Experimental analysis of a transcritical LOX/CH 4 swirl flame

This study presents an experimental investigation of the flow and combustion dynamics of a single LOX/CH 4 liquid-centered swirl injector under high-pressure transcritical conditions, conducted on the cryogenic MASCOTTE test bench. As a preliminary step, the interaction between hydrodynamic and combustion phenomena in a stable operating regime is examined. The structure and evolution of the flame and dense phase are analyzed using synchronized high-speed imaging at 13 kHz, with OH* chemiluminescence capturing the reactive zone and backlighting visualizing the dense phase. Time-averaged and instantaneous fields are provided to highlight the flame structure and flow behavior. Spectral analysis, conducted via Fast Fourier Transform (FFT), is performed, and complemented by video filtering using a combination of averaged Inverse Discrete Fourier Transform (IDFT) and phase-averaging techniques. The backlighting recordings reveal a dominant symmetric mode aligned with the preferred instability mode frequency of the dense jet. The corresponding Strouhal number, ranging between 0.25 and 0.5, is consistent with classical hydrodynamic predictions and is identified as a symmetric Kelvin–Helmholtz instability. Similar modal behavior is observed in the OH* chemiluminescence signal, sharing the same dominant frequency and mode shape. These coherent structures are confined to the reaction zone and dense phase region, with no significant modes persisting downstream. In the absence of significant pressure oscillations, it is inferred that the flame dynamics are primarily driven by the dense phase hydrodynamics. These findings enhance the understanding of coupled hydrodynamic-combustion instabilities in transcritical swirl injectors and offer insights relevant to the design and control of next-generation rocket propulsion systems. Novelty and significance statement The novelty of this work lies in its contribution to the very limited experimental data on transcritical LOX/CH 4 swirl flame dynamics available in the literature. Backlighting imaging and OH* chemiluminescence measurements reveal a dominant symmetric mode in the flow oscillations. Additionally, the state-of-the-art Strouhal number definition for non-reactive swirl flows provides a good estimate of the corresponding dominant frequency, which aligns with the preferred instability frequency of the dense jet. This analytical frequency estimation enhances understanding of the coupling between hydrodynamic and combustion instabilities in transcritical swirl injectors, representing a key step toward the development of safe and reliable liquid rocket engines.