Dimensionless correlation of peak soot volume fraction in MILD laminar counterflow diffusion flames: Effects of strain Rate, oxygen enrichment, and fuel blend composition

This study investigates the soot formation in laminar counterflow diffusion flames of multi component DME, H 2 , CO and C 2 H 4 blends under MILD combustion conditions, with the aim to develop a physicochemical -based, transferable soot prediction framework. Detailed simulations with full gas-phase chemistry, a PAH-based soot model, and radiative transfer are performed to quantify the combined effects of strain rate (a), oxidiser oxygen mole fraction (X O 2 ), and fuel-blend composition on aromatic precursors, radical fields, temperature, and soot volume fraction (f v,max ). From this database, a new correlation for the peak soot volume fraction is derived based on three dimensionless descriptors soot Damköhler number (Da s ), an effective local oxygen parameter (Chi_0 2 ), and a mixture sooting (S mix ) index that capture independently the competition between residence-time and kinetics, the local oxidising environment and the sooting tendency of the fuel-mixture. The correlation captures the detailed simulations over several orders of magnitude. This suggests that the strain-induced soot suppression is primarily controlled by residence-time limitation and reaction-zone compression rather than peak temperature changes. While oxidation increases concomitantly, oxygen enrichment strongly enhances precursor formation and soot growth, with mixture effects being mainly chemical in nature via modulation of PAH precursor pools. A regime map in the new dimensionless space segregates growth-dominated and oxidation-dominated operation regions, and serves as a compact design tool for low-soot operation of oxygenated fuel blends. This work provides mechanistic insight into the interplay of transport-chemistry-oxidation under MILD conditions and a predictive, dimensionless framework that can be extended to more complex configurations in future studies.