Probability density function calculations of local extinction and no production in piloted-jet turbulent methane/air flames

Abstract

The intense nonlinear interaction between turbulent fluctuations and finite-rate chemistry can cause local extinction and has a strong influence on NO production in non-premixed turbulent flames. Accurate predictions of local extinction and NO formation in turbulent flames require a rigorous means of representing such a strong coupling of turbulence and chemistry and hence are substantial challenges for turbulent combustion models. In this study, a self-contained joint velocity-composition-turbulence-frequency probability density function (PDF) method is used to make calculations of a series of piloted-jet nonpremixed flames of methane/air. The ingredients of the present model include the simplified Langevin model for velocity, a stochastic model of turbulence frequency, and the Euclidean minimum spanning tree (EMST) mixing model. An augmented reduced mechanism (ARM2) for methane oxidation, which involves 19 species and 15 reactions (including NO chemistry), is incorporated into the joint probability density function (JPDF) calculations using the in situ adaptive tabulation (ISAT) algorithm. The effects of radiative heat loss are studied using an optically thin limit model. The calculation results show good agreement with the experimental data, including the minor species NO and CO. The increase of local extinction with increasing jet velocity is accurately represented by the calculations.