The role of dilution level and canonical configuration in the modeling of MILD combustion systems with internal recirculation

Abstract MILD combustion regime emerged as one of the most promising technology to ensure low pollutant emissions and large fuel flexibility. Large-scale use of such technology requires important developments of its numerical modelling. The difficulty of this task lies in the recirculation of mass and heat that results in coupling chemistry and turbulence phenomena. In this work, the effect of the internal exhaust gas recirculation on the performances of a tabulated chemistry model was investigated by carrying out an experimental and numerical study on a cyclonic burner. FGM (Flamelet Generated Manifold) model was adopted. It makes two main assumptions: a turbulent flame can be considered as an ensemble of 1D laminar flames (Flamelets), and the space of compositions can be represented by a lower-dimensional manifold by defining few controlling variables. In this work, two different 1D flame configurations and two controlling variables (a mixture fraction and a progress variable) were used. The comparisons between model and experiments were reported for different dilution conditions at inlet and two diluent species. Results showed that the role of the internal EGR is crucial in the establishment of MILD regimes. In fact, for air conditions the model is not able to predict the low and more distributed reactivity of the system. On the other hand, inlet diluted conditions show a good agreement with the experiments due to burned product effects. CO2-dilution instead of N2, improves the model results due to the lower reaction rates. Finally, the Flamelet configuration does not strongly affect the simulation results.