Interaction between self-excited oscillations and fuel–air mixing in a dual swirl combustor

Abstract

A partially premixed gas turbine model combustor close to an industrial design is investigated using Large Eddy Simulation (LES). Two flames, one stable and another unstable with self-excited oscillations are computed. In particular, this study addresses the previously unexplained transition of flame shape in the experiments, from V-shaped to flat when the flame becomes acoustically unstable, suggesting a notable change of the important convective delay in the thermoacoustic feedback loop. The LES results show good agreement with the measured velocities, temperature and mass fractions. The acoustic power spectral density (PSD) obtained from the LES of the unstable flame also agrees well with the measured amplitudes in the air plenum and combustion chamber, and reasonably captures the frequency with a slight under-prediction. A comparison of the stable and unstable cases shows different mixing and reaction behaviours despite similar mean velocity fields. Further detailed analysis shows that the different mixing behaviour is driven by the significantly varying air mass split between the two air passages during a thermoacoustic oscillation cycle. This variation is due to the different impedances experienced by the pressure oscillations propagating through the two swirling injector passages with different internal geometries. This causes a periodic variation of the radial momentum of the fuel jets injected between the two swirling air flows. The resulting flapping of the fuel jets creates an enhanced radial fuel-air mixing that leads to a flattened flame in the unstable case. This provides a new physical explanation for the transitions of flame shape observed in the experiments.

Lean Flame Root Dynamics in a Gas Turbine Model Combustor

“…Hence, the modelling methodology for partially premixed combustion must account for both premixed and non-premixed modes. The combustion modelling framework is based on the partially premixed study by Chen et al (2017), which has proved to be successful for LES (Chen et al, 2019;Langella et al, 2018). The fuel-air mixing is described using a mixture fraction , as defined by Bilger et al (1990).…”

“…The SGS parameters Da Δ , C 3 , C 4 , and K c are described in detail by Dunstan et al (2013). This model has been successfully used in many previous studies (Chen et al, 2017(Chen et al, , 2019Langella et al, 2018Langella et al, , 2016Massey et al, 2018). The laminar flame speed s 0 L , thermal thickness δ 0 L and the heat release parameter τ, shown in Equation 8, depend on the local mixture fraction (Ruan et al, 2014).…”

“…The SGS velocity scale u 0 Δ is modelled using a scale-similarity approach (Pope, 2000). A value of 7:5 is used for β c , following the study by Chen et al (2019), although β c can be evaulated dynamically (Gao et al, 2015;Langella et al, 2015).…”

“…Extensive measurements using laser diagnostics for three operating conditions were made, which were for thermo-acoustically stable and unstable conditions, and for a flame close to blowoff (Meier et al, 2006;Weigand et al, 2006). The thermo-acoustically stable flame was investigated by See and Ihme (2015), Benim et al (2017), and Donini et al (2017) using LES, andChen et al (2019) have investigated the thermo-acoustically stable and unstable flames. The third case is of interest for this study, which has recently been investigated using CMC (Zhang and Mastorakos, 2018).…”

The Role of CFD in Modern Jet Engine Combustor Design

“…Here it is of particular practical importance that the change in the flame shape from a V-form in flame A to a flat shape in flame B is correctly reproduced by the LES because the location and distribution of the flame dictates many design factors such as combustor cooling and pollutant emission control, etc. The underlying physical mechanism for this flame shape change involves a fine interplay between the fluid dynamics of the fuel/air inflows and the combustor acoustics under the operating conditions of flame B [55]. This mechanism introduces a different fuel-air mixing pattern at the nozzle exit, and this is also reflected in the downstream temperature and fuel mass fraction distributions shown in Figure 2(b) and (c).…”