Assessment of Partially Premixed Flame by In-Situ Adaptive Reduced Mechanisms in OpenFOAM

The partially premixed flame was modelled using an open-source software based on finite volume method (FVM) of computational fluid dynamics (CFD), called OpenFOAM. The assessment of the tabulation dynamics adaptive chemistry (TDAC) algorithms for facilitating the computation was of interest. A total of seven models were performed, consisting of six models of the TDAC framework application and a direct computation model without TDAC. Simulation results were validated by comparing against the thermal flame height (HT) of Irandoost et al. [28]. The heat released rate was established from simulation results to identify the flame front and HT. This is a novel technique to illustrate the flame front, which agreed well with the experiment. Subsequently, it was found that all but one of the reduced mechanism methods agreed well in predicting the HT. The exception was DRGEP. Particularly, the CFD results were optimal. It was discovered that the TDAC based on the mechanism reduction called element flux analysis (EFA) was the second-fastest but optimal choice to solve the partially premixed flame model.

Relative Effects of Velocity- and Mixture-Coupling in a Thermoacoustically Unstable, Partially-Premixed Flame

Combustion instability, which is the result of a coupling between combustor acoustic modes and unsteady flame heat release rate, is a severely limiting factor in the operability and performance of modern gas turbine engines. This coupling can occur through different coupling pathways, such as flow field fluctuations or equivalence ratio fluctuations. In realistic combustor systems, there are complex hydrodynamic and thermo-chemical processes involved, which can lead to multiple coupling pathways. In order to understand and predict the mechanisms that govern the onset of combustion instability in real gas turbine engines, we consider the influences that each of these coupling pathways can have on the stability and dynamics of a partially-premixed, swirl-stabilized flame. In this study, we use a model gas turbine combustor with two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at an elevated pressure of 5 bar. The flow split between the two streams is systematically varied to observe the impact on the flow and flame dynamics. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence, and acetone planar laser-induced fluorescence are used to obtain information about the velocity field, flame, and fuel-flow behavior, respectively. Depending on the flow conditions, a thermoacoustic oscillation mode or a hydrodynamic mode, identified as the precessing vortex core, is present. The focus of this study is to characterize the mixture coupling processes in this partially-premixed flame as well as the impact that the velocity oscillations have on mixture coupling. Our results show that, for this combustor system, changing the flow split between the two concentric nozzles can alter the dominant harmonic oscillation modes in the system, which can significantly impact the dispersion of fuel into air, thereby modulating the local equivalence ratio of the flame. This insight can be used to design instability control mechanisms in real gas turbine engines.