Flamelet-based NOx radiation-integrated modelling of turbulent non-premixed flame using Reynolds-stress closure

A methodology of extending laminar flamelet model in its adiabatic form to a non-adiabatic form which can account for radiative heat loss as well as its effect on NO x pollutant has been developed. Coupling of radiation submodel with flamelet model is based on the enthalpy defect concept. Pollutant NO x has been calculated from solution of its transport equation containing source term which is derived from flamelet calculations. Flamelet calculations adopted GRI 2.11 reaction mechanism which accounts for detailed carbon and NO x chemistry. Depending on consideration of variation in scalar dissipation within flamelet calculations, the non-adiabatic form has been further divided into non-adiabatic model with single (NADS) and multiple scalar dissipation rates (NADM). Bluff-body stabilized CH4/H2 flame has been chosen as the test case to assess the capability of non-adiabatic models. Turbulence closure has been achieved with a Reynolds stress transport model. Calculations have also been carried out with a modified k-ε model for evaluation of relative performance of the two turbulence closures. Performance of non-adiabatic flamelet models in regard to the overall structure of the flame is reasonably good and the agreement is similar to that of the adiabatic flamelet model thereby indicating weakly radiating nature of the flame. However, the NADM model results in minor but encouraging improvement in NO mass fraction predictions by reducing the extent of overprediction observed with the adiabatic model. In contrast, the NADS model results in overprediction over and above the adiabatic predictions thereby showing that, it is imperative to consider variation in scalar dissipation rate in flamelet calculations to capture the effect of radiation on NO. The results also show that employing the modified k-ε model instead of the Reynolds stress transport model for turbulence closure in NADM calculations results in considerable overprediction in centerline NO mass fractions.