Flame transfer function analysis of hydrogen diffusion swirl flames

This paper investigates the first Flame Transfer Functions (FTFs) of hydrogen diffusion swirl flames, which are crucial for predicting and mitigating thermoacoustic instabilities. Given the need to develop new combustion technologies for hydrogen, it is therefore essential to accurately measure and analyze these FTFs. Employing acoustic and optical methods, we obtained the FTFs over a wide frequency range from 50 to 1000 Hz. Using the acoustic method, the FTFs are deduced from the flame transfer matrices. The FTFs exhibit a low-pass filter behavior with gains decreasing significantly above 150 Hz. Strouhal number normalization effectively collapses the FTFs across various thermal powers, bulk mass flow rates and global equivalence ratios. This result suggests that a generic flame response to acoustic perturbations exists and that the FTF can be interpolated over a range of operating conditions. This study identifies two dominant combustion modes in these hydrogen diffusion swirl flames: a diffusion-mode thin reaction layer near the nozzle and a partially premixed thicker reaction layer downstream. Phase-averaged OH* and OH-PLIF imaging revealed non-uniform transversal oscillations of the reaction zone, offering key insights into the complex swirling flow and the convective wavelength of the coherent heat release rate oscillations along these turbulent hydrogen diffusion swirl flames.