Measurement of temperature and water vapor concentration using laser absorption spectroscopy in kilogram-scale explosive fireballs

The temperature, water vapor concentration, and pressure within a kilogram-scale high-explosive fireball are probed using a custom, tunable diode laser absorption spectroscopy setup housed in a ruggedized gauge. Explosive fireballs are generated by the detonation of 2.2 kg spherical charges of C-4 high explosive at one end of a partially enclosed concrete tunnel structure. The 0.3 m fixed path-length absorption gauge is placed at varying stand-off distances from the charge at 6.4 m, 3.9 m, and 2.4 m, over several tests, to show survivability, measurement quality, and a repeatability. Changing the explosive composition to a 2.2 kg aluminized charge resulted in an explosive fireball that caused heavy beam attenuation at a 2.4 m stand-off distance due to the presence of added condensed-phase material. Reliable temperature measurements in the aluminized charge fireball were not possible due to the low signal-to-noise ratio and distortion of the background signal. Numerical simulations of the explosion in the hallway structure are performed using the CRAFT computational fluid dynamics code. While the simulations demonstrate general agreement with the hydrostatic pressure features measured in the experiment, the model predicts temperatures significantly higher than the temperature sensitivity limit of the probed spectral band feature and exhibits fluctuations of temperature on the order of several hundred kelvin due to turbulence.