Claude Sensiau

Acoustic modes in combustors with complex impedances and multidimensional active flames

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 stabilitybutisvalidonlyforcaseswherethecouplingbetweenthe flameandtheacousticwavesissmallinamplitude. 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 flametoacousticperturbationsislarge.Frequency-dependentboundaryimpedancesareaccountedforinbothcases. A parallel implementation of the Arnoldi iterative method is used to solve the large eigenvalue problem that arises fromthespacediscretization ofthe Helmholtzequation.Several academicandindustrial testcasesareconsideredto illustrate the potential of the method.

Assessing non-normal effects in thermoacoustic systems with mean flow

In this paper, non-normal interactions in a thermoacoustic system are studied, using a low-order expansion of the state variables in terms of eigenmodes. The thermoacoustic eigenmodes are determined as solutions of the Helmholtz equation or the linearized Euler equations, respectively, in the presence of a time-lagged heat source. Subsequently, non-normal effects are evaluated in a post-processing analysis based on the computed eigenmodes. In the case where the eigenmode analysis is based on the linearized Euler equations, effects of a non-zero mean flow velocity can be taken into account. The energy associated with the eigenmodes may then contain contributions of convected entropy and vorticity modes as well as the acoustic field. The notion of transient growth of perturbation energy is thus extended from an expression based on the classical acoustic energy density to a form based on a generalized disturbance energy. The expansion in terms of eigenmodes is computationally efficient, making the approach p...

A tool to study azimuthal standing and spinning modes in annular combustors

A methodology for the computation of azimuthal combustion instabilities which can occur in annular combustors is proposed in this work. A thermoacoustic numerical tool using the n – τ model for the coupling of acoustic and combustion is required to solve the Helmholtz equation in reactive media. The methodology is based on the Independence Sector Assumption in Annular Combustor (ISAAC) which states that the heat release fluctuations in a given sector are driven only by the fluctuating mass flow rates due to the velocity perturbations through its own swirler. This assumption is first discussed with respect to a Large Eddy Simulation of an annular combustor. The methodology is then applied to an academic annular test case which exhibits amplified or damped, standing or rotating azimuthal eigenmodes depending on parameters n and τ.

Combustion Instability Problems Analysis for High-Pressure Jet Engine Cores

The design of a clean combustion technology based on lean combustion principles will have to face combustion instability. This oscillation is often discovered late in engine development when, unfortunately, only a few degrees of freedom still exist to solve the problem. Individual component test rigs are usually not useful in detecting combustion instability at an early stage because they do not have the same acoustic boundary conditions as the full engine. An example of this unsteady activity phenomenon observed during the operation of a high-pressure core is presented and analyzed. To support the investigation, two numerical tools have been extensively used. First, the experimental measurement of unsteady pressure and the results of a multidimensional acoustic code are used to confirm that the frequency variations of the observed modes within the operating domain of the high-pressure core are due to the excitation of the first and second azimuthal combustor modes. The impact of acoustic boundary conditions for the combustor exhaust is shown to control the appearance and mode transition of this unsteady activity. Second, the three-dimensional reacting and nonreacting large eddy simulations for the complete combustor and for the injection system cup alone suggest that the aerodynamic instability of the flow passing through the cup could be the noise source exciting the azimuthal acoustic modes of the chamber. Based on these results, the air system (cup) was redesigned to suppress this aerodynamic instability, and experimental combustion tests confirm that the new system is free of combustion instability.

Impact of inlet distortion on fan tonal noise

Fan tonal noise is currently supposed to be mainly caused by the interaction of fan-blade wakes with Outlet Guide Vanes and by the shocks in transonic regimes. Evolution of air-craft engines towards Ultra High Bypass Ratio architectures may affect this noise with the additional distortion coming from the shortened and asymmetric air inlets. The influence of this distortion is studied here by means of simulations based on the Unsteady Reynolds-Averaged Navier-Stokes equations. The results show that the asymmetric air inlet is re-sponsible for a multiplication of the distortion level by four to six and for a deformation of fan-blade wakes. A filtering consisting in eliminating the flow patterns convected with the flow is achieved to extract only the acoustic fluctuations from the simulations. Acoustic power estimations are made and allow for evaluating the acoustic penalty induced by air inlet distortion.