Effect of NO on extinction and re-ignition of vortex-perturbed hydrogen flames

Abstract

The catalytic effect of nitric oxide (NO) on the extinction and re-ignition of a vortex-perturbed hydrogen/air diffusion flame is studied in a heated counter-flow configuration. Nitrogen-diluted hydrogen at room temperature is burned with air heated to approximately the auto-ignition temperature. Localized extinction is induced by a fuel-side toroidal vortex, and the recovery process of the extinguished region is monitored by planar laser-induced fluorescence of the hydroxyl radical (OH). The addition of a small amount of NO significantly alters the re-ignition process by shifting the balance between chain-termination and chain-branching reactions. Without NO doping, the flame recovery process is mainly governed by edge-flame propagation. With NO doping, however, auto-ignition occurs in the center of the extinguished region, and the flame recovers by a merging between the central re-ignition kernel and the annular edge-flame. The influence of NO on the re-ignition is investigated as a function of fuel concentration and air temperature. Experimental results are compared to direct numerical simulations. This combined experimental and numerical study provides detailed insight into the interaction between transient flows and ignition processes.