A computational fluid dynamics model of combustion of nitromethane propellant in a strand burner

Abstract

A computational fluid dynamics (CFD) model of unsteady combustion of liquid nitromethane (NM) propellant in a strand burner is developed. Two detailed chemical mechanisms of Boyer and Kuo (2007) and Brequigny et al. (2015) are employed. Ignition delays are computed by conducting flow reactor simulations. Based on the computed ignition delays, the activation energies are estimated to be in the range of 57–71 kJ/mol for higher initial temperatures (1111–1250 K) and in the range of 155–220 kJ/mol at lower initial temperatures (833–909 K). The effect of pressure on reaction rate is found to be more pronounced for Boyer and Kuo’s mechanism than for Brequigny et al.‘s mechanism due to the highly pressure-dependent NO→HNO→N2⁢O→N2 NO reduction pathway in the former case. Vapor-phase flame speeds are computed over a broad range of pressures using the propagation method. For Brequigny et al.’s mechanism, the flame speed remained nearly independent of pressure, suggesting overall nearly second-order kinetics. On the other hand, Boyer and Kuo’s mechanism predicted a non-monotonic pressure dependence, with a minimum flame speed at 1 MPa and a pressure exponent of 0.22 at higher pressures. Finally, the combustion of liquid NM in a strand burner in both inert and oxygen environments is simulated for a pressure range of 0.5–6 MPa. In inert environments, self-sustained combustion could not be achieved for pressures lower than 1 MPa due to low reaction rates and convection of supplied energy out of the domain. In oxygenated environments, the presence of oxygen led to the formation of a secondary flame, which enabled self-sustained combustion at lower pressures and enhancement in burning rates as high as 64%. The Boyer and Kuo’s mechanism yielded accurate burning rates with a pressure exponent of 1.27 over the pressure range of 1–6 MPa. It is observed that Brequigny et al.’s mechanism under-predicted the burning rates and yielded a lower pressure exponent of 1.10.