Transonic Flow Features in a Nozzle Guide Vane Passage

Abstract: The entropy noise in modern engines is mainly originating from two types of mechanisms.First, chemical reactions in the combustion chamber lead to unsteady heat releasewhich is responsible of the direct combustion noise. Second, hot and cold blobsof air coming from the combustion chamber are advected and accelerated throughturbine stages, giving rise to the so-called entropy noise (or indirect combustionnoise). In the present work, numerical characterization of indirect combustion noiseof a Nozzle Guide Vane passage was assessed using three-dimensional Large EddySimulations. The study was conducted on a simplified topology of a real turbinestator passage, for which experimental data were available in transonic operatingconditions. First, a baseline case was reproduced to validate a numerical finite volumesolver against the experimental measurements. Then, the same solver is used toreproduce the effects of incoming entropy waves from the combustion chamber andto characterize the additional generated acoustic power. Periodic temperature fluctuationsare imposed at the inlet, permitting to simulate hot and cold packets of aircoming from the unsteady combustion. The incoming waves are characterized bytheir characteristic wavelength; therefore, a parametric study has been conductedvarying the inlet temperature of the passage, generating entropy waves of greaterwavelengths. The study proves that the generated indirect combustion noise canbe significant. Moreover, the generated indirect combustion noise increases as thewavelength of the incoming disturbances increases. Finally, the present work suggeststhat, in transonic conditions, there might be flow features which enhance theindirect combustion noise generation mechanism.