Modeling midwave infrared muzzle flash spectra from unsuppressed and flash-suppressed large caliber munitions

Time-resolved infrared spectra of firings from a 152 mm howitzer were acquired over an 1800–6000 cm−1 spectral range using a Fourier-transform spectrometer. The instrument collected primarily at 32 cm−1 spectral and 100 Hz temporal resolutions. Munitions included unsuppressed and chemically flash suppressed propellants. Secondary combustion occurred with unsuppressed propellants resulting in flash emissions lasting ∼100 ms and dominated by H2O and CO2 spectral structure. Non-combusting plume emissions were one-tenth as intense and approached background levels within 20–40 ms. A low-dimensional phenomenological model was used to reduce the data to temperatures, soot absorbances, and column densities of H2O, CO2, CH4, and CO. The combusting plumes exhibit peak temperatures of ∼1400 K, areas of greater than 32 m2, low soot emissivity of ∼0.04, with nearly all the CO converted to CO2. The non-combusting plumes exhibit lower temperatures of ∼1000 K, areas of ∼5 m2, soot emissivity of greater than 0.38 and CO as the primary product. Maximum fit residual relative to peak intensity are 14% and 8.9% for combusting and non-combusting plumes, respectively. The model was generalized to account for turbulence-induced variations in the muzzle plumes. Distributions of temperature and concentration in 1–2 spatial regions demonstrate a reduction in maximum residuals by 40%. A two-region model of combusting plumes provides a plausible interpretation as a ∼1550 K, optically thick plume core and ∼2550 K, thin, surface-layer flame-front. Temperature rate of change was used to characterize timescales and energy release for plume emissions. Heat of combustion was estimated to be ∼5 MJ/kg.