Flame transfer function measurements of hydrogen-enriched reheat flames

This study experimentally investigates the dynamic behavior of hydrogen-natural gas reheat flames, which are stabilized by both flame propagation and autoignition. Measurements are conducted on an test rig which allows the acoustic excitation of reheat flames. The setup is equipped with acoustic and optical diagnostic techniques to capture the dynamic flame behavior in the form of flame transfer functions (FTF) and spatially resolved flame images. Acoustic FTFs are obtained using the Multi-Microphone Method combined with linearized Rankine–Hugoniot conditions. A comparison with FTFs from a hybrid approach using OH* chemiluminescence shows good agreement between the two approaches. FTFs of reheat flames with varying fuel compositions across two temperature ranges are measured, demonstrating the influence of mixture reactivity on flame dynamics. A novel three-zone approach is employed to describe the flames, enabling separate evaluation of the dynamic flame response in the shear layer flames, the autoignition core and the turbulent interaction zone. Zonal FTFs are extracted, which highlight significant differences in the response of these flame regions. An existing FTF model for autoignition flames is validated and the assumption verified, that the global flame behavior can be obtained through the linear superposition of transfer functions for the flame response to single parameters. The experimental FTFs are compared to the modeling approach confirming the observed sensitivities of the reheat flames. Additionally, flame images are analyzed to identify consistent patterns of acoustic response, which support the findings of the FTF analysis. Novelty and Significance Statement This study is the first to measure Flame Transfer Functions for reheat flames, representing a novel contribution that has not been previously published. The experimental data bridges the gap between modeling approaches and numerical studies describing the dynamic behavior of autoignition flames. Furthermore, by applying an FTF model that consists of transfer functions for the flame response to individual parameters, the study verifies the assumption of linear superposition in predicting overall flame behavior.