Promotion and inhibition of oxidation of rich hydrogen-air mixtures by nitric oxides (NO and NO$_2$) during adiabatic self-ignition

The tracer method was used to numerically study the effect of nitric oxides (NO and NO$_2$) on the oxidation of rich hydrogen-air mixtures during adiabatic self-ignition at low and high initial temperatures and a pressure of 0.1 MPa. At low temperatures, the added NO interacts with HO$_2$ to form NO$_2$, and NO$_2$ then interacts with H to form NO. When NO$_2$ is added at the same temperatures, a two-stage mechanism takes place: NO formed by the reaction NO$_2$ + H is not involved in the reaction until NO$_2$ is almost completely consumed. In the temperature range 900–1200 K, NO$_2$ inhibits self-ignition through participation in the reaction with H, leading to the replacement of part of the completely branched chain H $\to$ (O, OH) $\to$ 3H by the unbranched chain H $\to$ OH $\to$ H. At low initial temperatures, NO effectively promotes hydrogen oxidation due to replacement of the unbranched chain H $\to$ HO$_2$ $\to$ H$_2$O$_2$ $\to$ OH $\to$ H by a chain with branching.