Stretched Laminar Flamelet Modeling of a Turbulent Jet Diffusion Flame

A turbulent combustion model for a CO/H₂/N₂-air turbulent jet diffusion flame has been developed by combining a velocity-composition joint pdf turbulence closure, a Monte Carlo solution algorithm, and a two-scalar stretched laminar flamelet chemistry approach. In the flamelet model, it is assumed that the structure of the turbulent diffusion flame is locally that of a laminar diffusion flame at the same instantaneous value of the mixture fraction (ξ) and scalar dissipation (χ). A library of stretched laminar flamelets (density, temperature, and species concentrations vs, ξ and x) is generated for a laminar opposed-flow configuration including 15 chemical species, 32 reactions, and differential diffusion. A modeled transport equation for the joint probability density function of the velocities, ξ and x is then solved numerically by a Monte Carlo method. Calculated profiles of mean mixture fraction, temperature, and species concentrations are compared both with experimental data and with earlier modeling studies based on different turbulence closures and different chemistry schemes. Overall, the level of agreement between flamelet model and experiment is comparable to that obtained using a two-scalar partial equilibrium model. The flamelet approach shows some advantage in the fuel-rich regions of the flow, but yields an overly rapid approach to chemical equilibrium downstream in the jet. The flamelet model could not be rigorously justified in this turbulent diffusion flame because calculated laminar flame thicknesses are larger than the estimated smallest turbulence mixing scales. Still, we believe that a flamelet model is of value because it is computationally tractable and because it is one of the few approaches that directly includes the important coupling between chemical reaction and molecular diffusion.