Showing papers in "Particle & Particle Systems Characterization in 1990"

Drop Sizing by Laser Light Scattring Exploiting Intensity Angular Oscillation in the mie regime

Abstract: The intensity angular pattern of the light scattered by a drop at a scattering angle of ca. 90 o is related to the size of the scattering drop. A simple relationship between the number of fringes per degree and the size is presented. The possibility of measuring the drop size through visualization of the fringe pattern is demonstrated

Lagrangian Stochastic-Deterministic (LSD) Predictions of Particle Dispersion in Turbulence

Abstract: A Lagrangian model of particle motion in turbulence has been developed. The model is stochastic in the sense that the "instantaneous" fluid velocity field is generated from known turbulence energy and time scales of large eddies by using a random sampling. The particle motion during the interaction with the eddies is deterministic as being predicted from solutions of Lagrangian momentum equations. On the basis of a large number of calculated trajectories, the distributions of local particle mean and fluctuation velocity components and of the mean square particle displacement are evaluated in a flow domain. The results of the predictions for the case of particle dispersion in nearly homogeneous and isotropic turbulence behind a grid are compared with existing experimental data and satisfactory agreement is achieved. The model is also applied to the case of the particle dispersion from a point source in a nonhomogeneous turbulence of fully developed pipe flow. The agreement with experiments is satisfactory for both heavy and tracer particles. An explanation is given for the experimentally observed phenomenon that in some cases heavy particles disperse faster than fluid points.

Mean Particle Diameters. Part I: Evaluation of definition systems†

Abstract: Mean particle diameters are important for the science of particulate systems because they provide information on particlesize distributions in such a way that they can be related to physical or physiological processes or product properties. There are different notation systems for such mean diameters which may cause much confusion. This equally applies to their nomenclature. The present paper is concerned with comments on the “Moment-Ratio” and German (DIN) notations. The Moment-Ratio notation is recommended for standardization. A more extensive contribution of statistics to the science of particulate systems is recommended.

The kinetics and shape factors of ultrafine dry grinding in a laboratory tumbling ball mill

Abstract: The kinetics of batch grinding quartz from a feed of 600 by 425 mm to a product of 80% less than 10 mm have been determined using screening and laser diffractometer sizing for size analysis. The specific rates of breakage decreased by a factor of about three when the material became less than about 100 mm in size, but the primary breakage distribution function also changed to give proportionately more fine material, so that the grinding efficiency expressed as the development of surface area (B.E.T.) per unit of energy input did not decrease. Analysis of the shape of the particles in the 25 × 38 mm size range showed that particles of this size produced by roll crushing or by 8 minutes of grinding of a 425 × 600 mm feed were not different but at long grinding times the particles were rounded. This suggests that the breakage mechanism changes to give more chipping and abrasion and less disintegrative fracture. As the material approached the ultrafine size range it adhered to the mill case and there was no further size reduction. However, a technique for striking the mill case to dislodge the particles was successful in allowing further grinding to 40% by weight less than 2 μm.

Velocity and Drop Size Measurements in Fuel Sprays in a direct injection diesel engine

Abstract: High-pressure injection fuel sprays have been investigated in a single-cylinder research Diesel engine using the laser and phase Doppler anemometry techniques to measure fuel droplet velocities and sizes and their spatial and temporal distributions within the spray. The measurements relate to the determination of the droplets' axial velocity component and diameter on the spray centre line between 75 and 100 nozzle diameters downstream from a single-hole injector nozzle. The experimental program covered the use of both Diesel and a non-combusting alcohol fuel. Measurements at radial locations in the spray and at different fuel loads were also taken for the alcohol fuel. Temporally resolved measurements of droplet velocity and size are presented together with time — averaged values in such a way as to isolate the contributions from the leading and trailing regions of each spray and identify two different break — up mechanisms occurring as suggested by a conceptual model of the spray behaviour.

Particle Dispersion in Turbulent Flow: The effect of particle size distribution†

Abstract: The measured velocity fluctuation of particles in dispersed twophase flow systems is discussed and an interpretation of particle velocity statistics is given. Besides the turbulent velocity fluctuations resulting from the particle's response to the fluid turbulence, two additional apparent fluctuation components may be distinguished, caused by inertial effects of the particles and by a polydisperse particle phase. Especially for larger particles inertial effects may become important and therefore the particle motion in the flow field is strongly governed by the inlet conditions, namely the instantaneous particle velocity. Therefore, the measured axial particle velocity fluctuation at a given location may be considerably increased as particles from different starting locations with different initial velocities may cross the same measuring point. This dynamic particle behaviour is demonstrated by the numerical simulation of particle trajectories in a given flow field. The results show the importance of including appropriate inlet conditions for the numerical prediction of particulate two-phase flows. Further, in experimental realizations of particulate two-phase flows, the particles usually exhibit a certain size distribution. Owing to the different responses of the different sized particles to both the mean flow field and the turbulence characteristics of the flow, the measured particle velocity fluctuations are higher than in the comparable monodisperse case. Therefore, the particle size distribution was taken into account in the present numerical calculations in order to obtain results which are comparable to the corresponding experiments. In this numerical simulation the particle mean velocity and the rms value of the velocity fluctuation were obtained by averaging over the whole particle size distribution and then compared with the measured values. Calculations are presented for two different shaped particle size distributions with a different mean size and compared with calculations with a monosized particle material and experimental results. Further, the behaviour of the different sized particles in the particle size distribution is discussed by considering the changes in the particle size distribution and the particle velocity distribution throughout the flow field.

Calculated Characteristics of Droplet Size and Velocity Distributions in liquid sprays

Abstract: Predictions of the droplet size and velocity distributions in sprays under isothermal conditions are reported. The calculations are based on the maximum entropy formalism, complying with the conservation laws of liquid mass, momentum and energy. This theoretical approach considers only the macroscopic quantities about the atomization processes, without resorting to the details of the liquid breakup processes such as the onset and growth of instabilities. The derived joint droplet size and velocity distribution function depends on the Weber number as well as the liquid mass, momentum and energy source terms. These parameters represent the conditions under which the atomization occurs. The droplet velocity distributions are truncated Gaussian distributions for any specific sizes. The nondimensional Sauter mean diameter decreases slightly with the Weber number and then approaches an asymptotic constant. The calculated values of D21/D30 are very close to unity which agrees with the experimental observations. The computations also show that the atomization efficiency is very low; less than 2.6 percent.

Dual‐Cylindrical Wave Method for Particle Sizing

Abstract: A new laser Doppler method for particle sizing has been developed. In contrast to the standard phase Doppler technique, which uses scattering from plane waves for a measurement, the proposed method employs cylindrical waves of incident light. The main advantage is that signal frequency, instead of the signal phase, becomes a function of particle diameter and a standard frequency measuring device may be used as a signal processor. The advantages of spectrum analysis as the signal processing method are highlighted. The laws of geometrical optics applied to the present scattering problem provide a relationship for the frequency of the collected signal which is expressed as the sum of two terms, the conventional Doppler frequency and the "anisotropic frequency", which is directly dependent on the particle size. These theoretical assertions were examined experimentally. Measurements on glass and metal particles of known diameters showed good agreement with the theory.

Use of One‐Parameter Models for the Assessment of Particle Interactions by photon correlation spectroscopy

Abstract: The use of very simple one-parameter models of particle interactions for the analysis of the concentration dependence of the collective diffusion coefficient as determined by photon correlation spectroscopy is illustrated by measurements on two different systems. In one, a micro-emulsion, attractive interactions are dominant whereas in the other, a silica dispersion, repulsive interactions play the major role.

Particle Dispersion in a Swirling Confined Jet Flow

Abstract: Etude de la dispersion des particules dans un ecoulement turbulent tourbillonnaire du a une augmentation brusque du diametre de conduite. Comparaison des resultats experimentaux a ceux de simulations numeriques

A Statistically Conditioned Averaging Formalism for Deriving Two-Phase Flow Equations†

Abstract: A statistical formalism overcoming some conceptual and practical difficulties arising in existing two-phase flow (2 PHF) formulations and modelling techniques is introduced. Basic theorems for the case of dispersed 2 PHF are presented. Phase interaction terms with a clear physical meaning enter the equations and this formalism provides some guidelines to avoid closure assumptions or to close those terms rationally. Continuous phase averaged continuity, momentum, turbulent kinetic energy and turbulence dissipation rate equations can be rigorously and systematically obtained with this methodology in a single step. These equations display a structure similar to that for single-phase flows. It is also assumed that the dispersed phase is well described by a "Boltzmann-type" equation and Eulerian "continuity", momentum and fluctuating kinetic energy equations for the dispersed phase are obtained. A k-e turbulence model for the continuous phase is used. A gradient transport model is adopted for the dispersed phase fluctuating fluxes of momentum and kinetic energy. Closure assumptions are proposed for the phase interaction terms. This model is then used to predict the behaviour of an axisymmetric turbulent jet of air laden with solid particles varying in sizes and concentrations. Numerical results compare reasonably well with available experimental data.

Bipolar Ion and Electron Diffusion Charging of Aerosol Particles in high purity argon and nitrogen

Abstract: The bipolar diffusion charging process is studied theoretically for aerosol particles in high purity argon and nitrogen. The Fuchs theory is extended by the incorporation of the free electron charging of the aerosol particles. The ion parameters for positive and negative ions are assumed to be identical. The influence of the free electrons explains the differences between the mean mobility and mean mass of negative and positive ions. The ratio of the electron number concentration to the number concentration of negative ions is used to fit the curves, calculated by the extended model, to the experimentally determined bipolar charge distribution in argon and nitrogen. The extended Fuchs model was found to be rather insensitive to variations in the mobility and mass compared with the model with four different ion parameters. Further experimental studies of the bipolar charge distribution in a gas mixture of pure nitrogen and sulfur hexafluoride, SF6, indicate the importance of the free electrons in the bipolar diffusion charging process.

Optical Levitation Experiments for generalized lorenz‐mie theory validation

Abstract: Generalized Lorenz-Mie theory (GLMT) is validated by comparisons between theoretical results and experimental optical levitation data. Selected results are presented.

Use of the laser diffraction technique for particle size measurement at high suspension densities

Abstract: Developpement d'une technique de diffraction laser pour la mesure en ligne des vitesses de croissance cristalline

Axial‐View Measurements of Particle Motion in a Turbulent Pipe Flow

Abstract: This paper describes experimental measurements and numerical predictions of the motion of particles of size 500–800 mm diameter in a 20 m long (i. e. 620 pipe diameters) vertical tube. The numerical simulations suggest that the particles attain a fluctuating r.m.s. velocity in a direction normal to the axis of the tube which is at least one order of magnitude less than that of the gas phase turbulence fluctuations. However, the measured values are of the same order as the gas phase turbulence fluctuations (i. e. 0.5 to 1 m/s). This discrepancy is likely to occur because of particle/wall interactions and these were investigated in separate experiments. Although the cause of the measured coefficients of restitution in the normal direction being greater than unity was not finally demonstrated, the results indicated strongly that the effects were due to non-ideal bouncing of the particles in the presence of particle spin.

Prediction of gas-particle turbulent free or confined jet flows

Abstract: Numerical predictions of two-phase turbulent gas-particle flows are reported for two test cases: for a plane jet and for an axisymmetric free jet. The numerical algorithm uses an Eulerian approach for the fluid phase and the particulate phase is described by using a Lagrangian stochastic model. The k-e turbulence model was used to characterize the length and time scales of the fluid. In the plane confined jet the influence of the restitution coefficient on the statistical distribution of velocities was analysed. Particular attention was dedicated to the e equation corrections regarding the problem of the free jet spreading rate overprediction. The influence of the particle tangential momentum equation on the prediction of particle concentration near the centre line in axisymmetric free jets was also analysed. Comparisons between predictions and experimental data are presented and shown to be satisfactory regarding the incomplete knowledge of turbulent multiphase flows.

Improved Resolution and Accuracy in Electrical Sensing Zone particle counters through hydrodynamic focussing

Abstract: The electrical sensing zone technique is long established, well researched and widely used. It is often used as a reference method against which others are compared or calibrated. This position should not be allowed to disguise the potential which exists for still greater accuracy and versatility. In this paper the use of the hydrodynamic focussing technique is evaluated and an experimental comparison is made with a normal Coulter stand for several samples. It is clearly shown that hydrodynamic focussing enables higher resolution and accuracy to be obtained for materials with narrow or unusual particle size distributions.

The Origin of Turbulence Acquired by Heavy Particles in a round, turbulent jet

Abstract: The paper describes the application of a stochastic separated flow model for the dispersed phase to the prediction of a particle-laden turbulent air jet discharging at 13 m/s from a 15 mm nozzle into stagnant surroundings. Emphasis is placed on the stochastic element of the Lagrangian particle tracking part of the model and on the importance of particle initial conditions over the first 20 jet diameters. Calculations are presented for 80 mm sized glass particles which clarify how particles with turbulent Stokes number less than unity acquire axial turbulence much larger than radial. Far from being only a response to the gas-phase turbulence as implied by the model, the axial turbulence is shown to be also produced by an interaction between particle radial turbulence fluctuations and cross-stream spatial gradients in particle mean velocity, here referred to as "fanspreading". In addition, initial particle turbulence levels remain identifiable for about 10 jet diameters; the initial radial turbulence reinforces the fan-spreading contribution and leads to extra generation of axial turbulence farther downstream. In general, the results agree well with experimental measurements.

Application of Laser/Phase Doppler Anemometer to Dispersed Two‐Phase Jet Flow

Abstract: A confined jet flow involving glass particles was examined to clarify fundamental aspects of the turbulence structure of two-phase flow. Spherical glass particles of mean diameter 62.4 mm were loaded in a primary jet at a mass loading ratio of 0.3. The primary jet diameter was 13 mm and exit velocities at the centre line were set at 20 and 30 m/s. The confined pipe diameter was 60 mm and velocities were set at 10 and 15 m/s. The particle and gas flow characteristics, such as local mean velocities, their fluctuations and size distribution, are discussed for three flow conditions which were obtained by changing the velocities of primary and secondary flows. Laser Doppler anemometry was applied to measure the flows of both particles and tracer, discriminated by scattering light intensity, and the precise particle size, for examining the dependence of particle size on the flow, by phase Doppler anemometry. The signal processing of the phase Doppler anemometer was based on a fast Fourier transform method to detect not only Doppler frequency for velocity but also the phase shift of Doppler signals for particle sizing. The processing system consisted of conventional fast analogue to digital converters, multi-digital signal processor units and a host microcomputer.

Alignment Errors in Phase Doppler Receiving Optics

Abstract: It is shown that if the pinhole in a phase Doppler system' s collection optics is misaligned, even by a matter of micrometers, the effective collection aperture will be different from the aperture of the collection lens. This directly alters the phase characteristics of the resulting signal and can produce systematic and random errors in drop size measurements. The results of misalignment in a simple system are calculated, experimental evidence of the errors presented, and it is shown that replacing pinholes with vertiacl slits will reduce the sensitivity to such errors.

Standard Media for Particle Size and Number‐Density Calibrations in single, multiple and dependent scattering

Abstract: Two kinds of standard media for particle size and number-density measurements and calibration are described. Although the main concern is multiple scattering situations, they might also be useful for single and dependent scattering. One medium is made of particles embedded in a solid polymer matrix and the other of particles embedded in a gel. Transmittance measurements at two wavelengths (visible and far-infrared) are used to exemplify the use of these media.

Spectral properties of the regularized inversion of the Laplace transform

Abstract: Inversion of the Laplace integral equation, used in the laser scattering measurement of colloidal particle size distributions, presents sever numerical and experimental difficulties. In the presence of noise the variance of the inversion integral is infinite, indicating maximum uncertainty in the accuracy of the inversion. The regularized inversion of the Laplace intergral equation provides a convenient computational algorithm which requires no a priori knowledge of the unknown linewidth distribution. Using the eigenfunction decomposition of the Laplace kernel, the spectral properties of the regularized inversion may be seen. Regularized inversion represents a type of low pass filter which preserves the properties of the inversion spectrum at low frequencies, but provides a cutoff at a point controlled by the regularization parameter. This filtering reduces the variance of the inversion to a finite value. Regularized inversion is somewhere between optimal filtering and the abrupt truncation used in singular value decomposition and other similar methods. Two examples, a monodisperse and a bimodal linewidth distribution, are used to compare the performance of regularized inversion to that obtained through an optimal filter.

Characterization of Particle Swelling of Materials of Pharmaceutical Interest

Abstract: A method based on the employment of an electrical sensing zone instrument (Coulter Counter) has recently been proposed for the combined measurement of the increase in particle volume (particle swelling) and granulometric characterization of pharmaceutical tablet disintegrants. The performance of the method, as far as its applicability to limited swelling materials is concerned, has been assessed. Both inert materials (polystyrene latices) and limited swelling disintegrants (maize starch and crospovidone in different granulometric fractions) were examined for total particulate volume in aqueous media. The accuracy and repeatability of measurements of both total particulate volume and particle volume increase were defined. The results obtained indicate that the method may be adapted for measuring the increase in particle volume of so-called limited swelling disintegrants.

Pulse holography and phase-Doppler technique. A comparison when applied to swirl pressure- jet atomizers

Abstract: Pulse holography and the phase-Doppler technique are two of the very few methods with which the size and velocity of individual particles within a two-phase flow may be simultaneously measured. This enables the two-dimensional frequency distributions of particle size and velocity to be compiled and allows the existence of mutual correlations to be verified. The application of both methods to investigations into water droplet collections within the hollow spray cone of a swirl pressure-jet atomizer is described. When comparing the results of measurements obtained by the two methods under identical conditions, one fundamental difference must be taken into account: whereas pulse holography delivers space-averaged concentration-dependent frequency distributions, those derived from the phase-Doppler technique are time-averaged fluxdependent. The methods hence deliver different, but equivalent, results. Since the possibility of conversion exists, both representations are always available. As far as the particle size distributions are concerned, the results obtained from the two methods confirm the expected differences, whilst verifying the compliance of the interconverted size distribution data. Regarding the velocity distributions, however, some unexplained discrepancies still remain.

A Single Expression of Common Particle Size Distribution

Abstract: A new particle size distribution function based on a simple model is presented. Data generated by the normal, log-normal and Rosin-Rammler distribution functions were fitted with this new distribution function and the fitted curves were, for all practical purposes, indistinguishable from all three distributions. The new distribution function can replace all the three functions and thus a means of presenting and comparing the different size distribution patterns in terms of a single mathematical expression is obtained. Additionally, the new distribution function provides the possibility of developing the discussion on the physical meaning of the particle size distribution.

Technologically Relevant Particle Shape Analysis

Abstract: The current state in shape analysis is distinguished by a number of characterization methods, but the great variety of specific shapes complicates the selection of parameters that are relevant for a particular problem. Therefore, the preferred approach is to characterize single particles "free of presupposition" and to select technologically relevant parameters using cluster and discriminance algorithms. Parameter vectors including elongation, bulkiness, fractal dimension and area-equivalent diameter are calculated on the basis of image analysis. First applications to bacteria and agricultural freestuffs exemplify the concept and illustrate that technologically relevant particle shape analysis permits the classification of single particles and the quantification of property functions.

Dispersion of Monosized Heavy Particles in Grid Turbulence Effects of Body Forces

Abstract: Dispersion of monosized drops downstream a point injection in a grid-generated turbulence is studied. Influence of extra bodyforces is also investigated by use of ferrofluid drops and magnetic field. Datas are obtained through LDV and given for fluid and particles mean and fluctuant velocities. Presence probability repartition for particles downstream the injection is obtained by LDV counting.

The Use of Simulated Fractals in Particle Characterization

Abstract: The principal focus in this paper is the development of a general method to simulate irregulary shaped particles whose perimeters are fractal. It is shown that this method can produce a particle of almost arbitrary fractal dimension. The Fourier components of these fractals are then computed. It is shown that these components were either independent of the fractal dimension or that any dependence is less important than other properties of the cluster such as symmetry and aspect ratio.

Droplet Deposition from a Mist Flow on a Parallel Vertical Wall

Abstract: A primarily experimental investigation of the deposition of droplets from a turbulent two-phase suspension flow on a parallel vertical wall has been found to be closely related to the ratio of fluctuation velocity of the droplets to that of the flow and the size of the droplets. Data are obtained for an air-water mist flow parallel to a vertical flat plat at Re = 1.54 × 105 to 4.2 × 105 by the use of a particle-sizing two-dimensional reference-mode laser-Doppler anemometry technique. Although no rigorous theory can be formulated at present due to the complexities involved, however, an analytical attempt is made in the hope of providing an explanation to the physics of the phenomenon. It is based on an apparent turbulent viscosity of the fluid as felt by the moving particles in a turbulent two-phase suspension flow and the most energetic eddy frequency of the flow.

Improved Accuracy in the Measurement of Particle Size Distribution with a coulter counter equipped with a hydrodynamically focused aperture

Abstract: It is well known that broadening of the observed particle size distribution occurs both at the finer end and at the coarser end of the distribution when using the Coulter Counter. This problem can partly be overcome with the aid of electronic pulse editing. However, it has been found that the accuracy can be further improved when the Coulter Counter is equipped with a hydrodynamically focused (HDF) aperture. The standard deviations of the main peaks of samples with diameters of 5.0, 10.2, 15.1 and 20.0 mm decreased by 21%-49% with HDF. HDF also prevents artefacts at the fine side of the main peak that originate from particles re-entering the sensing zone of a standard aperture after they have passed through it. In the experimental model used i.e. a silica sample with a diameter of 15.6 mm, the estimate of small particles (>10 μm) decreased from 6.5% to 0.4%.