Combustion noise: modeling and prediction

Abstract

This paper reviews both direct and indirect combustion noise. For convenience, they will simply be referred to as combustion noise and entropy noise. Combustion noise has been studied for well over half a century. However, because of the large number of parameters involved and the complexities inherent in the combustion processes, a widely accepted theory has yet to be developed. For this reason, this review focuses primarily on experimental measurements, semi-empirical relations and empirical but practical prediction methodologies. Important characteristic features and empirical correlations of combustion noise based on open flames and engine noise data are highlighted. Plausible generation of entropy noise by the passage of entropy waves through a nonuniform mean flow was first predicted theoretically circa 1970s. But it took forty years for its existence to be confirmed experimentally. Since then, there have been numerous publications on this subject. They are the primary materials of this review. The fundamental experiment and noise generation mechanism will be discussed first. Of great practical importance is whether there is significant generation of entropy waves inside an engine. This issue and new methods for modeling and predicting internally generated engine entropy noise are items that are examined at some length. Recent advances in computational methods especially in large eddy simulation lead us to envision an important role to be played by high-fidelity numerical simulation in future combustion and entropy noise research and prediction. A critical evaluation of a strategy for investigating and predicting the generation and propagation of these noise components from the combustor of an engine to its exhaust is included in this review.

Sound generation by entropy perturbations passing through a sudden flow expansion

“…Entropy perturbations are silent when advected by uniform D. Yang, J. Guzmán-Iñigo and A. S. Morgans flows but generate sound when accelerated/decelerated (Marble & Candel 1977). This generated sound is termed entropy noise and is an important contributor to indirect combustion noise (Ihme 2017;Tam et al 2019), which is highly relevant to thermoacoustic instabilities in gas turbines and rocket engines (Keller, Egli & Hellat 1985;Morgans & Duran 2016) and noise from aeroengines (Ffowcs Williams 1977;Duran et al 2014).…”

Aircraft noise generation and assessment: executive summary

“…The chapter on engine noise is comprised of three contributions, i.e., Refs. [6][7][8], that are briefly summarized here.…”

Influence of viscosity on entropy noise generation through a nozzle

“…Direct combustion noise is associated with the acoustic fluctuations generated by the turbulent flame through heat release fluctuations [5,6], whereas indirect combustion noise is produced when flow heterogeneities, such as entropy (cold and hot spots), vorticity (turbulent spots) and compositional (mixture heterogeneities) fluctuations are accelerated by the mean flow through nozzles and turbine stages [7][8][9][10]. Such perturbations lead to a loss of balance, to which the flow reacts by emitting acoustic waves [11,12]. Relative contributions of direct and indirect combustion noise to global sound emission of aeroengines is still an open question [1,13] but recent analytical investigations suggest indirect noise may dominate direct noise in several practical situations [1,14], hence the need for its modelling and reduction.…”