PhD defense - E. Gullaud

Effect of multiperforated plates on the acoustic modes in combustors

(Confidential until December 2016)

Delivered by University of Montpellier II
Speciality: Mathematics

December 1, 2010 - CERFACS

Aeronautical engine constructors are using lean premixed regimes to deal with the necessity to cut down pollutant emissions. These regimes indeed help to prevent the emission of NOx but trigger on the other hand combustioninstabilities. Numerical simulation (which can consist of LES or Helmholtz solvers for example) has proven to be a usefool tool to predict these instabilities at the design stage. Acoustic modes can be well predicted only if geometrical details are taken into account. Multiperforated plates which equip combustion chambers with the purpose of cooling the inner walls must for instance be taken into account in a numerical calculation. These plates consist of several apertures with a diameter smaller than 1 millimeter, which makes their meshing impossible. The objective of this thesis is to take into account perforated plates in the numerical simulation of the acoustics of combustion chambers. The homogeneous model for the acoust ic behaviour of a perforated plate derived by Howe in 1979 is used. Provided some hypotheses, this model can predict the acoustic behaviour of a plate under an acoustic excitation. Howe's model, derived in an incompressible flow, is here adapted to be used in the case where the perforated plate in located between the casing (cold air) of a combustion chamber and the inner chamber (filled with a hot mixture). The model is well suited to be implemented in an existing 3D Helmholtz solver, because it appears under the form of an impedance. The coding is validated by comparing numerical results to analytical results on simple geometries. First results allow to show the damping behaviour of perforated plates and its dependance to geometric parameters or the speed of the incoming flow though the apertures.Acoustic instabilities can also be apprehended with an acoustic energy approach. Since industrial chambers are equipped with several pairs of multiperforated plates, it is interesting to show which of them are the most efficient at damping purposes. An acoustic energy budget allows to predict the percentage of the total damping a particular plate is responsible for. In the presence of a flame, the acoustic energy budget can also give information on the contribution of the flame on the triggering or damping of the instability.An industrial chamber designed by Turbomeca for a helicopter is computed. The acoustic energy budget on a computation taking into account the active flame and the multiperforated plates allows to predict the stability of the modes of the chamber. The elements responsible for the behaviour of the instability can be identified. This work has been funded by SNECMA and the code used to implement the model is AVSP, it co-belongs to CERFACS and SNECMA.

Y. Gervais	Senior reseracher - LEA, University of Poitiers	President
S. Ducruix	Researcher - EM2C, Ecole Centrale Paris	Referee
C. Schram	Assistant professor - Von Karman Institute for Fluid Dynamics, Rhode-St-Genèse, Belgium	Referee
B. Mohammadi	Professor - CERFACS, Toulouse	Member
S. Moreau	Researcher - University of Sherbrooke, Canada	Member
S. Roux	Engineer - SNECMA, Villaroche	Member
F. Nicoud	Professor - University of Montpellier II	Advisor