Entropy noise: A review of theory, progress and challenges

Abstract

Combustion noise comprises two components: direct combustion noise and indirect combustion noise. The latter is the lesser studied, with entropy noise believed to be its main component. Entropy noise is generated via a sequence involving diverse flow physics. It has enjoyed a resurgence of interest over recent years, because of its increasing importance to aero-engine exhaust noise and a recognition that it can affect gas turbine combustion instabilities. Entropy noise occurs when unsteady heat release rate generates temperature fluctuations (entropy waves), and these subsequently undergo acceleration. Five stages of flow physics have been identified as being important, these being (a) generation of entropy waves by unsteady heat release rate; (b) advection of entropy waves through the combustor; (c) acceleration of entropy waves through either a nozzle or blade row, to generate entropy noise; (d) passage of entropy noise through a succession of turbine blade rows to appear at the turbine exit; and (e) reflection of entropy noise back into the combustor, where it may further perturb the flame, influencing the combustor thermoacoustics. This article reviews the underlying theory, recent progress and outstanding challenges pertaining to each of these stages.

On the dissipation and dispersion of entropy waves in heat transferring channel flows

“…For instance, the results of wellcontrolled experiments of Bake et al [19,20] in an entropy wave generator were initially in fair agreement with the classical prediction of Marble and Candel [12]. The causes of disagreement were explained and mostly resolved in the recent refinements of the original theory [15,11]. Despite these theoretical developments, most acoustic models still assume entropy waves to be one-dimensional and preserved during advection in the upstream channel, while the validity of these assumptions is unclear.…”

“…p′ being negligible), it may be concluded that s′ c p ≅ T′ T ̅ and also ρ′ ρ ≅ T′ T , [11,28] which indicate that the temperature (density) and entropy disturbances scale similarly. This similarity is the rationale behind generating the entropy wave by a transient temperature function at the channel 4 inlet.…”

“…Hence, entropy noise is regarded as indirect combustion noise to be also differentiated from the direct combustion noise generated primarily by turbulence [3,9]. In recent years, the growing importance of clean and quiet combustion in land-based gas turbines and aero-engines has renewed the interest in entropy noise [10,11].…”

“…There is now a body of experimental [21,22] and theoretical [23,24,11] works showing that entropy noise could influence the thermoacoustic instability of laboratory scale combustors. Yet, the studies conducted in real engines revealed much less effects [25,26].…”

Flame thermometry using laser-induced-grating spectroscopy of nitric oxide

“…The ability to use NO as an absorber for product temperature measurements would enable the LITGS technique to measure the temperature uniformity around the nozzle guide vanes, an important measure for gas turbine performance [29,30]. Furthermore, it would create the ability to directly measure the amplitude of temperature fluctuations, and thus detect the emergence of localised regions of elevated temperature, called entropy spots, as a source of pressure fluctuations [31][32][33]. As a relatively stable species, NO provides a useful target molecule with which to monitor temperature in the burned-gas zones of flames and technical combustion devices.…”