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16 Chapter 1: General Introduction • Inside: A combustion chamber is the part of an engine in which fuel is burnt. In aeronautical engines, this chemical reaction should generate considerable levels of thermal power ∫ ˙Qdv and for that reason flames must be turbulent and generally stabilized by a swirled flow [74]. Noise is linked to fluctuations of thermodynamical quantities (density ρ, pressure p, temperature T, entropy s, ...) that are somehow generated by the unsteady velocity field and the turbulent flame. • Outside: Clearly, acoustic pressure waves exist both upstream (compressor stages and diffusor) and downstream (turbines and nozzle). These pressure oscillations might travel either with or against the mean flow reaching the combustion chamber. Subsequently, they interact with the turbulent flow/flame and the surroundings (walls, multiperforated plates, injectors, etc). • At the combustion chamber boundaries: Acoustic waves can be produced when either vortical or entropy waves reach zones of non-homogeneous mean flow [12, 55]. It usually happens at the ‘HPD’ (high pressure distributor) just after the combustion chamber. It is naive to believe that all these phenomena always happen independently of each other. In some cases, they are totally coupled and their study becomes clearly extremelly difficult. Classically, one can identify three different interactions that in the worst cases, when acoustic energy is not efficiently dissipated, lead to instabilities: • flame/entropy/acoustics In this case hot spots (entropy) are produced by the unsteady flame. These hot spots travel downstream at the flow velocity until reaching the HPD where acoustic waves are generated [12, 55]. These acoustic waves will travel upstream attaining the reactive region. They will modify, as a consequence, the flame dynamics and therefore the fluctuating entropy. When acoustic and entropy waves are coupled, a combustion instability called ‘rumble’ might appear. This instability is characterized by its low frequencies (50-150 Hz) and can take place during the startup phase of aeronautical engines [23]. • turbulence/acoustics/boundaries Everywhere where turbulence is enhanced, acoustic waves are generated. Acoustic waves can propagate until they are reflected on any boundary and travel back reaching the vortical zone. Turbulence, i.e. hydrodynamic perturbations, will in turn be modified by this acoustic field and the close loop may restart. If the hydrodynamic preferential frequency coincides with a multiple of the acoustic resonant frequency of the specific configuration, significant unstable interactions may occur.[81]. A ‘hydrodynamic instability’ would be said to have appeared. • flame/acoustics/boundaries A volumetric expansion due to the unsteady heat release is created. Acoustic waves are therefore generated and propagate until they reach reflecting boundaries where they will be sent back. These waves propagate back, reaching the
1.2 Scope of the present work 17 flame and consequently influencing it by modifying the heat release. When an unstable coupling of this nature takes place, one will talk about ‘thermoacoustics instabilities’ [78, 22]. This is a broad field of study within the combustion community [48]. Efforts have been done over several decades to understand the influence of acoustics on flame dynamics [78, 18, 19, 73, 87, 49, 86, 88, 63] Understanding combustion noise in real engines means understanding all these coupled phenomena. However, the most clear starting point to study combustion noise is to ‘un-couple’ all these mechanisms, focusing on one at a time. As a consequence, it is assumed that the sources of noise (flame, turbulence, etc) are independent of the acoustic field generated. In other words, that no instabilities occur. Still, the following noise generation phenomena remain and must be considered: • flame/entropy/acoustics → ‘indirect combustion noise’. The sources of noise are the entropy waves when crossing non-homogeneous regions (zones with mean flow gradients). • Turbulence/acoustics → ‘aerodynamic noise’. The unsteady turbulent field is the source of noise. The influence of walls as scattering/reflecting mechanism (ex: wing airfoils, cavities) are also usually considered into the physical formulations. • flame/acoustics → ‘direct combustion noise’. The unsteady heat release acts as a distribution of acoustic monopoles, which generate pressure fluctuations. 1.2 Scope of the present work Combustion noise has been widely studied in open flames. These predictions are based on what is known as hybrid methods. In these approaches the sources of noise are computed separately from the radiated acoustic field. Unsteady CFD methods such as LES or DNS are used to compute the sources of noise whereas wave equations coming from acoustic analogies are employed to compute the sound radiation produced by these sources. LES or DNS are rarely used to estimate acoustic fluctuations directly, since these fluctuations are much smaller in comparison to first order fluctuations, as velocity or temperature perturbations. The first objective of this thesis regards this concern. It is seen how much the computation resolution of LES influences the quality of the estimations of noise. A second objective is linked to the hybrid approach for the estimation of combustion noise. No significative studies within the combustion community have been performed to evaluate whether or not hybrid approaches are appropriate to compute noise of confined flames. Confinement of flames leads to possible complexities: one of them is that pressure fluctuations produced by the flame might contain an important contribution of hydrodynamics, since in confined flames turbulence might be significant everywhere. Another significant concern is the evaluation of acoustic-flow interactions. When
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Bibliography [1] AGARWAL, A., MORRI
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