Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session U25: Reacting Flows: Instabilities |
Hide Abstracts |
Chair: Sina Kheirkhah, The University of British Columbia Room: 233 |
Tuesday, November 22, 2022 8:00AM - 8:13AM |
U25.00001: Observations of vortex lock-in in an acoustically excited flame Eirik Æsøy, Girish K Jankee, Srikar Yadala Venkata, Nicholas A Worth, James R Dawson Understanding the characteristics of structures arising from vortex shedding can be of great importance in combustors, where avoiding thermoacoustic instabilities is primarily desired. An experimental study is conducted with focus on the interaction between vortex shedding off circular cylinders located upstream of a bluff body stabilised CH4/H2 flame, when excited by acoustic oscillations. Measurements of the flow and flame properties were carried out through hot-wire anemometry and planar PIV, and by photomultiplier tubes and high-speed imaging capturing OH*-chemiluminescence, respectively. These measurements reveal that the flame reacts to the cylinder vortex shedding with its response showing footprints of classical vortex lock-in as the heat release rate also locks into the sub-harmonic of the acoustic forcing frequency. The results highlight the potential in controlling the onset of thermoacoustic instabilities by leveraging/tuning the effect of interference between vortex shedding behind the cylinders and the acoustic excitation of the flame to suppress the global flame response. |
Tuesday, November 22, 2022 8:13AM - 8:26AM |
U25.00002: Effect of differential diffusion on the linear dynamics of premixed flames in swirling jets Christopher M Douglas, Wolfgang Polifke, Lutz Lesshafft Hydrogen's exceptionally low molecular weight causes its ratio of thermal to mass diffusivity (the Lewis number) to be less than one in lean flames. This property leads to a phenomenon known as thermal-diffusive instability, which is expected to have a significant impact on the response of hydrogen flames to flow disturbances. Recently, we have used modal and nonmodal stability analysis to show how such differential diffusion influences the linear dynamics of laminar Bunsen flames (EFMC, Athens, Sept. 2022). We now extend those results to swirling jet flames in order to explore how non-unity Lewis number effects interact with swirl. A low-Mach, single-step reacting flow model is used to study the three-dimensional linear flame dynamics via eigenmode and resolvent analysis, at a variety of swirl and Lewis numbers. The results reveal that swirl has a significant destabilizing influence on the flame behavior regardless of the Lewis number. However, the combination of swirl with differential diffusion leads to far greater nonmodal amplification of flow disturbances at low Lewis numbers compared to unity Lewis number flames. The implications of these findings towards the fundamental behaviors of lean-premixed hydrogen flames and their thermoacoustic stability characteristics is discussed. |
Tuesday, November 22, 2022 8:26AM - 8:39AM |
U25.00003: Acoustically Coupled Combustion Dynamics of Laminar Coaxial Methane-Air Jets Andres Vargas, Arin Hayrapetyan, Ann R Karagozian Acoustic excitation of coaxial methane-air laminar jet diffusion flames is explored as a means of studying fundamental non-linear coupling in combustion instabilities. Several alternative injector geometries and operating conditions are examined, with differing annular-to-jet area ratios, velocity ratios, wall thicknesses and excitation conditions. Flame dynamics are quantified using high speed visible imaging, with proper orthogonal decomposition (POD) to analyze various dynamical responses. With increasing excitation amplitudes, flames undergoing transition from sustained oscillatory combustion to periodic lift-off and reattachment to eventual blow-off may be correlated with POD mode coefficient plots having characteristic signatures, including strange attractor-like shapes. Such transitions are altered for different injector geometries and velocity ratios, suggesting that flame stabilization and dynamical response may be correlated with injector configuration in an acoustically resonant environment, and that phase portraits representing this response can aid in developing topology-based reduced order models. |
Tuesday, November 22, 2022 8:39AM - 8:52AM |
U25.00004: Image Denoising for Acoustically Coupled Combustion using Neural Networks and a Convolutional Autoencoder Arin Hayrapetyan, Andres Vargas, Ann R Karagozian The present study investigates the use of a convolutional autoencoder on dimensionality reduction of a coaxial methane-air laminar jet diffusion flame exposed to transverse forcing within a cylindrical acoustic waveguide. Proper Orthogonal Decomposition (POD) on high-speed imaging of the flame extracts spatial modes of the oscillating flame and is used as a baseline for comparison with a nonlinear mode-decomposing convolutional neural network autoencoder [Murata, et al., JFM, 2020]. This approach allows us to decompose the flow field into nonlinear low-dimensional modes with the use of nonlinear activation functions, unlike POD's linear orthogonal basis of modes. We apply this analysis to imaging data denoised using a multiscale Context Aggregation Network (CAN) [Chen, et. al., IEEE Conf. Comp. Vis., 2017]. This CAN is trained on low exposure input under steady conditions but enables production of a desired output represented by high exposure images for steady and unsteady cases. The CAN approach demonstrates a significant decrease in preprocessing time compared to conventional approaches and is able to process images directly for a range of different experimental conditions and flame dynamics while preserving image quality. |
Tuesday, November 22, 2022 8:52AM - 9:05AM |
U25.00005: Under-Sampled Data-Based Reduced Order Models for Periodic Dynamical Systems Davi S Lettieri, Andres Vargas, Ann R Karagozian, Leonardo Alves The Sparse Identification of Nonlinear Dynamics (SINDy) algorithm requires accurate data time derivatives. Hence, under-sampled data sets essentially prevents the use of SINDy due to time derivative inaccuracy. This issue is overcome here for periodic dynamical systems though a novel topological data analysis (TDA). It consists on a technique that folds data, originally available over many periods, into a single period based on its phase portrait analysis. Doing so enables the accurate reconstruction of the data temporal behavior. Under-sampled data from the Lorenz equations is used to demonstrate that the technique works. It is then applied to generate reduced order models (ROMs) for methane-air laminar jet diffusion flame dynamics under acoustic excitation. This sustained oscillatory combustion data consists of proper orthogonal decomposition (POD) coefficients over time obtained from high speed visible imaging using a sampling rate whose frequency is not high enough to satisfy the Nyquist theorem. Under-sampled data from single and triple jet injection systems are analyzed and highly accurate ROMs can be generated. |
Tuesday, November 22, 2022 9:05AM - 9:18AM |
U25.00006: Time-resolved prediction of Jet A spray using time-resolved flame chemiluminescence along with scarce Mie scattering and ILIDS data Liam Krebbers, Sajjad Mohammadnejad, Ali Rostami, Sina Kheirkhah Time-resolved spray characteristics were predicted using time-resolved flame chemiluminescence coupled with low-speed Mie scattering and Interferometric Laser Imaging for Droplet Sizing data. A gas turbine model combustor that featured self-excited thermoacoustics was employed. Jet-A was sprayed at the center of the combustor, generating a power of 10 kW. The experiments were performed at a fixed fuel-air equivalence ratio of 0.6. For time lags ranging from -25 to 25 ms, the relations between the flame chemiluminescence and the number as well as the diameter of the droplets were obtained using 20 repeats of the experiment. The number and diameter of the droplets are negatively correlated to the flame chemiluminescence for zero time lag between the measurements, but positively correlated for a time lag of 15 ms. The above correlations and the time-resolved flame chemiluminescence data were used to predict the time-resolved variations of both the number of droplets and their diameter. The above analysis and the developed tools are of significant importance, as they facilitate understanding the time-resolved spray characteristics in liquid-fueled gas turbine combustors featuring thermoacoustics. |
Tuesday, November 22, 2022 9:18AM - 9:31AM |
U25.00007: Classification of different regimes of ethanol pool fires Brandon Li, Wilfried Coenen, Sankaran Ramanarayanan, Antonio L Sanchez, Forman A Williams The characteristics of ethanol pool fires subject to ambient swirl were explored in this laboratory setup. Twenty vertically oriented thin acrylic vanes, 91.5 cm tall by 15.25 cm wide, were placed 49 cm from the center of an ethanol pool fire, which had diameters ranging from 1.9 cm to 7.6 cm. The vanes were all oriented at the same fixed angles from the radial direction for various different angles ranging from 0 degrees to 85 degrees, thereby imparting verious levels of circulation to the air entrained by the pool fire. It was found that different levels of swirl and values of the pool diameter, or associated Grashof number, resulted in different configurations of the flame. Configurations included a global puffing instability, a helical instability that generates a tall fire whirl, and a blue whirl that is associated with vortex breakdown and the edge flames becoming detached from the surface of the ethanol pool. The transition between these different regimes was recorded for varying ethanol pool diameters. |
Tuesday, November 22, 2022 9:31AM - 9:44AM |
U25.00008: Reaction-induced Kelvin-Helmholtz roll-ups Surya N Maharana, Manoranjan Mishra A simple (A+B→C)-type reaction can induce Kelvin-Helmholtz (KH) roll-up patterns at the reactive zone by a local viscosity modification [1, 2]. However, the instability mechanism is not understood so far, and a flat initial interface is required for better analysis. To do so, we consider laminar Poiseuille flow where a reactant fluid A shears on the top of another iso-viscous reactant B in a layered fashion within a two-dimensional channel. Following an (A+B→C)-type reaction kinematics, two reactants produce another differently viscous fluid C, creating an inflectional base flow. The inflection points of velocity within the reactive zone are generated; as such, they exacerbate a periodic perturbation to grow viscous wavy patterns at one reaction front while the other just stably diffuses. Streamlines oscillate and behave as in a phase lock system, amplifying the disturbance wave's amplitude. Through direct numerical simulations (DNS), we calculate the onset time (ton) of the instability for controlling parameters such as the log-mobility ratio (Rc), Damköhler number (Da), Péclet number (Pe), and Reynolds number (Re). The plots of ton demonstrate the existence of a critical log-mobility ratio (Da, Pe, and Re-dependent), below which the instability never onsets [3]. Further, an Orr-Sommerfeld equation-based linear stability analysis (LSA) is used to obtain the onset time of instability for all unstable wave numbers in the linear regime. The onset dynamics from LSA agree with those obtained from DNS, except for the Reynolds number effect. At the end of this talk, we shall discuss the possible reasons and more suitable advanced LSA techniques that may provide the DNS-matching onset dynamics. |
Tuesday, November 22, 2022 9:44AM - 9:57AM |
U25.00009: Influence of equivalence ratio on combustion instability of a low-swirl flame under lean-hydrogen conditions. Jun Nagao, Abhishek L Pillai, Takeshi Shoji, Shigeru Tachibana, Takeshi Yokomori, Ryoichi Kurose With targeting combustion instability in a low-swirl combustor using hydrogen as fuel, Large-eddy simulations are performed employing a dynamically thickened flame model and a detailed chemical reaction model considering 9 chemical species and 20 reactions. Especially in this study, the main discussion is focused on the impact of equivalence ratios. The equivalence ratio of the lean hydrogen premixture is changed from 0.33 to 0.45 in 0.03 increments referring to the experimental study (Shoji et al., Proc. Combust. Inst., 2020) to investigate the difference in combustion instability intensity and oscillation phenomena for different equivalent ratios. The experimental study reported that the combustion instability intensity has a maximum value at an equivalence ratio of 0.33, and the intensity did not increase at higher equivalence ratios. |
Tuesday, November 22, 2022 9:57AM - 10:10AM |
U25.00010: Three-dimensional structure of the underlying flow field of a bluff-body-stabilised flame forced by multiple convective modes Srikar Yadala Venkata, Girish K Jankee, Eirik Æsøy, James R Dawson, Nicholas A Worth An experimental study is conducted to understand the 3D flow structures produced by convective/acoustic interference of an acoustically forced bluff-body-stabilised CH4-H2 flame. Recently, the convective interaction from vortex shedding originating from cylinders installed upstream of the flame have been shown to passively suppress or amplify thermoacoustic instabilities. The structure of the flame and flow fields in such a scenario is highly 3D and therefore, needs to be determined with a three-dimensional measurement of the flow field downstream of the bluff body. This was achieved through a high-speed (10 kHz), scanning, stereoscopic PIV system. A total of 19 planes, spanning four times the fuel injector pipe's radius, was acquired. The flame structure was captured by imaging OH-chemiluminescence, simultaneously with the centre-plane PIV measurement. The mean flow velocity was ū = 10 m/s at the exit with acoustic forcing frequency and amplitude being 700 Hz and 20% of ū, respectively. Eigenmode decomposition of the velocity fields reveals two dominant frequencies in the flow field, one corresponding to the acoustic forcing frequency (700 Hz) and the other to the cylinders’ vortex shedding that is locked-in to the sub-harmonic of the acoustic frequency (350 Hz). The interplay of these modes is crucial for the global flame response. |
Tuesday, November 22, 2022 10:10AM - 10:23AM |
U25.00011: Large Eddy Simulation of Hydrogen Flame Stabilization in a Reheat Gas Turbine Combustor Ethan Cartwright, Chao Xu Reheat gas turbine combustors utilizing hydrogen-dense fuel blends serve as a potentially effective low-emission alternative energy system. These combustors operate under unique conditions that lack low-cost models for accurately predicting flame stabilization. A large eddy simulation (LES) with detailed chemistry is performed on a simplified geometry of the Ansaldo GT36 sequential combustor with an elevated temperature vitiated air-hydrogen flow in a rectangular mixing duct and combustion chamber, reflective of available direct numerical simulation (DNS) results. In particular, simulation parameters are assessed for relative importance in generating accurate flame characteristics against simulation cost. Additionally, adaptive mesh refinement at the junction of the mixing duct and combust chamber is employed to resolve the high reactivity flame zones of the H2 fuel. Detailed analyses will be performed to elucidate the importance of low-resolution inlet and outlet extensions for development of turbulent conditions and pressure dampening, respectively. Results gathered here validate the feasibility of accurate and relatively low-cost sequential combustor LES models which can complement fundamental experimental and DNS results in their application to the design process. |
Tuesday, November 22, 2022 10:23AM - 10:36AM |
U25.00012: Differential diffusion effects on turbulent premixed bluff-body stabilized flames near lean blow-off Tong Su, Nicholas A Worth, James R Dawson, Samuel Wiseman The effects of differential diffusion on the flow dynamics and structure of a turbulent premixed bluff-body stabilized flame near lean blow-off have been investigated experimentally. A range of different fuels and decomposed ammonia fuel blends with different Lewis numbers (larger or smaller than unity) and preferential diffusivities have been used to compare lean blow-off limits (LBO). Simultaneous high-speed PIV and OH-PLIF (operated at 10kHz) have been employed to obtain time-averaged and time-resolved flame images, in which flame front curvatures, stretch rates and other turbulent flow characteristics can be obtained. Lean blow-off occurred by increasing the air flow rates at a constant rate. The LBO of ethylene occurred at leaner conditions than that of methane. In addition, as the hydrogen volume fraction increases in the NH3/H2/N2-air blends, the flame becomes more stable due to preferential diffusion effects. The near blow-off flame structure is compared at the same blow-off velocity but varying equivalence ratios, and is observed to change depending on the fuel blend. The crucial role played by differential diffusion effects in the propagation of turbulent flames near lean blow-off will be highlighted, and the relevant physical mechanisms will be discussed. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700