Bruno Schuermans

Dynamic response of turbulent swirling flames to acoustic perturbations

The dynamic response of a turbulent, perfectly premixed flame, stabilized by means of an aerodynamic flameholder, to an upstream acoustic perturbation of the approaching flow is investigated by means of experimental and analytical tools, and simulated through a large eddy simulation of the reacting flow. It is found that the main contribution to the unsteady heat release rate is due to the fluctuation in area of the flame front, which in turn is strongly influenced by the corresponding response of the flow field to the acoustic perturbation. Numerical data show that perturbing a swirling flow that undergoes vortex breakdown results in a strong displacement of the breakdown position along its axis, while its outer part only weakly responds to the perturbation. This results in a translational motion of the flames anchoring point, which ultimately leads to an unsteady variation of the flame area and, therefore, of the amount of heat released. This unsteady heat release mechanism can be described in a way similar to that used for characterizing the dynamic behaviour of ducted flames, stabilized by means of a bluff-body flameholder; differently from these models, however, the anchoring point of the flame can now fluctuate freely in space, and the time delay of the system is no longer identified with the travelling time of a perturbation of the flame element along it, but is now associated with the oscillation of the breakdown position. Controlling the interaction between breakdown and acoustics should allow for obtaining optimal flame dynamics, so as to limit and possibly avoid the occurrence of strong pulsation peaks whenever the combustion device is operated in an acoustically closed system.

Flame imaging on the ALSTOM EV-burner: thermo acoustic pulsations and CFD-validation

The ALSTOM low emission swirl-induced premix EV-burner is investigated by OH-planar laser induced fluorescence (PLIF) and OH*-chemiluminescence (CL) imaging on a full-scale industrial burner test rig. Three different burner variants have been compared by their flame shape and position as well as emission and pulsation behavior. The flame images have been used to enable comparison and validation of thermo acoustic and computational fluid dynamics (CFD) models. The flame movement upstream inside the burner can be related to emissions and pulsation. Depending on the burner two different mechanisms dominate the acoustic pulsations: One is based on equivalence ratio fluctuations coupled to a sudden displacement of flame anchoring point into the burner. Another mechanism seems to be related to turbulence intensity fluctuations. The experimental images were compared with the results of Reynolds-averaged Navier-Stokes (RANS) CFD simulations for varying parameters for validation. The turbulence treatment in time-averaged RANS models is not sufficient to describe the flame movement properly and encourages to use a more sophisticated treatment.

Design for Thermo-Acoustic Stability: Procedure and Database

A design for thermo-acoustic stability (DeTAS) procedure is presented, that aims at selecting a most stable burner geometry for a given combustor. It is based on the premise that a thermo-acoustic stability model of the combustor can be formulated and that a burner design exists, which has geometric design parameters that sufficiently influence the dynamics of the flame. Describing the burner and flame dynamics in dependence of the geometrical parameters an optimization procedure involving a linear stability model of the target combustor maximizes the damping and thereby yields the optimal geometrical parameters. To demonstrate the procedure on an existing annular combustor a generic burner design was developed that features two geometrical parameters that can easily be adjusted. To provide the database for the DeTAS procedure static and dynamical properties of burner and flame were measured for three by three configurations at a fixed operation point. The data is presented and discussed. It is found that the chosen design exhibits a significant variability of the flame dynamics in dependence of the geometrical parameters indicating that a DeTAS should be possible for the targeted annular combustor.

Reduced-Order Modeling of Aeroacoustic Systems for Stability Analyses of Thermoacoustically Noncompact Gas Turbine Combustors

A methodology is presented to model non-compact thermoacoustic phenomena using Reduced Order Models (ROM) based on the Linearized Navier-Stokes Equations (LNSE). The method is applicable to geometries with a complex flow field as in a gas turbine combustion chamber. The LNSE, and thus the resulting ROM, include coupling effects between acoustics and mean fluid flow, and are hence capable of describing propagation and (e.g. vortical) damping of the acoustic fluctuations within the considered volume. Such a ROM then constitutes the main building block for a novel thermoacoustic stability analysis method via a low-order hybrid approach. This method presents an expansion to state-of-the-art low-order stability tools, and is conceptually based on three core features: Firstly, the multi-dimensional and volumetric nature of the ROM establishes access to account spatial variability and non-compact effects on heat release fluctuations. As a result, it is particularly useful for high frequency phenomena such as screech. Secondly, the LNSE basis grants the ROM the capability to reconstruct complex acoustic performances physically accurate. Thirdly, the formulation of the ROM in state-space allows convenient access to the frequency and time domain. In the time domain, non-linear saturation mechanisms can be included, which reproduce the non-linear stochastic limit cycle behavior of thermoacoustic oscillations.In order to demonstrate and verify the ROM’s underlying methodology, a test case using an orifice-tube geometry as the acoustic volume is performed. The generation of the ROM of the orifice-tube is conducted in a two-step procedure. As the first step, the geometrical domain is aeroacoustically characterized through the LNSE in frequency domain, and discretized via the Finite Element Method (FEM). The second step concerns the actual derivation of the ROM. The high-order dynamical system from the LNSE discretization is subjected to a modal reduction as order reduction technique. Mathematically, this modal reduction is the projection of the high-order (N ∼200,000) system into its truncated left eigenspace. An order reduction of several magnitudes (ROM order: Nr ∼100) is achieved. The resulting ROM contains all essential information about propagation and damping of the acoustic variables, and efficiently reproduces the aeroacoustic performance of the orifice-tube. Validation is achieved by comparing ROM results against numerical and experimental benchmarks from LNSE-FEM simulations and test rig measurements, respectively. Excellent agreement is found, which grants the ROM modeling approach full eligibility for further usage in the context of thermoacoustic stability modeling. This work is concluded by a methodological demonstration of performing stability analyses of non-compact thermoacoustic systems using the herein presented ROMs.Copyright

Parameter identification for a low-order network model of combustion instabilities

This paper describes the identification and modeling of a test rig designed to study thermoacoustic instabilities. Various operating conditions with different flame structures are identified. Physics-based models are used in a network model in order to advance the understanding of the phenomena involved. The parameters identified of the flame correlate well with the observed flame shapes. The transfer function from a loudspeaker input to a pressure reading is assembled and compared to measurements. Reflection coefficients are introduced to study the amplifying frequency ranges of the flame, which correspond to the highest peaks in the pressure spectrum.

EXTRACTION OF LINEAR GROWTH AND DAMPING RATES OF HIGH-FREQUENCY THERMOACOUSTIC OSCILLATIONS FROM TIME DOMAIN DATA

This paper presents a set of methodologies for the extraction of linear growth and damping rates associated with transversal eigenmodes at screech level frequencies in thermoacoustically non-compact gas turbine combustion systems from time domain data. Knowledge of these quantities is of high technical relevance as an required input for the design of damping devices for high frequency oscillations. In addition, validation of prediction tools and flame models as well as the thermoacoustic characterization of a given unstable/stable operation point in terms of their distance from the Hopf bifurcation point occurs via the system growth/damping rates. The methodologies solely rely on dynamic measurement data (i.e. unsteady heat release and/or pressure recordings) while avoiding the need of any external excitation (e.g. via sirens), and are thus in principle suitable for the employment on operational engine data. Specifically, the following methodologies are presented: 1) The extraction of pure acoustic damping rates (i.e. without any flame contribution) from oscillatory chemiluminescence and pressure recordings. 2) The obtainment of net growth rates of linearly stable operation points from oscillatory pressure signals. 3) The identification of net growth rates of linearly unstable operation points from noisy pressure envelope data. The fundamental basis of these procedures is the derivation of appropriate stochastic differential equations, which admit analytical solutions that depend on the global system parameters. These analytical expressions serve as objective functions against which measured data are fitted to yield the desired growth or damping rates. Bayesian methods are employed to optimize precision and confidence of the fitting results. Numerical test cases given by time domain formulations of the acoustic conservation equations including high-frequency flame models as well as acoustic damping terms are set up and solved. The resulting unsteady pressure and heat release data are then subjected to the proposed identification methodologies to present corresponding proof of principles and grant suitability for employment on real systems.Copyright

Optical Transfer Function Measurement for a Premixed Swirl-Stabilized Flame at Atmospheric Conditions

Thermoacoustic transfer functions of a generic swirl-stabilized gas turbine burner have been measured at atmospheric conditions. Loudspeakers were employed for the acoustic excitation of the combustion test rig. Arrays of condenser microphones have been used to record the acoustic response of the system to this excitation. The flame’s OH*, CH*, and C2* chemiluminescence response was measured by three photomultiplier tubes. In addition, a light spectrometer recorded the light spectrum emitted by the flame and an ICCD camera monitored the flame’s location in the combustion chamber. The thermoacoustic transfer function has been obtained by two different techniques: A widely used purely acoustic technique has been employed to determine the burner and flame transfer functions. This technique makes use of the well-known multi-microphone method. In addition, a combined acoustic-optical technique has been used, which makes use of the different response of the OH*, CH* and C2* chemiluminescence of the flame to acoustic excitation. The transfer function results obtained by the acoustic-optical and purely acoustic techniques have been compared and the benefits and limitations of the two method are discussed.