Modeling of nitromethane flame structure and burning behavior

Nitromethane was investigated in this study due to the push for higher performance and reduced toxicity monopropellant. A comprehensive detailed model for its flame structure and linear regression rate was developed and validated with experimental data. The model considered one-dimensional behavior with surface vaporization and detailed gas-phase kinetics based on the RDX mechanism of Yetter et al. combined with the nitromethane decomposition mechanism of Glarborg, Bendtsen, and Miller, resulting in a mechanism consisting of 47 species and 250 elementary reactions. The predictive model was implemented using a custom FORTRAN code wrapping the CHEMKIN 4 PREMIX gas-phase solver coupled with the condensed-phase solution. Predicted burning rates using the model showed good agreement with measured rates up to 15 MPa. Calculated species and temperature profiles showed three distinct regions based upon the appearance and consumption of certain species. The first region was marked by decomposition of nitromethane, the second region by consumption of all intermediate species except CH₄ and NO, and the third region by the rise to final temperature and species concentrations near the equilibrium values. Among the intermediate species, CH₄ and NO had higher concentrations than those of CH₂O, N₂O, HNO, and HONO. CH₄ served as fuel species and NO provided a portion of the oxidizer for the third-region reactions to reach equilibrium composition. Sensitivity analysis identified the importance of two elementary reactions involving HNO to the temperature profile, and therefore the burning rate. Although the absolute level of NH and HCO were low, they served as an important intermediate species transporting nitrogen and carbon, respectively, between other higher-concentration species. The calculated flame zone thickness is consistent with that measured by microthermocouples.