LES of a hydrogen flame controlled by mass flow oscillations and rotating shear layer disturbance

The paper presents the results of investigations on a low-Reynolds-number nitrogen-diluted hydrogen turbulent flame subjected to different types of inlet perturbations, including (1) high/low turbulence intensity and large/small turbulent length scales and (2) two-term excitation composed of mass flow oscillation and a radial velocity disturbance rotating along the nozzle lip. The ratio of axial to radial excitation frequency, $R$, is considered as the control parameter. The research is conducted using the Large Eddy Simulation (LES) method employing a high-order numerical code. A laminar chemistry approach and a detailed chemical reaction kinetic scheme (9 species, 21 reactions) are used to model the combustion process. In case (1), small-scale turbulent fluctuations prevent flame attachment to the nozzle, lifting it to a distance of $6.25D$ and $8.6D$ ($D$ - the nozzle diameter) for low/high turbulence intensity, respectively. The induced small-scale mixing process enhances combustion, leading to a doubling of the flame volume and an increase in the average temperature of approximately 100 K at a distance of $30D$. In case (2), three $R$ values are selected based on current knowledge of non-reacting jet control: $R=2.0,2.5,2.\left(27\right)$. The excitation markedly impacts the dynamics and the shape of the flame. For $R=2.0$ and $R=2.5$, bifurcating and fifth-armed flames form, respectively. The value $R=2.\left(27\right)$ is chosen based on a recent discovery by Y. Li et al. (J. Fluid Mech. 991 (2024)) - a rotating two-arm jet (double-helix). We demonstrate that all these patterns can be reproduced in reactive flows. The cases with five flame arms and rotating arms are presented and discussed for the first time. Analysis of lift-off heights reveals local flame front oscillations at frequencies exactly matching those of large-scale vortex shedding ($R=2.5$) and flame rotation ($R=2.\left(27\right)$). The oxidizer engulfing the separated arms, along with the induced large-scale mixing, results in a flame volume that is 2–4 times larger than in unexcited cases, accompanied by enhanced temporal stability. Radial profiles of the axial velocity and mixture fraction become more uniform and exhibit stronger off-axis mixing. This leads to a more uniform radial temperature distribution, as well as a rapid axial increase and subsequent stabilization of the spatially averaged temperature. In this respect, the excitation with $R=2.\left(27\right)$ proves to be superior, achieving the smallest deviations between local temperature and its spatial average - a combustion non-uniformity index. The presented results suggest that two-term excitation with carefully selected frequencies can serve as a highly effective control strategy, offering potential for more compact combustion chamber designs due to improved flame stability and simultaneous intensification of the combustion process.