Analysis of the Mixing and Emission Characteristics in a Model Combustor

Modern gas turbines in power systems employ lean premixed combustion to lower flame temperature and thus achieve low NO_x emissions. The fuel/air mixing process and its impacts on emissions are of paramount importance to combustor performance. In this study, the mixing process in a methane-fired model combustor was studied through an integrated experimental and numerical study. The experimental results show that at the dump location, the time-averaged fuel/air unmixedness is less than 10% over a wide range of testing conditions, demonstrating the good mixing performance of the specific premixer on the time-averaged level. A study of the effects of turbulent Schmidt number on the unmixedness prediction shows that for the complex flow field involved, it is challenging for Reynolds-Averaged Navier-Stokes (RANS) simulations with constant turbulent Schmidt number to accurately predict the mixing process throughout the combustor. Further analysis reveals that the production and scalar dissipation are the key physical processes controlling the fuel/air mixing. Finally, the NO_x formation in this model combustor was analyzed and modelled through a flamelet-based approach, in which NO_x formation is characterized through flame-front NO_x and its post-flame formation rate obtained from one-dimensional laminar premixed flames. The effect of fuel/air unmixedness on NO_x formation is accounted for through the presumed probability density functions (PDF) of mixture fraction. Results show that the measured NO_x in the model combustor are bounded by the model predictions with the fuel/air unmixedness being 3% and 5% of the maximum unmixedness. In the context of RANS, the accuracy in NO_x prediction depends on the unmixedness prediction which is sensitive to turbulent Schmidt number.