Indirect combustion noise

• Published in: Journal of fluid mechanics Vol. 659; pp. 267 - 288
• Main Author: HOWE, M. S.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 25.09.2010
• Subjects: Acoustics; aeroacoustics; Aeroacoustics, atmospheric sound; Aircraft; Applied sciences; Combustion; Energy; Energy. Thermal use of fuels; Engines and turbines; Entropy; Equipments for energy generation and conversion: thermal, electrical, mechanical energy, etc; Exact sciences and technology; Fluid dynamics; Fluid flow; Fundamental areas of phenomenology (including applications); gas dynamics; General theory; Inhomogeneities; instability; Noise; Nozzles; Physics; Slugs; Sound; Temperature effects; gas dynamics; aeroacoustics; instability; Flow separation; Swirling flow; Turbojet; Entropy; Sound source; Subsonic velocity; Noise level; Diffuser; Convergent duct; Aeroacoustics; Slug flow; Stretching; Mach number; Jet propulsion
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/S0022112010002466

An analysis is made of the noise generated during the passage of quiescent temperature/entropy inhomogeneities through regions of rapidly accelerated mean flow. This is an important source of jet engine core noise. Bake et al. (J. Sound Vib., vol. 326, 2009, pp. 574–598) have used an ‘entropy wave generator’ coupled with a converging–diverging nozzle to perform a series of canonical measurements of the sound produced when the inhomogeneity consists of a nominally uniform slug of hot gas. When flow separation and jet formation occur in the diffuser section of the nozzle, it is shown in this paper that the vortex sound generated by the jet is strongly correlated with the entropy noise produced by the slug and that the overall noise level is significantly reduced. Streamwise ‘stretching’ of the hot slug during high subsonic acceleration into the nozzle and the consequent attenuation of the entropy gradient in the nozzle are shown to significantly decrease the effective rate at which indirect combustion noise increases with the Mach number. Numerical predictions indicate that this is responsible for the peak observed by Bake et al. in the entropy-generated sound pressure at a nozzle Mach number near 0.6.