Flow and flame dynamics in a dual-bluff-body liquid spray combustor under combustion oscillations

This paper investigates the velocity-coupling mechanisms governing thermoacoustic instabilities in a dual-bluff-body stabilized liquid spray combustor under self-excited longitudinal and transverse modes. By employing simultaneous 10-kHz particle image velocimetry and OH* chemiluminescence, we characterize the coupled dynamics of the flow field, flame response, and vortex–flame interactions through qualitative and quantitative comparison. The spectral proper orthogonal decomposition results characterize the spatially coherent structures of both flow and flame under each instability mode. Further analysis of instantaneous fields, based on the fast Fourier transform of flame edge and heat release rate (HRR) fluctuations, elucidates the distinct coupling pathways. For longitudinal instability, velocity coupling manifests primarily as convection disturbances, with significant variation in the axial velocity phase. In contrast, transverse instability mode exhibits two main velocity-coupled pathways. In the near field, a varicose vortex shedding pattern with a slight phase offset between the two branches of each bluff body is identified. These vortical structures impose harmonic excitation on the flame, leading to periodic flame front rollup. The phases of velocity disturbances show a convection pattern in the near region, while the amplitudes exhibit superposition and interference of the acoustic field and vortices. Therefore, the velocity-coupled pathway manifests as a superposition of the transverse acoustic field and convective vortical disturbances. In the far field, however, vortices dissipate rapidly while unsteady heat release does not diminish accordingly. The amplitude and phase distribution of velocity fluctuations reveals that HRR is sustained not by vortical structures but rather by transverse acoustic waves. This work identifies the dominant velocity-coupling mechanisms governing flame dynamics in both longitudinal and transverse instability modes, providing new insights into the spatial–temporal evolution of thermoacoustic coupling pathways.Novelty and significance statement: The key contributions of this work are twofold. First, it presents an unprecedented experimental database for bluff-body stabilized liquid spray flames under both longitudinal and transverse self-excited instabilities, acquired via simultaneous 10-kHz PIV and OH* chemiluminescence diagnostics. Second, this work elucidates the dominant velocity-coupling mechanisms governing each instability mode by differentiating between convective and acoustic disturbances. These findings deliver fundamental new insights into the spatial–temporal evolution of thermoacoustic coupling, advance the comprehensive understanding of combustion instabilities and have direct implications for instability prediction and control.