A lagrangian model for predicting turbulent diffusion flames with chemical kinetic effects

Abstract

The present study is a new step in the development of a lagrangian turbulent combustion model devotedto the prediction of non-premixed flames in which kinetic effects occur. Taking into account, in a refined manner, the interaction between chemical reaction and turbulence, the model includes a spectrum of turbulent timescales instead of a single mean timescale. The more recent improvement of the model consists in including a library of chemical delay times derived from a six-step reduced reaction mechanism for methane-air combustion, which is valid over a wide range of equivalence ratio. The predictions of the combustion model in which the library was implemented were compared with experimental data on turbulent jet diffusion flames near extinction. These calculations were performed with a finite volume technique devoted to variable density Navier-Stokes equations. A standard two-equations turbulence model was used and provided reliable results for dynamic variables (mean velocity and kinetic energy of turbulence). Comparisons between computed and measured mean mixture fraction and mixture fraction root mean square (rms) are quite satisfactory, proving that scalar transport is correctly modeled. The more salient point is the prediction of partial extinction and reignition phenomena, in agreement with experiments. This result clearly demonstrates the ability of the turbulent combustion model to simulate the interaction between turbulence and chemical reactions, which occurs in flames displaying kinetic effects.