Combustor shape optimization and NO emission characteristics for premixed NH3-CH4 turbulent swirling flame towards sustainable combustion

Abstract

Ammonia is considered an attractive carbon-free fuel for energy generation in aerospace gas turbine engines. However, ammonia is often used with methane fuel blend due to its low combustion limits. The formation of nitrogen oxides (NOx) during the combustion of ammonia-methane fuel blend is a fundamental problem of interest in the scientific community. To date, there have been several studies of ammonia-methane fuel blend, but none of them have considered the effect of combustor shape on NO emission. In this work, large eddy simulations (LES) are carried out at stoichiometric condition to analyze the flame structure and NO emission characteristics of the ammonia-methane premixed turbulent swirling flame in box and cylindrical shaped combustors. The fuel blend used in this study is 70% ammonia and 30% methane by volume. The LES solver employs a dynamic thickened flame (DTF) combustion model along with Okafor's reduced mechanism, which consists of 43 species and 130 elementary reactions. The flame shape and stabilization are discussed for both combustors, along with the effects of flowfield on the flame. The characteristics of the swirl stabilized flame are illustrated using several time-averaged and instantaneous contours of parameters, including heat release rate, local equivalence ratio, axial velocity, and NO emission. The effect of mixing and reaction rate distribution in the premixed configuration is studied quantitatively, subsequently identifying several flame regions. It is identified from the LES results that the NO emission is 1.6 times higher in the cylindrical combustor compared to the box combustor. While a typical V-shape flame is observed for both combustors, a flame tail also co-exits for the cylindrical combustor. The heat release rate in the flame tail is about 33% higher than the main flame which reignites the unburnt gases for a secondary combustion and generates NO. Thus, a box combustion chamber design is beneficial to minimize the NO emission. The residence time is also analyzed in both combustor to understand the oxidation process and identify the key reaction pathways for NH3/CH4 premixed flames.