Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A02: Turbulent Combustion I: Reacting Turbulence |
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Chair: Jacqueline Chen, Sandia National Lab Room: Georgia World Congress Center B203 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A02.00001: Flame-vorticity interactions in DNS of a turbulent premixed flame stabilized over a backward facing step Aditya Konduri, Hemanth Kolla, Andrea Gruber, Jacqueline Chen Recirculation zones are often used to stabilize flames in high speed combustion, e.g. gas turbine and scramjet engines. In this work, a direct numerical simulation of a lean ethylene-air turbulent premixed flame, stabilized over the recirculation zone in a backward facing step configuration, is performed to study the interactions between the flame and the shear layer vorticity. Results show that the recirculation zone transports necessary hot radicals such as OH from the downstream region to the stabilization point. Periodic vortex shedding from the corner of the step, observed in the non-reacting flow, is absent in the presence of a flame. The vorticity which is present mainly on the reactant side of the flame near the stabilization point, gets advected into the products downstream. This leads to an enhanced heat transfer from the flame to the wall, and affects the CO and OH oxidation. Statistics of the enstrophy budget as a function of progress variable and streamwise distance will be presented. The turbulent flame structure, which is a function of the streamwise distance, will be compared to the laminar flame to identify the regions of flame thickening, arising due to the interaction with sheared turbulence. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A02.00002: Measurements of baroclinic enstrophy production in turbulent premixed flames Askar Kazbekov, Adam Michael Steinberg Reynolds averaged enstrophy transport budgets, conditioned on mean progress variable, are measured in a variety of turbulent swirl flames using tomographic particle image velocimetry (TPIV) and CH2O planar laser induced fluorescence (PLIF) across the Karlovitz number range Ka = 10-45. The combined contributions of baroclinic torque and dissipation to enstrophy transport are evaluated by subtracting the vortex stretching and dilatation terms from the Lagrangian derivatives. Enstrophy production through baroclinic torque dominated over dissipation towards the product sides of the flame brushes. Indeed, the net effect of baroclinic torque and dissipation in these regions was enstrophy production at a rate that generally exceeded production due to vortex stretching. In contrast to direct numerical simulations of flames in homogeneous isotropic turbulence, the significance of baroclinic torque relative to vortex stretching increased with increasing Karlovitz number due to effects of a realistic configuration. The observed flame-scale turbulence generation towards the combustion products has implications both for turbulence and combustion closure models. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A02.00003: On the Flame-Generated Turbulence of Highly-Turbulent Fast Flames Jessica Chambers, Laura O'Neill, Alexei Poludnenko, Vadim Gamezo, Kareem Ahmed The work explores the flame-generated turbulence production in highly-turbulent fast flames. Burning speeds of fast compressible flames are on the order of the Chapman–Jouguet deflagration velocity, which results in strong compressions forming ahead of the flame. Such compressible regimes result in the interactions of pressure-density gradients leading to baroclinicity-driven turbulence production. A Turbulent Shock Tube (TST) facility is used to explore the complex local reacting flow-field dynamics of these fast flames capable of transitioning to a detonation. In the current work, fast deflagrated flames are observed interacting with high levels of isotropic turbulence. Advanced diagnostics including high-speed Particle Image Velocimetry, high-speed OH chemiluminescence, and dynamic pressure transducers capture the evolution of the turbulence and compressibility dynamics. The analysis expands on the criterion for turbulence-driven DDT and validate existing computational DNS simulations. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A02.00004: Flame-generated turbulence in direct numerical simulations of the Turbulent Shock Tube facility Alexei Poludnenko, Laura O'Neill, Jessica Chambers, Kareem Ahmed, Vadim Gamezo This talk presents results of direct numerical simulations (DNS) of fast, highly compressible, premixed turbulent flames. DNS are designed to model the Turbulent Shock Tube (TST) facility developed at the University of Central Florida. Turbulent flames in the TST exhibit highly unsteady behavior undergoing rapid acceleration driven by significant turbulence amplification, which can ultimately result in a deflagration-to-detonation transition. Detailed comparison of the DNS results with the experimental data is presented. Furthermore, analysis of the dynamics of the unsteady flames in the TST is discussed with the primary focus on the properties of the flame-generated turbulence as well as the mechanisms of turbulence production. It is shown that fast turbulent flames with burning speeds comparable to that of a Chapman-Jouguet deflagration are capable of generating turbulence with r.m.s. Mach numbers approaching 0.4 - 0.5 and Karlovitz numbers well in excess of 100. Finally, turbulence generation by the flame also results in the significant up-scale transport of kinetic energy, which can greatly energize motions on scales well above the laminar flame thickness. The nature of such up-scale energy transport, as well as its implications for the LES combustion models, is discussed. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A02.00005: Lagrangian Analysis of Turbulent Flame Structure in High-Speed Combustor Cal Rising, Marissa Geikie, Kareem Ahmed The current study investigates the structural evolution of a turbulent flame and the effects induced by the flame-vortex dynamics. A bluff-body stabilized, premixed flame in a high-speed combustor is used to analyze the turbulent flame. Simultaneous high-speed CH* chemiluminescence and particle image velocimetry (PIV) are implemented to characterize the flame-flow fields and flame structure. The flame temporal evolution and structural dynamics are ascertained using a Lagrangian tracking and decomposition method. The flame initial position is determined from the CH* data and is convected spatio-temporally with the Lagrangian equations of motion with the PIV velocity data. The flame-vortex dynamics are investigated by decomposing the vorticity transport equation, and considering the individual vorticity mechanisms: vortex stretching, viscous diffusion, baroclinic torque, and dilatation. In doing so, the vorticity mechanisms are tailored to convect the flame spatio-temporally; the Lagrangian tracking results are compared with the CH* data. Comparison of the experimental data with the numerical tracking results allows the influence of each vorticity mechanism on the flame structural dynamics to be categorically determined. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A02.00006: Turbulent kinetic energy budget for n-dodecane flames at high Karlovitz numbers. Hsu Chew Lee, Xiao Yu Liu, Minping Wan Understanding of the underlying interactions between the premixed flame and the turbulence will enable us to design and develop cleaner and more efficient combustion engines. However, most of the DNS studies available in the literature are restricted to low Karlovitz number and low Lewis number (Le). Therefore, this presentation will be about the statistical behavior of the turbulent kinetic energy (TKE) transport for n-dodecane flames (Le = 4.2) at high Karlovitz numbers (Ka), ranging from 30 to 150, spanning the thin and broken reaction regimes. Direct Numerical Simulation (DNS) with detailed chemical scheme of 53 species and 289 reactions was employed to simulate the turbulent premixed flames. The terms in the TKE transport equation was analyzed and scaling terms proposed for thin reaction zone were examined to assess the suitability for broken reaction zone and also for high Lewis number fuel. Lewis number is found to have effect on several terms, where their statistical behaviors differed from unity Lewis number fuel such as methane. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A02.00007: Concurrent Relationships between Physical and Fourier Space Representations of 3D Flow Structures for LES of Premixed Turbulent Combustion. Yash Shah, Paulo Paes, James Brasseur, Yuan Xuan In LES of premixed turbulent combustion, the subfilter scale (SFS) motions surrounding the flame have major dynamical impacts on the resolved scales (RS) of motion. Closure models are needed to capture dominant interscale couplings in turbulent combustion. As representative of dominant turbulent eddy-flame interactions, we inject arrays of vortex rings with specified length and velocity scales into a laminar premixed flame with chemical reaction and heat release. The interactions between the vortex rings and flame are analyzed in 3D Fourier space after removing non-physical Fourier content associated with the boundary discontinuity in periodic extensions in inhomogeneous directions. We show one-to-one correspondences between specific structural SFS and RS features in 3D physical space and Fourier space associated with the flame front, its deformations and space-filling motions local and nonlocal to the flame with corresponding 3D scale representation. We analyze a mix of spectrally nonlocal and local relationships between 3D coherent structure in physical space and 3D coherent structure in Fourier space with implications to interscale RS-SFS dynamics for LES of premixed turbulent combustion. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A02.00008: Dominant Subfilter-Resolved Scale Flame-Turbulence Advective Nonlinearities in LES of Premixed Turbulent Combustion James Brasseur, Paulo Paes, Yuan Xuan Central to the LES framework is the need to model nonlinear interactions between variance-dominant resolved-scale (RS) motions and subfilter-scale (SFS) fluctuations. A central feature of premixed turbulent combustion is nonlinear flame-turbulence interactions, a key element of which is advective nonlinearity in momentum, energy and species generation. Here we extract the dominant SFS-RS advective nonlinear interactions to isolate SFS content that underlies the evolution of the resolved scales for encapsulation within mathematical forms for potential use in advanced modeling strategies. Our approach uses the triadic structure of the second-order advective nonlinearity within the multi-dimensional Fourier scale decomposition, applied here to “reduced physics” simulations of flame-vortex interactions with a new method for analysis of inhomogeneous turbulence in multi-dimensional Fourier-space. We determine the kinematic relationships between coherent physical-Fourier space structure. From these we extract the dominant triadic advective SFS-RS couplings in flame-turbulence dynamics which we find to be localized in spectral space to near the SFS-RS interface. In physical space we find the dominant SFS content to be localized near the thin flame heat-release zone. Supported by AFOSR |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A02.00009: An Investigation of Internal Flow Field and Swirling Jet Exiting a Cyclone Vortex Combustion Chamber Omer Khan, Anwar Ahmed Tangential injection of oxidizer near the nozzle attached to a cyclone vortex chamber gives rise to a distinct vortical-helical motion over the entire length of the Cyclone Vortex Combustion Chamber. Flow reverses its direction at the head wall and forms a core consisting of vortex filaments, and travels down the chamber before exiting through the nozzle. Primary advantages of combustion in the vortex chamber are prolonged fuel residence time, increased fuel/oxidizer mixing length and thermal shielding of chamber walls. Preliminary flow visualization results showed bifurcation of flow in the vortex chamber and unique swirling jet at the exit, and reversed flow under certain conditions. Both the axial flow and swirl of jet showed strong dependence on the injector geometry, chamber aspect ratio, state of boundary layer on nozzle walls, and inlet mass flow rate. Results of the connectivity between the internal and external flow, characteristics of jet, including transport of turbulent kinetic energy, and flow instabilities are presented and discussed. |
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