Advanced flamelet modelling of turbulent nonpremixed and partialy premixed combustion

Current work focuses on the development and performance evaluation of advanced flamelet models for turbulent non-premixed and partially premixed combustion in RANS and large eddy simulation (LES) based modelling. A RANS based combustion modelling strategy which has the ability to capture the detailed structure of turbulent non-premixed flames, including the pollutant NO, and account for the effects of radiation heat loss and transient evolution of NO, has been developed and incorporated into the in-house RANS code. The strategy employs an 'enthalpy-defect' based non-adiabatic flamelet model in conjunction with steady or unsteady nonadiabatic flamelets based NO submodels. The performanceo f the non-adiabaticm odel and its NO submodelsh asb eena ssessed against experimental measurements and steady flamelet model predictions for turbulent CH4/H2 bluff-body stabilized and CH4-air piloted jet flames. Appreciable improvements in the mean thermal structure predictions have been observed in the piloted jet flames by consideration of radiation heat loss through the non-adiabatic flamelet model. Since transient effects were weaker in the piloted jet flame, both unsteady and steady non-adiabatic NO submodels provided similar level of improvement in the pollutant NO predictions in comparison to their adiabatic counterpartsT. ransiente ffectsw ere, however,d ominanti n the bluff-body flame. The unsteady non-adiabatic NO submodel provided excellent agreement with measured NO distribution in comparison to the appreciably overpredicted distribution by its steadyc ounterpart.T he strategyo f non-adiabaticf lamelet model in conjunctionw ith unsteady non-adiabatic NO submodel seems to provide an accurate and robust alternative to the conventional strategy of steady flamelet model with steady NO submodel. While addressing the limitations of steady flamelet model in regard to radiation and slow chemistry of NO is one objective of this research, extending the applicability of the model to partially premixed combustion has been pursued as the second objective. Flamelet/progress variable (FPV) approach based combustion models, which have the potential to describe both non-premixed and partially premixed combustion, have been incorporated in the in-house RANS and LES codes. Based on the form of the PDF for reaction progress variable, two different formulations, FPV-8 function model and FPV-P function model, have been derived. (Continues...).