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Core noise in aeroengines is due to two main mechanisms: direct combustion noise, which is generated by the unsteady expansion of burning gases, and indirect combustion noise, which is due to the acceleration of entropy waves (temperature fluctuations generated by unsteady combustion) within the turbine stages. This paper shows how a simple burner model (a flame in a combustion chamber terminated by a nozzle) can be used to scale direct and indirect noise. An analytical formulation is used for waves generated by combustion. The transmission and generation of waves through the nozzle is calculated using both the analytical results of Marble and Candel (Marble, F. E., and Candel, S., 'Acoustic Disturbances from Gas Nonuniformities Convected Through a Nozzle,' Journal of Sound and Vibration, Vol. 55, 1977, pp. 225-243.) and a numerical tool. Numerical results for the nozzle verify and extend the analytical approach. The analytical relations for the combustion and the nozzle provide simple scaling laws for direct and indirect noise ratio as a function of the Mach number in the combustion chamber and at the nozzle outlet. DOI: 10.2514/1.43729

An adiabatic homogeneous model to account for multiperforated liners in combustion chamber flow simulations is described. It is based on a suction and an injection model to reproduce the average effect of effusion cooling on both sides of the plate. The coupled suction/injection model has been specifically designed to be used in industrial full- scale computations of gas turbine combustion chambers, where effusion cooling is commonly used for controlling the temperature of the liners. Notably, it can be used with a coarse grid, the real perforated plate being replaced by a homogeneous boundary condition where the model is applied. The new modeled boundary condition conserves the inviscid part of the wall fluxes, which are shown to be the main contribution, as evidenced by the analysis of former wall-resolved simulations. Conserving the wall fluxes allows reproduction of the global structure of the flow and leads to reasonable comparisons with experimental data. Hence, the proposed new model provides a practical way to account for multiperforated plates with inclined perforations without resolving the flow in the perforations.

This paper presents a method for computing the thermoacoustic modes in combustors. In the case of a nonisothermal reacting medium, the wave equation for the pressure fluctuations contains a forcing term related to the unsteady heat release. Depending on the phase relationship between the acoustics and the flame, certain linear modes may become unstable, leading to thermoacoustic instabilities. The relevant Helmholtz equation is derived and two approaches for solving the corresponding nonlinear eigenvalue problem are proposed. The first one is based on an asymptotic expansion of the solution, the baseline being the acoustic modes and frequencies for a steady (or passive) flame and appropriate boundary conditions. This method allows a quick assessment of any acoustic mode stability but is valid only for cases where the coupling between the flame and the acoustic waves is small in amplitude. The second approach is based on an iterative algorithm where a quadratic eigenvalue problem is solved at each subiteration. It is more central processing unit demanding but remains valid even in cases where the response of the flame to acoustic perturbations is large. Frequency-dependent boundary impedances are accounted for in both cases. A parallel implementation of the Arnoldi iterative method is used to solve the large eigenvalue problem that arises from the space discretization of the Helmholtz equation. Several academic and industrial test cases are considered to illustrate the potential of the method. DOI: 10.2514/1.24933

Large Eddy Simulations are used to study the heat-release response to fuel inflow acoustic harmonic oscillations, in a coaxial jet flame where both reactants (O 2 and CH 4) are injected in a dense cryogenic state. The geometry is that of the academic test rig Mascotte, operated by ONERA (France), which high pressure operating conditions are relevant for the characterization of flame dynamics in Liquid Rocket Engines (LREs). The simulations, which are carried out with a real-gas fluid solver using a detailed kinetic scheme for CH 4 high-pressure oxycombustion, provide a thorough insight into the flame response for a wide range of forcing frequencies, spanning from approximately 1kHz to 20kHz. Local Flame Transfer Functions (FTF) are computed and analyzed: regions of preferential heat-release response are observed to be highly dependent on the forcing frequency. An analysis based on a flame sheet assumption is conducted to distinguish the main sources of heat release fluctuations: the primary contribution comes from the species diffusivity oscillations, while the density variations have a negligible effect. The second largest contributor is either the mixture fraction gradient or the flame surface area, depending on the forcing frequency. The FTFs are expected to be useful for thermoacoustic Low-Order Models or Helmholtz solvers, and the subsequent analysis has the potential to guide future development of analytical models for flame dynamics in LREs. [ABSTRACT FROM AUTHOR]