Large eddy simulations of a partially premixed flame are performed with the purpose of predicting the reacting flow in a swirl-stabilised, low emissions industrial gas turbine combustor. The corresponding sub-grid scale turbulence-chemistry interactions are modelled using a probability density function transport equation, which is solved by the stochastic fields method. A 15-step reduced, but accurate, methane mechanism including 19 species is employed for the description of all chemical reactions. The test case involves a combustor with complex geometry and simulations are carried out for two different combustor operating conditions. Overall, results of the velocity, temperature and species mass fractions (including carbon monoxide) as well as the instantaneous thermochemical properties are shown to be in good agreement with experimental data, demonstrating the capabilities of the applied stochastic fields method. The inclusion of wall heat transfer in the combustion chamber is found to improve temperature and species predictions, especially in the near-wall regions. Comparisons between an oscillating and a 'stable' flame case furthermore highlight the influence of experimentally observed thermo-acoustic instabilities on the scalar fluctuations near the combustor centreline. None of the default model parameters were adjusted and the results showcase the accuracy and flexibility of the present large eddy simulation method for an application to complex, partially premixed combustion problems; this being particularly important for the designers of new generation low emission gas turbine combustors.

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This work predicts the evolution of self-excited thermo-acoustic instabilities in a gas turbine model combustor using large eddy simulation. The applied flow solver is fully compressible and comprises a transported sub-grid probability density function approach in conjunction with the Eulerian stochastic fields method. An unstable operating condition in the PRECCINSTA test case-known to exhibit strong flame oscillations driven by thermo-acoustic instabilities-is the chosen target configuration. Good results are obtained in a comparison of time-averaged flow statistics against available measurement data. The flame's self-excited oscillatory behaviour is successfully captured without any external forcing. Power spectral density analysis of the oscillation reveals a dominant thermoacoustic mode at a frequency of 300 Hz; providing remarkable agreement with previous experimental observations. Moreover, the predicted limit-cycle amplitude is found to closely match its respective measured value obtained from experiments with rigid metal combustion chamber side walls. Finally, a phase-resolved study of the oscillation cycle is carried out leading to a detailed description of the physical mechanisms that sustain the closed feedback loop.

The paper examines the combined effects of several interacting thermo-acoustic and hydrodynamic instability mechanisms that are known to influence self-excited combustion instabilities often encountered in the late design stages of modern lowemission gas turbine combustors. A compressible large eddy simulation approach is presented, comprising the flame burning regime independent, modelled probability density function evolution equation/stochastic fields solution method. The approach is subsequently applied to the PRECCINSTA (PREdiction and Control of Combustion INSTAbilities) model combustor and successfully captures a fully self-excited limit-cycle oscillation without external forcing. The predicted frequency and amplitude of the dominant thermo-acoustic mode and its first harmonic are found to be in excellent agreement with available experimental data. Analysis of the phaseresolved and phase-averaged fields leads to a detailed description of the superimposed mass flow rate and equivalence ratio fluctuations underlying the governing feedback loop. The prevailing thermo-acoustic cycle features regular flame lift-off and flashback events in combination with a flame angle oscillation, as well as multiple hydrodynamic phenomena, i.e. toroidal vortex shedding and a precessing vortex core. The periodic excitation and suppression of these hydrodynamic phenomena is confirmed via spectral proper orthogonal decomposition and shown to be controlled by an oscillation of the instantaneous swirl number. Their local impact on the heat release rate, which is predominantly modulated by flame-vortex roll-up and enhanced mixing of fuel and oxidiser, is further described and investigated. Finally, the temporal relationship between the flame 'surface area', flame-averaged mixture fraction and global heat release rate are found to be directly correlated.

An ultralow emission combustor concept based on “flameless oxidation” is demonstrated in this paper for aviation kerosene. Measurements of gas emissions, as well as of the size and number of nanoparticles via scanning mobility particle sizing, are carried out at the combustor outlet, revealing simultaneously soot-free and single-digit NO x levels for operation at atmospheric conditions. Such performance, achieved with direct spray injection of the fuel without any external preheating or prevaporization, is attributed to the unique mixing configuration of the combustor. The combustor consists of azimuthally arranged fuel sprays at the upstream boundary and reverse-flow air jets injected from downstream. This creates locally sequential combustion, good mixing with hot products, and a strong whirling motion that increases residence time and homogenizes the mixture. Under ideal conditions, a clean, bright-blue kerosene flame is observed, free of soot luminescence. Although soot is intermittently formed during operation around optimal conditions, high-speed imaging of the soot luminescence shows that particles are subjected to long residence times at O2-rich conditions and high temperatures, which likely promotes their oxidation. As a result, only nanoparticles in the 2–10 nm range are measured at the outlet under all tested conditions. The NO x emissions and completeness of the combustion are strongly affected by the splitting of the air flow. Numerical simulations confirm the trend observed in the experiment and provide more insight into the mixing and air dilution.