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Dry low emissions (DLE) systems are well-known to be susceptible to thermoacoustic instabilities. In particular, transverse, spinning modes of high frequency may appear, and lead to severe damage in a matter of seconds. The thermoacoustic response of an engine is usually specific to the combustor geometry, operating conditions and difficult to reproduce at the lab-scale. In this work, details of high frequency dynamics (HFD) observed during the early development phase of a new DLE system are provided, where a multipeaked spectrum was noticed during testing. Beginning with an analysis of the measured pressure spectra from three different concepts, an analytical model of the clockwise and anticlockwise transverse waves was fitted to the experimental data using a nonlinear curve fitting approach to produce a simple yet useful understanding of the phenomena. A flamelet-based large eddy simulation (LES) of the entire combustion system was used to complement this analysis and confirm the mode shapes using dynamic mode decomposition (DMD). Both approaches independently identified a spinning second-order mode as the dominant one in the high frequency regime. The LES indicates the coupling of a distortion of swirl profile with a precessing vortex core as a possible cause for the onset of instability. With regard to modeling sensitivities, it is shown that subgrid scale combustion modeling has a strong impact on predicted amplitudes. Ultimately, a thickened-flame model with a modified efficiency function provided consistent results. [ABSTRACT FROM AUTHOR]

Lean premixed flame stabilization at atmospheric conditions, in a linear array of five swirl injectors, was modeled using well-resolved large eddy simulation (LES) and chemical kinetics that were accurate both for ignition and flame speed. The effects of injector spacing were studied by selectively blocking injectors in the array to obtain five (F), two (T) and single (S) injector configurations. Each of these was simulated at well-anchored as well as near blow-off conditions. Experiments indicated a blow-off trend that was non-monotonic with spacing: the two-injector configuration exhibited the greatest resistance to blow-off, followed by the single-injector setup. The five-injector configuration proved to be the least resistant (by far) in comparison. In an earlier computational study [1], preferential blow-off in configuration F was successfully modeled and strong flame-flame interference could be investigated. This work is continued to assess the ability of a numerical model to study flames near blow-off, but with varying levels of flame-flame interaction. Passive scalar tracking was used to relate cross-injector transport of material to a given injector's flame-holding ability. The non-monotonic blow-off trend could not be explained by stretch and heat release rate trends, but well-stirred reactor (WSR) theory was found to be more relevant as trends in recirculation zone residence times correlate well with blow-off sequences. In addition, cross-injector transport was studied due to the multi-injector scenario to assess how flameholding zones may be diluted. This work is expected to be useful for analyzing part-load behavior in multi-injector power-generating gas turbine combustion systems, and helps to characterize injector performance towards extending the range of lean operability. [ABSTRACT FROM AUTHOR]

Sensitivity to stretch and differential diffusion of chemical species are known to influence premixed flame propagation, even in the turbulent environment where mass diffusion can be greatly enhanced. In this context, it is convenient to characterize flames by their Lewis number (Le), a ratio of thermal-to-mass diffusion. The work reported in this paper describes a study of flame stabilization characteristics when Le is varied. The test data are comprised of Le≪1 (hydrogen), Le≈1 (methane), and Le>1 (propane) flames stabilized at various turbulence levels. The experiments were carried out in a hot exhaust opposed-flow turbulent flame rig (HOTFR), which consists of two axially opposed, symmetric jets. The stagnation plane between the two jets allows the aerodynamic stabilization of a flame and clearly identifies fuel influences on turbulent flames. Furthermore, high-speed particle image velocimetry (PIV), using oil droplet seeding, allowed simultaneous recordings of velocity (mean and rms) and flame surface position. These experiments, along with data processing tools developed through this study, illustrated that in the mixtures with Le≪1, turbulent flame speed increases considerably compared to the laminar flame speed due to differential diffusion effects, where higher burning rates compensate for the steepening average velocity gradient and keeps these flames almost stationary as bulk flow velocity increases. These experiments are suitable for validating the ability of turbulent combustion models to predict lifted, aerodynamically stabilized flames. In the final part of this paper, we model the three fuels at two turbulence intensities using the flamelet generated manifolds (FGM) model in a Reynolds-averaged Navier-Stokes (RANS) context. Computations reveal that the qualitative flame stabilization trends reproduce the effects of turbulence intensity; however, more accurate predictions are required to capture the influences of fuel variations and differential diffusion. [ABSTRACT FROM AUTHOR]

Critical slowing down (CSD) is a phenomenon that is common to many complicated dynamical systems as they approach critical transitions/bifurcations. We demonstrate that pressure signals measured during the onset of thermoacoustic instabilities in a gas turbine engine test exhibit evidence of CSD well before the oscillation amplitude increases. CSD was detected through both the variance and the lag-1 auto-regressive coefficient in a rolling window of the pressure signal. Increasing trends in both metrics were quantified using Kendall's t, and the robustness and statistical significance of the observed increases were confirmed. Changes in the CSD metrics could be detected several seconds prior to changes in the oscillation amplitude. Hence, real-time calculation of these metrics holds promise as early warning signals of impending thermoacoustic instabilities. [ABSTRACT FROM AUTHOR]