Effects of pressure and dilution on the extinction of counterflow nonpremixed hydrogen-air flames

Abstract

To increase the understanding of the critical phenomena of ignition and extinction in high-speed propulsiondevices, an experimental and computational study has been conducted on the strain-induced extinction of nonpremixed counterflow flames of diluted hydrogen against air. The study reports laser-Doppler velocimetry-(LDV)-determined local extinction strain rates of these flames, with various amounts of dilution and at pressures of 0.5 and 1.0 atm. The measured data compare well with results obtained from computational simulations with detailed chemistry and transport. Additional computational study on the effects of dilution and pressure shows that the extinction flame temperatures and strain rates exhibit a nonmonotonic variation with increasing pressure, which is characteristic of the explosion limits of homogeneous hydrogen-oxygen mixtures. This behavior is explained on the basis of the intrinsic chain branching-termination kinetics of hydrogen oxidation. The similarity in the dominant kinetic steps responsible for both the ignition/explosion and extinction phenomena is discussed.