A numerical study of strain effects on hypergolic ignition of MMH/NTO

While hypergolic propellants (e.g., monomethylhydrazine (MMH)/dinitrogen tetroxide (NTO)), which are capable of spontaneous ignition upon contact, are widely used in rockets and spacecraft, the hypergolic ignition processes remain incompletely understood. To this end, numerical simulations of the hypergolic ignition of MMH/NTO are performed in this work using a detailed chemistry, with particular attention to the effects of flow strain. It is found that there are four distinct ignition stages: cold reaction, nitric acid reaction, edge flame, and final multilayer flame. Increasing the strain rate can advance all stages, thin the flame front, and increase the peak heat release rate. Furthermore, budget and flame displacement speed analyses indicate that the edge-flame stage exhibits a more source-dominated character and faster propagation than the multilayer stage. These results clarify the stage-wise evolution and strain-controlled propagation of MMH/NTO hypergolic ignition. Novelty and significance statement: This work builds on the MMH/NTO counterflow benchmark of Hayashi et al. and provides a mechanism-resolved description of the transient gas-phase route by which hypergolic ignition develops from low-temperature contact reactions to the multilayer flame structure reported in that prior study. Its novelty lies in combining homogeneous reactor analysis with fully resolved two-dimensional simulations to identify practical stage markers and a physically interpretable four-stage ignition sequence. The study further links the stage transitions to the evolving thermochemical structure of the reaction zone and examines how strain rate modifies stage transitions, heat release, flame front propagation, transport budgets, and displacement speed behaviors. These analyses clarify the transient formation pathway and strain-dependent propagation characteristics of MMH/NTO hypergolic ignition, thereby informing reduced models, ignition criteria, and safety-related simulations of hypergolic propulsion systems.