Chemical Kinetics of Hypergolic Ignition in N2H4/N2O4-NO2 Gas Mixtures

Abstract

A detailed chemical kinetic mechanism for hypergolic ignition of ${\mathrm{N}}_2{\mathrm{H}}_4$ in a ${\mathrm{N}}_2{\mathrm{O}}_4\text{-}\mathrm{N}{\mathrm{O}}_2$ gas mixture has been constructed. In this mechanism, the hypergolic ignition is mainly caused by the sequential reactions of H atom abstraction from ${\mathrm{N}}_2{\mathrm{H}}_m$ by $\mathrm{N}{\mathrm{O}}_2\text{:}{\mathrm{N}}_2{\mathrm{H}}_m+\mathrm{N}{\mathrm{O}}_2={\mathrm{N}}_2{\mathrm{H}}_{m-1}+\mathrm{HONO}/\mathrm{HN}{\mathrm{O}}_2(m=4\sim 1)$. Although the first step of the H atom abstraction ($m=4$) is endothermic, consecutive abstraction reactions for $m=3$, 2, and 1 are exothermic, and especially heat release by the reaction of ${\mathrm{N}}_2\mathrm{H}+\mathrm{N}{\mathrm{O}}_2={\mathrm{N}}_2+\mathrm{HONO}(m=1)$ is large because of ${\mathrm{N}}_2$ production. Temperature rise caused by the heat release accelerates the endothermic initiation reaction ($m=4$). This thermal feedback is responsible for the hypergolic ignition at low temperatures. Because no experimental and theoretical information is available on these reactions, rate coefficients were evaluated on the basis of transition state theory, unimolecular rate theory, and master equation analysis with quantum chemical calculations of potential energy curves. In addition, reactions of ${\mathrm{N}}_2{\mathrm{H}}_4$ with ${\mathrm{N}}_2{\mathrm{O}}_4$ isomers were also examined. The present kinetic mechanism can explain gas-phase hypergolic ignition of ${\mathrm{N}}_2{\mathrm{H}}_4/\mathrm{NTO}$ mixtures at temperatures down to 200 K.