Ni, Franchine. Accounting for complex flowacoustic interactions in a 3D thermoacoustic Helmholtz solver. PhD, Dynamique des fluides, Institut National Polytechnique de Toulouse, 2017

(Document in English)
PDF  Requires a PDF viewer such as GSview, Xpdf or Adobe Acrobat Reader 7MB 
Abstract
Environmental concerns have motivated turbine engine manufacturers to create new combustor designs with reduced fuel consumption and pollutant emissions. These designs are however more sensitive to a mechanism known as combustion instabilities, a coupling between flame and acoustics that can generate dangerous levels of heat release and pressure fluctuations. Combustion instabilities can be predicted at an attractive cost by Helmholtz solvers. These solvers describe the acoustic behavior of an inviscid fluid at rest with a thermoacoustic Helmholtz equation, that can be solved in the frequency domain as an eigenvalue problem. The flame/acoustics coupling is modeled, often with a first order transfer function relating heat release fluctuations to the acoustic velocity at a reference point. One limitation of Helmholtz solvers is that they cannot account for the interaction between acoustics and vorticity at sharp edges. Indeed, this interaction relies on viscous processes at the tip of the edge and is suspected to play a strong damping role in a combustor. Neglecting it results in overly pessimistic stability predictions but can also affect the spatial structure of the unstable modes. In this thesis, a methodology was developed to include the effect of complex flowacoustic interactions into a Helmholtz solver. It takes advantage of the compactness of these interactions and models them as 2port matrices, introduced in the Helmholtz solver as a pair of coupled boundary conditions: the Matrix Boundary Conditions. This methodology correctly predicts the frequencies and mode shapes of a nonreactive academic configuration with either an orifice or a swirler, two elements where flowacoustic interactions are important. For industrial combustors, the matrix methodology must be extended for two reasons. First, industrial geometries are complex, and the Matrix Boundary Conditions must be applied to nonplane surfaces. This limitation is overcome thanks to an adjustment procedure. The matrix data on nonplane surfaces is obtained from the welldefined data on plane surfaces, by applying nondissipative transformations determined either analytically or from an acoustics propagation solver. Second, the reference point of the flame/acoustics model is often chosen inside the injector and a new reference location must be defined if the injector is removed and replaced by its equivalent matrix. In this work, the reference point is replaced by a reference surface, chosen as the upstream matrix surface of the injector. The extended matrix methodology is successfully validated on academic configurations. It is then applied to study the stability of an annular combustor from Safran. Compared to standard Helmholtz computations, it is found that complex flowacoustic features at dilution holes and injectors play an important role on the combustor stability and mode shapes. First encouraging results are obtained with surfacebased flame models.
Item Type:  PhD Thesis 

Uncontrolled Keywords:  
Institution:  Université de Toulouse > Institut National Polytechnique de Toulouse  Toulouse INP (FRANCE) 
Laboratory name:  
Research Director:  Poinsot, Thierry and Nicoud, Franck 
Statistics:  download 
Deposited On:  20 Jun 2017 10:32 
Repository Staff Only: item control page