Application Note - PARTICLE ANALYSIS
AFR has developed the methodology, hardware, and software to perform on-line, at-line or off-line measurements of particulate samples. Infrared spectroscopic techniques are capable of probing gas suspended samples that are optically opaque at visible wavelengths due to particle scattering. The measurements can determine:
The methods have been demonstrated for the measurements of combustion products, ceramics, water vapor, chemical fumes and hot sprayed metal alloy particles. Measurements have been performed of:
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.
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.
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.
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.
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