Experiments on sound reflection and production by choked nozzle flows subject to acoustic and entropy waves

[Display omitted] The acceleration of entropy waves in the turbine stages of aero-engines and gas turbines produces sound, which can contribute to the feedback mechanism of thermoacoustic instabilities. These instabilities cause high cycle fatigue of the combustor components, which may lead to catastrophic engine failure. This work deals with the challenging problem of experimentally quantifying entropy-wave-induced sound production from nozzles under choked flow conditions, at frequencies and turbulence intensities that are relevant for practical applications. There is a substantial need for such experimental data in order to validate predictive models of thermoacoustic instabilities involving entropy waves. In this study, the transfer function linking incident entropy waves, produced by a pulsed hot jet in a non-reactive turbulent cross flow, and the resulting reflected acoustic waves from choked nozzle flows are identified by combining acoustic measurements, background-oriented Schlieren thermometry and particle image velocimetry. The transfer function is measured for frequencies comprised between 60 and 180 Hz in the case of entropy waves that undergo intense dispersion in the highly turbulent channel, and that exhibit, at their arrival at the supercritical nozzle, amplitudes ranging from −1 to 10 percent of the mean temperature. This work suggests that three-dimensional deformation of entropy spots upstream of the nozzle convergent is likely to play a key role in the sound generation process, and can be used for developing new models accounting for this phenomenon.