Response of a subsonic nozzle to acoustic and entropy disturbances

Bohn, M.

doi:10.1016/0022-460x(77)90647-2

Cited by 25 publications

(19 citation statements)

References 2 publications

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“…After much of the early work of the 1970's, 3,8,[12][13][14] entropy noise has seen a resurgence in research interest over the last decade. This is for two primary reasons:…”

Section: Introductionmentioning

confidence: 99%

Entropy noise: A review of theory, progress and challenges

Morgans

Durán

2016

International Journal of Spray and Combustion Dynamics

121

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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.

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“…After much of the early work of the 1970's, 3,8,[12][13][14] entropy noise has seen a resurgence in research interest over the last decade. This is for two primary reasons:…”

Section: Introductionmentioning

confidence: 99%

Entropy noise: A review of theory, progress and challenges

Morgans

Durán

2016

International Journal of Spray and Combustion Dynamics

121

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“…Acoustic response coefficients for the subsonic [10,11] and supercritical [11] nozzles were derived and found to be a function of Mach numbers alone. The compact coefficients provided a good analytical prediction at very low frequencies.…”

Section: Introductionmentioning

confidence: 99%

Phase prediction of the response of choked nozzles to entropy and acoustic disturbances

Goh

Morgans

2011

Journal of Sound and Vibration

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The development and transmission of sound through the exit of an aeroengine combustor is often investigated by modelling the complex geometry as a convergent-divergent nozzle. However, these analytical acoustic predictions are usually limited to the compact case, where the length of the nozzle is insignificant compared to the wavelength of the flow perturbations, or to cases where the variation of the mean velocity through the nozzle may be treated as linear, or piece-wise linear. Considering terms up to first order in frequency for the conservation of mass, momentum and energy, this paper investigates an alternative approach by deriving effective lengths for the passage of the flow perturbations through a supercritical convergent-divergent nozzle. The effects due to the presence of a normal shock wave are also studied using a linearised form of the Rankine-Hugoniot relations. The analyses lead to predictions for the phase and magnitude of the transmitted acoustic waves from finite-length nozzles, and are valid for low non-dimensional frequencies. It has been found that these predictions agree well with the numerical results from inviscid simulations.

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“…Multiple works dedicated to the characterisation of the nozzle impedance emerged in the 70's. Bohn (1977) for the subsonic case, as well as Marble & Candel (1977) for both subsonic and supersonic cases, derived the reflection/transmission coefficients for impinging acoustic and entropic waves in the zero frequency limit (compact nozzle assumption). This approach was later extended to the non-compact cases by Moase et al (2007) and Goh & Morgans (2011), and generalised for both subsonic and choked flows with and without shock wave by .…”

Section: Introductionmentioning

confidence: 99%

Mixed acoustic–entropy combustion instabilities in gas turbines

2014

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A combustion instability in a combustor terminated by a nozzle is analysed and modelled based on a low order Helmholtz solver. A Large Eddy Simulation (LES) of the corresponding turbulent, compressible and reacting flow is first performed and analysed based on Dynamic Mode Decomposition (DMD). The mode with the highest amplitude shares the same frequency of oscillation as the experiment (approx. 320 Hz) and shows the presence of large entropy spots generated within the combustion chamber and convected down to the exit nozzle. The lowest purely acoustic mode being in the range 700 − 750 Hz, it is postulated that the instability observed around 320 Hz stems from a mixed entropy/acoustic mode where the acoustic generation associated with entropy spots being convected throughout the choked nozzle plays a key role. The DMD analysis allows to extract from the LES results a low-order model that confirms that the mechanism of the low-frequency combustion instability indeed involves both acoustic and convected entropy waves. The Delayed Entropy Coupled Boundary Condition (Motheau et al. 2014) is implemented into a numerical Helmholtz solver where the baseline flow is assumed at rest. When fed with appropriate transfer functions to model the entropy generation and convection from the flame to the exit, the Helmholtz/DECBC solver predicts the presence of an unstable mode around 320 Hz, in agreement with both LES and experiments.

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Response of a subsonic nozzle to acoustic and entropy disturbances

Cited by 25 publications

References 2 publications

Entropy noise: A review of theory, progress and challenges

Entropy noise: A review of theory, progress and challenges

Phase prediction of the response of choked nozzles to entropy and acoustic disturbances

Mixed acoustic–entropy combustion instabilities in gas turbines

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