Multi-environment probability density function approach for turbulent partially-premixed methane/air flame with inhomogeneous inlets

The present study aims to systematically evaluate the capability of a multi-environment PDF approach using non-premixed and premixed tabulated chemistry to predict the fundamental characteristics of turbulent partially-premixed piloted flames with near-homogeneous and inhomogeneous inlets. For the near-homogeneous case, the non-premixed manifold yields better agreement with measurements in terms of conditional mean, unconditional mean, and rms scalars; however, the premixed manifold generates a narrower hot flame zone, which is mainly attributed to its inability to account for diffusion in the mixture fraction space. For the inhomogeneous flame, the premixed and non-premixed manifolds are limited in their ability to reproduce the measured flame at upstream locations with multiple combustion modes, while the non-premixed manifold is reasonably good at predicting the unconditional and conditional profiles of all scalars in the downstream regions with dominant non-premixed combustion. In terms of the predicted environment-conditioned scalar profiles, the present multi-environment PDF approach demonstrated the ability to realistically predict the near-vertical transition from the fuel-rich flammable mixture condition to the stoichiometric pilot condition. Unlike the near-homogeneous case, the inhomogeneous case generates a distinctly different distribution of CO conditional means, where the peak conditional CO level is leaning to the richer side along the downstream region. This tendency is a clear indication of the flame transition from a premixed-dominated flame to a diffusion-dominated flame. Based on our numerical results, detailed discussions about the essential features of turbulent partially-premixed flames with multiple combustion modes are presented, as well as the limitations of the proposed approach.