Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session R25: Thermoacoustics |
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Chair: Sarma Rani, University of Alabama in Huntsville Room: 251 C |
Monday, November 25, 2024 1:50PM - 2:03PM |
R25.00001: ABSTRACT WITHDRAWN
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Monday, November 25, 2024 2:03PM - 2:16PM |
R25.00002: Low Order Modeling for the Dynamics of Spray Flames Vishal Srinivas Acharya Spray flames are commonly used in aviation and land-based power-generation combustors. In order to model and understand combustion instabilities, system modeling tools use acoustic networks to capture the acoustic mode shapes and frequencies, especially those of the unstable modes. A critical piece is the model for the flame dynamics. A large body of work exists for modeling the dynamics of premixed and diffusion flames, with limited focus on spray flames. Prior work in the literature presented extensions to the Schvab-Zeldovich formulation by introducing the droplet phase through a mixture fraction that couples with the gaseous mixture fraction through vaporization physics. While several coupling mechanisms have been identified for thermoacoustic instabilities, a mechanism unique to spray flames is the dynamics of spray injection, oscillatory evaporation and atomization. This results in new control parameters related to a Damkohler number for vaporization, oscillatory droplet physics, spray injection, to name a few. First, we present results from this formulation within a linear framework for the global flame response to oscillatory spray and velocity dynamics. Furthermore, we assess the suitability of this reduced order framework against detailed Large Eddy Simulations from a model combustor. The formulation can be extended to study the non-linear flame response to build Flame Describing Functions, that can then be used in acoustic network tools to capture limit-cycles. |
Monday, November 25, 2024 2:16PM - 2:29PM |
R25.00003: Cluster-based control of quasiperiodic thermoacoustic oscillations Hiromi Kimishima, Bo Yin, Vikrant Gupta, Larry K.B. Li We demonstrate the application of cluster-based control to suppress two-frequency quasiperiodic oscillations in a prototypical thermoacoustic oscillator. This approach involves discretizing sensor measurements of pressure and heat-release-rate fluctuations into multiple clusters within a low-dimensional feature space. To determine the optimal closed-loop control laws, we use a Nelder-Mead simplex search on a cost function that balances the thermoacoustic amplitude (state cost) against the actuator power consumption (input cost). Our results show that this data-driven control strategy can effectively suppress various two-frequency quasiperiodic oscillations while minimizing actuator power usage. This study underscores the potential of cluster-based control in enhancing the stability of aperiodic thermoacoustic systems, paving the way for applications in energy conversion and combustion devices. |
Monday, November 25, 2024 2:29PM - 2:42PM |
R25.00004: Coherence between the global heat release rate and acoustic pressure fluctuations of a turbulent flame Sungyoung Ha, Tim Lieuwen In combustion noise studies, the coherence between temperature and pressure measurements are typically used to estimate the relative contribution of direct and indirect noise sources. In recent work, it was proposed that global chemiluminescence intensity could be used as an alternative or in conjunction. However, there has been no experimental evidence of unity coherence between global chemiluminescence and acoustic pressure fluctuations, even in direct noise dominant systems. It was discussed in previous work how nearfield and finite acoustic compactness could contribute to low coherence. Following this analysis, this work aims to present experimental measurements of coherence along with acoustic spectra of a turbulent Bunsen burner flame as a canonical baseline. The goal is to obtain explicit measurements of unity coherence in a direct noise dominant system and validate preceding theoretical studies. Results will deepen our understanding of combustion noise generation and serve as a basis for the use of chemiluminescence in combustion noise studies. |
Monday, November 25, 2024 2:42PM - 2:55PM |
R25.00005: Early detection of thermoacoustic instability via deep learning of recurrence plots Jungjin Park, Kang Eun Jeon, Jun Hur, Bo Yin, Jong Hwan Ko, Larry K.B. Li We present a deep learning framework for the early detection of thermoacoustic instabilities in laminar and turbulent flow systems. These instabilities, typically arising from Hopf bifurcations or via intermittency, manifest as large-amplitude pressure oscillations at a limit cycle. Predicting their onset is challenging owing to the nonlinearity and sensitivity of the thermoacoustic feedback loop, which involves complex multiscale interactions among combustion, hydrodynamics, and acoustics. Our approach leverages the pattern recognition capabilities of convolutional neural networks to forecast the onset of such instabilities using recurrence plots as input. These plots are two-dimensional topological representations of high-dimensional phase space trajectories. We train a ResNet-18 deep learning model on unbinarized recurrence plots generated from experimentally measured pressure data, producing a scalar output indicative of the proximity to the instability boundaries. We find that this hybrid framework is sensitive enough to extract evolving topological features from recurrence plots, generating reliable early warning indicators of impending thermoacoustic instabilities in various flow systems. |
Monday, November 25, 2024 2:55PM - 3:08PM |
R25.00006: Thermoacoustic oscillations of laminar premixed hydrogen-rich flames Meenatchidevi Murugesan, Manjunath Mailarappa Meti, Arvind Raj Sakthivel, ABHIJIT KUMAR KUSHWAHA Thermoacoustic oscillations are known to occur when the unsteady heat release rate from the flame is in phase with acoustic perturbations inside the combustor. A number of nonlinear dynamical states that arise due to the thermoacoustic feedback of the premixed flames have been reported. In this experimental study, we investigate the effect of fuel composition, particularly hydrogen-enrichment on thermoacoustic oscillations. We systematically increase the percentage of hydrogen (by energy and by volume) in the fuel and measure the system response in terms of acoustic pressure and high-speed chemiluminescence images of the flame. We find that the premixed flame undergo a sequence of transitions in nonlinear dynamical states when the hydrogen percentage is increased. At low hydrogen percentage, the system dynamics show limit cycle oscillations at a frequency closer to the fundamental acoustic mode. As the percentage of hydrogen is increased, we observe the emergence of quasiperiodic state characterized by oscillations at two incommensurate frequencies. We analyze the mode and the flame structure by using dynamic mode decomposition of high-speed chemiluminescence images. Further, the phase-averaged images corresponding to different nonlinear states are investigated. This study shows that moderate percentage of hydrogen modify the spatial distribution of heat release rate and cause the emergence of new acoustic modes. |
Monday, November 25, 2024 3:08PM - 3:21PM |
R25.00007: Genetic programming control of thermoacoustic oscillations in a hydrogen-methane-fueled turbulent combustor Bo Yin, Aksel Ånestad, Eirik Æsøy, Nicholas A. Worth, Larry K.B. Li We experimentally investigate the application of genetic programming (GP) to suppress self-excited thermoacoustic oscillations in a turbulent premixed combustor operating on hydrogen-methane blends. The GP framework uses an evolutionary data-driven algorithm to breed successive generations of control laws via genetic operations. Each control law is evaluated using a cost function that considers both the thermoacoustic amplitude (state cost) and the actuation power (input cost). We implement GP in both closed-loop and open-loop forms, and benchmark it against conventional open-loop control. Our results show that GP closed-loop control can achieve significant reductions in thermoacoustic amplitude with minimal input cost, outperforming other control strategies with the lowest cost function value. The suppression mechanism is identified as synchronous quenching, which occurs without resonant amplification. The GP algorithm is found to be effective across various operating conditions, including different bulk reactant velocities, hydrogen power fractions, and combustor lengths. This versatility highlights the potential of GP as a model-free control strategy for mitigating thermoacoustic oscillations in combustion systems, including those operating on hydrogen-enriched fuels. |
Monday, November 25, 2024 3:21PM - 3:34PM |
R25.00008: A Nonlinear Acoustic–Flame Synchronization Model for Thermoacoustic Instability in a Bluffbody-Stabilized Combustor Sarma L Rani, Swarnalatha K Vasanthakumar A nonlinear coupled-oscillator model is developed that can predict the various stages of synchronization between the acoustic and heat-release oscillations in the lead-up to thermoacoustic instability in a dump combustor with a bluffbody-anchored flame. The flame is perturbed by vortices shed from the shear layer formed at the combustor dump plane. The resulting heat-release-rate fluctuations generate acoustic waves that travel upstream and interact with the shear layer, thereby modulating the vortex shedding process and closing the feedback loop. Coupled nonlinear equations governing the temporal evolution of pressure and heat-release-rate fluctuations are derived and evolved along with an equation for the build-up of circulation at the dump plane, as well as an equation for vortex advection. The effects of vortex impingement on the flame are modeled as a localized, instantaneous source term in the flame oscillator equation. With the mean flow velocity $\Bar{u}$ as the control parameter, the nonlinear model is applied to investigate the acoustic--flame--vortex interactions. For smaller $\Bar{u}$, chaotic combustion noise is observed with essentially no synchronization between the pressure and heat-release-rate fluctuations, $p'$ and $\dot{q}'$, respectively. As $\Bar{u}$ is increased, intermittent bursts of periodic fluctuations are seen in $p'$ and $\dot{q}'$, which is followed by a weakly periodic state with only frequency synchronization. Eventually, when $\Bar{u}$ exceeds a critical value, limit-cycle oscillations are seen with both frequency and amplitude synchronization between $p'$ and $\dot{q}'$ (also known as generalized synchronization). Thus, the model qualitatively captures the successive stages of acoustic--flame coupling seen in the experiments of \citet{pawar2017}. Furthermore, the derived nonlinear equations accurately predict the natural frequencies of the pressure and heat-release oscillators, as well as the lock-on frequency of the two oscillators with vortex shedding. |
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