Three-dimensional direct numerical simulation of a turbulent lifted hydrogen jet flame in heated coflow: a chemical explosive mode analysis

TLDR

The study develops a chemical explosive mode analysis (CEMA) diagnostic and defines a Damköhler number to identify flame and ignition structures and assess auto‑ignition effects in complex turbulent lifted hydrogen jet flames. The authors performed a 3‑D direct numerical simulation of a turbulent lifted hydrogen–air slot jet flame in heated air coflow at Re = 11 000 with a detailed mechanism and 900 million grid points, then applied CEMA to extract explosive chemical modes, their time scales, species contributions, and an explosion index. CEMA reveals two premixed flame fronts and an upstream fork that marks the stabilization point, enabling differentiation between radical and thermal runaway in the lifted flame.

Abstract

A chemical explosive mode analysis (CEMA) was developed as a new diagnostic to identify flame and ignition structure in complex flows. CEMA was then used to analyse the near-field structure of the stabilization region of a turbulent lifted hydrogen–air slot jet flame in a heated air coflow computed with three-dimensional direct numerical simulation. The simulation was performed with a detailed hydrogen–air mechanism and mixture-averaged transport properties at a jet Reynolds number of 11000 with over 900 million grid points. Explosive chemical modes and their characteristic time scales, as well as the species involved, were identified from the Jacobian matrix of the chemical source terms for species and temperature. An explosion index was defined for explosive modes, indicating the contribution of species and temperature in the explosion process. Radical and thermal runaway can consequently be distinguished. CEMA of the lifted flame shows the existence of two premixed flame fronts, which are difficult to detect with conventional methods. The upstream fork preceding the two flame fronts thereby identifies the stabilization point. A Damköhler number was defined based on the time scale of the chemical explosive mode and the local instantaneous scalar dissipation rate to highlight the role of auto-ignition in affecting the stabilization points in the lifted jet flame.