Generalization of Oberlack’s definition of the MILD combustion regime

The Oberlack definition of the MILD combustion limit for premixed systems was derived under the assumption of lean combustion and a one-step reaction. In this study, a generalization of this definition is presented by removing the lean combustion assumption, which leads to a more comprehensive relation between the Damköhler number and temperature, defining the so-called S-curve. The transition of the S-curve to a monotonic function, indicating MILD conditions in the generalized formulation, reveals a dependency on the kinetic parameters of the reaction (reaction orders) and the equivalence ratio. Unlike the previous definition, the proposed solution applies across a broader range of conditions, from rich to lean mixtures, incorporating variations in combustion conditions and the reactivity of the analyzed system. Analytical solutions are not available due to the strong non-linearity of the model; therefore, the results are obtained numerically and are presented as plots and approximation functions, all valid in a wide range of parameter values and applicable to various fuels. The proposed methodology is adaptable to different parameter ranges if needed. Finally, two practical examples, based on hydrogen and methane, illustrate the findings. The results show that reaction orders and the equivalence ratio significantly influence the limit curve defining the MILD combustion regime, with dependencies on the combustion conditions and the chosen fuel. • Oberlack’s definition of the MILD combustion regime has been generalized. • The previously adopted assumption regarding lean mixture combustion has not been made. • A new, more general formula for the S-curve ( D a vs T ) is derived. • A methodology for finding the boundary of the MILD combustion regime is presented. • General and fuel-specific ( H 2 , CH 4 ) MILD combustion boundaries were determined.