E. Paganini

Modified version of the Chahine algorithm to invert spectral extinction data for particle sizing

A modified version of the nonlinear iterative Chahine algorithm is presented and applied to the inversion of spectral extinction data for particle sizing. Simulated data were generated in a λ range of 0.2-2 µm,and particle-size distributions were recovered with radii in the range of 0.14-1.4 µm. Our results show that distributions and sample concentrations can be recovered to a high degree of accuracy when the indices of refraction of the sample and of the solvent are known. The inversion method needs no a priori assumptions and no constraints on the particle distributions. Compared with the algorithm originally proposed by Chahine, our method is much more stable with respect to random noise, permits a better quality of the retrieved distributions, and improves the overall reliability of the fitting. The accuracy and resolution of the method as functions of noise were investigated and showed that the retrieved distributions are quite reliable up to noise levels of several rms percent in the data. The sensitivity to errors in the real and imaginary parts of the refraction index of the particles was also examined.

Commercial spectrophotometer for particle sizing

Particle-size distribution and the concentration of polystyrene particles suspended in water were accurately recovered from the inversion of spectral extinction data measured with a commercial spectrophotometer. The instrument was modified by placing a spatial filter in the collection optics to prevent low-angle scattered light from affecting the measurement of transmitted power. The data were inverted by use of a nonlinear iterative algorithm. When the extinction coefficient is measured in the lambda range of 0.3-1.1 microm, the particle distributions can be retrieved over a diameter range of 0.6-2.8 microm for a wide interval of sample concentrations. The average diameters are recovered with a precision of better than +/-1% and with accuracies consistent with the uncertainties by which the nominal diameters are known. The relative standard deviations of distributions corresponding to monodisperse samples are +/-5-10%, whereas the accuracy on the measured concentrations is approximately 5%.

Optical particle sizer based on the Chahine inversion scheme

An optical particle sizer utilizing an iterative procedure, proposed originally by Chahine for the inversion of scattering data, has been developed. The optical scheme is innovative and allows the measuring range to be selected without substitution of the Fourier transform lens. Numerical computer simulations and experimental results are presented to allow both the inversion procedure validity and the instrument performance to be evaluated.

Inversion of low-angle elastic light-scattering data with a new method devised by modification of the Chahine algorithm

A new inversion method, devised by modification of the nonlinear iterative method originally proposed by Chahine [J. Opt. Soc. Am. 58, 1634 (1968)] is applied to the inversion of low-angle elastic light-scattering data. The algorithm was tested by computer simulations carried out within the wave-vector range 2.5 x 10(2) -2.5 x 10(4) cm(-1). The particle-size distributions were recovered in the 0.70-77-mum range of radii. Compared with the original method of Chahine, this algorithm is much more stable and reliable with respect to random noise, improves the overall reliability of the fitting, and allows both number and weight particle-size distributions to be retrieved accurately. When the performances of the method are investigated with respect to the noise, the results of the simulations show that the particle-size distributions can be recovered accurately up to noise levels of several rms percent.

Low-angle elastic light scattering study of diffusion-limited aggregation

We have investigated the kinetics of the diffusion-limited aggregation of colloidal silica with low-angle elastic light scattering at 31 scattering angles covering the wave vector range 3.7 102 cm-1 <q<3.3 104 cm-1. We have studied both the asymptotic behaviour at larger q, which provides a measure of the clusters fractal dimension, as well as the roll-off region at small q, which gives information on the clusters size. Each set of data (scattered intensity vs. q vectors) has been analysed by means of a nonlinear iterative inversion scheme to provide the value of the number average gyration radius Rn which is found to grow in time with a power law tα. The exponent (average over a few runs) is α = 0.58 ± 0.02, consistent with the theoretical expectations.

Instrument for long-path spectral extinction measurements in air: application to sizing of airborne particles

A novel instrument that is capable of taking spectral extinction measurements over long optical paths (approximately 1-100 m) in the UV, visible, and IR ranges is described. The instrument is fully automated, and the extinction spectrum is acquired in almost real time (approximately 5-10 s) with a resolution of ~3 nm. Its sensitivity and accuracy were estimated by tests carried out in a clean room that showed that, for optical paths between 50 and 100 m, the extinction coefficient can be detected at levels of ~10(-5) m(-1). Tests carried out on calibrated latex particles showed that, when it was combined with an appropriate inversion method, the technique could be profitably applied to characterize airborne particulate distributions. By carrying out measurements over optical paths of ~100 m, the instrument is also capable of detecting extinction coefficients that are due to aerosol concentrations well below the limits imposed by the European Economic Community for atmospheric pollution (150 mug/m(3)). Scaled over optical paths of ~10 m, the limit imposed for particle emissions from industrial plants (10 mg/m(3)) can also be detected sensitively.

Interferometric system for precise submicrometer particle sizing

Following a recently exploited line of thought, we present an interferometric system for particle sizing in the submicrometer region. The phase of the field that results from the interference between the incident and the scattered waves is measured through a heterodyne detection scheme in a Mach-Zehnder-type interferometer. We explored the possibility of extending previous work on this subject to the case of larger particles, i.e., particles larger than 0.5 mum. Experimental results obtained with polystyrene spheres in the range of diameters 0.16-0.71 mum are reported and compared with theoretical predictions. We show that in this range univocal detection of the particle size is not successful because the phase versus particle-size plot exhibits a maximum in correspondence to diameters close to 0.5 mum.

Experimental and Numerical Characterization of Lean Hydrogen Combustion in a Premix Burner Prototype

The use of hydrogen as derived fuel for low emission gas turbine is a crucial issue of clean coal technology power plant based on IGCC (Integrated Gasification Combined Cycle) technology. Control of NOx emissions in gas turbines supplied by natural gas is effectively achieved by lean premixed combustion technology; conversely, its application to NOx emission reduction in high hydrogen content fuels is not a reliable practice yet. Since the hydrogen premixed flame is featured by considerably higher flame speed than natural gas, very high air velocity values are required to prevent flash-back phenomena, with obvious negative repercussions on combustor pressure drop. In this context, the characterization of hydrogen lean premixed combustion via experimental and modeling analysis has a special interest for the development of hydrogen low NOx combustors. This paper describes the experimental and numerical investigations carried-out on a lean premixed burner prototype supplied by methane-hydrogen mixture with an hydrogen content up to 100%. The experimental activities were performed with the aim to collect practical data about the effect of the hydrogen content in the fuel on combustion parameters as: air velocity flash-back limit, heat release distribution, NOx emissions. This preliminary data set represents the starting point for a more ambitious project which foresees the upgrading of the hydrogen gas turbine combustor installed by ENEL in Fusina (Italy). The same data will be used also for building a computational fluid dynamic (CFD) model usable for assisting the design of the upgraded combustor. Starting from an existing heavy-duty gas turbine burner, a burner prototype was designed by means of CFD modeling and hot-wire measurements. The geometry of the new premixer was defined in order to control turbulent phenomena that could promote the flame moving-back into the duct, to increase the premixer outlet velocity and to produce a stable central recirculation zone in front of the burner. The burner prototype was then investigated during a test campaign performed at the ENEL’s TAO test facility in Livorno (Italy) which allows combustion test at atmospheric pressure with application of optical diagnostic techniques. In-flame temperature profiles, pollutant emissions and OH* chemiluminescence were measured over a wide range of the main operating parameters for three fuels with different hydrogen content (0, 75% and 100% by vol.). Flame control on burner prototype fired by pure hydrogen was achieved by managing both the premixing degree and the air discharge velocity, affecting the NOx emissions and combustor pressure losses respectively. A CFD model of the above-mentioned combustion test rig was developed with the aim to validate the model prediction capabilities and to help the experimental data analysis. Detailed simulations, performed by a CFD 3-D RANS commercial code, were focused on air/fuel mixing process, temperature field, flame position and NOx emission estimation.Copyright

Diffraction based optical particle sizer for on-line monitoring in hostile environments of low concentration particle laden flows

We present an optical particle sizer conceived for the on-line analysis of low concentration particle-laden flows. It belongs to the wide class of instruments based on the detection of the light scattered at small angle in the forward direction. Innovative solutions have been adopted both in the optical configuration and in the detection scheme which enable the instrument to operate at very low concentration regimes (i.e., at extinction values as small as 10−5). Particle classification is made over 31 size classes in the range of diameters 0.9–90 μm. The whole system has been designed for applications in hostile environments. Preliminary measurements have been carried out in a coal fired power plant on a flue duct downstream the electrostatic precipitators (average temperature 150 °C, particle concentration smaller than 50 mg/Nm3).

Self-aligning optical particle sizer for the monitoring of particle growth processes in industrial plants

We describe a diffraction based optical particle sizer to be used on-line in an industrial plant for monitoring the growth process of polystyrene beads in the range (400 μm–4 mm). The instrument has been designed to perform elastic light scattering measurements at very small angles (from 8×10−5 to 8×10−3 rad) and is provided with an active servo system that controls the beam alignment during operations.