Two‐Phase Flow Characterization by Automated Digital Image Analysis. Part 2: Application of PDIA for Sizing Sprays

A series of experiments were conducted in order to assess the robustness and accuracy of a recently developed digital image analysis technique (PDIA). This paper investigates the application of the PDIA technique to the sizing of relatively small fuel droplets of diameters in the range 5 to 30 μm produced by a pressure‐swirl atomizer. The measurement performance of the PDIA system has been assessed in terms of individual object diameters and also number and volume probability density functions of diameter in comparison to phase Doppler anemometry (PDA) data obtained under identical conditions. PDIA measurements revealed good agreement with spray data obtained by PDA at a measurement location 36 diameters downstream from the nozzle orifice with differences in the arithmetic mean diameter, D₁₀ and volume mean diameter, D₃₀ of approximately 5 and 3% respectively. The PDIA technique was shown to detect the presence of very large, predominantly non‐spherical droplets whose diameters were in excess of 100 μm. These droplets, although few in number constitute a significant proportion of the total spray volume and would have otherwise been either erroneously measured or have passed through the probe volume undetected using PDA due to non‐sphericity. Smaller objects may also be measured correctly by both methods although sensitivity to signal‐to‐noise ratio, for both methods can generate spurious and contradictory errors.     

Simultaneous Droplet Size and Gas-Phase Turbulence Measurements in a Spray Flow using Phase-Doppler Interferometry

A recently developed method to increase the sensitivity of a phase-Doppler interferometer (PDI) has been applied to an annular air flow interacting with a spray under isothermal conditions in order to demonstrate the applicability of the method to obtaining velocity and turbulence characteristics of the gas-phase in a two-phase flow. A conventional PDI system operated in the first order refraction mode cannot be configured to efficiently detect small seed particles in the presence of large droplets due to the limited dynamic range of the photodetectors. It is therefore difficult or impossible to obtain seed particle detection rates sufficient for turbulence measurements. Doping the spray with a very small quantity of dye preferentially attenuates the light intensity scattered by large droplets, thus allowing the PDI system to detect small seed particles without damaging the photodetectors when large particles are present in the flow. Application of this method to a two-phase flow produced by a pressure-swirl atomizer coaxially located within an annular air jet has resulted in profiles of axial mean and fluctuating velocities and shear stresses as well as integral time scales and turbulent energy spectra.     

Sizing Polydisperse Dispersions by Focused Beam Reflectance and Small Angle Static Light Scattering

In this work, two different methods for particle characterization, namely focused beam reflectance and small angle static light scattering, are quantitatively compared. The results are presented in the form of moment ratios of the particle size distribution, i.e., the number weighted diameter, D_1/0, and the volume weighted diameter, D_4/3, for a broad range of particle size distributions ranging from 20 to 400 μm. Various aqueous dispersions including narrow, broad, and bimodal particle size distributions of spherical shaped ceramic beads were used in the comparison. It was found that the moment ratios obtained by focused beam reflectance measurements and small angle static light scattering correlate well, in the case of spherical particles. Furthermore, it was found that the D_1/0 values obtained by focused beam reflectance measurements are more sensitive to the presence of a small fraction of fine particles in a bimodal distribution than those obtained by small angle static light scattering.     

Permeability of mixed soft and hard granular material: Hydrogels as drainage modifiers

We measure the flow of water through mixed packings of glass spheres and soft swellable hydrogel grains, at constant sample volume. Permeability values are obtained at constant sample volume and at porosities smaller than random close packing, for different glass bead diameters D and for variable gel grain diameter d, as controlled by the salinity of the water. The gel content is also varied. We find that the permeability decays exponentially in n(D/d ) ^b , where n = N _gel/N _glass is the gel to glass bead number ratio and b is approximately 3. Therefore, flow properties are determined by the volume fraction of gel beads. A simple model based on the porosity of overlapping spheres is used to account for these observations.     

Planar Droplet Sizing for the Characterization of Droplet Clusters in an Industrial Gun‐Type Burner

An important problem in spray combustion deals with the existence of dense regions of droplets, called clusters. To understand their formation mechanism, the droplet dynamics and fuel concentration profile are investigated by means of planar laser techniques in an industrial gun‐type burner. The simultaneous measurement of elastic Mie scattering and Laser Induced Fluorescence (LIF) allows the instantaneous measurement of the Sauter Mean Diameter (SMD), after proper calibration. Using two different CCDs to get the two signals requires a detailed calibration of the CCD response before getting absolute diameters. Pixels are binned 6 by 6 to obtain the final SMD map, this is a compromise between spatial accuracy and noise. Velocity field is measured on both sets of images using standard Particle Image Velocimetry (PIV) algorithms. The comparison of cross‐correlation technique with PDA results shows that the velocity measured on the LIF images are close to the velocity based on D₃₀, whereas the Mie scattering results are similar to D₂₀. On Mie scattering images, regions of high interfacial area forming clusters can be detected. A special tracking scheme is used to characterize their dynamics in terms of velocity and diameters by ensuring that the same volume of fluid is tracked. It is shown that the clusters have a velocity similar to the velocity of droplets with the same diameter as the mean SMD of the cluster. It is also shown that an increase of pressure tends to trigger the appearance of such a group of droplets, due to a smaller diameter of the droplets caused by the increase of pressure discharge. Uncertainties for the different techniques used are discussed.     

Large eddy simulations of partially premixed ethanol dilute spray flames using the flamelet generated manifold model

The paper presents Large Eddy Simulations (LESs) for the Sydney ethanol piloted turbulent dilute spray flames ETF2, ETF6, and ETF7. The Flamelet Generated Manifold (FGM) approach is employed to predict mixing and burning of the evaporating fuel droplets. A methodology to match the experimental inflow spray profiles is presented. The spray statistical time-averaged results show reasonable agreement with mean and RMS data. The Particle Size Distribution (PSD) shows a good match downstream of the nozzle exit and up to x/D = 10. At x/D = 20 and 30 the PSD is under-predicted for droplets with mean diameter D10 > 20μm and over-predicted for the smaller size droplets. The simulations reasonably predict the reported mean flame structure and length. The effect of increasing the carrier velocity (ETF2–ETF7) or decreasing the liquid fuel injection mass flow rate (ETF2–ETF6) is found to result in a leaner, shorter flame and stronger spray–flow interactions. Higher tendency to local extinction is observed for ETF7 which is closer to blow-off compared to ETF2 and has higher scalar dissipation rates, higher range of Stokes number, and faster droplet response. The possible sources of LES-FGM deviations from the measurements are discussed and highlighted. In particular, the spray time-averaged statistical error contribution is quantified and the impact of the inflow uncertainty is studied. Sensitivity analysis to the pre-vaporized nozzle fuel mass fraction show that such small inflow perturbations (by ±?2% for the ETF2 flame) have a strong impact on the flame structure, and the droplets’ dynamics. Conditional scatter plots show that the flame exhibits wide range of mixing conditions and bimodal mixing lines particularly at upstream locations (x/D?相似文献   

Size of the Detection Area of a Phase-Doppler Anemometer for reflecting and refracting particles

Measurements of particle size distributions in multi-phase flows with a phase-Doppler anemometer yield incorrect results if polydisperse particles are investigated. For weighting biased size distributions, different in situ methods, requiring the size of the detection area, are known, but all of these weighting procedures are restricted to very small measuring volumes if off-axis instrument configurations are considered. Moreover, the weighting functions have some disadvantages in the case of poor statistics in single size classes or the results are not suitable for determining the size of the detection area for particles which are larger than the beam waist. Therefore, the intention in this work was to measure the size of the detection area for different kinds of monodisperse particles, different instrument configurations and varied instrument sensitivities experimentally and to develop an improved weighting procedure that copes with the above difficulties. The application of the results obtained from the investigations with monodisperse particles to measured particle size distributions and volume flux densities of polydisperse water droplets in a spray cone of an atomizer confirms the applicability of this weighting procedure. It is still restricted to directed flows, perpendicular to the fringes.     

On Different Approaches to Estimate the Mass Fractal Dimension of Coal Aggregates

Several methods to measure the structures of coal aggregates are compared. Loose and compact coal aggregates were generated through flocculation of ultrafine coal particles (mean volume diameter of 12 μm) under specific shearing conditions. Aggregate structure in terms of mass fractal dimension, D_f, was determined using various methods; namely 2D and 3D image analysis, interpretation of intensity patterns from small angle light scattering, changes in aggregation state through light obscuration, and settling behavior. In this study, the measured values of D_f ranged from 1.84–2.19 for coal aggregates with more open structures, and around 2.27–2.66 for the compact ones. All of these approaches could distinguish structural differences between aggregates, albeit with variation in D_f values estimated by the different techniques. The discrepancy in the absolute values for fractal dimension is due to the different physical properties measured by each approach, depending on the assumptions used to infer D_f from measurable parameters. In addition, image analysis and settling techniques are based on the examination of individual aggregates, such that a large number of data points are required to yield statistically representative estimations. Light scattering and obscuration measure the aggregates collectively to give average D_f values of the particulate systems; consequently ignoring any structural variation between the aggregates, and leaving possible small contaminations undetected (e.g. by dust particles or air bubbles). Appropriate utilization of a particular method is thus largely determined by system properties and required data quality.     

一种新型喷嘴的提出及流量特性的研究

本文在对各种气动喷嘴及其雾化机理分析基础上提出了一种新型的气动雾化喷嘴-"旋转型气-液雾化喷嘴"。在此喷嘴中,油与气分别从不同的槽道切向进入混合室,且油与气一一对应,油与气互相混合、旋转后从喷口喷出。其气液比在热态实验时为4%-6%(用压缩空气雾化),雾化状态良好。本文中对其流量系数及雾化角进行了系统的研究。主要考虑了喷嘴的结构参数,气液比(ALR),液体粘度等因素对流量系数的影响。通过实验测量与拟合,最后得到了喷嘴的流量系数和雾化角的表达式,可以用来指导喷嘴的设计。     

Microflow reactor synthesis of palladium nanoparticles stabilized with poly(benzyl ether) dendron ligands

A simple glass capillary microflow reactor system has been applied for the synthesis of palladium nanoparticles by thermal decomposition of palladium acetate (Pd(OAc)₂) in diphenyl ether in the presence of poly(benzyl ether) dendron ligands (PBED Gn-NH₂, n = 1–3) as a stabilizer. Effect of hydrodynamic parameters (capillary diameter, linear flow rate, volume flow rate, and reaction temperature) and concentrations (precursor and stabilizer) on the particle size was investigated. The particle size can be controlled by varying linear flow rate and temperature as well as ligand/precursor concentration ratio. Volume flow rate does not affect the particle size when the linear flow rate is held constant for different capillary diameters (150–320 μm). Unlike batch systems, in this microreactor system, smaller particles are produced at low ligand concentrations when the molar ratio of the ligand to metal precursor ranged from 1 to 5. As another characteristic of the microreactor synthesis, the concentration of the Pd precursor can be increased (up to 27 mM) with maintaining a constant particle size (3.1 ± 0.2 nm) and a good monodispersity, while in the batch system a significant increase and broadening in the particle size are observed with increasing precursor concentration.     

On the microstructure of electrodeposited Fe−P alloys investigated with SAXS

Small angle X-ray scattering of Fe_1–x P _x , 0.05<x<0.25 suggests the existence of some volume percent of small domains with an electron density different from that of the matrix. The experimental diagrams can be described assuming three kinds of scatterers with different diameters not only in amorphous but also in microcrystalline material or in material including both phases. The diameters of the scatterers of one class are independent of composition and the microstructural state. The effect of annealing on a suitably defined inhomogeneity parameter indicates a different nature of the three classes of scattering centers. More information in this respect results from additional measurements of other physical properties.     

New types of phase behaviour in binary mixtures of hard rod-like particles

The phase behaviour of binary mixtures of hard rod-like particles has been studied using Parsons—Lee theory (Parsons, J. D., 1979, Phys. Rev. A, 19, 1225); Lee, S. D., 1987, J. Chem. Phys., 87, 4972). The stability of the isotropic-nematic (I-N) transition with respect to isotropic—isotropic (I-I), and nematic—nematic (N-N) demixing is investigated. The individual components in the mixtures are modelled as hard cylinders of diameters D_i and lengths L_i (i = 1,2). The aspect ratios k_i = L_i/D_i of the components are kept fixed (with values of k ₁ = 15 and k ₂ = 150), and the phase behaviour of the mixtures is studied for varying diameter ratios d = D ₁/D ₂. When the diameter ratio is relatively large, e.g., for values of d = 50, component 1 may be considered a large colloidal particle, while the second component plays the role of a weakly interacting solvent. This mixture exhibits only an I-N phase transition which is driven by the excluded volume interaction between the large particles (no I-I or N-N demixing is seen). A decrease in the diameter ratio enhances the contribution of the smaller component to the free energy (especially in terms of the unlike excluded volume term), and I-I as well as N-N demixing transitions are observed. The character of the N-N transition is rather unusual, a single region bounded by a lower critical point (in the pressure—composition plane) is seen for a diameter ratio of d = 3.2, while two demixed nematic regions bounded by lower and upper critical points are observed for d = 3.13. A further decrease in the diameter ratio (e.g., to d = 3) leads to systems with a phase behaviour in which the two demixed N-N regions meet, giving rise to a large demixed region with very strong fractionation in composition, and no N-N critical points. The I-I demixing transition is always accompanied by a lower critical point and occurs for systems with intermediate size (diameter) ratios. A diameter ratio of d = 4.5 corresponds to systems with significant like and unlike excluded volume interactions, and in this case the I-N transition takes place over the whole composition range with weak fractionation and one azeotropic point. Surprisingly, the coexisting nematic phase is of lower packing fraction than the isotropic phase for some of the compositions, i.e., an inversion of packing fraction takes place. In addition to this, the longer rods can be less ordered that the shorter rods for certain values of the composition.     

Dispersion dipole between rare-gas atoms

Accurate calculations of the dispersion dipole coefficient D ₇ and its components are described for the systems HeH and HeHe, and used to check an approximate formula for D ₇ developed in a previous paper [1]. By means of a further approximation for a new atomic property, values of D ₇ are obtained for the rare-gas diatoms, based on semi-empirical Van der Waals C ₆ coefficients, Hartree-Fock expectation values and sum-rule values for the atoms. The dispersion contributions to the dipole are compared with the Hartree-Fock contributions [35], and at the conventional collision diameters are found to be of comparable magnitude but usually of opposite sign.     

Mean Particle Diameters. Part IV: Empirical Selection of the Proper Type of Mean Particle Diameter Describing a Product or Material Property

Mean particle diameters may be used to describe and to model physical, chemical or physiological properties of products or materials containing dispersed phases. An empirical method was developed to select the proper type of mean diameter from experimental data, if this mean diameter is not known a priori from theoretical reasoning. The present method uses mean diameters, D?_{p, q}, defined according to the Moment‐Ratio (M‐R) definition system. They are expressed as the 1/(p‐q)‐th power of the ratio of the p‐th and the q‐th raw moment of the number density distribution of the particle sizes. After calculation of the mean diameters, D?_{p, q}, the relationships between the product property and these mean diameters are investigated statistically. The selection method has been illustrated by four examples, three of which stem from a high shear granulation experiment in the field of detergent processing. The fourth example is concerned with a visual ranking of bubble size distributions of chocolate mousse samples. The data set of each example consists of a set of particle size distributions and the corresponding physical product properties that are influenced by the particle sizes. Hypotheses are formulated to explain the types of selected mean diameters. Application of the selection method gives mean diameters, D?_{p, q}, a clear physical look and identity, replacing their anonymity. Sharing worldwide results of applications of the newly developed selection method, will lead to a build‐up of knowledge of physical meanings and application areas for the types of the mean particle diameters. This will support decision‐making in product development. The examples used to develop the selection method clearly demonstrate the physical relevance of the previously developed nomenclature system for mean particle diameters, D?_{p, q}.     

Deriving the mean primary-particle diameter and related quantities from the size distribution and the gravimetric mass of spark generated nanoparticles

Spark generated carbon and iridium nanoparticles were characterised by their electrical-mobility diameter D and by the mass of particulate matter collected in parallel on filter. The particles exhibited slightly skewed lognormal size distributions with mean mobility diameters between 18 and 74 nm. The masses calculated from the measured distributions under the assumption that the particles were spherical (diameter D) and of bulk mass density turned out to be much higher than the gravimetric mass, by factors between 8 and as high as 340. This very pronounced difference initiated a search for an improved relation between particle size and mass. Data analysis suggested that the mass increases linearly with increasing D. Hence the measured distributions were evaluated under the assumption that the spark generated matter was composed of spherical primary nanoparticles of mean diameter d, aggregated in the form of chains of joint length βD, with β>1. Using reasonable values of β between 2 and 4, the mean diameter of carbon primary particles turned out to be 10±1.8 nm, in excellent agreement with size data recently obtained by transmission electron microscopy (TEM). The primary iridium particles were found to be distinctly smaller, with diameters between 3.5±0.6 nm and 5.4±0.9 nm. The comparatively small uncertainty is due to the fact that the primary-particle diameter is proportional to the square root of β. The calculated volume specific surface areas range between 500 and 1700 m²/cm³. These numbers are close to the ‘active’ surface areas previously measured by the BET method. The good agreement with TEM and BET data suggests that the novel approach of nanoparticle characterisation is meaningful. Accordingly, the number concentrations of all individual primary particles rather than the concentrations measured by the mobility analyser should be␣considered the correct dose metric in studies on animal exposure to spark generated nanoparticles. The␣evaluated data imply that the numbers quoted in the literature must be enlarged by factors ranging between about 10 and a maximum as high as 80. An erratum to this article can be found at     

Crystal Shape Characterisation of Dry Samples using Microscopic and Dynamic Image Analysis

A standard method to determine particle shape and size is by image analysis. This paper addresses microscopic image analysis (semi‐automated) investigations of two different organic crystalline chemicals generated by batch cooling crystallisation. The results generated from microscopic image analysis were compared with data obtained by dynamic image analysis (automated) because very few contributions are available in the open literature. The chemical systems were polymorphic L‐glutamic acid which crystallises into α (prismatic) or β (needle) form and the non‐polymorphic mono sodium glutamate which crystallises into needles. The images from these techniques were processed to generate information on crystal shape and size. It has been observed that shape effects can distort the size obtained in size characterization studies. In this study, comparisons were made of processing time, number of crystals and accuracy between microscopic and dynamic image analysis. For representative microscopic image analysis, 5000 crystals were analysed in an average of eight hours while several hundred thousand crystals were processed using dynamic image analysis within 15 minutes. Using the parameters D₁₀, D₅₀, D₉₀, span and aspect ratio for statistical comparison, it was found that the results obtained for D₅₀ by the two techniques were comparable and in accordance with other measurements (laser diffraction spectroscopy and ultrasonic attenuation spectroscopy) even though these non‐spherical particles had different orientations during measurement by the two methods. However, substantial differences in span of the distribution and aspect ratio were returned by the two techniques.     

X‐ray absorption measurements on an ultrasonic spray aerosol

Spray deposition of thin films and coatings is a widely used manufacturing process owing to its low cost, versatility and simple implementation. The objective of the presented experiments was to investigate whether X‐ray absorption measurements on solutes carried by aerosols are possible, and what count rates can be achieved depending on solution flow through and the resulting mass density in the interrogation volume. The investigated prototypical spray aerosol was InCl₃ dissolved in water or ethanol dispersed via an ultrasonic nebulizer. InCl₃ spray is essential for the ion layer gas reaction process used for the deposition of In₂S₃ buffer layers for highly efficient chalcopyrite solar cells. The discussed experiments demonstrate that measurements are possible, but that the achievement of good signal‐to‐noise ratios requires extended sampling times and concentrated solutions.     

Properties of NiO thin films deposited by intermittent spray pyrolysis process

NiO thin films have been grown on glass substrates by intermittent spray pyrolysis deposition of NiCl₂·6H₂O diluted in distilled water, using a simple “perfume atomizer”. The effect of the solution molarity on their properties was studied and compared to those of NiO thin films deposited with a classical spray system. It is shown that NiO thin films crystallized in the NiO structure are achieved after deposition. Whatever the precursor molarity, the grain size is around 25-30 nm. The crystallites are preferentially oriented along the (1 1 1) direction. All the films are p-type. However, the thickness and the conductivity of the NiO films depend on the precursor contraction. By comparison with the properties of films deposited by classical spray technique, it is shown that the critical precursor concentration, which induces strong thin films properties perturbations, is higher when a perfume atomizer is used. This broader stability domain can be attributed to better chlorides decomposition during the rest time used in the perfume atomizer technique.