Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session R40: Reacting Flows: Turbulent Combustion III |
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Chair: Michael Mueller, Princeton University Room: 204C |
Monday, November 20, 2023 1:50PM - 2:03PM |
R40.00001: Direct Numerical Simulations of Low-Emission Staged Ammonia Combustion Martin Rieth, Andrea Gruber, Evatt Hawkes, Jacqueline H Chen Ammonia is a promising carbon-free alternative to natural gas in dispatchable power generation. While carbon dioxide emissions are eliminated from the combustion process with ammonia, nitrogen oxide emissions (NOx) pose a significant challenge. One of the most promising strategies reducing such emissions in gas turbines is the rich-quench-lean staged combustion system, where the combustion process is divided into two stages. With ammonia-based fuels, typically the first stage features a fuel-rich flame while the second stage comprises air injection to consume the remaining fuel. While such a strategy has been shown experimentally to significantly reduce NOx formation, there is no fundamental understanding on how the combustion process occurs and how pollutants form in such a configuration. We present Direct Numerical Simulation (DNS) results of a simplified and downscaled setup of the second stage derived from a real gas turbine combustor, which retains relevant timescales of the actual combustor. The results reveal details of the turbulent inverted diffusion flame forming as fresh air reacts with hot combustion products from the first stage containing unburnt hydrogen. We will also discuss how emission formation processes are affected by turbulent mixing and residence time. |
Monday, November 20, 2023 2:03PM - 2:16PM |
R40.00002: DNS study of flame speed enhancement in turbulent premixed hydrogen flames Matthew X Yao, Guillaume Blanquart Hydrogen combustion is a desirable alternative to fossil fuel combustion due to the reduced emissions of pollutants such as greenhouse gases and soot. Lean hydrogen combustion has a propensity to develop thermodiffusive instabilities due to differential diffusion. In the presence of turbulence, these instabilities significantly enhance the flame speed, leading to safety concerns such as flame flashback. In this study, a suite of DNS are conducted across a range of Karlovitz numbers and integral length scales to study in detail this flame speed enhancement. The relationship between global effects (e.g. flame speed and area) and local effects (local flame structure) is studied through a generalized expression for the burning efficiency. The effect of detailed chemistry is discussed, in particular the effect of Soret diffusion. The integral length scale has a minor impact on the flame structure, whereas the turbulence intensity has a significant impact. As the Karlovitz number is increased, the flame structure asymptotes towards the unity Lewis flamelet solutions. Across the range of tested Karlovitz numbers, the flame speed and area are shown to increase before decreasing, while the burning efficiency continues to increase. |
Monday, November 20, 2023 2:16PM - 2:29PM |
R40.00003: A priori Analysis and modelling of Dilution Equation for MILD Combustion Using DNS Data Yuang Han, James C Massey, Xi Deng, Nilanjan Chakraborty, Nedunchezhian Swaminathan Addressing climate change requires innovative concepts and technologies for energy and transport sectors. A promising approach is MILD (Moderate to Intense Low-oxygen Dilution) combustion, offering enhanced energy efficiency, reduced pollutant emissions, and increased operational flexibility, especially with zero-C fuels like Ammonia. MILD combustion relies on naturally occurring fuel-air mixture dilution through flow recirculation in the combustor, which varies in time and space, influencing chemical conversion. However, understanding the dilution's impact on fuel consumption, particularly in the context of LES (Large Eddy Simulation), remains limited. In this study, a transport equation for the dilution factor is derived from fundamental principles and analyzed using DNS (Direct Numerical Simulation) data of turbulent MILD combustion. The goal is to develop simple models, focusing on various source terms in the filtered dilution equation. These sources arise from overall fuel consumption progress, chemical reactions affecting the dilution variable, and turbulent mixing represented by scalar dissipation rates. By treating each LES grid cell as a perfectly stirred reactor (PSR), simple models are developed, effectively capturing the background physics observed in the DNS data. Results from LES with the dilution equation and the tested sub-grid models will be discussed in the presentation. |
Monday, November 20, 2023 2:29PM - 2:42PM |
R40.00004: Numerical Investigation of High CO2 Dilution Effects on Methane-Hydrogen Flame Structure in Turbulent Swirl-Stabilized Flames Samuel Whitman, Chao Xu Lowering carbon emissions while meeting the demand for industrial power generation is a matter of great importance. Combustion of H2 fuels and high-H2 fuel blends has attracted interest in addressing this concern, especially due to the potential for use with minimal system re-design. To increase the efficiency of the carbon removal process from the exhaust, as well as to reduce the NOx emissions which increase with H2 addition, CO2 may be used as a diluent in industrial combustion strategies utilizing exhaust gas recirculation (EGR), whereby a portion of the hot gases exiting the combustor are mixed in with the unburned reactants. In the present study, we perform high-fidelity large eddy simulations (LES) of a simplified annular combustor geometry using the highly scalable spectral element code Nek5000 to capture the effects of varying levels of CO2 dilution on the flame structure, turbulence statistics and stability in the lean combustion regime near the blowoff limit. While flames can burn hotter and faster with higher H2 percentages in the fuel stock, increasing CO2 dilution has the opposite effect, decreasing flame stability and lowering blowoff limits of swirl-stabilized flames such as those found in typical combustors. In our numerical experiments, we investigate the flame and flow behavior in this highly diluted regime, using detailed chemical kinetics to enable new insights into distribution of key species, which play an integral role in the behavior of the flame under highly strained conditions. |
Monday, November 20, 2023 2:42PM - 2:55PM |
R40.00005: Modeling of turbulence-combustion interactions in a high-speed gas flow Anvar N Gilmanov, Ponnuthurai Gokulakrishnan, Michael Klassen Combustion is a complex physical phenomenon that couples chemical kinetics with turbulent flow. One of the most challenging and demanding applications for turbulent combustion is high-speed airbreathing systems, where combustion occurs in supersonic gas flow. A modeling approach based on the OpenFOAM® library has been developed to solve a high-speed, multi-component mixture of a reacting, compressible gas flow. This work presents comprehensive validation of the solver (compressibleReactiveFoam) with different supersonic flows, including shocks and expansion waves. The comparisons of the simulation results with experimental/analytical data confirm the fidelity of this solver for problems involving reactive multi-component high-speed flows. Modeling turbulent combustion using large-eddy simulation coupled with a partially-stirred reacting formulation provided promising results and an understanding of the complex physics involved in supersonic combustors. |
Monday, November 20, 2023 2:55PM - 3:08PM |
R40.00006: Vorticity Dynamics in Bluff Body Stabilized Premixed Flames with External Pressure Gradients and Free-Stream Turbulence Kelsea J Souders, Peter E Hamlington Previous studies based on direct numerical simulations of statistically planar premixed flames have shown that flame-generated turbulence is small compared to flow-generated turbulence for highly turbulent (i.e., high Karlovitz, low Damkohler) conditions. However, recent computational and experimental studies have indicated that the flame-generated component can be significant in configurations featuring large background pressure gradients. In this study, we use PeleC, a fully compressible reacting flow code featuring adaptive mesh refinement, to study vorticity dynamics in the near-wake region of a bluff-body stabilized premixed flame at near-blowoff conditions. We consider the combined effects of free-stream turbulence and mean background pressure gradients, as well as their cumulative effect, on the balance between flame-generated turbulence attributed to heat release from combustion and flow-generated turbulence inherent to the flow configuration. We describe how different terms in the vorticity magnitude transport equation vary with turbulence intensity and mean pressure gradient and connect these results with observed flame phenomena. Ultimately, this study will inform the development of improved models for large eddy simulations used to model real-world combustion systems. |
Monday, November 20, 2023 3:08PM - 3:21PM Author not Attending |
R40.00007: Irregular configurations of shock wave impingement on reactive mixing layers. Nereida G. de Codina, Ignacio Sánchez-Ojeda, Daniel Martínez-Ruiz, Daniel Mira, Pedro J Martínez-Ferrer, Cesar Huete This study investigates the impact of an oblique shock wave at a specific angle on a hydrogen-air mixing layer formed by two supersonic streams with different Mach numbers. The interaction between the shock wave and the reactive mixing layer can result in the formation of regular or irregular structures. The regular configurations, which aim to achieve equilibrium following the incident and transmitted shock waves, can be formed through a reflected wave or a Prandtl-Meyer expansion. These regular configurations occur within critical limits, and exceeding these limits prevents the flow from reaching equilibrium downstream of the oblique shock. In such cases, the flow exhibits irregular configurations characterized by new structures, including slip lines, triple points, curved shocks, or subsonic regions. |
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