Spray Droplet Size Distribution Research Articles

Droplet Size Distribution in Sprays Based on Maximization of Entropy Generation

The maximum entropy principle (MEP), which has been popular in the modeling of droplet size and velocity distribution in sprays, is, strictly speaking, only applicable for isolated systems in thermodynamic equilibrium; whereas the spray formation processes are irreversible and non-isolated with interaction between the atomizing liquid and its surrounding gas medium. In this study, a new model for the droplet size distribution has been developed based on the thermodynamically consistent concept - the maximization of entropy generation during the liquid atomization process. The model prediction compares favorably with the experimentally measured size distribution for droplets, near the liquid bulk breakup region, produced by an air-blast annular nozzle and a practical gas turbine nozzle. Therefore, the present model can be used to predict the initial droplet size distribution in sprays.

A Predictive Model for the Initial Droplet Size and Velocity Distributions in Sprays and Comparison with Experiments

AbstractThe distribution of sizes and velocities of droplets initially formed in sprays is an important piece of information needed in the spray modelling, because it defines the initial condition of the spray droplets in the predictive calculations of the downstream two‐phase flow fields. A predictive model for the initial droplet size and velocity distributions in sprays is formulated in this study. The present model incorporates both the deterministic and the stochastic aspect of spray formation process. The deterministic aspect takes into account of the unstable wave motion before the liquid bulk breakup through the linear and nonlinear instability analysis, which provides information for the liquid bulk breakup length, the mass‐mean diameter and a prior distribution for the droplet sizes corresponding to the unstable wave growth of various wavelengths. The stochastic aspect deals with the final stage of droplet formation after the liquid bulk breakup by statistical means through the maximum entropy principle based on Bayesian entropy. The two sub‐models are coupled together by the various source terms signifying the liquid‐gas interaction, the mass mean diameter and the prior distribution based on the instability analysis. The initial droplet size and velocity distributions are measured experimentally by phase‐Doppler interferometry for sprays generated by a planar research nozzle and a practical gas turbine airblast nozzle. For the two nozzles, the liquid bulk sheet is formed before its breakup in a coflowing air stream. It is found that the model predictions are in satisfactory agreement with the experimental data for all the cases measured. Hence the present model may be applied to a variety of practical sprays to specify the initial conditions for the spray droplets formed in practical spray systems.

Patch spraying: future role of electronics in limiting pesticide use.

Developments relating to the control of application equipment can deliver improvements in pesticide use by better matching applications to target requirements. This may have components relating to the spatial distribution of a weed, pest or disease or methods by which the target, particularly a crop canopy, can be described with respect to a given application. Changes in application can relate to the dose and/or volume applied, but may also concern the way in which a treatment is delivered in terms of parameters such as spray trajectory angle and droplet size distribution. For many weed species there is evidence of patchy distributions in field situations. Studies have shown that savings of typically up to 40% in herbicide use can be achieved by adopting patch spraying approaches in such situations. Weed patch detection is key to the performance of such patch spraying systems. In widely spaced rowcrops such as vegetables, there is considerable scope for developing fully automated detection systems based on image analysis, and for the development of accurate guidance systems that apply pesticides only to the crop row. In crops with a relatively high plant density, weed detection in the medium/short term is likely to be based on manual discrimination. The costs of labour for manual weed patch mapping have been estimated at less than 1.50 ha(-1) pounds sterling. Potential savings in pesticide use can also be made if applications are matched to crop canopy structure. This is most important in bush and tree crops where savings of up to 75% in pesticide use could be achieved. In crops such as cereals, studies have shown that savings in fungicide use may be possible, particularly at earlier stages of growth by adjusting spray delivery to measured canopy characteristics. Key components of the performance of application systems concern the ability to deliver over a dose rate range of more than 3:1 while maintaining control of variables such as delivery trajectory angle and spray quality. Traceability and the effective monitoring of applications is likely to be a major driver influencing the uptake of more sophisticated control systems. Methods of labelling pesticides with systems that can be read by the application unit will be an important step in the development of recording and data handling systems that will operate safely with the minimum of operator input and enable the environmental advantages of targeted pesticide application to be monitored.

Nanoparticle synthesis at high production rates by flame spray pyrolysis

Scaling-up of nanoparticle synthesis by the versatile flame spray pyrolysis process at production rates up to 1.1 kg/ h is investigated. Product silica powder is collected continuously in a baghouse filter unit which is cleaned periodically by air-pressure shocks. The effect of powder production rate, dispersion gas flow rate and precursor (hexamethyldisiloxane, HMDSO) concentration on product particle size, morphology and carbon content is investigated. Droplet size distributions of the cold spray are measured by laser diffraction, while N 2 adsorption (BET), transmission electron microscopy and thermogravimetric analysis coupled with a mass spectrometer are employed to characterize the product powder. The product primary particle size was precisely controlled from 10 to 75 nm and compared to a well-established vapor-fed flame aerosol reactor.

Measurement of droplet size distribution of gasolinedirect injection spray by droplet generator and planar imagetechnique

To understand the spray characteristics that are related tothe characteristics of emissions and the combustion of directinjection engines, the planar image technique was applied to measurethe distribution of droplet size. This technique was based on theprinciples of laser-induced fluorescence (LIF) and the Mie scatteringof droplets in the spray. We decided to apply the LIF and Miescattering intensity indices for unleaded gasoline to this techniquewith a droplet generator. The fluorescence and scattering signalswere imaged simultaneously by an optical system composed of adoubling prism, optical filters and an ICCD camera. Relative Sautermean diameter distributions according to space and time were obtainedby calculating the ratio between two light intensities, and thedistributions were quantified with a laser diffraction particleanalyser. Experimental results have shown that the characteristics ofthe gasoline direct injection injector varied with injectionpressure.

Scale-up of mixer granulators for effective liquid distribution

There is considerable anecdotal evidence from industry that poor wetting and liquid distribution can lead to broad granule size distributions in mixer granulators. Current scale-up scenarios lead to poor liquid distribution and a wider product size distribution. There are two issues to consider when scaling up: the size and nature of the spray zone and the powder flow patterns as a function of granulator scale. Short, nucleation-only experiments in a 25L PMA Fielder mixer using lactose powder with water and HPC solutions demonstrated the existence of different nucleation regimes depending on the spray flux Ψ a—from drop-controlled nucleation to caking. In the drop-controlled regime at low Ψ a values, each drop forms a single nucleus and the nuclei distribution is controlled by the spray droplet size distribution. As Ψ a increases, the distribution broadens rapidly as the droplets overlap and coalesce in the spray zone. The results are in excellent agreement with previous experiments and confirm that for drop-controlled nucleation, Ψ a should be less than 0.1. Granulator flow studies showed that there are two powder flow regimes—bumping and roping. The powder flow goes through a transition from bumping to roping as impeller speed is increased. The roping regime gives good bed turn over and stable flow patterns. This regime is recommended for good liquid distribution and nucleation. Powder surface velocities as a function of impeller speed were measured using high-speed video equipment and MetaMorph image analysis software. Powder surface velocities were 0.2 to 1 ms −1—an order of magnitude lower than the impeller tip speed. Assuming geometrically similar granulators, impeller speed should be set to maintain constant Froude number during scale-up rather than constant tip speed to ensure operation in the roping regime.

Droplet Size and Velocity Distributions of a Transient Hollow-Cone Spray for GDI Engines

An experimental investigation of a gasoline direct injection (GDI) spray, emerging from an electronically controlled swirl-type injector, was carried out at an injection pressure and duration of 7.0 MPa and 3.0 ms, respectively, in an optically accessible vessel, at atmospheric pressure and ambient temperature. The temporal and spatial spray evolution was investigated in terms of global spray structure, interaction with the external gas, time-resolved droplet size and velocity distribution. The measurements were carried out with an AVL Engine Video System with a CCD camera, a frame grabber and a strobe flash triggered by the injection apparatus. Digital image processing software for the study of the global parameters of the spray was used. A particle Doppler analyzer (PDA) system was used to estimate the local droplet size and velocity as function of the radial coordinate and distance from the nozzle. A laser light extinction technique was applied to investigate the region close to the nozzle up to 5 mm. The spray emerging from the nozzle, at an early time, showed a hollow-cone type of structure because of the swirl motion of the fuel in the orifice that produces a rotational momentum at the nozzle exit. The spray starts with a central pre-injection phase followed, later, by the main body characterized by a large cone angle. The interaction of the fuel with the gas in the spray chamber was evidenced; curls on the external boundary and re-filling of the central part of the cone, during the second part of the injection, were observed. Measurements at different locations within the spray showed high values of the droplet velocity close to the nozzle exit at an early time. These high values were found downstream in the final stage of injection. Radial velocities were present along the entire injection process with an outer direction in the first stage, while an inner direction was observed in the second part. The droplet size distribution showed increasing values along the injector axis and towards the periphery of the spray.

On the Theoretical Prediction of Fuel Droplet Size Distribution in Nonreactive Diesel Sprays

Droplet size distribution function and mean diameter formulas are derived using information theory. The effects of fuel droplet evaporation and coalescence within combustion chamber on the droplet size are emphasized in nonreactive diesel sprays. The size distribution function expressions at various spray axial cross sections are also formulated. The computations are compared with experimental data and KIVA-II code. A good agreement is obtained between numerical and experimental results. Droplet size distribution and mean diameter at various locations from injector exit and at various temperature conditions are predicted. The decreases of droplet number and variations of mean diameter are computed at downstream and higher temperature.

Average Droplet Size Distribution of a GDI Spray by Simultaneous Fluorescence/Scattering Image Technique

The objective of this study is to investigate the average droplet size distributions of a GDI spray by simultaneous fluorescence/scattering image technique. GDI engine is recently very popular because of high engine efficiency and low emissions. However, the injectors must have good spray characteristics because the fuel is directly injected into the cylinder. The fuel mixtures used in this study were 2% of fluorobenzene, 9% of DEMA(diethyl-methyl-amine) and 89% of hexane by volume. The system for obtaining 2-D fluorescence/scattering images of fuel spray was constituted of a laser sheet, a doubling prism, optical filters, and an ICCD camera. Using the ratio of the fluorescence to the scattering intensities, SMD distributions were obtained. SMD measured by the technique was compared with that obtained by PDA. It was found that average droplet size was bigger at spray center in the early stage of injection and at the outer periphery of the spray in the late stage of injection.

Optical patternation: a technique for three-dimensional aerosol diagnostics

A novel technique based on optical patternation is described for three-dimensional diagnostic studies of aerosols used in analytical spectroscopies. The aerosol is illuminated with a thin laser light sheet to capture images of the fluorescence and Lorenz-Mie light-scattering signals from the aerosol field with a charge-coupled detector. These measurements allow for the rapid and nonintrusive elucidation of two-dimensional spray structures, planar mass distributions, and spatial droplet size distributions. The ratio of the fluorescence image to the Lorenz-Mie image is then utilized to construct a spatially resolved map of the volume-to-surface area mean of the aerosol (Sauter mean diameter). Three-dimensional maps of spray structure, mass distribution, and droplet size distribution are obtained for the entire aerosol field by image stacking. The technique is applied to the measurement of the droplet size over the aerosol field at distances of 5-30 mm from the nebulizer tip where droplet sizes ranged from 6 to 12 microm for a direct injection high efficiency nebulizer used in inductively coupled plasma spectrometries.

Characterization of spray atomization of 3003 aluminum alloy during linear spray atomization and deposition

Linear spray atomization and deposition is an attractive technique to produce near-net-shape deposits, such as aluminum sheet and strip. In the present study, phase Doppler interferometry (PDI) was used in a backscatter mode to characterize, in situ, the droplet size and velocity distributions during linear spray atomization and deposition of a 3003 aluminum alloy. The PDI measurements were obtained along axes corresponding to the direction parallel to the nozzle slit and to the direction perpendicular to the slit. The PDI results delineate the temporal and spatial distribution of the droplet size and velocity during the metal spray. Both point and “line” measurements were obtained and are reported. The line measurements resulted from the integration of measurement made along a line scan obtained in real time (i.e., not ensemble averaged). Postrun analysis of the droplet size distribution using laser diffraction and sieving techniques is also reported. The PDI point measurements revealed that droplet size and velocity distribution were relatively invariant with time. The line measurements of droplet velocity showed that the droplet axial velocity exhibits a bimodal behavior, which becomes more apparent with increasing atomizing gas pressure, a result of droplet recirculation inside the spray chamber. In addition, the peak in the droplet axial velocity distribution increased as atomizing gas pressure increased. The line characterization also showed that the droplet size distribution became more homogeneous with increasing gas pressure, and that the distribution characteristic diameters of droplets decreased consistently with increasing gas pressure. Postrun characterization of the droplet size distribution of the entire metal spray using diffraction and sieving methods indicated that the mass (volume) median diameter D 50 and the Sauter mean diameter (SMD) D 32 decreased with increasing gas pressure in a manner consistent with PDI results.

Cooling of a heated surface with an impinging water spray

Several important parameters, such as liquid mass flux, droplet size distribution, droplet velocity, and heating target conditions (roughness and surface temperature) are involved in the industrial spray cooling heat transfer process. In this study, we investigated the effect of liquid mass flux, heating target roughness, and the droplet size on the droplet wall direct contact heat transfer in spray cooling phenomena. Three different conditions of surface roughness were investigated. The measurement of test surface temperature was performed using a non-intrusive method, i. e., using an infrared thermometer. The droplet size distribution of water spray was measured with Malvern 2600. The results indicated that the most influential parameters were the liquid mass flux and the surface roughness. The droplet size and the velocity played a less important role in the direct contact heat transfer because the interactions between droplets were very strong in a dense spray. The smooth surface showed the highest heat transfer among the surfaces tested. At high air pressure ([7] kPa), however, the degree of roughness did not affect much the heat transfer rate.

Emulsions from Aerosol Sprays

An electrostatic emulsification apparatus has been designed for the purpose of studying diffusion from oil droplets which have a mean size in the range of ∼1.5–3.5 μm, with standard deviations of 40–50%. The emulsification technique involves the collection of a spray of electrically charged oil droplets onto a rotating water film which is sustained from a reservoir. In this way, emulsions with volume fractions of approximately 10−3are produced within several minutes at oil flow rates of around 10−2ml min−1. Phase-Doppler anemometry (PDA) was used to assess droplet size distributions for the sprays and emulsions. Results show that the mean emulsion droplet size was smaller than the mean spray droplet size by several orders of magnitude. At flow rates around 10−2ml min−1, the spray droplet size distribution was little affected by the applied potential between about −4.20 and −4.65 kV (mean droplet size between ∼7.6 and 7.8 μm, with standard deviations of ∼20%), whereas the mean droplet size of the corresponding emulsion decreased more rapidly with applied potential. Above an applied potential of ∼–4.30 kV, which corresponded to an emulsion droplet size below approximately 2 μm, the measured volume fraction of the emulsion decreased with respect to the volume fraction as calculated on the basis of total amount of injected oil.

Techniques for measurement of droplet size and velocity distributions in agricultural sprays

Two techniques for measuring the size and velocity of droplets in agricultural sprays produced by hydraulic flat fan nozzles have been evaluated for their ability to produce consistent results. These were a one-dimensional phase Doppler particle analyser (PDA) and a two-dimensional imaging probe of a Particle Measuring Systems (PMS) instrument, both measuring velocities in a single plane and operated in conjunction with a computer-controlled nozzle transporter to enable the whole spray to be sampled. The different operating principles of the instruments resulted in different droplet size and velocity distributions in relatively dense, polydispersed sprays. PDA gave consistently lower values for the volume median diameter and higher droplet velocities than PMS. PMS gave lower percentages of spray volume in droplets less than 100 μm in diameter, while the PDA indicated some large droplets that were not detected by PMS. The required sample size was smaller for PMS (3000 droplets) than for PDA (13,000 droplets). Both instruments are useful for measuring the characteristics of agricultural sprays, providing that their limitations are recognized. Appropriately configured, such systems enable droplet size and velocity distributions, entrained air velocities, droplet trajectories and the spatial structure of sprays to be determined.

The effect of some adjuvants on sprays produced by agricultural flat fan nozzles

Six commercially-available agricultural spray adjuvants were mixed with water and sprayed through a medium quality flat fan nozzle to evaluate their effect on the spray formation process and droplet size distributions. Droplet size and velocity distributions were significantly changed. Ethokem, which reduced droplet sizes and LI-700, which increased them, were selected for more detailed study with medium and fine quality flat fan nozzles. The adjuvants significantly affected the variation of droplet size within the spray and the thickness of the spray fan. Photographs showed that the mechanism of spray formation was changed by the addition of LI-700 and that droplets containing Ethokem had air inclusions.

Spray characterization: Numerical prediction of Sauter mean diameter and droplet size distribution

A simplified equation of the Nukiyama-Tanasawa type for droplet size distribution in sprays is obtained from the synergetic concept of entropy information, assuming spherical droplets and zero and infinity as their limit sizes. The introduction of Sauter mean diameter (SMD) definition in that equation yields a new distribution function dependent solely on SMD, which is calculated from available correlations for pressure-jet and pre-filming airblast atomizers. For plain-jet airblast atomizers a new and dimensionally consistent correlation is determined. Several droplet size distributions are then predicted. Experimental data are compared with predictions of SMD; the agreement is satisfactory.

Automated Detection of Fluorescent Spray Deposits with a Computer Vision System

An automated computer vision system was developed to characterize spray deposit features. This system characterized water-soluble fluorescent tracer spray deposits on artificial targets and also on leaf images on which tracer deposit images had been digitally superimposed. In an automated fashion, the system positioned samples, focused the camera, acquired images, and processed and analyzed the images. Components for the computer vision system were selected to discern small and low intensity particles under low level UV light conditions. After an image was acquired, a set of feature parameters related to spray coverage, tracer intensity, droplet shape, and droplet size distribution were extracted by using custom-developed image processing and analysis algorithms. In the performance evaluation of this system, relatively good statistical correlations were found between the known deposit feature parameters and deposit feature measurements extracted from digitally treated leaf surfaces. This acquisition system represents an improvement on previously reported deposit analysis systems because it can view a wide range of feature sizes within the same sampling area. It is also better able to work in conditions of low contrast between water-soluble tracer deposits and a plant leaf background. The system hardware and software could be applied as a means to compare spray quality on plant targets treated with either a water-soluble or water-insoluble tracer.

Evaluation of carbon black slurries as clean burning fuels

Experiments were performed to examine the pumpability, atomization and combustion characteristics of slurries made of mixtures of carbon black with No. 2 fuel oil and methanol. Carbon black-No. 2 fuel oil and carbon black-methanol slurries, with carbon black contents of up to 50 and 45 wt%, respectively, were pumped and atomized by means of a peristaltic pump and air atomizing scheme, and burned in an 82 kW laboratory combustor. Measurements of slurry spray droplet size distributions indicated mean droplet diameters of approximately 100 and 30 μm for the carbon black-No. 2 fuel oil and carbon black-methanol mixtures, respectively. Particulate emissions from the combustion of slurries containing 47 wt% carbon black in No. 2 fuel oil and 42 wt% carbon black in methanol were approximately 40 and 28 mg dm −3, respectively. These paniculate emissions are significantly higher than corresponding emissions from ‘base case’ No. 2 fuel oil and methanol tests (0.75 and 0 mg dm −3, respectively). However, in spite of the increased particulate emissions, carbon monoxide emissions from all tests were similar (less than 50 ppm dry, corrected to 0% oxygen, for furnace stoichiometric ratios of 1.05 or greater). In addition, at 20% excess air, nitric oxide emissions from the combustion of the carbon black-No. 2 fuel oil and carbon black-methanol (approximately 50 and 15 ppm, respectively) were approximately half of those measured from the combustion of No. 2 fuel oil and methanol (105 and 30 ppm, respectively). Although not examined here, the use of dispersants, stabilizers and modifications to the atomization equipment could improve the burning characteristics of carbon black slurries.

The characteristics of the shore-line sea spray aerosol and the landward transfer of radionuclides discharged to coastal sea water

The droplet size distribution and mass of sea spray blown onto the Cumbrian coast have been measured on a beach at a wide range of wind speeds. The results indicate that the aerosol generated immediately above the surf zone has a size spectrum (plotted as dN/d log r) that peaks between 12 and 17 μm diameter and shows little variation within the wind speed range 5–10 m s−1. The observed peak diameter is around the upper limit of the range of peak sizes observed in oceanic aerosol spectra. The concentration of airborne sea salt was substantially greater in on-shore winds over the beach than has been observed over the open ocean, and there was some evidence that it increased more rapidly with wind speed. The Al to Na ratio, a measure of the concentration of fine sediment suspended within the droplets of sea spray, appears to increase with increasing wind speed. This finding has implications for assessing the transfer of particulate-associated pollutants such as plutonium, bacteria and some heavy metals.

On the Prediction of Droplet Size and Velocity distributions in sprays through maximum entropy principle

AbstractBased on the conservation laws of mass, momentum and energy, the droplet size and velocity distributions in liquid sprays are formulated through the maximum entropy principle, without taking into account the details of liquid atomization processes such as the characteristics of disturbances and behavior of liquid bulk preceding the droplet formation. The joint probability distribution function in size and velocity of the droplets can be predicted from the atomization conditions which are represented by the Weber number and the source terms in the formulation. The model prediction agrees well with the experimental data measured by a phase Doppler particle analyzer.