Near-GHz scanned-wavelength-modulation spectroscopy for MHz thermometry and H $$_2$$ 2 O measurements in aluminized f
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Near‑GHz scanned‑wavelength‑modulation spectroscopy for MHz thermometry and H 2 O measurements in aluminized fireballs of energetic materials Garrett Mathews1 · Christopher Goldenstein1 Received: 27 June 2020 / Accepted: 27 September 2020 / Published online: 29 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract This manuscript presents the development of a two-color laser-absorption-spectroscopy (LAS) sensor capable of providing calibration-free measurements of temperature and H 2 O at 1 MHz in particle-laden combustion environments. This sensor employs scanned-wavelength-modulation spectroscopy with first-harmonic-normalized second-harmonic detection (scannedWMS-2f/1f) with two distributed-feedback (DFB) tunable diode lasers (TDLs) emitting near 1392 nm and 1469 nm. The wavelength of each laser was modulated at 35 or 45.5 MHz to frequency multiplex the lasers and, more importantly, enable simultaneous wavelength scanning across the peak of each H 2 O absorption transition at 1 MHz. This method provides an absolute, in situ wavelength reference which improves measurement accuracy and robustness. Methods to characterize the lasers’ wavelength and intensity modulation at frequencies above 10 MHz are presented. Measurements of temperature and H 2 O mole fraction within 0.3–2.5% and 2–10%, respectively, of known values were acquired in a static-gas cell at temperatures of 700–1200 K. The sensor was applied to measure the path-integrated temperature and H 2 O column density in fireballs produced by igniting 0.75 g of grade 3, class B HMX with and without H-5 micro-aluminum powder (20% by mass). Temperature measurements were acquired in the fireballs with a 1–𝜎 precision of 50 K, 30 K, and 15 K for measurement rates of 1 MHz, 250 kHz, and 25 kHz, respectively. The results are the first to demonstrate that calibration-free measurements of gas properties can be acquired at 1 MHz using WMS-2f/1f.
1 Introduction Fireballs of detonated and/or suspended energetic materials are utilized in a variety of important defense applications (e.g., to destroy chemical and biological weapons of mass destruction). Such fireballs represent an extremely complex and hostile combustion environment, often consisting of strong shock waves and turbulent, multi-phase combustion. A variety of high-speed optical diagnostics are needed to understand the combustion chemistry and physics governing these environments, as well as provide validation data for models used to predict the performance and behavior of such applications. Unfortunately, fireballs present several challenges which complicate the use of optical diagnostics, namely large optical transmission losses (e.g., from * Garrett Mathews [email protected] 1
School of Mechanical Engineering, Purdue University, 585 Purdue Mall, West Lafayette, IN 47906, USA
particulate scattering), high luminosity and opacity, beamsteering and, in field-scale tests, the need to utilize fibercoupled optical equipment or large standoff distances [1, 2]. The
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