Local Droplet Research Articles

Electrical and chemical consequences of point discharges in a forest during a mist and a thunderstorm

The first part of this paper is devoted to the electrical characteristics of the air below the canopy of a pine and spruce forest. In fair weather conditions, the site influence, i.e. the filtering effect of the trees on air conductivity and electric field, is evidenced. Under disturbed weather conditions, the meteorological influence is depicted to show: (i) that electric fields sufficiently high to produce `point discharges' occur not only during thunderstorms but also during mist; (ii) that, by taking into account the gaseous ions produced by the point discharges, it becomes possible to understand on the one hand the field divergence with height observed in the mist, and on the other hand the detection of alternatively positive and negative gaseous clouds of charges during the thunderstorm. The second part presents the results of chemical analysis performed below and just above the canopy on the same site and for the same period. Increased ozone and hydrogen peroxide concentrations were measured during the thunderstorm. It is shown that these chemical species were not only locally produced by photochemistry and/or transported from different (industrial and traffic) sources, but also arose from transient electrical point discharges in the forest under high electric field conditions. Their local concentration is shown to be influenced by the electrical discharge current density on the one hand, and by the local conditions of atmospheric stability and of water content, determining the evolution of the chemical products, on the other hand. Actually, after their production, the initially gaseous chemical products were shown to be involved in the local droplet chemistry. Under specific weather conditions, ionic densities as well as chemical by-products of a forest therefore depend both on the electrodynamical characteristics of the lower atmosphere and on the local environmental conditions (liquid water content of the air and atmospheric stability) associated with the two different situations investigated, a mist and a thunderstorm.

Holographic Investigations of a Diesel Jet injected into a high‐pressure test chamber

AbstractA Diesel spray injected into a high‐pressure test chamber was investigated with two different holographic techniques. The usual off‐axis recording geometry was used to investigate the very early beginning of the injection. Double pulsing of the recording laser facilitated velocity measurements. By using two different reference angles for the two recording the two images can be separated upon reconstruction of the holograms. Thus superior image quality could be achieved. Processes leading to jet formation could be identified: the spray tip is periodically axially decelerated and radially accelerated. Consequently, liquid moves at the tip off axis. The spray consists of fast central region surrounded by a slower outer jet region. The outer jet region exhibits periodic droplet concentration fluctuations which are interpreted as a result of the periodic processes at the spray tip. Some high concentration regions in the outer jet region were investigated using a dark‐field holographic technique. This technique permits single droplet velocity measurements and local droplet concentration determination at extremely high droplet concentrations. The most important result of these investigations is that no correlation between the local droplet concentration and the droplet velocity could be found.

Sprays in radially spreading shear‐layer flows

Two radially spreading adjacent streams, which differ in their radial velocities and thus form a radially spreading shear‐layer flow, are considered here. The theory presented by the authors for sprays suspended in unidirectional [Katoshevski and Tambour, Phys. Fluids A 5, 3085 (1993)] shear layers is extended here for radially spreading shear layer flows. The behavior of a multisize (polydisperse) evaporating spray, which is suspended in one of the streams, is studied. The spray spreads in the lateral direction towards the other coflowing stream, resulting in lateral changes in spray densities and in local droplet size distributions across the shear layer. These effects are analyzed here via similarity solutions of the governing equations. A comparison between the behavior of the multisize sprays and their vapors in radially spreading versus unidirectional shear‐layer flows is also presented and discussed here. The dynamics of the radially spreading spray is essentially different from that of the unidirectional spray. In the radial case, streamlines of the host‐gas flow become more crowded with radial distance, and thus, it is shown here how lateral evolution in size histograms and lateral Sauter mean diameter (SMD) profiles are affected by this feature of the radially spreading flow. The effects of initial drop‐size histograms on lateral distributions of: droplet SMD, overall spray densities, and vapor are also studied here for three basic initial drop‐size distributions: monodisperse, bimodal, and polydisperse. It is shown how the behavior exhibited by polydisperse (and bimodal) sprays differs intrinsically from the behavior of monodisperse sprays. For example, for sprays which are initially monodisperse the lateral profile of the spray’s SMD across the shear layer always decreases, whereas for polydisperse or bimodal sprays it may increase or assume an ‘‘S’’ shaped curve.

Fractal Analyses of High-Resolution Cloud Droplet Measurements

Abstract Fractal analyses of individual cloud droplet distributions using aircraft measurements along one-dimensional horizontal cross sections through clouds are performed. Box counting and cluster analyses are used to determine spatial scales of inhomogeneity of cloud droplet spacing. These analyses reveal that droplet spatial distributions do not exhibit a fractal behavior. A high variability in local droplet concentration in cloud volumes undergoing mixing was found. In these regions, thin filaments of cloudy air with droplet concentration close to those observed in cloud cores were found. Results suggest that these filaments may be anisotropic. Additional box counting analyses performed for various classes of cloud droplet diameters indicate that large and small droplets are similarly distributed, except for the larger characteristic spacing of large droplets. A cloud-clear air interface defined by a certain threshold of total droplet count (TDC) was investigated. There are indications that this inte...

APPLICATION OF PHASE DOPPLER ANEMOMETRY IN PAINT SPRAYS

For the first time, simultaneous measurements of droplet size and velocity in paint sprays using a phase Doppler anemometer are presented. In agreement with the results of Mie calculations of the scattered light field, first-order refracted light could be used successfully for phase Doppler measurements of two different water-based paints, a laquer and a silver metallic paint. Preliminary measurements, including a comparison with results from digital image processing of stroboscopic light pictures of paint droplets, were performed to demonstrate the accuracy and repeatability of the measurements. Furthermore, comparative measurements in original paint spray guns using an optically homogenous water-sucrose-alcohol solution with similar viscosity and surface tension as the specific paints used herein have shown almost identical droplet size distributions. Extended measurements in a spray booth were performed, changing a variety of parameters such as gun type, air cap, and paint and airflow rates. Profiles of mean droplet velocity and droplet size were taken at three different distances, 5, 15, and 30 cm downstream from the nozzle. The results have shown some of the expected dependencies of the local droplet velocity and size distributions on global parameters, e.g., an increase of the mean sizes with decreasing airflow.

Dynamics and transport of a localized soluble surfactant on a thin film

The flow induced by a localized droplet of soluble surfactant on the surface of a thin film is analysed, motivated by an interest in the interaction between inhaled droplets and the lung's thin liquid lining after an aerosol lands on its surface. The spreading is driven by gradients of surface tension and results in flow of the droplet and underlying liquid film. This induced flow field plays an important role in the transport of dissolved species from the droplet, through the film, and to the tissue for absorption.Evolution equations for the film thickness, surface and bulk liquid concentrations are derived using lubrication theory, since the depth of the film is much smaller than the characteristic radius of the droplet. Solutions are obtained numerically using the method of lines for a variety of surface Péclet numbers.We find that the effect of solubility is to decrease both film disturbances and surface concentrations, and to induce an absorption-driven backflow. In addition, there is a gravity-driven backflow from hydrostatics. At large surface Péclet numbers, large film disturbances are obtained and more surfactant is able to diffuse across the rigid permeable wall, while surface diffusion causes more rapid spreading and decreases film disturbances. Gravity acts as a restoring force by creating a bidirectional flow, and hence disenhances the vertical flux of surfactant across the air-liquid interface. This model may have implications for the delivery of drugs by aerosol inhalation.

Local drop size variation in an agitated vessel

AbstractThe local droplet diameters were measured at two positions inside the circulation region of a stirred tank. The Sauter mean diameter near the top of the vessel was always 1.4 times greater than the one just outside the impeller region for rotational speeds of 250, 300, 350 and 400 rpm and for hold‐up fractions of 0.01, 0.025 and 0.05. The drop size distributions at a hold‐up fraction 0.01 had one mode for all impeller speeds studied and at both positions. At higher hold‐up fractions the distributions usually were bimodal.

Significance of Local Group Combustion Number Introduced.

For sprays of practical interest, the region where vigorous vaporization occurs is confined to a thin layer on their outer edges. The cubical array of droplets occupying a semi-infinite space provides a microscopic prototype to investigate this vaporization layer structure, upon which interactive droplet vaporization/combustion characteristics are examined to derive rational boundary conditions to be imposed at the spray edge when the spray is regarded as a continuum medium on large scales. It is found that not only the local droplet number density but also the gas-to-liquid density ratio plays an important role in excitation of prevailing vaporization/combustion modes, leading to their criteria in terms of the droplet separation distance as a function of the gas-to-liquid density ratio.

ONSET OF DRY-WALL HEAT TRANSFER IN LOW-MASS-FLUX SPRAY COOLING

Abstract An experimental investigation was performed to study a low-mass-flux, spray nucleate boiling phenomenon termed dry wall during which all the impinging liquid is immediately vaporized upon contact with the heated surface. Measurements of wall heat flux and spray coolant mass flux were performed together with a characterisation of spray parameters (such as local droplet size and velocity), using a laser-based diagnostic technique. Two different atomizing nozzles were used and the effect of liquid subcooling on the transition was also studied. The transition to the dry-wail heat transfer regime was found to correlate well with the average global concentration of the spray obtained by the ratio of the spray mass flux to the average spray velocity. The experimental results showed that above a concentration of approximately 0.20 kg/m3, no evidence was seen of transition to dry wall. This concentration corresponding to transition was found to be independent of the two different nozzle types used in the study.

Primary afferents evoke excitatory amino acid receptor-mediated EPSPs that are modulated by presynaptic GABAB receptors in lamprey

1. The primary afferent neurons (dorsal cells) are of two types in lamprey, which are fast (touch) and slowly adapting (pressure), respectively. Intracellular stimulation of such sensory neurons evokes mono- and polysynaptic excitatory postsynaptic potentials (EPSPs) in spinobulbar neurons (giant interneurons) and in unidentified interneurons. Paired intracellular recordings between identified sensory cells and spinobulbar neurons made it possible to study the synaptic transmission in detail. It is shown that both touch and pressure primary afferents utilize excitatory amino acid (EAA) transmission and, furthermore, that these effects are subject to a presynaptic GABAB receptor modulation. 2. The monosynaptic mixed electrical and chemical EPSPs in giant interneurons had a mean peak amplitude of 3.2 +/- 1.3 (SD) mV, a time to peak of 4.7 +/- 1.2 ms, and a duration at one-half peak amplitude of 9.4 +/- 3.2 ms. Corresponding results were obtained with dorsal root or dorsal column stimulation. Seventy percent of the fast-adapting dorsal cells of the "touch" type evoked monosynaptic mixed EPSPs in giant interneurons, whereas only 3% of the slowly adapting "pressure" dorsal cells did. 3. The chemical part of the monosynaptic EPSPs evoked in giant interneurons was, in all cases tested, blocked by application of EAA antagonists, like the nonselective antagonist kynurenic acid (KYAC; 2 mM). The selective kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5 microM) had a similar effect, whereas the selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-aminophosphono-5-valeric acid (AP-5; 200-400 microM) did not change the EPSP, even in the absence of magnesium ions. 4. The monosynaptic excitatory synaptic transmission was modulated by application of the selective GABAB receptor agonist L-baclofen (5-10 mM local droplet application or 100-1,000 microM bath applied) or by gamma-aminobutyric acid (GABA; 100-1,000 microM), also when GABAA receptor-evoked effects were blocked by bicuculline (10 microM). L-baclofen or GABA in combination with bicuculline did not evoke any effects in the postsynaptic neuron on membrane potential, input resistance, or spike threshold. Therefore the effects of the GABAB receptor activation most likely occurs at the presynaptic afferent level. 5. In conclusion, the monosynaptic excitation from skin mechanoreceptors evoked in spinobulbar neurons is mediated by EAA receptors of the kainate/AMPA type. GABAB receptor activation causes a depression of this EPSP, most likely because of a presynaptic action. GABA interneurons are known to form close appositions on sensory axons in the lamprey.

APPLICATION OF LASER DIAGNOSTIC TECHNIQUES FOR THE EXAMINATION OF LIQUID FUEL SPRAY STRUCTURE

Results are presented on the spatial distribution of droplet mean size and number density obtained from a hollow-cone kerosene spray, introduced into nonswirling and swirling flow fields. An ensemble light scattering technique, based on measurement of the polarization ratio, has been employed to determine local droplet characteristics in both dense and dilute regions of the spray. The measurements are complemented with Lorenz-Mie calculations of the scattering characteristics for a polydispersion of droplets; the calculations were carried out for different mean sizes and refractive indices. The results reveal that the degree of swirl imparted to the surrounding air flow has a strong influence on spray structure. For all conditions examined the droplet mean size is found lo be larger on the spray boundary than towards the centerline. Droplet mean size is also found to increase with axial distance at all radial positions of the spray; this trend is attributed to the vaporization of smaller droplets and/or p...

Local droplet diameter variation in a stirred tank

AbstractThe local variation of droplet diameter in a stirred tank was measured in the vicinity of the impeller and at another region. The degree of difference in droplet diameter between regions increases with the impeller speed.A correlation equation between the local difference in droplet diameter and the frequencies of coalescence and circulation of the droplet was derived according to the circulation interaction model. The degree of local difference in droplet diameter was found to be controlled strongly by the ratio of coalescence to circulation frequency.

Mixed Convective Droplet Combustion with Internal Circulation

Abstract The combustion of a fuel droplet in a mixed convective environment once the initial transient heating of the droplet has been completed is analyzed considering the internal motion of the liquid. The coupled gas and liquid phase analyses are solved simultaneously by a series expansion approach in terms of the polar angle. A previously developed mixed convection formulation is used to provide solutions ranging from the forced to the free convective limit. Detailed results are presented for the forced convective combustion of a hexadecane droplet in a standard atmosphere. The pure evaporation of the hexadecane droplet is presented as a particular case of the analysis. The effect on the local mass burning and droplet regression rates of the liquid motion and mixed flow intensities is presented in terms of the parameters that represent the respective processes. It is shown that the internal circulation of the liquid has only a moderate influence in enhancing the evaporation rate of the droplet. A sing...

An Investigation into Laser Diagnostic Technique of Liquid Droplets : Part 1 Measurements in high temperature streams

A novel laser diffraction technique is applied to an investigation of spray vaporization phenomena in hot gas stream. Measurements are made of local droplets size and their distribution in volume element within a quasi two dimentional spray generated from Fan Spray nozzle. The results indicated that the uniformity of the size distribution is improved under the vaporizing conditions in hot down gas stream. The presented technique provides information and useful data on spray vaporizing processes and it will be useful in studying the spray vaporization model and combustion model.

Transit timing velocimetry (TTV) for two-phase reacting flows

A simple single-beam transit timing velocimetry (TTV) method has been developed for determining particle size and mean speed measurements. The method uses a single-beam light scattering system with the addition of two commercial nuclear instrumentation modules to obtain timing information from the trailing edge of the scattering pulses. Results of test studies show that the modal value of transit time remained constant over a wide range of particle sizes, even for particles approaching 50% of the total beam width. Applications of the TTV method to combustion systems include studies of liquid-fuel laminar flames, where integrated reaction times, and thus local droplet velocities, are needed in addition to particle size information.

The Effect of Heat Transfer Coefficient, Local Wet Bulb Temperature and Droplet Size Distribution Function on the Thermal Performance of Sprays

Energy balance considerations indicate that the droplet heat transfer coefficient, local wet bulb temperature, and droplet size distribution function are the basic parameters affecting spray system thermal performance. Within the range of available experimental data, results indicate that the Ranz-Marshall correlation gives an agreement to within ±5.0 percent of measured droplet temperatures at the pond surface for a medium wind range of between 2.5 and 5 m/s. The local wet bulb temperature is taken as the arithmetic mean of the initial and final wet bulb temperatures. For wind speeds greater than 3.5 m/s, the local wet bulb can be taken as the ambient. The modified log normal distribution of Mugele and Evans provides the best description of the droplet size distribution. Further, through the introduction of a correction term, the Spray Energy Release (SER) can be deduced from single droplet information.

Photomicrographic analysis of a spray: Determination of droplet size and velocity spectra by means of diamant's rotating mirror microscope

This paper presents a new application of the rotating mirror microscope described by S. W. Diamant for the study of sprays. One predicts the theoretical picture of a spherical droplet moving in the focal plane at a velocity different from that of synchronism with the rotating mirror. This study makes it possible to infer from the photograph not only a local droplet size spectrum but also local droplet velocity distributions.

A Monte Carlo coalescence model for reaction with dispersion in a tubular reactor

Deviations from plug flow are handled by a Monte Carlo model which simulates mixing and segregation effects in a tubular chemical reactor. The model superimposes dispersion, which is simulated by local droplet coalescence, on the plug-flow mean velocity. Both the residence time distribution and the micromixing details are determined by the local coalescence-mixing phenomena. The model is applied with success to the experiments of Vassilatos and Toor. Comparisons are drawn between this and other models. Use of the model is suggested whenever deviations from plug flow are present in conjunction with segregation effects and rapid of two miscible fluids with mixed or unmixed feed. Further use should be possible with coalescence two-phase extraction and reaction systems.

Liquid Atomization in Electrostatic Field

The effect of static electricity on the disintegration of a liquid jet and on the droplet size of a distilled water is investigated by using an injection syringe. The following results are obtained : (1) Static electricity increases the rate of atomization and creates peculiar phenomena such as sinuous, zigzag, winding and wavy flows. (2) At the jet stream velocity of about 1 m/sec, the effect of static electricity on atomization is the most remarkable. (3) In the regions of sinuous and wavy flow, droplets descend in a circular spiral motion. (4) In the regions of zigzag and winding flow, the droplets descend in an elliptic spiral motion. The local mean droplet diameter measured along the major axis is greater than the one along the minor axis. (5) The local mean droplet diameter becomes smaller as the distance from the center becomes greater.