Effects of NO2 addition on hydrogen ignition behind reflected shock waves

Ignition delay time measurements of H2/O2/NO2 mixtures diluted in Ar have been measured in a shock tube behind reflected shock waves. Three different NO2 concentrations have been studied (100, 400 and 1600 ppm) at three pressure conditions (around 1.5, 13, and 30 atm) and for various H2–O2 equivalence ratios for the 100 ppm NO2 case. Results were compared to some recent ignition delay time measurements of H2/O2 mixtures. A strong dependence of the ignition delay time on the pressure and the NO2 concentration was observed, whereas the variation in the equivalence ratio did not exhibit any appreciable effect on the delay time. A mechanism combining recent H2/O2 chemistry and a recent high-pressure NOx sub-mechanism with an updated reaction rate for H2 + NO2 ⇄ HONO + H was found to represent correctly the experimental trends over the entire range of conditions. A chemical analysis was conducted using this mechanism to interpret the experimental results. Ignition delay time data with NO2 and other NOx species as additives or impurities are rare, and the present study provides such data over a relatively wide pressure range.