Operando Spectral Imaging of OH A²Σ⁺ → X²Π 1D Emission in Elevated-Pressure Nitromethane Flames

Optical emission spectroscopy was used to spatially resolve excited OH (OH*) emission as a function of height along the flame for nitromethane (NM) combusting in both inert and oxidative environments at 34 bar. Emission spectra from OH A²Σ⁺ → X²Π relaxation show that electronically excited OH* emission persists over the length of a nitromethane flame in air, but emission diminishes abruptly within several millimeters of nitromethane flames burning in inert atmospheres (N₂). These findings mark the first reported instances of spatially resolved spectral images of NM combustion at elevated pressures and provide important benchmarks for monopropellant combustion models, where number densities of products and intermediates are often reported as a function of distance from a nominal flame front. Vibrational and rotational temperatures calculated as a function of distance along the flame are compared to Cantera-simulated adiabatic flame temperatures. OH* rotational temperatures appear thermalized, although vibrational temperatures suggest that the radical possesses excess vibrational energy. Additionally, vibrational temperatures are used to infer the formation pathway─chemical vs thermal─of OH* as a function of height within the flame.