Low-frequency streaky structures in turbulent hydrogen jet flames

This study investigates the dynamics of a lean technically premixed, turbulent, hydrogen jet flame at a Reynolds number of 10’000 in a multi-jet combustor configuration. The focus is on the natural, unforced dynamics observed in line-of-sight integrated OH* and mono-PIV measurements. Long time series are recorded in order to analyze the dynamics in a broad spectrum that extends down to very low frequencies. The discussion of the experimental results is supported by a resolvent model based on linearized Navier–Stokes equations and the experimental mean velocity field. The model allows to investigate the underlying mechanisms of the dominant dynamics observed in the flow. It is found that the dynamics of the investigated hydrogen jet flame are mainly driven by two mechanisms. At intermediate frequencies, the Kelvin–Helmholtz mechanism dominates the dynamics and generates axis-symmetric oscillations in the OH* data. In the low Strouhal number regime, large-scale structures of coherent axial velocity fluctuation generated by the lift-up mechanism are observed. These energetic structures are known as streaks in boundary layer flows and non-reacting jets. Here, the streaks are found to play a significant role in the low-frequency dynamics of the turbulent hydrogen jet flames. Most notably, the OH* signal, which is indicative of heat release, is also significantly influenced by the large-scale structures. It is therefore to be expected that streaks are of high technical relevance. Since streaks are associated with strong fluctuations in the axial velocity component, it can be assumed that they are of particular importance for triggering flashback events.