| Abstract | This paper investigates the dynamic response of flames, considering H₂-enriched CH₄ fuels that are submitted to upstream mechanical perturbations in a model gas turbine combustor, resulting in fuel velocity oscillations. Flame transfer function (FTF) and cold-flow transfer function (CTF), which represent, respectively, the dynamic responses in the flow of the flame and fuel induced by a velocity modulation in the fuel feed line (FFL), are derived. The gain of FTF shows a low-pass behavior and gradually increases until a specific modulation frequency is reached, which is identical to the frequencies of the corresponding peak gains of the CTF. The periodic variation of OH* intensity is observed from the images collected with a high-speed OH-PLIF. The high oscillation near the injector indicates that the fuel velocity perturbation leads to the amplification of the heat release fluctuation at specific modulation frequencies. The singularity frequencies corresponding to the peaks of the gain of both transfer functions coincide with the resonance frequencies in the FFL. The phase of FTF and CTF decreases as the H₂ ratio increases and the phase of FTF is well fitted into a single line by nondimensionalization through Strouhal number. Most of these findings on the characteristics of combustion instability and acoustical characteristics of FFL can be used as a crucial methodology of H₂ combustor design, such as an anti-resonance fuel/air feed line, which can ensure the stable operation of combustion system by avoiding H₂ induced combustion instability failure. |
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