A NUMERICAL STUDY OF EMISSION CHARACTERISTICS IN BLUFF BODY STABILIZED TURBULENT PREMIXED METHANE-AIR FLAMES IN CONFINED ENVIRONMENTS

Emissions and blow-off are two of the most pertinent issues in the operation of gas turbine combustors. Typical combustors are operated heavily fuel lean to optimize performance. As a result of this, lean blow-off (LBO) becomes an important phenomenon to address. Leaner combustors are cleaner in terms of emissions. However, finding a good operating regime between fuel-lean conditions and allowable emissions can be complicated. This problem is further exacerbated due to turbulent combustor operating conditions. Simulating these problems requires high enough fidelity to capture the detailed flame dynamics and emission profiles. In this study, we reduce this problem to study emission profiles of very lean methane-air flames in a bluff-body stabilized combustor at conditions similar to those studied by Pathania et al (2019). Large eddy simulations are performed using OpenFOAM (an open source, fully compressible reacting flow solver). Two chemical kinetic models - the reduced 17-species and 73-reaction model by Sankaran (2007) and the detailed GRI 3.0 model - are compared in their ability to capture CO emissions in a confined environment. The turbulent Reynolds number associated with the bluff-body flameholder diameter is 38,000. The combustor operates in ambient conditions with no inlet preheat. Simulations are carried out over a computational domain of 4.5 million grid points, and the dynamic k-equation model is used to compute sub-filter stresses. Various flow-flame dynamics are analyzed to assess characteristics such as heat release rates and emission profiles. A comparative analysis of flow field regions (such as upstream, recirculation/flameholding and downstream regions) and their turbulence properties is done. A comprehensive assessment of the spatio-temporal evolution of key intermediate species is also carried out for both models. Finally, the reasons for their respective behaviors and effects on CO production are discussed.