An iterative methodology for REDIM reduced chemistry generation and its validation for partially-premixed combustion

Partially-premixed flames (PPFs) incorporate effects of both premixed and non-premixed types of reaction zones. The modelling of PPFs using manifold-based model reduction methods faces some inherent difficulties due to the underlying assumptions of a-priori identification of the type of combustion system. In this work, the reaction–diffusion manifold (REDIM) model reduction method is applied to study PPFs. The REDIM method requires minimal prior knowledge about the type of combustion system, which makes it a suitable method for studying PPFs. It allows incorporating system-specific diffusion (gradients) terms in a generic way so that the manifold can evolve according to the diffusion related information provided by the combustion system. In this way, a prior identification of the type of combustion system is no longer needed. This work utilises an iterative methodology to generate REDIM chemistry tables so that the reduced manifold can be iteratively converged very close to the detailed manifold according to the gradients of the reduced coordinates provided by the physical combustion system in each iteration step. In addition, a new method is proposed to provide the gradient estimates of the reduced coordinates during the generation of REDIM from the scattered gradient data in REDIM reduced CFD calculations. Laminar triple flames, a special case of PPFs, with two types of mixture fraction gradients are selected as the target cases to assess the presented iterative methodology. REDIM reduced calculations are compared with simulations based on detailed finite-rate kinetics. It is found that in the final iteration steps, temperature and all considered major and minor species mass fraction profiles are very well predicted by the REDIM reduced calculations.