Experimental and numerical investigations of forced response of multi-element lean-premixed hydrogen flames

Abstract

Understanding the distinctive thermoacoustic properties of multi-element lean-premixed hydrogen flames is crucial for the development of hydrogen-capable gas turbines. Extending our earlier investigations of the self-excited dynamics of a cluster of lean-premixed pure hydrogen-air flames, here we explore the dynamic response to harmonic velocity perturbations, using an integrative approach combining loudspeaker-forced direct measurements and LES-based numerical simulations. Electronically-excited OH intensity distribution, generally assumed to be equivalent to the flame's heat release rate, shows an anchored conical reaction zone. Numerical simulations of heat release rate contours, however, reveal the formation of a more concentrated thin annulus region resembling an inverted V shaped layer, created by preferential diffusion of hydrogen molecules. This difference between OH-intensity distribution and heat release rate contours suggests consequential uncertainties in surrogate-dependent flame transfer function evaluations; the transfer function gains from measured data are well defined by a fast-decaying low-pass filter behavior, but the simulation result exhibits slowly-decaying large gain with slight overshoot over the entire range of frequency. Because the systematic uncertainties associated with the estimation of the flame's heat release rate cannot be accurately quantified, we rely on a reduced-order network modeling framework combined with direct measurement to test the accuracy of the prediction of the growth of self-sustained pressure fluctuations. We demonstrate that LES-based transfer functions predict the system's stability more accurately, and this suggests that the calculated heat release rate data are closer to the hydrogen flame's true heat release response than the wavelength-specific light intensity measurement. This assessment enables us to identify structurally simple moderate transverse oscillations of local regions containing atypically concentrated energy as pivotal processes controlling high-frequency hydrogen combustion dynamics.