Predicting no formation in flameless combustion regime

The direct implementation of large kinetic mechanisms for the prediction of pollutant emissions into CFD codes is still unfeasible, due to computer time limitations which become particularly relevant when considering the typical scale of the industrial applications. Therefore, simplified modeling approaches are generally adopted, as they allow reducing the computational effort associated with the numerical simulations. With regard to NO formation, simple one-step rates are used to describe each of the relevant routes contributing to the overall generation of NO, i.e. thermal, prompt, etc... The main drawback associated to a simplified NO formation approach lies, however, in the extreme sensitivity of the lumped rates on the thermo chemical state which characterize the combustion system of interest. Then, a proper description of turbulence-chemistry interactions must be performed in the CFD model, to provide a realistic background for the estimation of NO emissions. This becomes particularly important in flameless combustion regime, which generally requires an accurate description of the gas-phase oxidation, due to the kinetic control on the overall combustion process. The present paper discusses main key aspects and requisite for predicting NO formation in flameless combustion regime. The approach is based on the direct coupling of simplified NO mechanisms to the CFD calculation and is applied to different flameless conditions. An a priori analysis for the selection of the best available CFD model for the system of interest is also proposed. In particular, Principal Component Analysis (PCA) is employed for the extraction of the parameters controlling the thermo-chemical state of the system and to identify the CFD modelling requirements, from the point of view of turbulence/chemistry interaction models.