Analysis of a Turbulent Lifted Hydrogen/Air Jet Flame from Direct Numerical Simulation with Computational Singular Perturbation

Abstract

The theory of computational singular perturbation (CSP) was employed to analyze the near-field structure of the stabilization region of a lifted hydrogen/air slot jet flame in a heated air coflow simulated with three-dimensional direct numerical simulation (DNS). The simulation was performed with a detailed hydrogen–air mechanism and mixture-averaged transport properties at a jet Reynolds number of 11,200 with approximately 1 billion grid points. Explosive chemical processes and their characteristic time scales, as well as the species involved, were identified by the CSP analysis of the Jacobian matrix of chemical source terms for species and temperature. An explosion index was defined for explosive modes, indicating the participation of species and temperature in the explosion process. Radical explosion and thermal runaway can consequently be distinguished. The CSP analysis of the simulated lifted flame shows the existence of two premixed flame fronts, which are difficult to detect with conventional methods. The upstream fork separating the two flame fronts thereby identifies the lift-off point. A Damkohler number was defined with the time scale of the chemical explosive mode and the scalar dissipation rate to show the role of auto-ignition in affecting the lift-off point and in stabilizing the flame.