Intrinsic thermoacoustic instability in hydrogen enriched partially premixed flames

The influence of hydrogen addition on the intrinsic thermoacoustic instability in swirl stabilized partially premixed methane flames is investigated using large eddy simulation (LES) and reduced order modeling (ROM). The LES results compare well with the measurements. Hydrogen addition results in a significant deviation of the fundamental frequency from the cavity acoustic modes. The thermoacoustic behavior is analyzed using an ROM with the classical n–τ model for flame dynamics, where the interaction index n and time delay τ are deduced from the LES results. A parametric sweep of n and τ with ROM reveals a broad variation of the acoustic eigenvalues in the eigenspectrum. The eigenspectrum shows two distinct regions corresponding to the classical acoustic and intrinsic modes. These intrinsic modes are characterized by a distinct change in the sign of axial pressure gradient across the flame. In the hydrogen-enriched cases, the pressure mode shapes obtained from the ROM and LES analyses show this change in the axial pressure gradient across the flame zone. This is due to flame-induced pulsation dominating the near-field acoustics, resulting in the emergence of intrinsic modes. Further, general results obtained using the one-dimensional acoustic model on the influence of geometry on the stability of acoustic and intrinsic modes are discussed.