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.