Near-GHz scanned-wavelength-modulation spectroscopy for MHz thermometry and H$$_2$$O measurements in aluminized fireballs of energetic materials

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 (scanned-WMS-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– $$\sigma$$ 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.