Extinction of stratified counterflow H 2 / air premixed flames under intense turbulence and strain

Highly turbulent lean premixed hydrogen-air flames stabilized against counter-flowing non-adiabatic stoichiometric combustion products in chemical equilibrium are investigated in a joint numerical and experimental effort. The effects of turbulence, strain rate, non-adiabaticity and composition stratification on flame structure, local flame quenching and re-ignition are studied using three-dimensional Direct Numerical Simulations (DNS) and laser induced fluorescence (LIF) imaging of OH. Combustion was established at an elevated turbulent Reynolds number of 1,500 and a bulk strain rate of 2400 s in a compact cylindrical volume delimited by the two co-axial nozzles of diameter 12.7 mm and separation distance of 12 mm. The reactant and equilibrium product streams are at temperatures of 294K and 1475K respectively, and at 1 atm. Computationally, the compressible Navier-Stokes, total energy and species conservation equations are solved using a high order, low-dissipative finite difference scheme. An explicit Runge-Kutta scheme is used for time integration. A detailed chemical kinetics mechanism for hydrogen-air combustion is used, that involves 9 species and 19 elementary reaction steps. The code is parallelized with MPI message passing routines. The analysis of the combined numerical and experimental data aims to quantify the local extinction levels and examine the morphology of the extinguishing and re-light events. The joint approach focuses on a detailed presentation of the turbulence-flame interactions and an assessment of the driving mechanism that leads to the appearance of non-reactive regions along with the main re-ignition patterns. The main objective is to provide a combined numerical and experimental database and achieve in-depth physical understanding of the fundamental and complex mechanisms associated with local extinction and flame healing in turbulent stratified combustion.