Effects of H_2O and NO on extinction and re-ignition of vortex-perturbed hydrogen counterflow flames

摘要

The influence of water vapor (H_2O) and nitric oxide (NO) on the dynamics of extinction and re-ignition of a vortex-perturbed non-premixed hydrogen-air flame is investigated. A steady non-premixed flame is established in an axisymmetric counterflow configuration with a fuel stream of nitrogen-diluted hydrogen flowing against heated air containing 160 ppm of NO and various amounts of H_2O ranging from 1.2% to 7.1% by volume. Local extinction is induced by a fuel-side vortex, and the temporal evolution of the hydroxyl radical (OH) field is measured during the extinction and subsequent re-ignition processes using planar laser-induced fluorescence (PLIF). The additives modify the flame recovery mode by affecting the competition between autoignition and edge-flame propagation. Doping with 160 ppm of NO can significantly enhance the re-ignition of hydrogen due to its catalytic effect, while H_2O addition inhibits re-ignition because of its high specific heat and large third-body chaperon efficiency. In the flames considered here, direct numerical simulations are performed using a detailed H_2-air mechanism and are compared with experiments. The simulations show that a small amount of OH from NO-catalyzed reactions enhances autoignition in the extinguished region, whereas adding H_2O inhibits re-ignition. Although flame propagation and extinction are largely unaffected by these additives, the induction time associated with the flame's recovery mode by autoignition is strongly affected. Hence, these additives can be used to control the recovery time following local extinction.