Particle Sizing by Planar-Phase Doppler System

Conventional phase Doppler systems are useful for sizing particles in the order of microns, but sensitive to the Gaussian beam defect which can cause sizing errors. The defect can be significant when a large size is measured. In this paper, we present a new phase Doppler system using a planar optical layout which permits large particles to be measured in a forward scattering scheme without the Gaussian beam errors. The optical system design is discussed by numerical simulation based on the Mie theory.     

Coherent Scattering by Multiple Particles in Phase Doppler Interferometry

A theoretical model has been developed for studying the response of the phase Doppler interferometer when multiple particles are simultaneously present within the measurement probe volume. The developed model incorporates the geometrical optics theory for describing the coherent interaction between the scattered light signals of multiple particles, eachhaving different size, velocity, trajectory, and arrival time. The resulting Doppler signal is processed by a theoretical signal processor which can simulate the performance characteristic of different signal processing schemes that are widely used in phase Doppler interferometry, namely, zero-crossing counter, covariance, autocorrelation and DFT parocessors. The application of the developed model for studying the coherent scattering by two particles has been specifically addressed in this paper. It has been shown that a DFT processor can be used to simultaneously measure the size and velocity of the two particles in most instances. However, for more than two particles, the signal processing scheme becomes more complex because of a quadratic increase in the beat frequency components.     

Trajectory Ambiguities in Phase Doppler Systems: Study of a near forward and a near-backward geometry

Generalized Lorenz-Mie theory for the scattering of arbitrarily shaped beams by spherical particles has been applied to two standard phase Doppler layouts, employing receiving units at 30° and 150° off-axis locations. It is shown that the particle trajectory effects may lead to inaccurate size measurements for the near-forward receiver and may make the near-backward measurements totally misleading when a large particle size range (1:40) needs to be covered. Only limited improvements can be achieved by using two phase-shift signals from a single receiving unit for discrimination. The errors associated with the trajectories are also detrimental to the concentration measurements based on the existing criteria. However, an extended optical system employing two identical receiving units, located symmetrically about the plane of the laser beams, provides a robust solution to the trajectory ambiguity. It can be used to measure correctly the particle size and the particle location in the measuring volume. The difficulties associated with estimating the effective size of the measuring volume as a function of the particle diameter (in order to determine the true size distribution and the particle number density) may also be resolved by employing an extended system. Hence, despite a higher cost, this arrangement is attractive, at least for obtaining some benchmark simultaneous measurements of sizes and velocities in two-phase particulate flows.     

Absorption and Scattering of Light by Highly Concentrated Two‐phase Flows

The spray cone emerging during an extended metal atomization process (called spray forming) has been investigated in order to quantify the influence of highly concentrated multiphase flows on phase‐Doppler‐anemometry (PDA) measurements. Using this non‐intrusive, optical measurement technique not only the local particle size and velocity distributions of the spray can be obtained but also additional information about the mass flux in the multiphase flow. Since standard phase‐Doppler systems can be easily applied to low concentrated particle systems (spherical particles with smooth surfaces and an optical transparent continuous phase taken for granted) the application of this measurement technique to highly concentrated multiphase flows is more complex. Both the laser light propagating from the PDA device to the probe volume and the scattered one going backward to the PDA receiving system are disturbed by passing the highly concentrated multiphase flow. The resulting significant loss in signal quality especially concerns the measurement of the smaller particles of the spray because of their reduced silhouette (in comparison with the bigger ones). Thus, the detection of the smallest particles becomes partially impossible leading to measurement of a distorted diameter distribution of the entire particle collective. In this study the distortions of the measured distributions dependent on the particle number concentration as well as on the path length of the laser light are discussed.     

Modelling study on the three-dimensional neutron depolarisation response of the evolving ferrite particle size distribution during the austenite–ferrite phase transformation in steels

The magnetic configuration of a ferromagnetic system with mono-disperse and poly-disperse distribution of magnetic particles with inter-particle interactions has been computed. The analysis is general in nature and applies to all systems containing magnetically interacting particles in a non-magnetic matrix, but has been applied to steel microstructures, consisting of a paramagnetic austenite phase and a ferromagnetic ferrite phase, as formed during the austenite-to-ferrite phase transformation in low-alloyed steels. The characteristics of the computational microstructures are linked to the correlation function and determinant of depolarisation matrix, which can be experimentally obtained in three-dimensional neutron depolarisation (3DND). By tuning the parameters in the model used to generate the microstructure, we studied the effect of the (magnetic) particle size distribution on the 3DND parameters. It is found that the magnetic particle size derived from 3DND data matches the microstructural grain size over a wide range of volume fractions and grain size distributions. A relationship between the correlation function and the relative width of the particle size distribution was proposed to accurately account for the width of the size distribution. This evaluation shows that 3DND experiments can provide unique in situ information on the austenite-to-ferrite phase transformation in steels.     

Shadow Doppler Velocimetry for Simultaneous Size and Velocity Measurements of Irregular Particles in Confined Reacting Flows

We report the application of the Shadow Doppler Velocimeter (SDV) for spatial precise, simultaneous measurement of the size and velocity to assess the particle retention performance of a laboratory, 1/6 scale, 10 kW vertically-fired atmospheric model of the pressurised pulverised-coal furnace of Reichert et al. [1]. The SDV is based on the imaging of a conventional LDV probe volume onto a linear photodiode array and has the advantage over other sizing methods for irregular particles that it is tolerant of the optical misalignment and fouling which are inevitable when passing laser beams through windows in such furnaces. The size and two components of velocity of burning coal particles were measured in the present geometry which has 172 mm furnace diameter and 40 mm lateral exit duct diameter and a calculated exit bulk velocity of 4 m/s, evaluated at 300 K. The Sauter mean diameter of the particles is, within the experimental error, uniform at about 40 μm in the vertical profile normal to the axis of the exhaust pipe, 34.5 mm upstream of the exit. Coal particle velocities in the near-exit region are directed towards the exit, closely following the gas-phase velocities. Both these observations imply that particle retention efficiency due to streamline curvature is low and extrapolation suggests that there will be even less at large scales.     

Critical Angle Refractometry for Simultaneous Measurement of Particles in Flow: Size and Relative Refractive Index

The principle of the optical technique critical angle refractometry, used to determine the size and refractive index of spherical particles (with relative refractive index below unity) in liquid flows, was investigated. This technique is based on the observation of the particle scattering pattern around the critical angle. Similarly to the recent technique developed for rainbow scattering pattern analysis for droplet temperature and size measurements, it is shown that the relative particle refractive index (m_r<1) and size can be determined from the position of the primary diffraction fringe and from the angular spacing between two fringes. Explicit equations for refractive index and particle size measurement were derived from the first-order term of the physical optics approximation. An experimental validation test and numerical computations based on the Lorenz-Mie theory were used to validate the principle of the proposed technique and to estimate its sensitivity, which was shown to be of the same order as that of the rainbow technique. This technique is considered to be useful for various applications in liquid multiphase flows where the particles size and material are to be characterized.     

Simultaneous Measurement of Particle Size and Particle Velocity by the Spatial Filtering Technique

The objective of this study was to compare the measuring results of a fiber‐optical probe based on a modified spatial filtering technique with given size distributions of different test powders and also with particle velocity values of laser Doppler measurements. Fiber‐optical spatial filtering velocimetry was modified by fiber‐optical spot scanning in order to determine simultaneously the size and the velocity of particles. The fiber‐optical probe system can be used as an in‐line measuring device for sizing of particles in different technical applications. Spherical test particles were narrow‐sized glass beads in the range 30–100 μm and irregularly shaped test particles were limestone particles in the range 10–600 μm. Particles were dispersed by a brush disperser and the measurements were carried out at a fixed position in a free particle‐laden air stream. Owing to the measurement of chord lengths and to the influence of diffraction and divergent angle, the probe results show differences from the given test particle sizes. Owing to the particle‐probe collisions, the mean velocity determined by the probe is smaller than the laser Doppler mean velocity.     

Monte Carlo Simulations of Reflected Spectra Derived from Tissue Phantom with Double-Peak Particle Size Distribution

We made a tissue phantom with double-peak particle size distribution, which has polystyrene particles of cell nuclear size and mitochondrial size, and measured the spectrum from the tissue phantom using a single optical fiber. In this paper we investigate the characterization method for the tissue phantom with double-peak particle size distribution by comparing the measured spectra with the calculated ones using the Monte Carlo (MC) method. It is first shown that the Mie phase function characterizes better than the Henyey-Greenstein (H-G) phase function in MC calculation. Next, we compare the measurement spectra with those obtained by modeling as single-peak, conventional modeling for particle size distribution, and for double-peak particle size distribution. The single-peak modeling is found to cause considerable error for the tissue phantom with double-peak particle size distribution, which seems to simulate a biological tissue. We suggest that if one simulates the particle size distribution of a biological tissue by conventional modeling, the accuracy of estimation will be lower.     

Optical properties of silver colloidal centers in KCl crystals

The Ag colloidal centers in additively colored KCl: Ag crystals have been studied by optical and electron microscope methods. The transmission electron microscope studies reveal that the colloidal particles are spherical in shape and have a size distribution which can be described by the skewed zeroth order logarithmic distribution (ZOLD) function. The band shape of the experimental absorption spectra of colloidal particles agrees well with that calculated on the basis of Mie theory, if the optical constants are suitably modified and if the particle size distribution is taken into account. If the particle size is so small that the size distribution cannot be measured by electron microscope the optical absorption bands can be used to determine the size distribution parameters provided the distribution is well described by ZOLD, which in turn is used to calculate theoretical extinction bands.     

Gold Nanoparticles Obtained by Bio-precipitation from Gold(III) Solutions

The use of metal nanoparticles has shown to be very important in recent industrial applications. Currently gold nanoparticles are being produced by physical methods such as evaporation. Biological processes may be an alternative to physical methods for the production of gold nanoparticles. Alfalfa biomass has shown to be effective at passively binding and reducing gold from solutions containing gold(III) ions and resulting in the formation of gold(0) nanoparticles. High resolution microscopy has shown that five different types of gold particles are present after reaction with gold(III) ions with alfalfa biomass. These particles include: fcc tetrahedral, hexagonal platelet, icosahedral multiple twinned, decahedral multiple twinned, and irregular shaped particles. Further analysis on the frequency of distribution has shown that icosahedral and irregular particles are more frequently formed. In addition, the larger particles observed may be formed through the coalescence of smaller particles. Through modification of the chemical parameters, more uniform particle size distribution may be obtained by the alfalfa bio-reduction of gold(III) from solution.     

DDA simulations of light scattering by small irregular particles with various structure

We present DDA investigations of light scattering by irregular particles whose size is comparable with wavelength. We consider four types of randomly irregular particles: strongly damaged spheres, rough-surface spheres, pocked spheres, and agglomerated debris particles. Each type of particle is generated with a well defined algorithm producing an ensemble of stochastically different particles that have a common origin. The different types of irregular particles produce different angular dependencies of intensity and linear polarization degree. Transformation of phase curves of intensity and polarization with changing size parameter for irregular particles tends to be more monotonic, unlike spheres. We find that the magnitude of the negative polarization branch (NPB) tends to shrink as particle absorption increases; whereas, the maximal value and position of the positive polarization branch tends to increase. The most frequently observed shape of the negative polarization at small phase angles is asymmetric with a shift of the minimum position towards the angle of polarization sign inversion. All types of considered irregular particles reveal such asymmetry at x<10. Symmetric negative polarization branches occur seldom. The necessary conditions for their appearance are a relatively large size parameter x?10 to 12 and low absorption.     

Investigation of the Flow Field in the Upper Part of a Cyclone with Laser and Phase Doppler Anemometry

The cyclone is a well known apparatus for separating particles out of a gas stream. With the modern laser diagnostic technologies of laser and phase Doppler anemometry (LDA and PDA), there is the potential to measure the flow and particle field inside the cyclone. The gas phase only measurements used micron‐sized oil seeding droplets, whereas the solid phase, chosen for the PDA particle size measurements, was limestone powder. To assess the possibility of measuring milled limestone particles with PDA, the measured size distribution was compared with those obtained by laser diffraction. The measurements inside the cyclone showed that the flow field in the upper part of the cyclone was different to that commonly thought. Therefore, the vertical height of the cyclone's vortex finder could be shortened without deterioration of the separation efficiency. The particles found in the hold‐up of the cyclone air flow were considerably larger than the average particle size in the feed pipe.     

层流等离子体制备球形氧化铝粉末的实验研究

为研究非转移弧层流等离子体制备面向新材料领域的μm级球形氧化铝粉末的能力,使用自制的非转移弧分段式阳极层流等离子体球化设备,以载气送粉的方式,对η相的不规则μm级三氧化二铝粉末进行等离子体球化处理,并采用均匀设计法,研究等离子体发生器和送粉器不同的工作参数对氧化铝粉末球化率的影响规律。结果表明,实验所采用的直流非转移弧层流等离子体发生器能有效制备球化率接近100%的高球化率球形氧化铝粉末。实验发现,高球化率、高分散性和粒径大小均匀的球形氧化铝粉末可在不同工艺参数组合下制备,并证明了采用非转移弧分段式阳极层流等离子体发生器可实现较低功率下制备较高球化率球形氧化铝的可行性。实验还通过XRD与PDF卡片索引技术对球化前后的氧化铝粉末进行了物相定性分析,发现η相的氧化铝粉末经射流作用转化成了Corundum型的氧化铝粉末。     

Irregularly shaped ice aggregates in optical modeling of convectively generated ice clouds

We propose a model of irregular shaped ice particles for satellite and ground-based cloud remote sensing applications. Microphysical observations have shown that ice particles generated in convective clouds tend to have highly irregular structures as a result of aggregation process. To simulate such complex structures, we used spatial Poisson–Voronoi tessellations. Furthermore, we adopted fractal-like shapes that were consistent with the proposed mass-dimension and area ratio-dimension relationships of measured cirrus particles. Single-scattering properties of the modeled “Voronoi aggregates” at visible wavelengths with size parameters up to 2246 were estimated from numerical calculations using the finite-difference time-domain method and the geometrical-optics integral-equation method. The phase functions for randomly oriented Voronoi aggregates showed features with no halos in the forward-scattering direction and a flat angular dependence in the side-to-backscattering directions. These characteristics and resultant asymmetry factors agreed with those of measured ice particles. Moreover, we confirmed the weak size and shape dependences of these scattering properties for the Voronoi aggregates, as well as high backscattering depolarization ratios and low linear polarizations.     

Light Scattering by Homogeneous Axisymmetric Particles for PDA Calculations to Measure Both Axes of Spheroidal Particles

Mathematical tools are provided for the computation of the scattered field produced by a non-spherical particle moving through the measurement volume of a phase Doppler anemometer. The Doppler signal is modeled by using the rigorous extended boundary condition method and an approximate ray theory. The region of applicability of ray theory is discussed. In particular, a pure refraction model is considered for large particles. It is shown that under some circumstances the phase Doppler technique may be used for sizing of spheroidal particles.     

Effects of Ionospheric Radio Wave Scattering in the Decameter Interferometry

We consider some theoretical issues concerning diffraction of radio waves in a randomly irregular ionosphere with application to the problems of long-wavelength interferometry of cosmic objects. The statistical characteristics of intensity fluctuations of the decameter radio emission from discrete sources in the case of ground-based observations by two-element interferometers with very long and small baselines are analyzed. Analytical expressions are obtained for the autocorrelation function of the radiation intensity and for the scintillation spectrum of a point source in the limiting cases of large and small phase increments in an irregular ionospheric plasma. We find that in the case of radio interferometric reception, the scintillation spectrum corresponding to observations of a source by a single antenna is transferred from the zero-frequency region to the region of the Doppler frequency of the interferometer. It is shown that decameter ground-based and space-borne radio interferometers can be used to study the angular distribution of the radio brightness of cosmic sources under conditions of both quiet and disturbed ionosphere.     

Size effect in the optical properties of small metallic particles: A solvable model for cubic symmetry

Summary A solvable tight-binding model for the electronic states of cubic crystals of arbitrary size is used to compute the real and the imaginary part of the dielectric function of metallic particles. Two types of microscopic processes are discussed: first-order transitions between electronic states and second-order transitions due to collisions with vibrational states and impurities. Interband and intraband transitions are both taken into account fors andp bands. The results display very strong size effects due to the lack of translational symmetry and to the presence of surface states; the Lorentz-Drude behaviour of the bulk metal is approached as the number of atoms in the particles increases. The size effects in the optical functions of the metallic particles are shown to depend on the environment and this is made to account for the basic features in the optical properties, in particular for the resonance plasma absorption. The case of silver particles in a glassy environment is investigated, experimental trends of the optical absorption with varying particle sizes are explained and reasonable agreement with experimental data is obtained. Based on work supported in part by the Italian Research Council (C.N.R.) through a contract G.N.S.M. and by the French Ministry of National Education (Ministère de l'Education Nationale).     

Can one hear the shape of an electrode? II. Theoretical study of the Laplacian transfer

The flux across resistive irregular interfaces driven by a force deriving from a Laplacian potential is computed on a rigorous basis. The theory permits one to relate the size of the active zone to the derivative of the spectroscopic impedance with respect to the surface resistivity r through: . It is shown that the macroscopic transfer properties through a system of arbitrary shape are determined by the characteristics of a first-passage interface-interface random walk operator . More precisely, it is the distribution of the harmonic measure (or normalized primary current) on the eigenmodes of this linear operator that controls the transfer. In addition, it is also shown that, whatever the dimension, the impedance of a weakly polarizable electrode for any irregular geometry scales under a homothety transformation as L^d-1, L being the size of the system and d its topological dimension. In this new formalism, the question addressed in the title is transformed in a open mathematical question: “Knowing the distribution of the harmonic measure on the eigenmodes of the self-transport operator, can one retrieve the shape of the interface?” Received 3 November 1998     

Studies of the dynamic behaviour of charged dust particles in high electric fields using laser Doppler velocimetry

As a preliminary study for the collection of dust particles in an electrostatic precipitator, laser Doppler velocimetry has been used to measure the velocity of small dust particles charged in the high electric field of a model electrostatic precipitator. A differential optical system is adopted for the velocimeter. The period-measuring system is used to analyse Doppler beat signals and to obtain velocity data from them. By means of the laser Doppler velocimeter, the dynamic behaviour of the velocity and direction of moving dust particles is fully revealed as a function of the applied electric voltage in the entire collecting space of the model electrostatic precipitator, and is shown in vectorial flow diagrams of particle velocity.