LES of turbulent partially-premixed flames using reaction–diffusion manifold-reduced chemistry with a consistent gradient estimate determined “on the fly”

This paper devises and applies a method for determining consistent, system-adapted reaction–diffusion manifolds (REDIM) for turbulent partially-premixed combustion. The method is an extension of a previous technique which was limited to laminar flames. It adapts a REDIM to the scalar gradients present in a given reacting flow by an iterative procedure, in which gradients from REDIM-reduced simulations are used to create a new and improved REDIM. An application of the method is demonstrated using large eddy simulations (LES) of a turbulent partially-premixed methane/air flame as the “gradient estimate generating” flame simulation. Compared to the previous laminar flame studies, the turbulent flame features less sharp gradient-scalar correlations because of the strong scattering of data. Methods for capturing the scatter in scalar-gradient correlations are applied, allowing us to assess the influence of the scatter on the resulting REDIM. Also, the fact that turbulent flame simulations often involve spatial filtering operations which tend to reduce the magnitude of the gradients is taken into account by comparing the REDIM output to quasi-direct numerical simulation (qDNS) data. It is found that the gradients devised from the filtered simulations are by a factor of 2–3 below those of the more fully resolved qDNS. Nevertheless, the REDIM devised from the LES predicts the states of the qDNS with good accuracy. Overall, observations show that the method can produce realistic reduced models also for systems with complex interactions of chemical reaction, molecular transport and flow, like partially-premixed turbulent flames.