The role of detailed kinetics in the double flame structure of nitromethane droplet combustion

Abstract

Although nitromethane (NM) kinetics has been long examined, either as a fuel or as an intermediate in the carbon-nitrogen interaction, the coupling between its gas-phase chemistry and liquid droplet evaporation still needs further fundamental investigation. To address this, this work performs the first detailed numerical comparison of existing nitromethane kinetic mechanisms applied to single-droplet combustion, providing quantitative assessment against available experimental data. The numerical model accounts for transient heat and mass transfer, vapor-liquid equilibrium, radiative effects, and the influence of the supporting rod. It was observed that existing literature kinetic mechanisms for NM often contain inaccurate parameters for NM transport properties, which strongly affect droplet evaporation predictions, particularly through overestimated thermal conductivity values. Comparisons with experimental data show that the CRECK mechanism has the best trade-off in reproducing both droplet burning rates and flame propagation when corrected transport properties are applied, although the predicted flame speeds are slightly overestimated. All models capture the characteristic double-flame structure typical of hybrid monopropellant combustion, with an inner decomposition zone and an outer oxidation layer. Only the Caltech mechanism exhibits an endothermic peak within the inner flame region. These results provide insight into droplet combustion behavior and the performance of different kinetic mechanisms.

Novelty and significance statement

This study presents the first simulation of nitromethane (NM) droplet combustion in air with variable oxygen concentration, using transient one-dimensional modeling with six detailed kinetic mechanisms. Unlike earlier studies focused on gaseous flames or simplified droplet models, the approach couples detailed chemistry with accurate transport and thermodynamic properties, including a correction to the previously overestimated NM thermal conductivity. Radiation and buoyancy-driven convection under normal gravity are also considered. Comparison with two experimental datasets identifies mechanisms capable of reproducing both droplet burning rates and flame speeds. Analysis of the characteristic double-flame structure provides new insight into hybrid combustion behavior and kinetic-mechanism validation for green propulsion applications. A comparison with a methanol droplet flame under identical conditions is also included. Overall, the work improves understanding of NM droplet combustion, highlights the influence of uncertainties in transport data, and provides a validated framework for modeling energetic monopropellants and related combustion systems.