Entropy noise: A review of theory, progress and challenges

Combustion noise comprises two components: direct combust ion noise and indirect combustion noise. The latter is the lesser studied, with entropy noise believed to be its main co mponent in combusting flows. Entropy noise is generated via a sequence involving diverse flow physics. It has enjoyed a re surgence of interest over recent years, due to its increasin g importance to aero-engine exhaust noise and a recognition t hat it can affect gas turbine combustion instabilities. Entropy noise occurs when unsteady heat release rate genera tes temperature fluctuations (entropy waves), and these subsequently undergo acceleration. Five stages of flow phys ics have been identified as being important, these being (i) generation of entropy waves by unsteady heat release rate; ( ii) advection of entropy waves through the combustor; (iii) acceleration of entropy waves through either a nozzle or bla de row, to generate entropy noise; (iv) passage of entropy noise through a succession of turbine blade rows to appear at the turbine exit and (v) reflection of entropy noise back into the combustor, where it may further perturb the flame, in fluencing the combustor thermoacoustics. This paper reviews the underlying theory, recent progress and outstan ding challenges pertaining to each of these stage