Optical emission from nitrogen containing intermediates in high pressure nitromethane flames

Abstract

Liquid monopropellants are fuels capable of sustained combustion without a separate oxidizer, making them attractive for applications where oxygen is scarce, such as underwater or in space. However, these fuels require high pressures to combust reliably, making experimental measurements technically challenging. As a result, the chemical processes governing monopropellant combustion at elevated pressures remain poorly understood, and existing computational models describing these processes lack experimental validation under realistic operating conditions. This dissertation presents spectroscopic measurements of nitromethane flames burning at elevated pressures under both air-independent and air-supported conditions. Experiments were conducted using a custom-built high-pressure combustion assembly capable of sustaining stable flames at pressures relevant to propulsion applications. Optical emission spectroscopy, a technique that measures the light emitted by a flame to identify the chemical species present, was used to spatially resolve individual intermediates in the flame under both atmospheric conditions. Measurements revealed the presence of three nitrogen-containing intermediates, CNꞏ, NHꞏ, and NH 2ꞏ, that have not been previously reported in significant quantities in high-pressure nitromethane combustion and are largely absent from existing computational kinetic models. All three species were detected in air-independent flames and in the decomposition region of air- supported flames, but were suppressed in regions where oxygen was available, consistent with known oxidation chemistry. Temperature measurements extracted from the spectroscopic data provided additional characterization of the flame environment. These experimental observations were found to be in qualitative agreement with new kinetic modeling simulations performed by collaborators, lending further support to the observed trends. These findings provide the first spatially resolved experimental measurements of nitrogen-containing intermediates in high-pressure nitromethane combustion and establish critical benchmarks for the validation and refinement of computational combustion models. The experimental platform demonstrated here is broadly applicable to other liquid propellant fuels, offering a path toward more comprehensive experimental characterization of monopropellant combustion chemistry under application-relevant conditions.