Acoustic waves in combustion devices : interactions with flames and boundary conditions

Combustion devices are prone to combustion instabilities. They arise from a constructive coupling between the unsteady heat release rate of the flame and the resonant acoustic modes of the entire system. The occurence of such instabilities can pose a threat to both performance and integrity of combustion systems. Although these phenomena have been known for more than a century, avoiding their appearance in industrial engines is still challenging. The objective of this thesis is threefold: (1) study the dynamics of the resonant acoustic modes, (2) investigate the flame response of a liquid rocket engine under unstable conditions using Large Eddy Simulation(LES) and (3) derive, use and study Time Domain Impedance Boundary Conditions (TDIBCs), i.e. boundary conditions modeling complex acoustic impedances

Delayed-time domain impedance boundary conditions (D-TDIBC)

Defining acoustically well-posed boundary conditions is one of the major numerical and theoretical challenges in compressible Navier–Stokes simulations. We present the novel Delayed-Time Domain Impedance Boundary Condition (D-TDIBC) technique developed to impose a time delay to acoustic wave reflection. Unlike previous similar TDIBC derivations (Fung and Ju, 2001–2004 [1], [2], Scalo et al., 2015 [3] and Lin et al., 2016 [4]), D-TDIBC relies on the modeling of the reflection coefficient. An iterative fit is used to determine the model constants along with a low-pass filtering strategy to limit the model to the frequency range of interest. D-TDIBC can be used to truncate portions of the domain by introducing a time delay in the acoustic response of the boundary accounting for the travel time of inviscid planar acoustic waves in the truncated sections: it gives the opportunity to save computational resources and to study several geometries without the need to regenerate computational grids. The D-TDIBC method is applied here to time-delayed fully reflective conditions. D-TDIBC simulations of inviscid planar acoustic-wave propagating in truncated ducts demonstrate that the time delay is correctly reproduced, preserving wave amplitude and phase. A 2D thermoacoustically unstable combustion setup is used as a final test case: Direct Numerical Simulation (DNS) of an unstable laminar flame is performed using a reduced domain along with D-TDIBC to model the truncated portion. Results are in excellent agreement with the same calculation performed over the full domain. The unstable modes frequencies, amplitudes and shapes are accurately predicted. The results demonstrate that D-TDIBC offers a flexible and cost-effective approach for numerical investigations of problems in aeroacoustics and thermoacoustics

Study of flame response to transverse acoustic modes from the LES of a 42-injector rocket engine

The Large-Eddy Simulation of a reduced-scale rocket engine operated by DLR has been conducted. This configuration features 42 coaxial injectors fed with liquid oxygen and gaseous hydrogen. For a given set of injection conditions the combustor exhibits strong transverse thermo-acoustic oscillations that are retrieved by the numerical simulation. The spatial structure of the two main modes observed in the LES is investigated through 3D Fourier analysis during the limit cycle. They are respectively associated with the first transverse and first radial resonant acoustic modes of the combustion chamber. The contributions of each individual flame to the unsteady heat release rate and the Rayleigh index are reconstructed for each mode. These contributions are in both cases low in the vicinity of velocity anti-nodes and high near pressure anti-nodes. Moreover it is noticed that these pressure fluctuations lead to large velocity oscillations in the hydrogen stream. From these observations, a driving mechanism for the flame response is proposed and values for the gain and phase of the associated flame transfer function are evaluated from the LES

Ondes acoustiques au sein des systèmes de combustion : interactions avec les flammes et les conditions limites

Les systèmes de combustion sont sujets aux instabilités de combustion (IC). Elles résultent d'un couplage constructif entre le taux de dégagement de chaleur instationnaire et des modes acoustiques du système. Les IC peuvent mettre en danger la performance et l'intégrité des systèmes de combustion. Même si ces phénomènes sont connus depuis plus d'un siècle, éviter quelles aient lieux dans les chambres de combustions industrielles reste difficile. Les objectifs de cette thèse sont les suivants : (1) étudier la dynamique des modes acoustiques, (2) analyser la réponse de flamme d'un moteur de fusée à propergol liquide H2/O2 (appelé "BKD"), sujet aux IC, à l'aide de la Simulation aux Grandes Echelles (SGE) et (3) dériver, utiliser et étudier des conditions limites permettant d'imposer des impédances acoustiques complexes en SGE.Combustion devices are prone to combustion instabilities. They arise from a constructive coupling between the unsteady heat release rate of the flame and the resonant acoustic modes of the entire system. The occurence of such instabilities can pose a threat to both performance and integrity of combustion systems. Although these phenomena have been known for more than a century, avoiding their appearance in industrial engines is still challenging. The objective of this thesis is threefold: (1) study the dynamics of the resonant acoustic modes, (2) investigate the flame response of a liquid rocket engine under unstable conditions using Large Eddy Simulation(LES) and (3) derive, use and study Time Domain Impedance Boundary Conditions (TDIBCs), i.e. boundary conditions modeling complex acoustic impedances

Influence of straight nozzle geometry on the supersonic under-expanded gas jets

International audienceThe sodium-cooled fast nuclear reactor (SFR) is one of the most promising designs of the fourth generation (Gen IV) nuclear power reactors. Sodium-gas heat exchangers (SGHE) using nitrogen is being investigated as an alternative to improve operational safety associated with the use of steam Rankine cycles. This alternative eliminates the potential risk of chemical reactions. It is known that cracks inside an SGHE can cause the accidental leakage of nitrogen into the sodium-side. Due to the pressure difference between the secondary and tertiary loops, this nitrogen jet is therefore under-expanded. When the nitrogen leak is strong enough to flush the liquid sodium outside the SGHE channel, the nitrogen jet can be considered as single-phase. In this context, this work focuses on the influence of geometrical parameters of cracks (size, cross-section shape, transverse localization and inclination angle) on the impinging under-expanded nitrogen jet and its shock-wave system. A numerical study of impinging under-expanded nitrogen jet has been carried out using large eddy simulation (LES) technique. We applied a stagnation pressure upstream of the crack of 180 bar while the nozzle pressure ratio (NPR) ranged from 6.0 to 9.2. We were able to identify the link between the nozzle geometry and the Mach disk diameter and its localization. The vorticity distribution at the nozzle can be used to explain the structure of the jets and the entrainment. The central cross-section of the gas jet tends to turn 45° and 90° for square and rectangular cross-section nozzles respectively. The Taylor-Görtler instability is enhanced with a reduction in the nozzle diameter. These instabilities are also increased with square, rectangular and inclined nozzles

Joint experimental, LES and Helmholtz analysis of self-excited combustion instabilities in a hydrogen-oxygen rocket combustor

Self-excited combustion instabilities of the first tangential mode have been observed in a research combustor operated with hydrogen and oxygen in cryogenic state as propellants. A detailed analysis of this unstable mode is conducted and the experimental observations are compared with numerical simulations. A high fidelity Large-Eddy Simulation together with an acoustic eigenmode computation performed with a Helmholtz solver give complementary information on the mechanisms driving the instability. A methodology for the 3D reconstruction of the acoustic field is presented