METHODOLOGY
Particle Size Distributions
The diffraction of radiation from small particles leads to a wavelength dependent
extinction which contains particle size information. Measurements of the wavelength
dependent extinction are employed to extract size distributions. The analysis is based on
the Mie theory of electromagnetic radiation scattering from spherical and ellipsoidal
particles. Distributions are calculated by fitting simulated size distributions to
measured extinction spectra.
Particle Temperature
The particle stream temperature is determined by an innovative patented
emission/transmission (E/T) measurement technique. For opaque particles, the analysis
yields precise values for the spectral emittance as well as the temperature, independent
of the particle size distribution and interfering gaseous species.
INSTRUMENTATION
Instruments can be configured for on-line, at-line, or off- line measurements. A system
includes an FT-IR spectrometer, a particle sampling optical interface, and analysis
software. Data collection times range between 1 and 30 seconds, depending on the sampling
noise due to particle motion. Typical optics provide a spot size between 1 mm and 5 cm.
Sized distribution calculation times can be as short as 1 sec. The technique is applicable
to particle sizes ranging from 0.2 µm to ~20 µm. Tomographic reconstruction of streams
to obtain spatial distribution can also be applied.
RESULTS
Metal particle production
On-line measurements of the production of fine alloy particles by atomization of molten
metal were performed at the National Institute of Standards and Technology in
Gaithersburg, MD. Figure 1 shows the result of E/T measurements of an atomized hot spray
of nickel alloy 625, with superimposed black body spectra calculated for different
temperatures. The results show that the radiating metal particles were at approximately
1550¡ C. Discrepancies between the measured and calculated spectra indicate that a
distribution of particle temperatures provides a better description of the particle stream
radiance than does a single temperature. Figure 2 shows a series of particle extinction
measurements sampled from a downstream position in the reactor. The measurements clearly
show time variations in the particle extinction that can be related to particle size and
number density. Refinements to the numerical analysis are being developed to perform
deconvolutions of arbitrary size distributions from extinction spectra in the presence of
particle noise.
Figure 1. E/T Measurements of Hot Metal Spray to Determine Temperature.
Theoretical Spectra Calculated for Different Particle Temperatures are Shown for
Comparison.
Silica, coal and fly ash
Extinction measurements of air suspended streams of silica spheres, coal and fly ash
were performed. The results are shown in Figure 3, comparing measured and theoretical
extinction based on Mie theory calculations. Extracted particle size distributions are
shown on the right. Theoretical curves for silica, labeled a and b, were calculated
assuming solid and 65% sphere porosity, respectively. Theoretical curves for fly ash,
labeled a and b, were calculated assuming solid and 57% porosity, respectively. Coal data
were taken from two size cuts (sieved) 200 x 325 and 325 x 400 mesh.
Figure 2.
On-line Measurements of Particle Extinction. Measurements Performed at
the N.I.S.T. Supersonic Inert Gas Molten Atomization (SIGMA) Facility. The Measurements
were taken Before, During and After an Atomization Run of a Nickel Based Alloy.
Figure 3. FT-IR particle size analysis for silica, fly ash and coal.