A Helmholtz resonator is an important passive control device to abate excessive sound pressure levels. Because of its effectiveness and simplicity (in principle, it consists of an arbitrarily shaped volume and a small duct), it is of high relevance for industrial applications relying on robustness, availability, and reliability. It is the method of choice to counteract high-amplitude combustion-induced pressure oscillations often exhibited in lean premixed gas turbine combustors. Traditional Helmholtz resonators feature an efficient damping performance only in a narrow frequency bandwidth. To derive an efficient design for the full-scale engine, it is key to accurately predict its resonance frequency. In addition to the geometrical dimensions and the speed of sound, this is a function of a length correction term that accounts for inertia effects of the oscillating fluid. In case the Helmholtz resonator is connected to an enclosure of equal density, several empirical correlations for this correction term are available in the literature. However, if density differences between resonator and enclosure exist, as is the case for gas turbine combustion or acoustic liners in aeroengines, the end correction is affected. To the knowledge of the authors, the influence of nonisopycnic conditions on the length correction was not systematically investigated before. In this work, dedicated cold-flow experiments featuring density differences are conducted to investigate this phenomenon, and a density-dependent correction term is derived. A wide range of different densities are realized by blending air and krypton.
Impact of Density Discontinuities on the Resonance Frequency of Helmholtz Resonators
AIAA Journal ; 53 , 4 ; 877-887
01.04.2015
Aufsatz (Zeitschrift)
Elektronische Ressource
Englisch
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