Assessment of steady and unsteady flamelet models for MILD combustion modeling

Moderate or intense low-oxygen dilution (MILD) combustion is a novel combustion technology with the comparable chemical and turbulent mixing timescales. In the most of literatures, relatively expensive volume-based models are recommended for this combustion regime while this regime is not completely idealized homogenized reactor and nor strong flamelet like. This paper is focused on the assessment of the lower cost, flamelet approach for MILD condition. In this way, simplifying JHC burner of Dally et al. is considered to model using RANS approach. The effects of inclusion of unity versus non-unity Lewis numbers, radiation heat transfer, and various scalar dissipation rates are evaluated. Results show that the flamelet model may not be totally rejected for the MILD condition because it could still capture flame characteristics relatively acceptable. Choosing an appropriate scalar dissipation rate value is discussed for single flamelet and higher values are recommended by O2 increment. Moreover, considering non-unity Lewis numbers improved species concentration at the jet centerline for low O2 levels. Furthermore, the unsteady flamelet model with radiation had a better prediction for NOx especially for higher O2 Levels.