Experimental Study on the Role of Entropy Waves in Low-Frequency Oscillations in a RQL Combustor

Eckstein, J.; Freitag, E. H.; Hirsch, Christoph; Sattelmayer, Thomas

doi:10.1115/1.2132379

Cited by 81 publications

(34 citation statements)

References 8 publications

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“…The findings of this paper appear to be in keeping with the experimental observations made in heat transferring combustors of real gas turbines [25]. They are also in agreement with the pervious simulations in adiabatic flows [28].…”

Section: Discussionsupporting

confidence: 79%

“…Yet, the studies conducted in real engines revealed much less effects [25,26]. This was such that some authors considered entropy waves to be of little significance to thermoacoustics of gas turbines [27].…”

Section: Introductionmentioning

confidence: 99%

“…Further, in Eq. (25), is the longitude position of an arbitrary cross section of the channel in Fig. 1a and is an arbitrary frequency.…”

Section: Dissipation and Dispersion Of The Wavementioning

confidence: 99%

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

2017

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This paper investigates the hydrodynamic and heat transfer effects upon the dissipation and dispersion of entropy waves in non-reactive flows. These waves, as advected density inhomogeneities downstream of unsteady flames, may decay partially or totally before reaching the exit nozzle, where they are converted into sound. Attenuation of entropy waves dominates the significance of the subsequent acoustic noise generation.Yet, the extent of this decay process is currently a matter of contention and the pertinent mechanisms are still largely unexplored. To resolve this issue, a numerical study is carried out by compressible large eddy simulation (LES) of the wave advection in a channel subject to convective and adiabatic thermal boundary conditions. The dispersion, dissipation and spatial correlation of the wave are evaluated by post-processing of the numerical results. This includes application of the classical coherence function as well as development of nonlinear quantitative measures of wave dissipation and dispersion. The analyses reveal that the high frequency components of the entropy wave are always strongly damped. The survival of the low frequency components heavily depends upon the turbulence intensity and thermal boundary conditions of the channel. In general, high turbulence intensities and particularly heat transfer intensify the decay and destruction of the spatial coherence of entropy waves. In some cases, they can even result in the complete annihilation of the wave. The current work can therefore resolve the controversies arising over the previous studies of entropy waves with different thermal boundary conditions.

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Section: Discussionsupporting

confidence: 79%

Section: Introductionmentioning

confidence: 99%

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

2017

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“…Complemented by experiments (Hield et al 2009;Yu et al 2010;Sisco et al 2011) these low order models show that a new family of instability modes that depends on the mean flow velocity may exist. However, as pointed out by Eckstein et al (2006), one important issue is that the influence of indirect combustion noise is not systematic and a low-frequency instability may appear even without the presence of a nozzle, and vice-versa. Some elements of response were provided by Polifke et al (2001) who showed that entropy and acoustic waves may have a constructive or destructive phase dependency, as well as Sattelmayer (2003) who argued that entropy spots are submitted to strong dilution (mixing) due to the highly turbulent nature of the flow in practical combustors.…”

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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“…Thus, several studies have been conducted to understand and alleviate combustion instabilities. Previous research contributed greatly to the understanding of combustion instability mechanisms, such as flame-vortex interaction [1,2], feedback on fluctuations in acoustic pressure, mixture velocity, equivalence and heat release [3,4], perturbation of entropy [5,6], processing This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/bync/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.…”

Section: Introductionmentioning

confidence: 99%

Effects of High-harmonic Components on the Rayleigh Indices in Multi-mode Thermo-acoustic Combustion Instability

Song¹,

Yoon²,

Yoon³

et al. 2016

International Journal of Aeronautical and Space Sciences

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This paper presents the characteristics of non-fundamental multi-mode combustion instability and the effects of highharmonic components on the Rayleigh criterion. Phenomenological observations of multi-harmonic-mode dynamic pressure waves regarding the intensity of harmonic components and the source of wave distortion have been explained by introducing examples of second-and third-order harmonics at various amplitudes. The amplitude and order of the harmonic components distorted the wave shapes, including the peak and the amplitude, of the dynamic pressure and heat release, and consequently the temporal Rayleigh index and its integrals. A cause-and-effect analysis was used to identify the root causes of the phase delay and the amplification of the Rayleigh index. From this analysis, the skewness of the dynamic pressure turned out to be a major source in determining whether multi-mode instability is driving or damping, as well as in optimizing the combustor design, such as the mixing length and the combustor length, to avoid unstable regions. The results can be used to minimize errors in predicting combustion instability in cases of high multi-mode combustion instability. In the future, the amount of research and the number of applications will increase because new fuels, such as fast-burning syngases, are prone to generating multi-mode instabilities.

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Experimental Study on the Role of Entropy Waves in Low-Frequency Oscillations in a RQL Combustor

Cited by 81 publications

References 8 publications

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

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

Mixed acoustic–entropy combustion instabilities in gas turbines

Effects of High-harmonic Components on the Rayleigh Indices in Multi-mode Thermo-acoustic Combustion Instability

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