Competing Effects of a Vortex on Non-Premixed Autoignition of H 2 /air in a Mixing Layer: A DNS Parametric Study

Ignition characteristics of non-premixed hydrogen and heated air in a rolled-up vortex are investigated through a series of two-dimensional direct numerical simulations with a detailed chemical kinetic mechanism. A comprehensive parametric study is conducted by varying three control parameters: vortex intensity, vortex core radius, and hot air temperature. Chemical explosive mode analysis (CEMA) is employed to detect the onset of the ignition kernel, and Lagrangian particle tracking is used to examine the temporal evolution of scalar dissipation rate within the vortex. Two distinct ignition modes are identified based on the ignition kernel location: vortex tip ignition (VTI) and outside vortex ignition (OVI). The ignition delay of OVI remains largely unaffected by the vortex structure, while that of VTI exhibits a non-monotonic “U-shaped” dependence on vortex intensity. This behavior arises from two competing effects: the vortex initially reduces the scalar dissipation rate at the vortex tip by locally extending the mixing layer (advancing effect), but the progressive two-dimensional tangential diffusion subsequently increases the scalar dissipation rate (retarding effect). The advancing effect prevails at moderate vortex intensities, while the retarding effect becomes dominant at higher intensities. The same non-monotonic behavior is observed with respect to vortex core radius and air temperature, confirming that the ignition characteristics of VTI are governed by the interplay between convective and chemical time scales. The ignition delay of VTI is characterized by the Damköhler number, Da, defined as the ratio of convective to chemical time scales, which identifies three regimes: the convection-dominated regime (Da<1), where VTI is retarded or suppressed; the balanced regime (Da≈1), where VTI is maximally advanced; and the chemistry-dominated regime (Da≫1), where the vortex effect diminishes. This framework suggests that in turbulent flows, vortices with Da≈1 are most likely to govern the overall ignition process.