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Une approche numérique de l'amortissement acoustique dans les moteurs-fusées à propulsion liquide

Year: 2024
Type: dissertation
Abstract: Combustion instabilities, which appear in combustion devices when the flame couples with the device's chamber's acoustics, remain an issue in developing new liquid rocket engines (LRE). The process of instability growth can be viewed as a competition between noise sources and damping. Quantifying these processes is critical to understanding the physics and modelling the phenomena for industrial applications. This work focuses on damping phenomena.The first part of this thesis highlights the advantages of using numerical simulations to achieve that goal. It presents the disturbance energy balance (DEB), which formulates the sources and damping terms in an energy equation by decomposing the fluid variables into mean and perturbation. The DEB is implemented in the CFD code AVBP from CERFACS, and simple validation test cases are shown. The budget closes well on non-reactive cases, but closure errors remain important on reactive cases.The second part is a detailed application of this methodology on a non-reactive test rig dedicated to studying damping processes in liquid rocket engines: the NPCC (New Pressurized Coupled Cavities) bench. The bench, operating air at 3.5 bar, was built and tested during a previous PhD thesis at the EM2C laboratory. It mimics the geometry of an LRE thrust chamber assembly with a feeding dome linked to a chamber by three injectors. Air exits the domain by two nozzles at the chamber's end. The rig is equipped with an excitation device, the VHAM (Very High Amplitude Modulator), developed previously to excite high-pressure rigs operating rocket engine injectors. It can modulate the rig's eigenmodes with a perforated wheel, alternatively opening and closing the nozzles at its eigenfrequencies. This functioning is reproduced by Large Eddy Simulation (LES) in AVBP, necessitating a detailed geometrical and physical development to mimic the functioning of the VHAM using the boundary conditions imposed in the simulation. The results are extremely satisfying with the limit-cycle pressure signals of three experimental cases perfectly reproduced by the numerical simulation: the excitation of the 1T mode of the chamber, the 1T1L mode of the chamber coupled with the 1T of the dome and the 1T2L mode of the chamber. The study of the spectral content of the pressure signal reveals higher-order harmonics, especially for the 1T case, which are also present in the experiment. A first linear model for the damping is presented, able to reproduce the beginning of the excitation but overestimating the limit cycle in all cases. Noticing that the harmonic content in the limit cycle comes from a steepening of the acoustic waves inside the chamber, a second model is derived from non-linear acoustics theory to represent the energy transfer from the fundamental (here excited by the VHAM) to the first harmonic frequency. The model encompassing linear and non-linear terms can retrieve the limit-cycle amplitude of the 1T case (1-dimensional) but fails for the 2-dimensional cases.Simulation of a second operating mode of the rig with the VHAM equipped with a semi-perforated wheel is attempted but fails to reproduce the experimental results. Finally, the DEB is applied to the numerical simulation of the rig at a lower excitation amplitude to remain in a linear regime not obtained in experiments. The balance study highlights one damping mechanism: the formation and dissipation of vortices in the jets formed at the end of the injectors when submitted to acoustic modulation. Integration of the budgets on subdomains of the rig (dome, injectors, jets) gives a quantitative value for the damping in each zone, with the sum being coherent with results provided by the linear model. This last part of the work highlights the potential of the DEB to quantify the physical phenomena leading to combustion instabilities.
Language: English
Open Access status: green
OpenAlex ID: W7080185848
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