Compositional noise in nozzles with dissipation

TL;DR

This paper introduces a physical model to predict indirect noise caused by compositional inhomogeneities in viscous, non-isentropic nozzles, validated with experimental data and offering a fast estimation method for gas turbine noise analysis.

Contribution

The paper develops a semi-analytical, low-order model for compositional noise in nozzles, incorporating dissipation effects and providing a quick estimation tool with less than 1% error.

Findings

Friction significantly influences indirect noise generation.

The model accurately predicts transfer functions with less than 1% error.

Physical mechanisms of compositional noise are identified and explained.

Abstract

We propose a physical model to predict indirect noise generated by the acceleration of compositional inhomogeneities in nozzles with viscous dissipation (non-isentropic nozzles). First, we derive the quasi-one-dimensional equations from the conservation laws of multicomponent flows. Second, we validate the proposed model with the experimental data available in the literature for binary mixtures of four gases. Third, we calculate the nozzle transfer functions for different Helmholtz numbers and friction factors, in both subsonic and supersonic flows with/without shock waves. We show that friction has a significant effect on the generation of indirect noise, for which the physical mechanism is identified and explained. Fourth, we find a semi-analytical solution with path integrals, which provide an asymptotic expansion with respect to the Helmholtz number. Fifth, we introduce the compositional noise scaling factor, which is applied to quickly estimate compositional noise from the knowledge of only one single-component gas transfer function. The approximation error is less than $1\%$. The proposed low-order model provides accurate estimates of the transfer functions and physical insight into indirect noise for multicomponent gases. This opens up new possibilities to accurately predict, and understand, sound generation in gas turbines.