A Strategy to Tune Acoustic Terminations of Single-Can Test-Rigs to Mimic Thermoacoustic Behavior of a Full Engine

Abstract

Thermoacoustic properties of can-annular combustors are commonly investigated by means of single-can test-rigs. To obtain representative results, it is crucial to mimic cancan coupling present in the full engine. However, current approaches either lack a solid theoretical foundation or are not practicable for high-pressure rigs. In the present study we employ Bloch-wave theory to derive reflection coefficients that correctly represent can-can coupling. We propose a strategy to impose such reflection coefficients at the acoustic terminations of a single-can test-rig by installing passive acoustic elements, namely straight ducts or Helmholtz resonators. In an iterative process, these elements are adapted to match the reflection coefficients for the dominant frequencies of the full engine. The strategy is demonstrated with a network model of a generic can-annular combustor and a 3D model of a realistic can-annular combustor configuration. For the latter we show that cancan coupling via the compressor exit plenum is negligible for frequencies sufficiently far away from plenum eigenfrequencies. Without utilizing previous knowledge of relevant frequencies or flame dynamics, the test-rig models are adapted within a few iterations and match the full engine with good accuracy. Using Helmholtz resonators for test-rig adaption turns out to be more viable than using straight ducts.

Interplay of Clusters of Acoustic and Intrinsic Thermoacoustic Modes in Can-Annular Combustors

“…The generic combustor consists of N identical cans placed in an annular arrangement. Upstream the cans, we neglect the impact of the plenum, as it often shows little influence [33]. The cans are acoustically decoupled and the inlet reflection coefficient is set to R i = −1.…”

“…Fournier et al [24] and Haeringer et al [33] showed that, under some assumptions discussed later in the paper, a can-annular combustor can be fairly well represented by an equivalent Rijke tube. The latter is one of the simplest thermoacoustic system and has been extensively studied for decades.…”

“…al. [24] proposed the modeling of the gap as a thin annulus and the method was applied by Haeringer et al [33] in a strategy to tune single-can test-rigs to mimic full engines. Von Saldern et al [34] proposed to model the crosstalk between cans with the Rayleigh conductivity, which relates the acoustic flux through the aperture to the pressure gradient between cans.…”

Coupling-induced instability in a ring of thermoacoustic oscillators

“…The same theory was also applied in [53] in the time domain to reduce the computational cost of fluid dynamics simulations for the modelling of limit cycle oscillations in (can-)annular combustors. In [54], Bloch wave theory is employed to derive equivalent reflection coefficients that represent the can-to-can coupling. The authors propose a strategy to impose such reflection coefficients at the acoustic terminations of a single-can test rig by installing passive acoustic elements such as straight ducts or Helmholtz resonators, to mimic the thermoacoustic behaviour of a full engine.…”

Coupling-Induced Instability in a Ring of Thermoacoustic Oscillators

“…The theory was also applied by Haeringer and Polifke [37] in the time domain to reduce the computational cost of computational fluid dynamics (CFD) simulations for the modelling of limit cycle oscillations in (can-)annular combustors. In [38], Bloch wave theory is employed to derive equivalent reflection coefficients that represent the can-can coupling. Therein, a strategy is proposed to impose such reflection coefficients at the acoustic terminations of a single-can test-rig by installing passive acoustic elements such as straight ducts or Helmholtz resonators, to mimic the thermoacoustic behavior of a full engine.…”