A combined longitudinal-transverse combustion instability in a hydrogen jet in crossflow configuration

Rajendram Soundararajan, Preethi and Sampath, Ramgopal (2025) A combined longitudinal-transverse combustion instability in a hydrogen jet in crossflow configuration. Symposium on Thermoacoustics in Combustion, , Trondheim, Norway. 08 Oct 2024 - 09 Oct 2025 .

Abstract

In the transition toward zero-carbon combustion, understanding thermoacoustic instabilities in hydrogen fuelled combustors is crucial for stable engine design. This work investigates such instabilities in a canonical combustor operating with pure hydrogen within a jet-in-crossflow (JICF) configuration. Experiments are conducted in a square cross-section channel, with hydrogen injected either as a single jet or as three jets. High-speed OH∗ chemiluminescence imaging is used to capture flame dynamics from two perspectives—the side view and front view. The wavelet map of the total OH∗ fluctuation for the single-JICF case reveals two distinct frequency bands: one corresponding to intermittent acoustic oscillations, and a lower-frequency band that may be attributed to hydrodynamic fluctuations. The proper orthogonal decomposition (POD) technique is applied to the high-speed images to identify the dominant oscillatory modes. For the single-JICF case, the side-view POD modes reveal a spectral peak at the quarterwave acoustic mode of the channel, along with a low-frequency flapping mode characteristic of the JICF configuration. The corresponding spatial mode shows flame elbow oscillations, likely driven by velocity fluctuations in the crossflow. In the fourth mode, shorter-wavelength undulations appear due to the response of the incoming hydrogen jet to pressure fluctuations in the crossflow channel. The corresponding spectrum shows a peak at the quarter- and three-quarterwave modes, indicating a varicose-type response of the flame to pressure and velocity fluctuations. The highest energy mode on the front view indicates a sinuous mode with transverse flapping of the two flame wings. However, this mode is absent in a stable case, indicating that this motion is likely triggered by the thermoacoustic coupling. In the multi- JICF configuration, the side-view modes remain similar to the single-jet case. However, the front-view modes show the flames exhibiting synchronous and anti-synchronous motion in the transverse direction, in addition to the sinuous mode exhibited by a single-JICF case. This transverse motion can be understood in the context of coupled-oscillator theory, where each flame behaves analogously to a pendulum. When the flames oscillate asynchronously, their interaction leads to the formation of localised pockets of unsteady heat release from the JICF flames. The spectral POD analysis reveals that localised flame pockets form primarily at the sub-harmonic and fundamental frequencies.

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Contributors

Author: Preethi Rajendram Soundararajan

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