stabilization. In this research, various factors, such as oil types (olive oil and sesame oil) and oil content for their effects on particle size and stability of protein nanoemulsion were first investigated. Secondly, optimized formulation of protein nanoemulsion was characterized by droplet size and zeta potential analysis, Transmission electron microscopy (TEM), viscosity, pH, and refractive index. Thirdly, stability studies were done to select the best formulation. The results of our experiments showed that an increase in the concentration of olive oil and sesame oil led to a nanoemulsion with smaller size of droplets and with higher stability, respectively. However; slight variations in droplet size were observed in case of nanoemulsion with olive oil. As a result of various experiments, a Nanoemulsion with 25% olive oil was selected as optimized formulation owing to its much smaller droplet size (143.1 nm), lower polydispersity index, higher zeta potential (−33.3 mV). No considerable changes in droplet size, viscosity and pH occurred during a 30-day storage period at 4 °C. This procedure also suggested that our selected nanoemulsion was physically stable. Additionally, TEM revealed that particles were morphologically spherical. In conclusion, our analyses of the experiments proved the fact that nanoemulsions are promising novel formulations for protein drugs and can greatly enhance stability of protein drugs.