Nonpremixed ignition of H2/air in a mixing layer with a vortex

The ignition of a laminar non-premixed H2/air mixing layer with an embedded vortex was computationally studied with detailed chemistry and transport. The initial vortex velocity and pressure fields were specified based on the stream function of an incompressible nonviscous vortex. The fuel side is pure hydrogen at 300 K, and the oxidizer side is air at 2000 K. The vortex evolution process was found to consist of two ignition events. The first ignition occurs in a diffusion mode with chain branching reactions dominating. The second ignition takes place in the premixed mode, with more chemical reactions involved, and is significantly affected by the heat and species generated in the first ignition event. The coupling between the most reactive mixture fraction and scalar dissipation rate was verified to be crucial to the ignition delay. The effects of the vortex strength, characteristic size, and its center location were individually investigated. For all vortex cases, the ignition delay was shorter than that of the 1D case. Furthermore, the ignition delay has a nonmonotonic dependence on all the vortex parameters. 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.