Driving mechanism of spray combustion oscillations in a backward-facing step combustor from viewpoints of complex systems

We numerically investigate the driving mechanism of spray combustion oscillations in a backward-facing step combustor. We adopt the analytical methods based on symbolic dynamics and complex networks for the acoustic pressure, flow velocity, heat release rate, and fuel evaporation rate fluctuations obtained by the large-eddy simulation [Kitano et al., Combust. Flame, vol. 170, p. 63, 2016; Pillai et al., Combust. Flame, vol. 220, p. 337, 2020]. The organized vortex separated from the step induces a local flow near the side wall of the step and a reverse flow in the region near the top wall of the step. The reverse flow promotes the evaporation in the upstream region, resulting in the expansion of the flame formation region. The expanded flame formation region nearly corresponds to the driving region of combustion oscillations. This is reasonably identified by the local Rayleigh index. The symbolic transfer entropy indicates that the evaporation rate fluctuations drive the heat release rate fluctuations in the downstream region, while the opposite trend is observed in the upstream region. These processes play an important role in driving spray combustion oscillations.