Acoustic and Entropy Waves in Nozzles in Combustion Noise Framework

Abstract

A low-order model is presented to study the propagation and interaction of acoustic and entropic perturbations through a convergent-divergent nozzle. The calculations deal with choked, unchoked, as well as compact and non-compact nozzles. In the choked case, a normal shock exists in the divergent section of the nozzle. To validate the models developed, cylindrical configurations corresponding to Entropy-WaveGenerator (EWG) and Hot-Acoustic-Testrig (HAT) of DLR are studied. For the EWG, an entropy wave is generated upstream of a nozzle by an electrical heating device, and for the HAT a speaker is used to generate pressure waves. In these two configurations and for the choked case, the supersonic region between the nozzle throat and the normal shock is assumed to be acoustically compact. The results of the low-order model are found to give excellent agreement with the experimental results of the EWG and HAT rigs. To give insight into the physics, the model is used to undertake a parametric study for a range of nozzle lengths and shock strengths. The low order model is finally used to calculate the direct to indirect (entropy and vorticity) combustion noise ratio for an idealised thin-annular combustor. For this model combustor the direct acoustic noise is found to dominate within the combustor while, the entropy indirect noise is found to be the main source of noise downstream of the choked nozzle. The indirect vorticity noise has a negligible contribution.

Entropy noise modelling in 2D choked nozzle flows

“…Excellent agreement is observed between the 2D model and CAA results for all the forcings considered and the compact solution of Marble & Candel [19] is correctly recovered by the model as well. With entropy forcing, 2D transfer functions collapse well with the quasi-1D solution classically used in the literature [28,29,[31][32][33] in the low frequency range but discrepancies increase as frequency rises because the radial distortion of the entropy wave is not taken into account by the quasi-1D approach. Of interest, quasi-1D modelled transfer functions collapse with CAA data in the case of acoustic forcings, which indicates that 2D mean flow effects are negligible for noise scattering through the nozzle.…”

“…This approach proved efficient to design a nozzle with optimal indirect noise generation through the use of a genetic algorithm, for instance [30]. In a different way, Mahmoudi et al [31] discretised the nozzle as a succession of ducts of constant radii and used the compact solutions to link acoustic, entropy and vorticity waves (including azimuthal modes) between successive elements.…”

Effect of Film Cooling on Entropy Noise Generation in a Stator Blade Row

“…It is suggested that the impact of perturbations in the mixture composition is important for advanced low-emission combustors with supersonic flows, and can reach amplitudes comparable to the direct noise and entropy noise [7]. Without considering the contribution of noise due to the mixture composition, the entropy noise is believed to be the main mechanism of noise generation downstream of the combustion chamber [8]. It also has been shown that, without considering entropy wave attenuation in the turbine stage, engine noise is over predicted by almost two orders of magnitude [9].…”

A comparative analysis of the evolution of compositional and entropy waves in turbulent channel flows

“…The indirect noise, however, is emanated from the convecting disturbances of temperature, chemical composition, or vorticity passing through a region of strong pressure gradient and thus flow acceleration [7,8]. Such disturbances are generated by the flame while their conversion to sound occurs in the combustor exit nozzle and the first stage of the turbine blade [9,10]. It follows that the occurrence of indirect combustion noise depends upon the survival of convecting disturbances throughout their journey from the flame to the downstream nozzles.…”