Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session A07: Acoustics: Thermoacoustics |
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Chair: Vishal Acharya, Georia Tech Room: North 122 C |
Sunday, November 21, 2021 8:00AM - 8:13AM |
A07.00001: Model selection by maximizing the marginal likelihoods of candidate physics-based models of a thermoacoustic experiment. Matthew P Juniper, Matthew J Yoko We automate an experiment consisting of an electrical heater inside a vertical tube. We examine 16 heater positions, 8 powers, and various cold configurations. For each configuration we force acoustically and measure the response with 8 probe microphones. We introduce prior knowledge that the acoustic response depends on how the heat release rate depends on the velocity at the heater and propose several physics-based models. We find the most likely values of each model's parameters, given the 17600 datapoints, using first order adjoints. We obtain the uncertainties in each model's parameters using Laplace's method using second order adjoints. We identify the most plausible model by calculating the integral of the posterior probability over parameter space (marginal likelihood). This penalizes models with too many parameters, which over-fit the data. We find the maximum marginal likelihood, allowing the measurement noise to vary, in order to account for model error elsewhere. This method successfully ranks the candidate models. The most plausible model is a simple n-tau model, with two parameters, which fits the data well across the entire range without overfitting. The physical insight from this method exposes systematic experimental error that otherwise might not be noticed. |
Sunday, November 21, 2021 8:13AM - 8:26AM |
A07.00002: Real-time data assimilation of nonlinear thermoacoustics. Andrea Nóvoa, Nicolas Noiray, Luca Magri Thermoacoustic instabilities are commonly modelled using low-order models that provide qualitatively accurate solutions at a low computational cost. We design an ensemble data assimilation algorithm to perform state and parameter estimation, which enables model learning on the fly and makes the model more quantitatively accurate. The algorithm is tested and validated through a multimicrophone twin experiment in a Rijke tube. Bifurcation analysis evidenced the extreme sensitivity of thermoacoustic models to small changes in the parameters, which give rise to complex nonlinear regimes. Hence, a small update in a parameter during the filtering can give rise to large changes in the state that in turn may result in unphysical solutions to the thermoacoustic parameters. We overcome this issue by a combined increase, reject, inflate strategy. The filter is shown to be robust and capable of recovering the true state even for chaotic regions with large uncertainties. Current efforts focus on the application of this technology for state and parameter estimation in the stochastic differential equations governing the slow-varying amplitude and phase of a stochastic model of an axial combustor with real experimental data. |
Sunday, November 21, 2021 8:26AM - 8:39AM |
A07.00003: Modelling Coupled Thermoacoustic Systems: Synchronization and Amplitude Death Sneha Srikanth, Samadhan A Pawar, Krishna Manoj, R I Sujith Control of thermoacoustic (TA) instabilities in multiple combustion systems, such as those in can-annular combustors of gas turbines, has been a challenging problem in propulsion and power generation industries. Recent experiments show that coupling two TA systems can lead to synchronization, switching between in-phase and anti-phase synchronized states via phase-flip bifurcation (PFB), or even complete suppression of TA oscillations in both systems through amplitude death (AD). However, the mechanism through which TA systems attain AD or exhibit PFB is not yet well understood. In this study, we examine the model of two time-delay coupled prototypical TA systems called Rijke tubes. Through numerical simulations and analytical approximations, we predict the critical parameter values for the occurrence of PFB and AD in the model. The results from the model qualitatively match with previous experiments on coupled Rijke tubes. Using bifurcation analysis, we show the system to transition from oscillatory state to AD through fold bifurcation, and vice-versa through subcritical Hopf bifurcation. These findings may provide insight into the coupled interaction and suppression of oscillatory instabilities in engineering systems, ecological networks, financial markets, and other systems. |
Sunday, November 21, 2021 8:39AM - 8:52AM |
A07.00004: Influence of Heat Losses on Thermo-Acoustic Coupling Frequencies Daniel Martínez-Ruiz, Enrique Flores-Montoya, Mario Sánchez-Sanz, Victor Muntean A simplified model to predict the coupling frequencies of self-induced thermo-acoustic instabilities of premixed flames propagating from the open to the closed end of a tube is here addressed in the presence of heat-losses. Non-adiabatic tubes affect the coupling frequency by modifying the temperature distribution of burned gases and thereby the acoustic modes. |
Sunday, November 21, 2021 8:52AM - 9:05AM Not Participating |
A07.00005: Exceptional points in a thermoacoustic model of a can-annular combustor Frida M Cronqvist, Jonas P Moeck Gas turbines for power generation often feature can-annular combustors. This combustor architecture consists of a set of nominally identical cans, which host individual, spatially separated flames; the cans are, however, acoustically coupled at the turbine inlet. The thermoacoustic dynamics in this type of configuration can be modeled as a system of weakly coupled identical oscillators with a time delayed feedback. Such a system generally exhibits eigenvalue clustering and, as a result, high eigenfunction sensitivity and possibly mode localization under asymmetric perturbations. We investigate the existence and parameter dependence of exceptional points (EPs) in this type of system. EPs are generally found for every azimuthal mode order, and because of the weak coupling, frequency and growth rate are typically close. As in single-flame systems, the eigenvalue collision at the EP is linked to modes of intrinsic and acoustic origin. We show that the location of all EPs as a function of the number of cans, the Bloch wavenumber, and the can-to-can coupling strength can be obtained from solutions of a one-parameter problem. This allows for an effective prediction of all EPs, given initial solutions from a corresponding single-can system and assuming weak coupling. |
Sunday, November 21, 2021 9:05AM - 9:18AM Not Participating |
A07.00006: Three-dimensional investigation of flame dynamics of azimuthally forced flames in annular combustor using Scanning PLIF Dirren Govender, Samuel Wiseman, James R Dawson, Nicholas A Worth In combustion systems, self-excited thermo-acoustic instabilities are a well-known problem and many studies are aimed at better understanding the phenomenon. Three-dimensional techniques are useful when investigating asymmetrical responses in such systems. In this study, a three dimensional Scanning OH* Laser Induced Fluorescence (LIF) method is used to investigate the response to forced azimuthal oscillations in an annular combustor. Two speaker pairs were used to control the orientation and nature of the acoustic mode inside the combustor. The flame of interest was subjected to spinning modes and standing modes, in the latter case the flame was positioned at either a pressure node, an anti-node, or an intermediate position between these. The measurements consisted of scanning a laser sheet across the flame with the use of a galvanometric mirror. Flame edges extracted from the imaging were then conditionally averaged based on the forcing cycle and projected to real space, from which spatially resolved three dimensional Flame Surface Density (FSD) is calculated. FSD was then used to investigate the flame structures under different forcing conditions. The scanning method made use of two separate scan directions to overcome bias errors in determining FSD at the flame edges. The results provide insight into asymmetries in the flame dynamics and response due to both the azimuthal excitation and annular confinement geometry. |
Sunday, November 21, 2021 9:18AM - 9:31AM |
A07.00007: The effect of asymmetric can-to-can coupling on thermoacoustic modes in a can-annular laboratory combustor Philip E Buschmann, Nicholas A Worth, Jonas P Moeck Can-annular combustors are equipped with a set of nominally identical cans, circumferentially arranged around the shaft. Adjacent cans are coupled acoustically via a small gap at the downstream end, where the circular cross-section transitions into the annular turbine inlet. Recent experimental and theoretical work has shown that the coupling strongly affects thermoacoustic system stability and, hence, may give rise to damaging pressure oscillations that originate from a constructive interference of pressure waves and heat-release rate fluctuations. A laboratory-scale can-annular combustor has been operated with premixed CH4-H2-air mixtures to study these instabilities. The combustor consists of eight identical cans, connected acoustically to their respective neighbours via size-adjustable side branches; the latter allow for a variation of the coupling strength. Experimental results are presented in which five asymmetric sets of coupling strengths are investigated. The sets are chosen such that the symmetry is gradually reduced until a fully asymmetric configuration is realized. The can-to-can coupling strength patterns strongly affect the amplitude and structure of the dominant mode. |
Sunday, November 21, 2021 9:31AM - 9:44AM |
A07.00008: Azimuthal Instabilities of Non-swirling bluff body stabilised Methane/Hydrogen Flames in a Pressurized Annular Combustor Byeonguk Ahn, Thomas Indlekofer, Håkon T Nygård, Girish K Jankee, Dirren Govender, Nicholas A Worth The present study experimentally investigates the modal dynamics of the azimuthal mode with twelve methane/hydrogen blended non-swirl bluff body stabilised flames in a lab-scale pressurized annular combustor. The hydrogen power fraction was set to 40% which corresponds to 67% per volume. Equivalence ratio was varied between 0.5 to 1.1. Air mass flow was maintained at 85.0 g/s. The chamber pressure was ranged between 2.0 to 2.4 bar, which leads to fully reflecting boundary conditions. Longitudinal modes were observed for the lower equivalence ratio conditions. As the equivalence ratio is increased, the system couples to the azimuthal mode with a pressure oscillation amplitude of around 4.2 kPa. The fundamental mode at around 1680 Hz features a standing mode with pressure amplitude of 1.6 kPa. Interestingly, harmonic components are characterized by a time varying nature of the modes, with both counterclockwise and clockwise spinning mixed modes observed. Flame imaging analysis reveals that a periodic strong transverse flapping motion clearly dominates the flame structure and dynamics. This implies the presence of a significant effect of the flame structure on the nature of the azimuthal mode. |
Sunday, November 21, 2021 9:44AM - 9:57AM |
A07.00009: Mean Flame Shape Effects on the Non-linear Response of Premixed Flames to Helical Flow Disturbances Vishal S Acharya This work considers a model swirl-stabilized premixed flame responding to helical disturbances in swirling flows. Hydrodynamic instabilities manifest themselves with an axisymmetric mode (m = 0) and/or with helical modes (m ≠ 0) where the mode number m is a signed integer. Prior research considered an axisymmetric framework, showing both the linear and non-linear response of premixed flames to the m = 0 mode. Majority of swirling flows show dominant amplitudes for helical modes. Prior work has shown that the linear response is determined purely by the m = 0 mode for axisymmetric mean flames regardless of their strength compared to helical modes. This work was extended to the non-linear regime to show that one or more helical modes can interact under certain conditions to result in a non-zero flame response. In multi-nozzle can combustor configurations, flame-flame interactions result in asymmetric mean flame shapes. In contrast with axisymmetric flames in the linear regime, when mean flame asymmetries are introduced, helical modes in the flow interact with asymmetric modes in the mean flame shape to result in non-zero flame response. In this study, we extend the framework to show that mean flame asymmetries have a strong impact on non-linear flame response. |
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