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 P11: Reacting Flows: Turbulent Combustion |
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Chair: Peter Hamlington, CU Boulder Room: North 125 AB |
Monday, November 22, 2021 4:05PM - 4:18PM |
P11.00001: Sustained Cavity Combustion in a M = 3 Flow Esteban Cisneros-Garibay, Carlos Pantano, Jonathan B Freund Cavities provide a means for holding flames in high-speed combustion applications, such as scramjets. Detailed simulations of sustained combustion and a corresponding inert mixing cavity, based on recent experiments, are analyzed. Entrainment into the cavity is characterized by tracking of Lagrangian trajectories. These show the importance of large-scale unsteadiness and, particularly, how this is altered by combustion. Insufficient oxidizer within the cavity leads to a low-speed, fuel-rich region near the front wall. This region supplies fuel to the mixing shear layer above the cavity where most of the combustion occurs. This shear-layer combustion is self-sustaining, not reliant on the jet flame in the cavity. The deflection of the shear layer by the expansion associated with combustion in the cavity leads to a virtual throat and complex shock structure above the cavity, which couples with the mixing layer, altering its dynamics. |
Monday, November 22, 2021 4:18PM - 4:31PM |
P11.00002: A computational study on flame extinction limits and behavior of a heated air slab burner Andrew Gonzalez, Joseph M Kalman, Ehsan Madadi A solid-fueled ramjet (SFRJ) is a type of air-breathing engine that provides suitable advantages for supersonic flight, primarily in the aspects of simplistic design and performance. The simplicity of a ramjet engine lies in the minimal usage of moving parts and coupled with the usage of solid fuel, further progresses towards a theoretically simpler design. Furthermore, ramjets have a higher specific impulse than chemical rockets making them an advantageous engine for atmospheric supersonic flight. However, because of the high Mach numbers attained and shockwaves generated in supersonic flight, substantial discontinuities in flow characteristics exist. One such discontinuity is temperature, where the oxidizer (air) inlet temperature directly behind the generated shockwave becomes larger than the freestream temperature. There has been little investigation into the implications of these high inlet temperatures on the flame characteristics and combustion stability of an SFRJ. As a result, the goal of this study is to examine the phenomenon through computational modeling. The present study utilized the open-source software OpenFOAM to model the in-house slab burner combustion chamber using the UCSD combustion mechanism to model the reactions and combustion process. The oxidizer inlet temperatures to be studied are 300 K to 1000 K in increments of 100 K; with the fuel held at a constant 300 K. The present research desires data pertaining to flame strain rates to observe the correlation between species formation, extinction strain rates, and inlet temperature. |
Monday, November 22, 2021 4:31PM - 4:44PM |
P11.00003: Impact of turbulence on the response of flames to external coherent excitation Ashwini Karmarkar, Jacqueline O'Connor Large-scale coherent oscillations in flow fields, which are often a consequence of hydrodynamic instabilities, can significantly modulate the response of flames by creating oscillations in flame surface area and the unsteady rate of heat release. Such oscillations in the heat release rate have been shown to be the drivers of thermo-acoustic instabilities in some combustor systems, and occurrence of thermo-acoustic instabilities can be detrimental to combustor performance and operability. In this study, we examine the impact of varying in-flow turbulence on the response of rod-stabilized flames to external coherent excitation. We systematically vary the amplitude of the longitudinal coherent excitation to simulate coherent perturbations created by hydrodynamic flow instabilities at varying strengths and the frequency of forcing is set to match the natural frequency of vortex-shedding from the cylindrical bluff body. The in-flow turbulence is varied using turbulence-generating screens upstream of the flame. High speed, stereoscopic, particle image velocimetry is used to obtain three-component velocity fields to characterize the flow response and Mie scattering scattering images are analyzed to obtain the flame location at each condition. Our results show that, in conditions with low levels of in-flow turbulence, high amplitudes of coherent excitation can modulate not only the flame response, but also the time-averaged base flow. By contrast, the conditions with high levels of in-flow turbulence exhibit a significantly damped flame response and no modulation of the base flow can be seen. The results indicate that turbulence intensity can not only impact the response of flames to flow perturbations, but can also impact the underlying flow field. |
Monday, November 22, 2021 4:44PM - 4:57PM |
P11.00004: Data-Based Progress Variable Dissipation Rate Modeling for Turbulent Premixed Combustion Cristian E. Lacey, Sankaran Sundaresan, Michael E Mueller Manifold-based turbulent combustion models substantially reduce computational cost by projecting the thermochemical state onto a lower-dimensional space and reconstructing the state from a set of manifold equations. Closure of the one-dimensional premixed manifold equations in progress variable requires a profile for the progress variable dissipation rate with respect to the progress variable. In LES, the filtered progress variable dissipation rate can be obtained by convoluting this model profile against a presumed subfilter PDF. These profiles are typically ad hoc and derived analytically with crude assumptions, and the resulting thermochemical state can be sensitive to these profiles. In this work, deep neural networks (DNN) are leveraged to model both the filtered progress variable dissipation rate and its profile. The particular focus of this work is the role of the selection of model inputs and the pre-processing of these inputs using simple physical principles. Consideration of the progress variable source term as a model input for the filtered progress variable dissipation rate, for instance, is shown to substantially reduce model error. The data-based models are assessed using a priori analysis of a DNS database of a turbulent premixed jet flame. |
Monday, November 22, 2021 4:57PM - 5:10PM |
P11.00005: Pressure Gradient Tailoring Effects on Simulated Flow Behind a Ballistic Bluff Body Tyler J Souders, Samuel Whitman, Kareem Ahmed, Peter E Hamlington We present simulations and analysis of non-reacting flow and reacting stabilized flames anchored on a ballistic bluff body in a confined channel. The simulations are performed with PeleC, a fully compressible, highly parallelized reacting flow code with adaptive mesh refinement (AMR) based on the AMReX framework. This code is used here to fully resolve all spatial scales in areas of interest (e.g., regions of high vorticity or intermediary combustion species), while relying on coarse grids in less dynamically intensive regions. Following an experimental validation case, we tailor the mean pressure gradient in the channel using embedded boundaries to systematically study the magnitude and dynamical significance of baroclinic torque resulting from misalignments between pressure and density gradients, the latter of which results from combustion and heat transfer processes. The high-resolution simulations allow for additional three-dimensional insights and analysis of highly localized interactions, fully resolved here with AMR in the absence of any subgrid modeling for turbulence interactions. |
Monday, November 22, 2021 5:10PM - 5:23PM |
P11.00006: Chemical Kinetic Skeletal Reduction with Forced Optimally Time Dependent Modes Arash G Nouri, Yinmin Liu, Daniel Livescu, Peyman Givi, Harsha Chelliah, Hessam Babaee Skeletal model reduction based on local sensitivity analysis of time dependent systems is presented in which sensitivities are modeled by forced optimally time dependent (f-OTD) modes. The f-OTD factorizes the sensitivity coefficient matrix into a compressed format as the product of two skinny matrices, i.e. f-OTD modes and f-OTD coefficients. The modes create a low-dimensional, time dependent, orthonormal basis which capture the directions of the phase space associated with most dominant sensitivities. These directions highlight the instantaneous active species, and reaction paths. Evolution equations for the f-OTD modes and coefficients are derived, and the implementation of f-OTD for skeletal reduction is described. For demonstration, skeletal reduction is conducted of the constant pressure ethylene-air/methane-air burning in a zero-dimensional reactor, and new reduced models are generated. The laminar flame speed, the ignition delay, and the extinction curve as predicted by the models are compared against some existing skeletal models in literature for the same detailed model. The results demonstrate the capability of f-OTD to eliminate unimportant reactions and species in a systematic, efficient and accurate manner. |
Monday, November 22, 2021 5:23PM - 5:36PM Not Participating |
P11.00007: Fluid dynamics and photoionization contribution to plasma-assisted combustion models Rajath Shetty, Luca Massa The coupling between the plasma fields and ionization waves has two fundamental modes, one driven by preionization, which generally supports smooth and deterministic fronts, and one driven by photoionization, which supports the stochastic process. High-speed aerodynamics affects (weaken) the first path but not the second. This research investigates how the coupling between fluid and plasma wave is affected by Mach number variations for regimes (altitude) of interest to scramjet combustion. We will be studying the effect of photoionization on the propagation of streamers with combustion to improve the combustion ignition probability inside scramjets. We have modified a code typically used in astrophysics simulations to fully couple emission, photoionization, and combustion. The emissions are calculated using Bolsig, a code that calculates the electron energy distribution function using a two-term expansion of the Boltzmann equation. Photoionization is handled with a diffuse ray trace solver using Monte Carlo ray tracing. We have done some tests against a simplified method based on approximations of Green's function for shear layers relevant to cavity flow in scramjet condition. We will be investigating the effect of electrode geometry on shock formation during ignition. |
Monday, November 22, 2021 5:36PM - 5:49PM |
P11.00008: Laser-induced ignition of a methane-oxygen turbulent shear layer Jonathan M Wang, Mario Di Renzo, Hai Wang, Javier Urzay Laser-induced optical breakdown of a gas is a versatile means of depositing energy and seeding ignition in a combustible mixture. The expanding plasma kernel generates a complex flow that can affect subsequent flame growth, though the details of these dynamics in a turbulent and otherwise inhomogeneous flow are not fully understood. We analyze the flow and ignition dynamics in a subsonic methane-oxygen turbulent shear layer using direct numerical simulation. The vorticity fluctuations in the hot kernel are significantly attenuated by its supersonic expansion, whereas the fluctuations immediately behind the resulting shock wave do not exhibit a comparable change. At the flow conditions considered, the vorticity generated by the deposition alone is weaker than that of the turbulence, indicating that the hydrodynamic development of the kernel is dominated by the shear flow. The initial high temperature leads to rapid dissociation into monatomic radical species, which persist as the kernel becomes distorted and corrugated and cools to the flame temperature. Comparison with a corresponding unsheared, yet non-premixed, configuration indicates that vorticity generated due to the fuel-oxidizer density gradient also leads to distortion of the flame kernel. |
Monday, November 22, 2021 5:49PM - 6:02PM |
P11.00009: Heterogeneous multiscale modelling of dodecane drop vaporization in nitrogen Arijit Majumdar, Nguyen Ly, Matthias Ihme Understanding vaporization and mixing of hydrocarbon fuels under supercritical conditions is crucial for designing rocket engines and gas turbines. The challenging task is to accurately study the liquid-vapor interface, which is of molecular thickness, while also accounting for the mass, momentum and energy transfer at a macroscopic scale. Computational models rely on macroscopic methods or consider molecular approaches in the absence of scale separation. Heterogeneous multiscale methods have the capability of bridging the gap between macroscopic and microscopic scales. This study presents the development of a multiscale method in which a diffuse interface method is used to capture the flow around a planar interface while the fluid properties at the interface are evaluated using non-equilibrium molecular dynamic (NEMD) simulations. Different models for dodecane are compared and the interface structure obtained from NEMD simulations is validated against MD results and experimental data. |
Monday, November 22, 2021 6:02PM - 6:15PM |
P11.00010: Influences of heat release, blockage ratio and swirl on the recirculation zone behind a bluff body Dimitrios P Kallifronas, Pervez Ahmed, James C Massey, Midhat Talibi, Andrea Ducci, Ramanarayanan Balachandran, Nedunchezhian Swaminathan The recirculation zone created through vortex breakdown mechanisms in swirling flows plays a vital role for aerodynamic stabilisation of turbulent flames in practical combustion systems. This zone interacts with the wake of an upstream bluff body and leads to a complex flow behaviour that depends on the blockage ratio and swirl number. The vortex breakdown bubble merges with the wake at large swirl number or blockage ratio. Although these behaviours were studied well for isothermal flows, the effects of heat release on these flow patterns are not understood sufficiently. In this study, the influences of heat release on this flow structure and their physical mechanisms are explored through a series of large eddy simulations of bluff body stabilised swirling premixed flames. Detailed analyses of the results show that the bubble moves upstream leading to its stronger interaction with the wake in combusting flows which can affect the thermo-acoustic characteristics of the combustor. Comparisons of simulation results with measurements are observed to be good. |
Monday, November 22, 2021 6:15PM - 6:28PM |
P11.00011: Direct numerical simulation of a turbulent diffusion flame impinged by an external electric field Mario Di Renzo, Bénédicte Cuenot It is well-known that electric fields are very efficient tools to actively controlling the multiple aspects of combustion. Laboratory experiments have shown that the appropriate use of an external electric field can extinguish a flame, reduce its soot, CO, and nitrogen oxide emissions, and vary its lift-off height. The numerical simulation of these phenomena has remained elusive to the scientific community because of the physical and numerical complexity of the physical phenomena that regulate this interaction. In this study, we present new calculations performed using a multi-component compressible Navier--Stokes solver, called HTR solver (Di Renzo et al., Comp. Phys. Comm. 2020), which has been modified in order to include the effects of the ion transport due to electric fields. A peculiar aspect of this solver is its implementation in the task-based environment provided by the Legion runtime system, which makes the solver highly scalable and portable on machines with heterogeneous architectures. The presented calculation will focus on the modifications induced by an external electric field on a diffusion flame of methane and air developing in a turbulent temporal shear layer. |
Monday, November 22, 2021 6:28PM - 6:41PM |
P11.00012: Self-similarity and small-scale intermittency in turbulent premixed flames Amitesh Roy, Jason R Picardo, Timothy C Lieuwen, R I Sujith We reveal the presence of self-similarity and intermittency in the small-scale fluctuations of a turbulent premixed flame. From temporal measurements of the position of a methane-air V-flame, we find that the power spectrum has as a power-law dependence on frequency, over an intermediate range of time-scales, with a scaling exponent close to -2. This exponent is explained as a consequence of the flame fluctuating passively in response to perturbations from inertial-range turbulent eddies. Next, we calculate the moments of the temporal structure function, and find that they scale anomalously—a signature of small-scale intermittency—showing that higher frequency fluctuations have increasingly non-Gaussian, wide-tailed probability distributions. Our results have important implications for modelling turbulent flame speeds and heat release rate in premixed flames, while also showing how a flame reflects the characteristic features of the turbulent carrier flow. |
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