Jonas P. Moeck

Modeling the response of premixed flame transfer functions - Key elements and experimental proofs

A wide variety of analytical models has been proposed to describe the transfer function of premixed flames submitted to flow disturbances. These models generally rely on a kinematic description of the flame reaction surface which is perturbed around its steady state. The response then depends on the type of flow perturbation considered, the mean flow characteristics and flame properties. Systematic comparisons between model predictions and experimental results are however less numerous. This study revisits some of these theoretical descriptions and their underlying hypotheses to delineate their domain of application. The main features of the response of premixed flames to flow disturbances are highlighted. Different mechanisms are identified and their analytical representation is discussed. It is shown that experimental observations provide useful guidelines in the flame transfer function modeling. The cases investigated include laminar and turbulent configurations in conical or inverted V-flame configurations. Effects of swirl, flame root dynamics and confinement are emphasized.

Exceptional points in the thermoacoustic spectrum

Abstract Exceptional points are found in the spectrum of a prototypical thermoacoustic system as the parameters of the flame transfer function are varied. At these points, two eigenvalues and the associated eigenfunctions coalesce. The systems sensitivity to changes in the parameters becomes infinite. Two eigenvalue branches collide at the exceptional point as the interaction index is increased. One branch originates from a purely acoustic mode, whereas the other branch originates from an intrinsic thermoacoustic mode. The existence of exceptional points in thermoacoustic systems has implications for physical understanding, computing, modeling and control.

Thermoacoustics of can-annular combustors

Can-annular combustors consist of a set of independent cans, connected on the upstream side to the combustor plenum, and on the downstream side to the turbine inlet, where a transition duct links the round geometry of each can with the annular segment of the turbine inlet. Each transition duct is open on the sides towards the adjacent transition ducts, so that neighbouring cans are acoustically connected through a so called cross-talk open area. This theoretical, numerical and experimental work discusses the effect that this communication has on the thermoacoustic frequencies of the combustor. We show how this communication gives rise to axial and azimuthal modes, and that these correspond to particularly synchronised states of axial thermoacoustic oscillations in each individual can. We show that these combustors typically show clusters of thermoacoustic modes with very close frequencies and that a slight loss of rotational symmetry, e.g. a different acoustic response of certain cans, can lead to mode localization. We corroborate the predictions of azimuthal modes, clusters of eigenmodes and mode localization with experimental evidence.