Flame transfer function characterization of a hydrogen jet flame via enhanced microphone array placement

Although hydrogen combustion has been extensively investigated in recent years, several challenges remain in accurately assessing jet flame transfer functions (FTFs) under intermediate thermal load conditions with a thermal power over ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>P</mml:mi> <mml:mtext>th</mml:mtext> </mml:msub> <mml:mo>≥</mml:mo> <mml:mn>100</mml:mn> <mml:mspace width=".1em"/> <mml:mtext>kW</mml:mtext> </mml:math> ). The well-established multi-microphone technique, commonly used for swirl-stabilized methane flames, faces several difficulties when applied to jet hydrogen combustion. These challenges are connected to the non-acoustically compact extension of the mixing tube in hydrogen jet burners and to significant changes in the gas properties under non-reactive (air) and reactive (air-hydrogen) conditions. In previous studies, the impact of changes in fuel properties on the multi-microphone method and burner transfer matrix (BTM) reconstruction was demonstrated. However, this approach was limited to well-mixed configurations. In the present work, we extend the methodology to technically premixed single-jet burner systems by introducing an array of four microphones within the mixing tube. This configuration significantly reduces the complexity of the BTM. We demonstrate this experimentally on a technically premixed hydrogen jet burner. Using low-order acoustic modeling of the BTM, we show through sensitivity analysis that the relocated microphone array enables a more robust assessment of the flame transfer matrix for the considered jet–burner configuration, and thus the FTF. Finally, a distributed time-delay model is fitted to the measured FTF to analyze flame dynamics across the relevant frequency range.