Monodisperse Spray Research Articles

Polydisperse sea spray effect on the vertical momentum transport in hurricanes

Abstract This study focuses on the influence of the sea spray polydispersity on the vertical transport of momentum in a turbulent marine atmospheric boundary layer in high-wind conditions of a hurricane. The Eulerian multifluid model treating air and spray droplets of different sizes as interacting inter-penetrating continua is developed and its numerical solutions are analyzed. Several droplet size distribution spectra and correlation laws relating wind speed and spray production intensity are considered. Polydisperse model solutions have confirmed the difference between the roles small and large spray droplets play in modifying the turbulent momentum transport that have been previously identified by monodisperse spray models. The obtained results have also provided a physical explanation for the previously unreported phenomenon of the formation of thin low-eddy-viscosity “sliding” layers in strongly turbulent boundary layer flows laden with predominantly fine spray.

Optimizing the parameters of the air disc atomizer for the low volume desalination spraying of cotton crops

The article presents the results of a multi-factorial experiment to justify the parameters of a rotating sprayer. Optimization of the parameters of the pneumatic disk atomizer shows that with the correct choice of the specific liquid flow rate (qi), disk radius (r) and the number of radial channels (η r) at a constant air flow rate, it leads to obtaining the required median-mass droplet diameter. Therefore, when justifying the diameter of sprayed drops, it is necessary to consider a combination of the specific flow rate of the liquid and the radius of the disk. To obtain a monodisperse spray of drops with a median-mass diameter d= 80…120 µm, the disc radius should be in the aisles r=75.8…83.4 mm, the specific liquid flow rate qi= 0.61…0.96 l/min and the number of radial channels for pairs of disks in the aisles η p = 4.59… 5.75 things.

Numerical simulation of TTIP/[formula omitted]-xylene precursor solution spray flames in the counterflow configuration

Sprays of precursor solutions are used in flame spray pyrolysis (FSP) to produce functional nanoparticles with specific properties. The present study concerns the precursor titanium(IV) isopropoxide (TTIP) in p-xylene. A dilute monodisperse spray carried by an air stream is directed against an air stream to establish a counterflowing laminar spray flame. The complex thermophysical properties of the bi-component droplets are incorporated into the code as well as the detailed chemical reactions of the system with 213 chemical reactions among 52 species; the chemical reaction mechanism includes thermal decomposition of the TTIP and the combustion of p-xylene. The droplets are modeled using the multicomponent evaporation model. Drag and gravity forces are considered in the equation of droplet motion. Monodisperse spray flame structures are presented depending on the gas strain rate, the initial droplet size, the precursor mass loading as well as the equivalence ratio. The preferential evaporation of the more volatile p-xylene provides the gaseous fuel for combustion, and the thermal decomposition of the precursor TTIP considerably reduces the flame temperature.

ЭКОЛОГИЧЕСКИЕ АСПЕКТЫ ОПТИМИЗАЦИИ ПРИМЕНЕНИЯ СРЕДСТВ ЗАЩИТЫ РАСТЕНИЙ

Within the “From Field to Counter” Strategy framework, the European Parliament calls for a review of the procedure for the admission of plant protection products (PPP) in the EU. The aim is to strengthen control over the PPP use and reduce their use. The development of energy- and resource-saving agrochemical technologies for the PPP use implies designing monodisperse spray devices for specific chemicals, specifying their consumption rates per unit area, and selecting compatible spray equipment and relevant agricultural technologies. The elaboration of spray technology requires a fundamentally new approach to its testing and determining the dispersed composition of spray droplets. To elaborate an innovative methodology for optimizing the operation of spraying devices based on the dispersive analysis of spray droplets, the authors have developed and manufactured a test bench that provides a representative aerosol sampling. Express analyzers of spraying quality have been developed, made in the form of cards with dimensions 90 by 40 mm or tape 40 mm wide and 3 m long, impregnated with a solution of potassium hexacyanoferrate (III) at a concentration of 5…7%. A computer program has been made for counting and ranking spray drops in a real-time mode. The article presents study results on the optimization of several parameters: the consumption rates of the products and the hydraulic medium, the density (concentration) of the power fluid droplets on the surface of the treatment object, the dispersed composition of the spray droplets (the droplet size, the degree of productivity polydispersity and the possibility of adjusting the technological process parameters). The developed experimental complex and the technology of experimental testing of spraying devices will facilitate comparative tests of spraying devices.

Contactless electrostatic shaping of a capillary jet for drop-on-demand purposes

The contactless electrostatic shaping of a capillary jet can be adjusted so as to generate a monodisperse spray in the jetting regime. To demonstrate this, an actuator based on stacked electrodes is developed to generate a spatially modulated electric stress. The latter promotes the parametric excitation of a wavelength along a deformed capillary jet. The objective is to better control the breakup length of the capillary jet and to phase lock drop ejection. Experiments are carried out based on light absorption and fast imaging. The breakup length and the drop size distribution are measured. The comparison with a stability model allows us to identify resonant frequencies which monitor the size or the number density of drops. The concept of synchronization frequencies is introduced here with possible use in future developments of electrodynamic actuators.

A universal lookup table for determining sprinkler spray patterns

Sprinkler systems are effectively used to protect people and reduce property loss during fire events. While historically their performance has been evaluated experimentally, recent progress in spray modelling open the door to new applications in increasingly popular performance-based design. In this context, we propose a new approach built upon the description of the spray through a lookup-table, useful for engineering applications such as Building Modelling Information. This approach considerably reduces the calculation time and provides unprecedented access to spray dispersion characteristics. An efficiently parameterized lookup table with detailed information of the spray, here referred to as Universal Table (UT), is proposed based on a discretization of the spray phase-space. The spray is mapped through evaluation of the UT, using high-fidelity measurements of the initial characteristics of the spray from the Spatially-resolved Spray Scanning System device. Quantitative view of the spray including spray patterns, drop size distribution and drop velocity in quiescent conditions are achieved at any location of interest in few seconds. The model is validated by comparing experimental and predicting spray patterns produced by a commercial sprinkler. A new methodology to predict spray discharge characteristics from pre-specified spray patterns is also presented and demonstrated in mono-dispersed and multi-dispersed spray.

Morphology and formation of crystalline leucine microparticles from a co-solvent system using multi-orifice monodisperse spray drying

The purpose of this study was twofold: to investigate leucine crystallization kinetics in co-solvent spray drying for a deeper understanding of the morphology and formation process of spray-dried crystalline particles, and also to develop multi-orifice plates using gallium focused ion beam milling in order to increase the production rate of monodisperse spray drying. A monodisperse spray drying technique using a vibrating orifice atomizer equipped with a gallium focused ion beam-manufactured dual-orifice plate was used to produce model particles in this study. The dual-orifice plate achieved an average production rate of 23.8 ± 2.0 mg/hr for the model particles, which allowed sufficient production of monodisperse powders for microscopic and spectroscopic analysis as well as fragility testing, within a reasonable spray drying time period. Leucine was dissolved in a water-ethanol co-solvent at ratios of 0.25/0.75 w/w, 0.5/0.5 w/w, and 1/0 w/w, and then spray dried at inlet temperatures of 20, 40, and 80 °C. Crystallinity of the spray-dried particles was confirmed by Raman spectroscopy. Numerical models were used to predict the crystallization and drying kinetics for the different co-solvent ratios and drying temperatures. Increasing the time available for crystallization correlated qualitatively with a larger crystal size. Changes in particle morphology affected the fragility of the particles, as illustrated in electron microscope images. This work highlights the importance of controlling crystal size and particle morphology via solvent environment and drying temperature in the design and manufacture of microparticles for desired powder properties.Copyright © 2021 American Association for Aerosol Research

Leucine enhances the dispersibility of trehalose-containing spray-dried powders on exposure to a high-humidity environment

This study investigates the ability of various shell-forming excipients to preserve the dispersibility of dry powder dosage forms, e.g., nasally administered vaccines, upon exposure to a high-humidity environment. Trehalose combinations using leucine, pullulan, or trileucine were selected as the candidate excipient systems, and the powder dispersibility of these systems was compared with that of pure trehalose particles. Scaled-up monodisperse spray drying was used to produce sufficient quantities of uniform-sized particles for powder dispersibility analysis. Particle size, crystallinity, and morphology of the powders before and after exposure to moisture were characterized by an aerodynamic particle sizer, Raman spectroscopy, and scanning electron microscopy, respectively. Three two-component particle systems composed of trehalose/trileucine (97/3 w/w), trehalose/pullulan (70/30 w/w), and trehalose/leucine (70/30 w/w) were first formulated and their dispersibility, characterized as the emitted dose from dry powder inhalers, was then compared with that of trehalose particles. The formulation containing 30% leucine maintained the highest emitted dose (90.3 ± 10%) at a 60 L/min flow rate after 60 min exposure to 90% RH and 25 °C, showing its superior protection against exposure to humidity compared with the other systems. Further investigations under more challenging conditions at a 15 L/min flow rate on the trehalose/leucine system with various compositions (70/30, 80/20, 90/10 w/w) showed that a higher leucine concentration generally provided better protection against moisture and maintained higher powder dispersibility, probably due to higher surface coverage of crystalline leucine and a thicker leucine shell around the particle. The study concludes that leucine may be considered an appropriate shell-forming excipient in the development of dry powder formulations in order to protect the dosage forms against humidity during administration.

ПЕРСПЕКТИВЫ РАЗВИТИЯ МЕТОДОВ И ТЕХНИЧЕСКИХ СРЕДСТВ ЗАЩИТЫ СЕЛЬСКОХОЗЯЙСТВЕННЫХ РАСТЕНИЙ

Modern intensive highly productive agricultural production should be based on a combination of various types of plant protection measures – quarantine, agrotechnical, chemical, breeding, biological, mechanical and physical methods, taking into account the balance between their effi ciency, minimal negative impact on the environment and economic feasibility. The paper discusses the advantages and disadvantages of the listed methods. The chemical method of plant protection, despite its shortcomings, is one of the most widely used productive and eff ective methods. The negative eff ect of the chemical method can be signifi cantly reduced by tightening the requirements for the safety of using pesticides for the environment and human health, ensuring targeted application of the product directly to the treated object, increasing the quality of application and reducing the loss of working fl uid, using productsw with a narrower range of action and short decomposition time. The authors stress that in order to achieve maximum effi ciency of chemical treatment, sprayer nozzles must provide a monodisperse spray and, at the same time, be able to adjust the size of monodisperse aerosol droplets in the range from 10 to 150 microns. Technical means for plant protection should be equipped with means that allow eliminating or minimizing the working fl uid drift as a result of wind exposure and evaporation, and ensuring the forced deposition of aerosol droplets on the treated objects to reduce the loss of working fl uid.

Determination of kinematic parameters of falling drops of standard solution of pesticides during spraying, taking into account geometric dimensions variability

Expansion of range of applied pesticides and liquid mineral fertilizers necessitates continuous improvement of spray nozzle design, allowing to create a monodisperse spray and ensure high-quality application of chemical agents at low doses and minimal losses. The issue of studying the process of falling drops with varying geometric dimensions remains sore. Studies of drop movement in air environment make it possible to determine the falling speed and coordinates on the treated surface, to substantiate the design, dimensions, optimal operating modes and parameters of sprayers and devices for protecting the spray cone from direct exposure to wind, which is especially important at the design stage of sprayer for field spraying machines. The paper presents simulation of process of falling drops of pesticide standard solution in resisting environment, considering geometric dimensions variability. An equation for drop radius variability depending on the unit motion horizontal transverse variability, formula for variability of intensity of drop decrease depending on the initial conditions and state of environment are obtained. Dependence between coefficients of drop displacement along the horizontal transverse to the unit movement axis and time is obtained, expressions for variability of drop radius depending on the horizontal displacement and the equation for variability of velocity and vertical coordinate of drop movement on time are presented. The coefficient of mass transfer from the drop surface is determined depending on the resistance coefficient, initial velocity, medium density at the border of drop and plant medium subjected to treatment. The results obtained can be used in mechanical engineering for design and testing of sprayers and nozzles, design of wind protection devices for spray cones of standard solutions of pesticides in field sprayers, in simulation of process of drop movement with varying mass.

Surface coverage by impact of droplets from a monodisperse spray

A novel pneumatic monodisperse spray was fabricated to validate the results of a probabilistic model and a stochastic model that were developed to predict surface area coverage of the monodisperse spray. Multiple droplets of a highly viscous 87 wt% aqueous glycerin solution were deposited on a flat, transparent, plexiglass substrate. The uniformly sized droplets (2.5 mm in diameter) impacted, spread, and coalesced on the substrate which led to the creation of a liquid sheet. Using a high-speed camera placed underneath the transparent substrate, droplet impact and resultant area coverage were recorded. When impinging droplets overlapped those already on the surface, surface tension forces pulled impacting droplets towards them, agglomerating the liquid, in a phenomenon known as drawback. The area covered by the liquid was measured from photographs using image analysis software. In the numerical models, droplets were set to impinge at random locations and drawback effects between droplets were accounted for by using a semiempirical correlation. While both models showed good agreement with the experimental results at lower area fractions, the stochastic model gave better estimates at larger area fraction due to its incorporation of drawback effects.

Closure of the scalar dissipation rate in the spray flamelet equations through a transport equation for the gradient of the mixture fraction

In this paper, the closure of the Scalar Dissipation Rate (SDR) in the Spray Flamelet Equations (SFE) is addressed. For this purpose, the gradient gξ of the mixture fraction ξ is used instead of the SDR itself. A transport equation for this variable is derived and transformed from physical into mixture fraction space. Moreover, the spray flamelet equations of the species mass fractions and of gas temperature are re-derived in terms of gξ for consistency, considering differential diffusion effects.Numerical simulations of different axi-symmetric counterflow ethanol/air flames are carried out in physical space using a well established model and the results are employed for the validation and analysis of the newly proposed set of SFE. In particular, a non-premixed gas flame is established as a base case and then perturbed by means of different mono-disperse sprays injected from the air side of the configuration.In the newly proposed SFE, two different kind of unclosed quantities appear: The spatial gradient of the product of gas velocity and gas density, a^, and sources of mass and energy due to evaporation. In the present work, different alternatives for the closure of a^ are presented and analyzed, where the following approaches are proposed: 1) Introducing a stream-like function and using the global mass and axial momentum balance equations to derive an expression for a^ and 2) Assuming a constant value for this variable. Moreover, two different constant values of a^ are considered: its value at the stoichiometric mixture fraction and its value at the air side of the counterflow configuration. The evaporation-related source terms are closed through projections of the numerical results from physical into mixture fraction space. The suitability of the proposed approaches is tested in terms of the ability of the SFE of properly predicting both gξ and the spray flamelet structure of the reference counterflow flames. Additionally, the contributions of the individual terms in the SFE are analyzed, with special focus on the effects of evaporation. The validation confirms that the new set of SFE accurately reproduces the counterflow profiles when the stream-like function is employed, whereas employing the value of a^ at stoichiometry still adequately describes the relative importance of the different physical and chemical phenomena taking place in the flames studied (the contribution of the different terms in the SFE), even when the flame structure itself shows non-negligible deviations. Finally, using the value of a^ at the left inflow boundary leads to poor predictions in most situations.The present results represent a significant advance towards the development of a comprehensive and self-contained spray flamelet theory.

Numerical simulation of mono-disperse droplet spray dryer under the influence of nozzle motion

With the increasing demand of uniform particles in the fields of medicine etc., the research on monodisperse droplet spray dryer (MDSD) becomes important. The drying efficiency of the current MDSD facility has to be improved to realize industrial applications. In this work, the influence of several kinds of nozzle movement on the drying process has been thoroughly explored by silico experiments. It is found that the velocity of the nozzle movement plays a key role in drying efficiency improvement. The movement of the nozzle leads to a wider distribution of the final moisture content of particles, and this effect becomes less significant with the increase of the velocity of nozzle motion. For all motion types under investigation, the circular motion is superior to the other motion types. It is revealed that the change of droplet dispersion state due to nozzle motion is the main factor that improves drying performance.

Sprays from droplets impacting a mesh

In liquid spray applications, the sprays are often created by the formation and destabilization of a liquid sheet or jet. The disadvantage of such atomization processes is that the breakup is often highly irregular, causing a broad distribution of droplet sizes. As these sizes are controlled by the ligament corrugation and size, a monodisperse spray should consist of ligaments that are both smooth and of equal size. A straightforward way of creating smooth and equally sized ligaments is by droplet impact on a mesh. In this work we show that this approach does however not produce monodisperse droplets, but instead the droplet size distribution is very broad, with a large number of small satellite drops. We demonstrate that the fragmentation is controlled by a jet instability, where initial perturbations caused by the injection process result in long-wavelength disturbances that determine the final ligament breakup. During destabilization the crests of these disturbances are connected by thin ligaments which are the leading cause of the large number of small droplets. A secondary coalescence process, due to small relative velocities between droplets, partly masks this effect by reducing the amount of small droplets. Of the many parameters in this system, we describe the effect of varying the mesh size, mesh rigidity, impact velocity and wetting properties, keeping the liquid properties the same by focusing on water droplets only. We further perform lattice Boltzmann modelling of the impact process that reproduces key features seen in the experimental data.

Hysteresis in the Vaporization-Controlled Inertial Regime of Nonpremixed Counterflow Spray Combustion

ABSTRACTThis numerical study uses a Lagrangian–Eulerian formulation employing the point-source approximation to investigate nonpremixed axisymmetric spray diffusion flames stabilized between a stream of hot air and a stream of nitrogen carrying a monodisperse spray of fuel droplets injected at a distance zl from the stagnation plane. The computations consider moderately large values of the relevant thermal Peclet number, such that the thickness of the mixing layer separating both streams is much smaller than zl. A one-step irreversible reaction is assumed between the fuel vapor and the oxygen of the air. Most of the computations consider the thermochemical, vaporization, and transport properties of dodecane, with additional results for the properties of methanol used to assess effects of higher fuel volatility. The analysis considers moderately small values of the liquid mass loading ratio at the injection plane, consistent with the stoichiometric value of the fuel-to-air mass ratio of the global oxidation reaction of typical hydrocarbon and alcohol fuels. Attention is mainly focused on inertial droplets with associated Stokes numbers St of order unity or larger, for which the droplets penetrate far into the oxidizer stream, vaporizing over distances of order zl and leading to combustion occurring outside the mixing layer. In this inertial regime, besides solutions involving a diffusion flame separating the fuel and oxygen with a well-defined peak temperature of the order of the adiabatic flame temperature, the analysis reveals, for the first time, the existence of flameless solutions with distributed combustion and small temperature increments from the air-side value. The two solutions are found to exist for the same sets of boundary conditions. Integrations for increasing values of St identify a value of the Stokes number St2 above which the vaporization rate is too small to support a diffusion-flame solution, while integrations for decreasing St lead to the identification of another value of the Stokes number St1 < St2 below which a distributed reaction cannot occur. In the intermediate hysteretic range St1 ≤ St ≤ St2 where two different steady solutions may exist, the type of solution encountered depends on the initial conditions imposed. The preliminary results suggest that future efforts targeting quantification of the flameless regime, which is found to be relevant in applications with larger droplets, would benefit from consideration of different possible gas-phase chemical kinetics, including cool-flame chemistry.

Numerical simulation of mono-disperse droplet spray dryer: Coupling distinctively different sized chambers

The discrete phase model (DPM) has been widely used in CFD simulations to track discrete particles or droplets in a continuum fluid filed. This powerful technique, however, may face tremendous difficulty in tracking droplets flying through chambers with significantly different sizes. In such cases, it becomes very challenging to develop effective mesh for the big and small chambers together with the transition zone that can ensure solution convergence within reasonable computational time. In this work, a systematic method is introduced to tackle this problem. This method allows simulation of different sized chambers separately to avoid the meshing difficulty. A unique coupling approach offers smooth transition of droplets from one chamber to the other with rigorous conservation of their mass, energy and momentum. The effectiveness of the method is demonstrated through the simulation of skim milk droplet drying in a mono-disperse droplet spray dryer (MDSD), where droplets must pass through a small dispersion chamber before entering the main big drying chamber. For the first time, droplet drying process from exiting nozzle to the arrival at the dryer outlet can be simulated. The new model can be used in the future to optimize the spray drying process.

Numerical investigation of the effects of polydisperse water sprays on extinction conditions of counterflow methane non-premixed flames

Water, sprayed in the form of tiny droplets, has emerged as a potential fire suppressant after the halon compounds such as trifluorobromomethane (CF3Br, Halon 1301) were banned by the Montreal protocol. The size distribution of the water droplet plays a crucial role in the effectiveness of the water spray in fire suppression. A numerical investigation of the influence of size distribution of a polydisperse water spray on extinction of counterflow diffusion flames is presented in this paper. This study uses laminar finite rate model with reduced CHEMKIN chemistry for numerical simulations. The discrete phase, namely the water spray, is simulated using Lagrangian Discrete Phase Modelling approach. In this work, the polydispersity of water spray is taken into account in the numerical simulation by a suitable Rosin–Rammler distribution. Results obtained from numerical simulation are validated with the experimental results reported in the literature. This study demonstrates that the representation of the polydisperse spray by a monodisperse spray (with droplet diameter same as the SMD of the polydisperse spray) in numerical simulations is not always justified and it leads to deviation from the experimental results. The effects of number mean diameter and spread parameter on the efficacy of flame suppression are investigated for polydisperse sprays. A comprehensive comparison is done between the effectiveness of monodisperse and polydisperse water sprays. An optimum droplet diameter is obtained for monodisperse sprays for which the effectiveness of the spray is maximum. The effects of evaporation Damköhler number and Stokes number of water droplets on flame suppression have also been explained.

Experimental and Analytical Investigations of Aerosol Processes—Wash-Out and Wash-Down

Timely evolution of radioactive airborne aerosols, the mass of particles deposited on structures or transported with condensing water into the sump, and any influence of safety systems on the aerosol distribution will have a crucial influence on the potential aerosol source term into the environment. Different phenomena affect the main aerosol processes, and important ones besides gravimetrical and diffusive settling require experimental investigations to improve and validate modeling assumptions. Experimental investigations in the THAI (Thermal-hydraulics, Hydrogen, Aerosol, Iodine) test facility have been performed to investigate (a) insoluble silver aerosol wash-down behavior from vertical steel and horizontal decontamination paint-coated surfaces by condensing steam (test AW-3, supported by a laboratory-scale wash-down test series), and (b) the depletion of the airborne aerosol concentration by wash-out due to the use of a nuclear power plant typical water spray system (test AW-4). The paper discusses experimental findings of both tests supported by analytical analyses using the containment code system COCOSYS developed by Gesellschaft für Anlagen- und Reaktorsicherheit (GRS). To validate and further improve the new model Abwaschmodell für unlösliche Aerosole (AULA) in COCOSYS, used for the wash-down of insoluble aerosols from containment typical structures, the AW-3 laboratory tests related to the AW-3 test were used. Building upon these results, the AW-3 wash-down test is simulated. The results of the calculation for the AW-3 test show that the wash-down of insoluble silver particles at least qualitatively resembles the experimental results, though generally the washed-down aerosol mass is lower compared to the experiment. In test AW-4, it was in question if the modeling of aerosol wash-out with spray systems is adequately treated by assuming monodisperse spray droplets or if a droplet distribution has to be applied. Posttest calculation of AW-4 indicates that the wash-out of CsI aerosols by spray systems can be captured qualitatively. However, it is also shown that the calculated wash-out rate is too large and the depletion of the CsI aerosols during the dry phase is underestimated.

Experimental investigations of particle formation from propellant and solvent droplets using a monodisperse spray dryer

Experimental studies of particle formation from solution droplets were conducted using a newly developed monodisperse spray drying process. Solutes beclomethasone dipropionate and caffeine were dissolved in ethanol, pressurized hydrofluoroalkane propellant 134a, and mixtures thereof. Solutions were atomized into monodisperse microdroplets using a custom droplet generator installed in a laboratory scale spray dryer, enabling drying and collection of the resulting monodisperse microparticles. The effects of droplet diameter, solution concentration, solvent composition, and drying rate on the physical properties of the dried particles were evaluated. Particle morphology and size were assessed using ultramicroscopy and image analysis of micrographs. Extent of crystallinity and polymorphism were investigated using Raman spectroscopy. The drying temperature was found to have a large effect on the morphology of amorphous beclomethasone dipropionate particles. Particles dried near room temperature were spheroidal to ellipsoidal with prevalent surface concavities and evidence of shell buckling; increasing the drying temperature for fixed droplet size and composition resulted in a transition to more spherical, smooth-surfaced particle morphologies. Crystalline caffeine microparticles were made up of assemblies of multiple crystallites. The measured length and breadth of these crystallites was found to be correlated with the time available for crystal nucleation and growth as calculated using a particle formation model. The results highlight the abilities and limitations of currently available particle formation models in elucidating the relationships between the size, composition, and evaporation rate of drying solution droplets and the physical properties of the resulting particles. The work demonstrates the suitability of monodisperse spray drying as an experimental technique for investigating the fundamentals of particle formation from solution droplets.© 2018 American Association for Aerosol Research

A two-angle far-field microscope imaging technique for spray flows

Backlight imaging is frequently used for the visualization of multiphase flows, where with appropriate microscope lenses, quantitative information on the spray structure can be attained. However, a key issue resides in the nature of the measurement which relies on a single viewing angle, hence preventing imaging of all liquid structures and features, such as those located behind other fragments. This paper presents results from an extensive experimental study aimed as a step forward towards resolving this problem by using a pair of high speed cameras oriented at 90 degrees to each other, and synchronized to two high-speed diode lasers. Both cameras are used with long distance microscope lenses. The images are processed as pairs allowing for identification and classification of the same liquid structure from two perspectives at high temporal (5 kHz) and spatial resolution (∼3 μm). Using a controlled mono-disperse spray, simultaneous, time-resolved visualization of the same spherical object being focused on one plane while de-focused on the other plane 90 degrees to the first has allowed for a quantification of shot-to-shot defocused size measurement error. An extensive error analysis is performed for spheroidal structures imaged from two angles and the dual angle technique is extended to measure the volume of non-spherical fragments for the first time, by ‘discretising’ a fragment into a number of constituent ellipses. Error analysis is performed based on measuring the known volumes of solid arbitrary shapes, and volume estimates were found to be within ∼11% of the real volume for representative ‘ligament-like’ shapes. The contribution concludes by applying the ellipsoidal method to a real spray consisting of multiple non-spherical fragments. This extended approach clearly demonstrates potential to yield novel volume weighted quantities of non-spherical objects in turbulent multiphase flow applications.