Analysis of a Quasi-Two-Dimensional Flamelet Model on a Three-Feed Non-premixed Oxy-Combustion Burner

Three-feed combustion systems in which fuel gas, oxygen, and diluent (CO ₂) are issued into a combustor are key components to realize an oxy-fuel type gas turbine in a zero-emission plant. Yet, simulations of such systems using mixture fraction-based models are difficult, since multiple mixture fractions are required to describe the system. In this study, large-eddy simulations (LES) with different formulations of non-adiabatic quasi-two-dimensional flamelet (Q2DF) models were performed on a three-feed non-premixed swirl burner. The Q2DF models are derived based on the treatments regarding the third stream; the diluent stream is put in the oxidizer side and/or in the fuel side, giving rise to three models called Q2DF1, Q2DF2, and Q2DF3 models. Results show that the three Q2DF models can predict the results of the experiment well; however, the deviations could not be overlooked. The analysis shows that the differences between the three models become apparent as the mixture fraction of the inactive third stream (Z₃) evolves very large, otherwise, the three models give almost the same results. It is confirmed that for a pure inactive diluent third stream when Z₃ is quite large, its scalar dissipation rate (χ₃) plays an important role and the mixing way (premix or non-premix) of the third stream with other streams should be taken into account, however, the influence of χ₃ on the performance of the three models is quite limited in the condition of a smaller Z₃, for instance, less than 0.8, and thus the mixing way of the third stream in the three models will not affect the system.