Finite Rate Chemistry Effects in Turbulent Reacting Flows

The classical “fast chemistry” analysis by Damköhler remains a common basis for calculation methods aimed at turbulent reacting flows. Perturbation approaches can be used to introduce finite rate chemistry effects, particularly where a distinct chemical time-scale separation is present, though more comprehensive techniques, e.g. based on a transported joint probability density function (JPDF), are typically required. Potential difficulties with the JPDF technique include issues related to the intrinsic structure of turbulent flames, particularly at low Reynolds numbers, and models for molecular mixing. The ability to predict the formation of NO is particularly interesting in this context given the strong sensitivity to chemical kinetic and non-adiabatic effects. The current work initially provides an assessment of uncertainties in the formation chemistry of NO in the context of new quantitative measurements, obtained in non-premixed laminar methane/air counterflow flames using ps-LIF, and subsequently explores how these translate to turbulent flames. A consistent systematically reduced (16 independent, 4 dependent and 28 steady state scalars) reaction mechanism is applied to model the turbulent flames of Barlow and co-workers (8200 ≤Re≤ 44000). The highest Re number flame additionally permits an investigation into the ability of the transported JPDF technique to deal with emissions of nitric oxide in flames close to global extinction. The work shows that the technique has the potential to reproduce NO levels and conditional PDFs under conditions with significant local extinction/re-ignition to within the uncertainties associated with the principal elementary reaction steps.