Role of intermediate temperature kinetics and radical transport in the prediction of leading edge structure of turbulent lean premixed flames

Abstract

The stabilization of turbulent lean premixed flames in a backward-facing step combustor is investigated using large eddy simulations, with a reduced 2-step reaction kinetics mechanism and one of two detailed mechanisms. Although all reaction mechanisms predict similar laminar flame velocities and adiabatic flame temperatures, significant instabilities are observed only when the detailed mechanisms are employed, as supported by experimental observations. The flame structure, location of its leading edge away from the step, and primary recirculation zone are generally in better agreement with the experimental data when using the detailed mechanisms. These observations suggest that key kinetic steps missing in a reduced mechanism may be crucial for modeling the complex interactions between the turbulent flow and chemical reactions in these reacting flows. Indeed, by examining the flame structure in some detail, we find that intermediate temperature chemistry in the recirculation zone region contributes to the flame response and hence the evolution of instabilities downstream. We examine the results obtained using the detailed mechanisms to show that radical transport within the flame anchoring standoff distance play an important role in intermediate temperature reactions, initiated by local mixing of reactants and products. While flamelet like behavior was observed downstream, departure from that structure in the upstream recirculation region where reactants mix with products makes the flames more receptive to large-scale vorticity fluctuations. While global reduced mechanisms reproduce macro targets such as flame speed and temperature, they need not necessarily reproduce the correct flame anchoring location, and the interactions with shear layers.