Spectrally‐ and Temporally‐Resolved Optical Depth Measurements in High Explosive Post‐Detonation Fireballs

Abstract

Temporally‐and‐spectrally‐resolved‐optical‐depth‐measurements were made in fireballs from five common high explosives. Chambered explosive detonations were probed with 405 nm, 532 nm, 650 nm, 1310 nm, and 9600 nm laser light to quantify optical attenuation during the first 5 ms after initiation. Attenuation lengths are reported for each wavelength and high explosive as a function of time. Two charge sizes were studied to show optical depth scaling for charge mass. Trends in optical depth between type of explosive and probe wavelength are presented and discussed. The spectral dependence of attenuation departed strongly from the λ4 prediction from Rayleigh scattering, indicating significant absorption in addition to scattering. Beam steering effects were determined to be of minimal influence for the scale of explosives tested. Blast scaling is considered in attempt to generalize the results presented.

Extinction Imaging Diagnostics for In Situ Quantification of Soot within Explosively Generated Fireballs

“…For example, using exploding bridgewire detonators similar to the devices used here, Lodes et al. [33] observed attenuation at several different wavelengths, finding optical depths around 1.1 and 2.5 for long‐wave infrared and visible wavelengths, respectively. Similar attenuation experiments in aluminized high explosives showed optical depths of 2.3–3.0 at visible wavelengths [32].…”

“…Finally, during periods of luminosity, post‐detonation fireballs are known to be optically dense [32]. Consequently, spectral emission measurements can be biased to phenomena occurring near the surface of the fireball [33], and the spatial, temporal, and spectrally resolved optical depth of the fireball must be known to correctly interpret measured spectral emissions. This has motivated some previous measures of optical depths in post‐detonation fireballs.…”

Cepstral Analysis for Baseline-Insensitive Absorption Spectroscopy Using Light Sources with Pronounced Intensity Variations

“…In the case of measuring a spectral absorbance of 0.05 (typical of near-infrared LAS applications), achieving a 2% error in the spectral absorbance of the target species requires achieving an effective error in I o of only 0.1%. This is especially challenging to achieve when characterizing harsh combustion environments where beamsteering, window fouling, mechanical vibration, and scattering off particulates frequently cause the transmitted light intensity to vary on the order of 1 to 10% [6] and, in extreme applications (e.g., coal gasifiers, explosive fireballs), by several orders of magnitude [7][8][9].…”