Analysis and comparison of the thermoacoustics of freely-propagating CH 4 and H2 flames in narrow channels

Laminar premixed flame propagation in a narrow channel open at one end and closed at the other end is a fundamental but physically rich phenomenon. Prior experimental studies have shown that in this configuration, self-excited thermoacoustic instabilities due to the interaction of the premixed flame with the acoustic field first manifest as small-amplitude oscillations (primary instability), which subsequently transitions to large-amplitude oscillations (secondary instabilities) in the flow quantities and flame position. While these works mainly focused on natural gas, in the present paper we characterize and compare the instability behavior of natural gas and hydrogen flames at conditions of identical laminar flame speed ( S l 0 ) and adiabatic flame temperature ( T a d ) using experiments and computations. A ‘stepped’ mesh approach is adopted to perform time-efficient high-fidelity computations of the self-excited instabilities associated with the propagating flame, showing good qualitative agreement with the experimental data. The CH 4 and H 2 flames at identical S l 0 exhibit similar thermoacoustic behavior, while the CH 4 and H 2 flames at identical T a d are very different in their thermoacoustic response. Furthermore, the thermo-diffusive effects associated with the H 2 flame have a significant impact on the thermoacoustic response. The transition from a stable to an unstable flame is accompanied by distinct transitions in the flame shape. The results also reveal that the onset of primary instabilities occurs at selective flame front positions along the duct, a finding also supported by thermoacoustic network model analyses. Novelty and significance statement Thermoacoustic instabilities associated with freely-propagating CH 4 flames in narrow channels have been reported previously in slender-tube experiments (Flores-Montoya et al., 2022; Gatón-Pérez et al., 2025). The key novelty of this work is to compare the thermoacoustic behavior of CH 4 and H 2 flames at identical laminar flame speed and adiabatic flame temperature to elucidate the role of chemical kinetics and mass diffusion. Furthermore, this study fills the scarcity of high-fidelity computations of freely-propagating flame instabilities in the literature by performing numerical simulations and comparing these with experimental data. The multi-pronged experimental, computational and analytical approach used in this work gives three significant contributions: (a) time-efficient methodology to compute freely-propagating flames in long channels, (b) insight into the role of thermo-diffusive effects in governing self-excited instabilities associated with freely-propagating flames, and (c) analytical prediction of the instability behavior.