Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session P06: Thermoacoustic Instability |
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Chair: Nicholas Worth, Norwegian Univ Science and Technology Room: 205 |
Monday, November 25, 2019 5:16PM - 5:29PM |
P06.00001: Three-dimensional Scanning Laser Induced Fluorescence for the investigation of flame dynamics on annular combustor instabilities Dirren Govender, Samuel Wiseman, James R Dawson, Nicholas Worth The need for three dimensional techniques is of importance when investigating turbulent asymmetrical flame structures. In combustion systems, self-excited thermo-acoustic instabilities are a prevalent problem and many studies are aimed at better understanding the phenomena. In this study, a Scanning OH* Laser Induced Fluorescence method is introduced to spatially resolve three dimensional flame structures during forced azimuthal oscillations in an annular combustor. The scanning setup consisted of a galvanometer mirror that sweeps a laser sheet through the flame while imaging at 10kHz. The obtained images were phase averaged based on the forcing cycle and the results were used to reconstruct the three dimensional flame structure. A variety of forced standing and spinning modes were investigated. Phase-averaged Flame Surface density (FSD) was used to characterize the three-dimensional flames. A series of calibrations were performed and evaluated for the transformation of image space to object space. This consisted of an additional camera to characterize the laser sheet positions and track any movements of the calibration plate while calibrating the in-plane camera. The results shows the scanning method's ability to resolve three-dimensional flame structures and dynamics. [Preview Abstract] |
Monday, November 25, 2019 5:29PM - 5:42PM |
P06.00002: Exploration of Unstable Spray Flames Using Reduced-Order Models Abdulla Ghani, Thomas Steinbacher, Alp Albayrak, Wolfgang Polifke We investigate low-frequency thermoacoustic instabilities of a swirling spray flame. This study is based on an aeronautical lab-scale experiment, for which a reduced-order model (ROM) has been generated. A parametric study of the ROM suggests that the instability mechanism is caused by an intrinsic thermoacoustic (ITA) feedback loop. Further analysis such as separation of acoustic and ITA modes or the scaling of the ITA frequency with the bulk velocity confirm the ITA feedback loop as the instability driver. Results of the ROM agree well with experimental observations and demonstrate the effectiveness of ROMs. [Preview Abstract] |
Monday, November 25, 2019 5:42PM - 5:55PM |
P06.00003: Effect of Global Acceleration on the Stability of a Thermoacoustically Coupled Shear Layer in a Backward-Facing Step Combustor Joel Vasanth, Satyanarayanan Chakravarthy The impact of the acoustic field on variable density flow in a step combustor is explored via a linear stability analysis of the low Mach number Navier-Stokes equations. In the equi-time scales limit, the acoustic feedback to the flow reduces to a global acceleration (GA) as a momentum source. The velocity profiles are parameterized by shear layer thickness $\delta$ and reverse flow ratio $\beta$. To close the equation set, the $n$-$\tau$ flame model is used. A local spatio-temporal perturbation analysis shows a shift in the absolutely/convectively unstable (AU/CU) transition boundaries towards the AU zone in the $\beta$-$\delta$ space as the flame response gain is increased. Further, the AU modes are less responsive to the acoustic feedback than the CU modes, affirming the semi-open/fully-closed loop mechanisms of acoustic feedback advanced in recent literature. Competing contributions of the vorticity equation source terms to the overall vorticity generation show that the GA is always destabilizing unlike the density gradient, which could be stabilizing under some conditions. The GA and the resultant of vorticity production and baroclinic vorticity are in phase under all conditions, which implies the forcing nature of the GA that unconditionally promotes combustion instability. [Preview Abstract] |
Monday, November 25, 2019 5:55PM - 6:08PM |
P06.00004: Intrinsic thermoacoustic modes in systems with two axially separated heat sources Philip Buschmann, Jonas Moeck Modern gas turbines for power generation are required to provide high operational flexibility while complying with emission limits. Axially staged flames or sequential combustors are advantageous for this purpose. However, the distributed heat release rate in these systems gives rise to new thermoacoustic interaction phenomena. Recently, it was shown that a mechanism exists that does not require acoustic wave reflection at the boundaries. Instead, this so-called intrinsic feedback loop rests on the velocity fluctuation associated with the flame-emitted acoustic wave traveling upstream. We study intrinsic thermoacoustic modes in a generic combustor with two axially separated heat sources. Computations of the spectrum under parameter variations show that both flames have individual intrinsic feedback mechanisms but also interact directly, without acoustic reflection. Furthermore, allowing for a convective response mechanism from the first flame to the second gives rise to an additional intrinsic feedback loop. A combustor with two sequential heat sources, therefore, exhibits a dramatically increased number of intrinsic modes. [Preview Abstract] |
Monday, November 25, 2019 6:08PM - 6:21PM |
P06.00005: Large-eddy simulations of a heated and combusting Rijke tube configuration Wei Xian Lim, Wai Lee Chan Due to its potential to cause catastrophic damages to combustion engines, thermoacoustic instability is an active area of research that demands further understanding. The process to experimentally identify or eliminate thermoacoustic instability effects can be costly, so the ability to employ numerical simulation techniques will be advantageous. In this work, a canonical thermoacoustic configuration, known as the Rijke tube, was chosen to study the coupling effects between the heat-addition and acoustic pressure fluctuations. To this end, two large-eddy simulation cases were studied, one with heating wire and the other with combustion represented by the flamelet/progress-variable model. In all simulations, proper boundary conditions and buoyancy force were accounted for to represent the actual conditions of Rijke tube in a laboratory space. Good results were obtained for the heating-wire case when comparing the theoretical and numerical resonant frequency. Meanwhile, additional flame-acoustics interactions were also observed in the flamelet/progress-variable case. [Preview Abstract] |
Monday, November 25, 2019 6:21PM - 6:34PM |
P06.00006: Receptivity of a transversely forced jet Jose G. Aguilar, Eirik Asoy, James Dawson, Nicholas Worth Annular combustion chambers are often used in gas turbines. These configurations are prone to thermoacoustic instabilities which emerge as standing or spinning azimuthal modes among others. During the instability the flow conditions of typical flames resemble swirling annular jets which are transversely forced by an acoustic wave. Previous studies suggest that in acoustically compact configurations the flame response to transverse flow excitation is strong if it is seen locally (within sections of the flame), but it is weak if it is seen globally given that there is negligible contribution to the total fluctuations of the heat release. By means of experimental analysis and numerical simulation the present study aims to quantify the receptivity of a more simplistic flow configuration, an isothermal jet subject to transverse forcing and elucidate the modes responsible for its strong local excitation. The jet is forced with a standing wave at different nodal positions. The experiments show that the response of the jet depends on the position of the acoustic wave and is described by a mixture of the symmetric and antisymmetric modes. Receptivity analysis is carried out using numerical simulation in order to quantify the strength of each mode at the different forcing positions. [Preview Abstract] |
Monday, November 25, 2019 6:34PM - 6:47PM |
P06.00007: Structure of thermoacoustic modes in can-annular combustion systems Jonas Moeck, Giulio Ghirardo, Mirko Bothien Large gas turbines for power generation employ a can-annular combustor architecture. In contrast to annular combustors, in a can-annular system, the flames are isolated and burn in individual cans; however, there is acoustic communication between adjacent cans at the turbine inlet through a small gap. The thermoacoustic modal structure of a can-annular combustor significantly differs from that of the extensively studied annular combustor. We consider a can-annular configuration as a system composed of weakly coupled, nominally identical subcomponents. This provides a consistent framework for studying the spectral properties and explains prevalent phenomena in these systems, such as eigenvalue clustering, and mode localization when the symmetry is perturbed. Based on a generic can-annular model system, we illustrate the general modal structure and derive the equivalent downstream boundary condition that links the multi-can resonances to a single-can model. [Preview Abstract] |
Monday, November 25, 2019 6:47PM - 7:00PM |
P06.00008: Time-Accurate Calibration of a Thermoacoustic Model on Experimental Images of a Forced Premixed Flame Hans Yu, Ushnish Sengupta, Matthew Juniper, Luca Magri Thermoacoustic instabilities are a persistent challenge in the design of jet and rocket engines. The time-accurate calculation of thermoacoustic instabilities is challenging due to the presence of both aleatoric and epistemic uncertainties, as well as the extreme sensitivity to small changes in certain parameters. We extend our previous work (Yu et al., CTR summer program 2018; Yu et al., J. Eng. Gas Turbines Power 2019) by applying our recently published level-set data assimilation framework (Yu et al., J. Comput. Phys. 2019) to experimental images of a forced premixed flame. We force a Bunsen flame with a loudspeaker and record videos at different frequencies and amplitudes. Data assimilation provides an optimal estimate of the true state of a system, and improves the predicted shape and location of the flame. Parameter estimation uses the data to find a maximum-likelihood set of parameters for the model while simultaneously quantifying their uncertainty and identifying deficiencies in the model. We demonstrate our level-set data assimilation framework using both the ensemble Kalman filter and smoother. More generally, we take a physics-informed, reduced-order model and use statistical learning techniques to make it quantitatively accurate. [Preview Abstract] |
Monday, November 25, 2019 7:00PM - 7:13PM |
P06.00009: Forced and mutual synchronization of periodic and aperiodic oscillations in a self-excited thermoacoustic system Yu Guan, Vikrant Gupta, Larry K.B. Li Recent studies have shown that external forcing is effective in controlling both periodic and aperiodic thermoacoustic oscillations. We examine the mutual synchronization of pressure ($p'$) and heat-release-rate ($q'$) fluctuations in a prototypical thermoacoustic system undergoing forced synchronization by periodic acoustic forcing. When unforced, the system can oscillate periodically, quasiperiodically or chaotically. For all three types of oscillations, we find several common features, including (i) the presence of a $\mathbb{T}^2$ quasiperiodic state before lock-in in which asynchronous quenching occurs without mutual synchronization between $p'$ and $q'$, (ii) the emergence of synchronization between $p'$, $q'$ and the forcing signal at lock-in, and (iii) the destruction of synchronization beyond lock-in. As well as providing new insight into the way external forcing affects the mutual synchronization of $p'$ and $q'$, this study shows that, regardless of its initial unforced state, a thermoacoustic system synchronized to external forcing does not necessarily remain synchronized if the forcing becomes too strong. For effective control, this implies that the forcing amplitude should be limited to a value just sufficient to cause the onset of lock-in. [Preview Abstract] |
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