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 A24: Reacting Flows: LES and DNS |
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Chair: Chao Xu, Argonne National Labs Room: 233 |
Sunday, November 20, 2022 8:00AM - 8:13AM |
A24.00001: Effect of elevated pressure in the multi-scale behavior of turbulent ammonia/hydrogen/nitrogen-air premixed flames Jacqueline H Chen, Martin Rieth, Myoungkyu Lee Spectral behaviors of turbulent ammonia/hydrogen/nitrogen-air premixed flame at different pressures are investigated to understand the effect of elevated pressures. We analyze datasets from direct numerical simulations at 1 atm and 10 atm with high mean shear velocities. We calculate the spectral densities derived from density weighted two-point correlation function defined by (Kolla et. al. JFM 2014) of scalar variance and turbulent kinetic energy (TKE). Product- and reactant-species and TKE exhibit k-5/3 scaling where heat release rate is maximum at both pressures. However, intermediate species such as OH and HO2 exhibit k-5/3 region only at 1 atm. Also, TKE spectra show a stronger contribution in the dissipation range at 10 ATM. In this presentation, we will examine the causality of the scaling for intermediate species and TKE which exhibits unique multi-scale behavior at elevated pressure by performing spectral analysis of transport equations. |
Sunday, November 20, 2022 8:13AM - 8:26AM |
A24.00002: Reactive decontamination of toxic chemicals within porous media Ellen K Luckins Following a chemical weapons attack, it is crucial for both public health and the environment that the toxic chemical agent is properly cleaned up. One particular issue is when the agent has contaminated porous materials, such as brick or concrete; in such cases, decontamination is typically achieved by reacting the agent with a cleanser, which neutralises it in a chemical reaction. The agent and cleanser fluids are usually immiscible, so that the chemical reaction occurs only at fluid--fluid interfaces within the pores of the porous medium. While it is relatively straightforward to write down a model that describes the interplay of the agent and cleanser on the scale of the pores, it is computationally expensive to solve such a model over realistic spill sizes. In this talk we present homogenised models for the reactive decontamination of porous media, which are computationally efficient to simulate while still taking the pore-scale behaviour into account. Solutions of these homogenised models show how differences in the initial distribution of agent affect both the decontamination time and the amount of cleanser required to fully decontaminate the porous material. |
Sunday, November 20, 2022 8:26AM - 8:39AM |
A24.00003: Fractal characteristics of turbulent premixed hydrogen-enriched flames at different pressures Martin Rieth, Andrea Gruber, Jacqueline H Chen The combustion of ammonia/hydrogen/nitrogen blends and pure hydrogen in air presents an attractive carbon-free alternative to conventional natural gas firing in gas turbines. However, lean combustion of such fuels poses challenges due to the thermo-diffusively unstable nature of hydrogen-enriched fuel burning. We present results from Direct Numerical Simulations (DNS) of premixed ammonia/hydrogen/nitrogen-air flames in temporally-evolving shear layer configurations at atmospheric and elevated pressure under lean conditions and in a planar jet configuration at different equivalence ratios and atmospheric pressure. In addition, we present results of various lean hydrogen-air flames subjected to isotropic turbulence. These DNS show a fundamentally different turbulent combustion behavior at different pressures, with cases at elevated pressure exhibiting stronger effects of preferential diffusion and enhanced thermo-diffusively unstable behavior. The analysis will focus on the influence of pressure, preferential/differential diffusion and intrinsic instabilities on the fractal dimension of those flames, which is an important parameter in turbulent premixed flame modeling. The analysis clearly shows increased fractal dimensions as pressure increases and instabilities are amplified. |
Sunday, November 20, 2022 8:39AM - 8:52AM |
A24.00004: Towards DNS of spray flame stabilization processes with Sustainable Aviation Fuels Bruno S. Soriano, Landon Owen, Tuan Nguyen, Jacqueline H Chen Concern for emission reduction and energy security have motivated the development of new cost-effective alternative sustainable aviation fuels (SAFs). Drop-in SAFs are beneficial because they are made from renewable biomass and waste resources and do not require engine modifications for use in current aviation engines. However, any new fuel must characteristically match commercial Jet-A (or A2) fuel in order to meet the criteria for drop-in SAFs. Combustion characteristics are of particular concern when comparing a new fuel with Jet-A. Therefore, a fundamental understanding of the differences in combustion dynamics of new fuels is necessary to identify the impact of SAFs on current combustors. The objective of our research is to perform Direct Numerical Simulations (DNS) of spray flames under gas turbine cruise operating conditions (high pressure and temperature). Specifically, flame stabilization dynamics will be investigated using a jet flame configuration at representative thermodynamic conditions of the central recirculation zone (CRZ) in a swirl-stabilized combustor. The simulations will be performed using PeleLMeX, the low-Mach solver of the Pele suite. Lagrangian multi-phase modeling will capture the liquid spray injection and Adaptive Mesh Refinement (AMR) will allow for higher resolutions in targeted regions of the domain. The proposed DNS configurations will closely follow laboratory experiments performed at Sandia National Laboratories Combustion Research Facility. Two different SAFs (C1 and C9) will be compared with the reference Jet-A fuel. The impact on flame stabilization dynamics from 'turbulence-chemistry' interactions and low-temperature chemistry dependency on fuel properties are investigated. |
Sunday, November 20, 2022 8:52AM - 9:05AM |
A24.00005: Effect of radial flame position on the flow topology of a reacting jet in crossflow Abin Krishnan, Vedanth Nair, Subodh Adhikari, Vishal Acharya, Tim Lieuwen Reacting jet in crossflow (RJICF) is a canonical flow topology encountered in various relevant applications. In the current study, we investigate the effect of the radial flame position on the near-field and far-field flow dynamics of a reacting jet in crossflow using Large Eddy Simulation (LES). We present here two reacting cases – a) a pure methane jet into a crossflow of air, where the flame is present radially outside the jet shear layer (the R1 case), and b) a diluted methane jet into a crossflow of oxygen, where the flame is present radially inside the shear layer (the R2 case). For the R1 case, we observe the growth of the shear layer vortices and the development of the counter-rotating vortex pair. For the R2 case, the vorticity does not concentrate into discrete vortices and the counter-rotating vortex pair lacks the characteristics double-lobed coherent structure. These results show that the variation in the radial flame location introduces significant and novel changes in the flow topology of reacting jet in crossflow dynamics. |
Sunday, November 20, 2022 9:05AM - 9:18AM Author not Attending |
A24.00006: Large Eddy Simulation of Reacting Flow in a Hydrogen Jet into Supersonic Cross-Flow with Adaptive Mesh Refinement Robert C Sheerin, Chao Xu This work focuses on large eddy simulation of transverse hydrogen jet mixing and combustion process in supersonic crossflow. Combustion is modeled using a finite rate chemistry model with a 9-species hydrogen/air reaction mechanism. A large eddy simulation (LES) model is developed in the CONVERGE code and is validated against experimental data using mean wall pressure and OH mass fraction contours. Adaptive mesh refinement (AMR) is employed to replicate results within the desired accuracy while limiting the model to fewer computational cells. The AMR process taken involves refining the local grid size to allow for capturing the thin flow, flame, and shock wave structures. A detailed analysis of the shock and turbulent structures is conducted to determine their effects on combustion enhancement and flame stabilization. The high reactivity of hydrogen gas presents a large danger regarding industrial applications and low-cost simulations can help mitigate that risk. |
Sunday, November 20, 2022 9:18AM - 9:31AM |
A24.00007: Adjoint-based neural network optimization for sub-grid-scale combustion modeling Seungwon Suh, Jonathan F MacArt, Luke Olson, Jonathan B Freund We consider sub-grid-scale models that leverage machine learning methods. Unlike a direct approach, in which training of the machine learning model attempts to reduce the mismatch between the sub-grid-scale model and an accepted model (e.g., DNS), our approach is to expose the training process to the governing equations by using the adjoint equations. Specifically, we embed a standard deep neural network model into the governing equations and train by backpropagating the model itself and solving the adjoint equations, yielding end-to-end gradients of the neural network parameters. This leverages the physics embodied in the governing equations, which provides greater robustness to the learned model. In addition, the training target is distinct from the closure, thereby allowing training for any flow observable. The approach has been demonstrated for simulation of isotropic turbulence and free-shear-flow turbulence; we extend it to the greater coupled-physics challenge of a planar premixed flame propagating through isotropic turbulence. |
Sunday, November 20, 2022 9:31AM - 9:44AM |
A24.00008: DNS of Lean Premixed Combustion Assisted by Partial Fuel Stratification for Assessment of G-Equation Modeling Approach Chao Xu, Pinaki Pal, Muhsin Ameen, Sibendu Som This study focuses on understanding the detailed flame structures associated with partial fuel stratification (PFS)-assisted lean premixed combustion using two-dimensional direct numerical simulation (DNS). The DNS is performed using the spectral element CFD solver Nek5000 with detailed chemistry, experimental pressure trace, and realistic initial conditions mapped from a separate large-eddy simulation (LES), replicating a lean spark ignition engine condition. Numerical results indicate the prevalence of conventional triple flame structures during the initial stage of flame kernel growth. Both premixed and non-premixed combustion modes are present with the premixed mode contributing dominantly to the total heat release. Detailed analysis reveals non-negligible effects of flame stretch and fuel pyrolysis on the flame displacement speed. Based on the DNS findings, the accuracy of a hybrid G-equation/well-stirred reactor combustion model is assessed for PFS-assisted operation in the LES context. Implications of the DNS analysis for other advanced engine combustion concepts with low-carbon fuels, such as H2, are further discussed. |
Sunday, November 20, 2022 9:44AM - 9:57AM |
A24.00009: Modeling the boundary-layer flashback of premixed CH4/H2/air swirling flames Shiming Zhang, Zhen Lu, Yue Yang We model the boundary-layer flashback of CH4/H2/air swirling flames via large-eddy simulations with the flame-surface-density model (LES-FSD), in particular, at high pressures. A local displacement speed model depends on the flame stretch, curvature, and heat loss is applied. For the CH4/air flames at 1 atm, the LES-FSD results agree with those in experiments on the shape and propagating speed of the turbulent flame brush during flashback. For lean CH4/H2/air flames at 2.5 bar, the LES-FSD simulations yield lower critical equivalence ratio for flashback with the increase of H2 volume fraction, consistent with the experiments. This is due to the improved modeling of the flame stretch and heat loss effects on the local displacement speed. We also propose a simple theoretical model to predict the boundary-layer flashback limit of the swirling flames. The model estimates the critical bulk velocity for given reactants and swirl number, via the balance between the flame-induced pressure rise and adverse pressure for boundary-layer separation. We validate the model against 11 datasets of CH4/H2/air swirling flame experiments, with the H2 enrichment in fuel varying from 50% to 85%. Results show that the proposed model well estimates the flashback limits over a wide range of conditions. |
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