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Modelling Turbulent Transport Effects on the Formation Probability of Flame Kernel in Ignition Process

In this study, the one-dimensional model for kernel formation accounting for turbulent transport (Kong et al., 2019) is first calibrated by the experimental data of turbulent expanding spherical flames of premixed ethylene/air mixtures. It further reveals that the ranges of equivalence ratio for successful kernel formation in turbulent flows can be much narrower than that in laminar flows, and in general, turbulent transport has more pronounced effects on fuel rich mixtures. In addition, the range decreases with increased turbulent intensity. The dependence of flammability limits for successful kernel formation on turbulent intensities is thereafter tabulated for methane/air flames and serves as the flammability criterion for kernel formation in a methane/air bluff-body flame. The kernel formation probability map for the bluff-body flame is computed with the flow and fuel/air mixing characteristics being extracted from a non-reacting Reynolds-Averaged Navier-Stokes (RANS) simulation. Results show that the computed probability map agrees qualitatively with experimental results, and it confirms that the turbulence has pronounced inhibitive effect on the formation of flame kernel before the stage of flame propagation in the ignition process. INTRODUCTION The development of advanced low-emission gas turbine combustor such as those in aero-engines demands more careful design of ignition devices and their placement in the combustion chamber to ensure reliable ignition or relighting in case of flame blow-out particularly at high altitudes. It is of practical interest to investigate physiochemical processes in the ignition process of gas turbine combustion and thereafter develop physical models and numerical approaches for the ignition prediction. Four stages can be generally identified in a successful ignition event with electric spark (Mastorakos,2009; Eyssartier, 2013): (1) energy deposition, (2) formation of a viable flame kernel, (3) flame kernel growth and flame propagation, (4) flame stabilization. The forced ignition process in laboratory turbulent flames and gas turbine combustor have been extensively studied experimentally and numerically, showing that the forced ignition process is highly transient in nature and is intrinsically stochastic. Various factors can affect the development of spark kernels from the moment of energy deposition until the complete flame establishment. The most favourable ignition spots for sparking are influenced not only by local equivalence ratio, but also by the time history of the velocity fluctuations. The establishment of an ignition probability map for practical burner by experimental means is practically challenging and expensive. Unsteady three-dimensional simulations could fairly describe the associated physics and give insights into the experimentally observed ignition phenomenon. Numerical work has explored various aspects of the behaviour of the kernel and the subsequent flame evolution with gaseous fuels (Chakraborty and Mastorakos, 2006, 2008) and liquid fuels (Neophytou, et al., 2010, 2011). For example, large eddy simulation (LES) have been performed to study the transient ignition process in laboratory flames such as methane/air bluff-body flames (Ahmed et al., 2007) and jet flames (Ahmed and Mastorakos, 2006). The ignition sequence has been studied in a full annular combustion chamber of gas turbine by means of massively parallel LES computations (Boileau et al., 2008). However, it is still impractical to compute the ignition