Abstract
The physical mechanism leading to flame local extinction remains a key issue to be further understood. An analysis of large eddy simulation (LES) data with presumed probability density function (PDF) based closure (Chen et al., 2020, Combust. Flame, vol. 212, pp. 415) indicated the presence of localised breaks of the flame front along the stoichiometric line. These observations and their relation to local quenching of burning fluid particles, together with the possible physical mechanisms and conditions allowing their appearance in LES with a simple flamelet model, are investigated in this work using a combined Lagrangian-Eulerian analysis. The Sidney/Sandia piloted jet flames with compositionally inhomogeneous inlet and increasing bulk speeds, amounting to respectively 70 and 90% of the experimental blow-off velocity, are used for this analysis. Passive flow tracers are first seeded in the inlet streams and tracked for their lifetime. The critical scenario observed in the Lagrangian analysis, i.e., burning particles crossing extinction holes on the stoichiometric iso-surface, is then investigated using the Eulerian control-volume approach. For the 70% blow-off case the observed flame front breaks/extinction holes are due to cold and inhomogeneous reactants that are cast onto the stoichiometric iso-surface by large vortices initiated in the jet/pilot shear layer. In this case an extinction hole forms only when the strain effect is accompanied by strong subgrid mixing. This mechanism is captured by the unstrained flamelets model due to the ability of the LES to resolve large-scale strain and considers the SGS mixture fraction variance weakening effect on the reaction rate through the flamelet manifold. Only at 90% blow-off speed the expected limitation of the underlying combustion model assumption become apparent, where the amount of local extinctions predicted by the LES is underestimated compared to the experiment. In this case flame front breaks are still observed in the LES and are caused by a stronger vortex/strain interaction yet without the aid of mixture fraction variance. The reasons for these different behaviours and their implications from a physical and modelling point of view are discussed in this study.
Highlights
The development of efficient, ‘green’ engines requires strong understanding of the physical processes involved along the whole gas path
Some overprediction of reaction rate was reported in Chen et al (2020) for x∕D > 30, causing an excess consumption of reactants and slightly higher values of flame temperature, which was argued to be due to the dominant non-premixed combustion mode at downstream locations not accurately interpreted by the premixed flamelets approach
The large eddy simulation (LES) was shown to predict the correct levels of mixture fraction scalar dissipation rate (SDR) in the flammable region, which was experimentally observed to play a crucial role in the formation of extinctions (Barlow et al 2015; Cutcher et al 2018)
Summary
The development of efficient, ‘green’ engines requires strong understanding of the physical processes involved along the whole gas path. More recent experimental (Watson et al 1999; Stroomer et al 1999; Lyons et al 2005; Baudoin et al 2013; Tuttle et al 2013; Kariuki et al 2015; Chowdhury and Cetegen 2018) and numerical (Zhou et al 2015; Wang et al 2017) works found that local extinctions occur when the flame front moves on high vorticity regions in the shear layer due to the high strain rates breaking the flame front. Whether this leads to further extinctions or reignition, remains less understood. The mechanism for the so-defined extinctions to occur in first place, and how the combustion model can predict them despite its limitations, is unclear, and is the focus of this work
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