Measuring acoustic transfer matrices of high-pressure hydrogen/air flames for aircraft propulsion

Destructive thermoacoustic instabilities may potentially slow down significantly the ongoing development of hydrogen combustors for decarbonizing aviation. Their early prediction requires the knowledge of the heat release rate response of individual flames to acoustic perturbations. Obtaining this response at engine conditions is very challenging as it requires the development of sophisticated acoustic actuation and sensing techniques for harsh temperature and pressure environment. To date, experimental measurements of the response of single-flames to upstream and downstream acoustic excitation have been limited to academic burners operated at atmospheric condition. Moreover, to the authors knowledge, the response of turbulent non-premixed H2/air flames has not been experimentally investigated yet, not even at atmospheric pressure. Our experiments address this challenge by determining the acoustic transfer matrix of rich-quench-lean H2 flames anchored on an industrial prototype burner at engine-relevant conditions, including high-altitude flight. The response of the flame is measured up to 2 kHz by using the multi microphone method (MMM). It is shown that the MMM becomes more sensitive to temperature estimations at high frequency and we outline a strategy to improve the method. It is found that the acoustic response of these H2/air non-premixed flames exhibit large gains with non-monotonic trends over a wide frequency range. Different fuel-to-air ratios and flow velocities are considered up to nearly 7 bar. We show that the equivalence ratio and operating pressure do not alter significantly the acoustic flame response, while the flow velocity does, although the flame shape is nearly unchanged when the latter parameter is varied. Furthermore, we extend the classic model of low-Mach-number flame transfer matrices to the relevant case of RQL combustors. Novelty and Significance The ability to accurately measure, at relevant mean pressure, the transfer matrix linking acoustic pressure and velocity across a single burner and its turbulent H2/air flame is key for the development of H2 powered medium-range civil aircrafts. This is because such measurement enables predictions of potential thermoacoustic instabilities in the full annular combustor featuring a large number of burners and flames, and therefore it offers possibilities for burner prototype selection and optimization before full engine tests. The present study is the first demonstration of such challenging measurement, revealing the peculiar acoustic response of non-premixed H2/air flames.