A R Jones

Light scattering for particle characterization

Abstract Light scattering has proved to be one of the most powerful techniques for probing the properties of particulate systems. The purpose of this paper is to review the status of elastic light scattering. The emphasis is on recent developments, rather than being over-repetitive of earlier reviews, but considerable background is included with the aim of making the paper self-contained. There is an extensive summary of theoretical treatments, including both historical work and new. On the experimental side, while there is some discussion of well-tried methods, the emphasis is on recent techniques for measuring other properties as well as size. This includes, among others, the fractal treatment of agglomerates, determination of particle shape and measurement of refractive index. The discussion is broad rather than deep to provide a wide-ranging review of an extremely active field.

The production of charged monodisperse fuel droplets by electrical dispersion

The electrical dispersion of a jet of kerosene into a spray of monodisperse droplets is discussed. It is found that monodisperse spraying can be achieved within a small finite range of applied voltages, within which droplet size is almost independent of voltage and is a function of the flow rate of kerosene. An approximate theory is developed which shows that the controlling parameter is the field strength at the surface of the jet, and correctly predicts the specific charge acquired by the droplets as a function of this field strength.

The principles of the detection of flammable atmospheres by catalytic devices

Abstract Most instruments used for measuring the explosibility of fuel/air atmospheres use catalytic oxidation as a method of measurement. The detailed mechanism of this method has been examined and equation describing the output from these devices have been derived. The output V(LEL) at the lower explosive limit of a fuel has the general form V (LEL) =K D 12 δH[LEL], where D12, ΔH, and [LEL] are respectively the diffusion coefficient, heat of oxidation, and the lower explosive limit of the fuel in air, and K is a constant. Calculations have been made which enable the responses to explosive gas/air mixtures to be predicted and correction factors to be derived for practical devices. A new method for the measurement of explosiveness is discussed based on the empirical correlation between the heat of oxidation of the fuel and its lower explosive limit.

Scattering efficiency factors for agglomerates for small spheres

Scattering and absorption efficiencies are calculated for chains of spheres in the Rayleigh approximation, and compared with individual particles. For aligned chains significant differences are found for the scattering case which arise due to multiple scattering and coherent phase effects. The absorption efficiency is not influenced by the latter, but the changes are found to be of the same proportion as those in the scattering efficiency due to multiple scattering alone. For randomly oriented chains the effects of multiple scattering are small within the range considered.

The effect of nonsteady electric fields on sooting flames

The effects of transient dc and 50-Hz ac fields acting on a sooting acetylene diffusion flame were compared with those of steady dc fields. The object was to separate the velocities of charged soot particles from those of the ionic wind by making use of the difference in their relative evolution times. This has not been considered previously, largely because much of the research was carried out on flat counterflow flames, but is here shown to be important for conventional flame shapes. Laser Doppler and scattering methods were used to determine particle velocities and sizes. Soot particles of the order of 10 {delta} m diameter were found to have mobilities of the order of 5 {times} 10 {sup -6} m {sup 2} s {sup -1} V {sup -1} and zero for particles exposed to a flux of positive and negative charge carriers, respectively. Major perturbations to flow due to ionic wind effects set in after a delay of the order of 10 ms. Similar results were obtained whether the transients followed the switching on of a dc field or occurred during the 50-Hz cycling of the ac field.

A light scattering technique for particle sizing based on laser fringe anemometry

It is demonstrated that the visibility, or modulation depth, of the AC signal produced in the light scattered by a particle crossing an interference pattern can be used to obtain particle size. A method is described for direct determination of size distribution which is not strongly dependent upon refractive index.

A drop tube furnace study of coal combustion and unburned carbon content using optical techniques

The combustion of pulverized coal in a drop tube furnace has been studied optically. The coal passes through an excess air flame to ensure ignition and then burns as it passes down a segmented ceramic tube electrically heated to 1100 °C. The method permits the optical measurement of particle size distribution as a function of residence time. The results of this demonstrate the relative reactivity of the coals used and also indicate that fragmentation may occur. Samples of the particles are taken at each furnace stage for Carbon-Hydrogen-Nitrogen (CHN) analysis. Comparison of the carbon content by mass with the cross-polarization in the backscattered light shows that a linear correlation can be established for particles up to at least 100 μm diameter. This implies the possibility of a simple optical tool for the measurement of unburned carbon content and, thus, for monitoring burnout and combustion efficiency.

A study of the influence of absorption on the spatial distribution of fluorescence intensity within large droplets using Mie theory, geometrical optics and imaging experiments

The dependence of energy distributions within droplets on internal absorption effects has been investigated by calculations based on Mie theory and the geometrical optics approximation and experiments. The objective was to evaluate the accuracy of the geometrical optics approximation in calculating droplet volume to fluorescence intensity proportionality, required for planar droplet sizing measurements in sprays based on Mie scattering and fluorescence intensity from droplets. A geometrical optics approach was used to calculate the energy-density distribution in the meridional plane of a droplet and this was compared to the Mie theory solution for a range of imaginary refractive indices mi = 1×10-5 to mi = 5×10-4. Integration of the energy density distribution over the droplet volume provided a method to compare experimental and theoretical results. Good agreement was found for the energy density and volume integrated energy distribution patterns obtained from both calculation methods and the experimental results. Quantitative comparison of the volume integrated energy results shows that for the investigated range of absorptivity Mie theory calculations lead to results that are ≈30% higher than in the geometrical optics case. This discrepancy is independent of light absorption and droplet size. Tunnelling waves were identified as the cause for the discrepancies between Mie theory and geometrical optics; these contribute to high energy density in the rim region of the droplet images.

Error contour charts relevant to particle sizing by forward-scattered lobe methods

Error contour charts are derived from a comparison of the Mie theory for light scattering by spheres with the Fraunhofer diffraction prediction for the forward-scattered lobe. It is concluded that forward lobe particle sizing methods are accurate for quite small particles, particularly if they are absorbing, but that problems may arise with refractive indices close to unity.

Design of an optical instrument to measure the carbon content of fly ash

Abstract The polarisation ratio resulting from the backscattering of linearly polarised incident light by fly ash particles is indicative of the carbon content. Determination of this parameter is useful for characterising the efficiency of coal burning furnaces. This paper describes the development of an instrument to make these measurements, which is designed to be attached furnace ducts. The performance of the instrument is tested in laboratory measurements on a range of ashes from different coals. It is concluded that if the mineral content of the ashes is not known then the carbon mass fraction can be determined to within ±1%. If the mineral content is known then the use of neural network analysis can reduce this to ±0.5%.