Session M09: Reacting Flows: DNS

Spectral analysis of turbulent ammonia/hydrogen/nitrogen-air premixed flames at ambient and elevated pressure

Multi-scale spectral analysis of reactive scalar (co)-variance and turbulent kinetic energy is presented as a function of pressure for recent direct numerical simulation (DNS) of ammonia/hydrogen/nitrogen-air premixed flames in preheated strong shear layer turbulence. The objective is to understand the length scale-dependent behavior of different reactions and transport mechanisms for diffusive-thermally unstable flames. In particular, we compare two cases at 1 atm and 10 atm. Overall, the contribution of small-scale modes (SSMs) with wavelength smaller than 10δ_L is stronger in all state variables for the 10 atm case. The spectral densities of the mass fraction variations, E_Y², of products and reactants show peaks at large-scale modes (LSMs) greater than 10δ_L corresponding to the peaks of the turbulent kinetic energy spectra. The spectral behavior of E_Y² of intermediate species is found to vary with the molar weights. E_Y² of heavy molecules including H₂O₂, NO, NO₂, for example, shows peaks at LSM at 1 atm but at SSM at 10 atm. Also, E_Y²s for some species, such as HO₂, exhibit multiple peaks at different length scales and locations. The potential mechanisms causing species-specific spectral behaviors, e.g. mixing, mass-diffusion, and reaction rates will be discussed.    

Not Participating

Flame-flame interaction statistics are analysed for a series of lean methane-air flames in a slot jet configuration.  Direct Numerical Simulation (DNS) was used to simulate these flames, for which the turbulence integral length scale was systematically increased across the cases while keeping the Karlovitz number and the Kolmogorov scales constant. The Reynolds number Re, defined using the bulk velocity, slot width and the reactant properties, was ~ 22,000 for the case with the largest turbulence integral length scale. The frequency of flame-flame interactions is evaluated for the different cases, and also for the different regions within each flame. These regions are the flame base region, the fully developed turbulent flame region and the flame-tip region. The corresponding flame-flame interaction topology is also evaluated in these regions using the Morse theory of critical points. The differences in the results for the different regions, as well as across the dataset are compared and discussed. The findings of this analysis are used to explain the factors contributing to flame-flame interactions and their effect on overall flame propagation.    

The role of intense reactant-product mixing in distributed turbulent premixed combustion

Distributed turbulent premixed combustion (DTPC) is a combustion regime in which reduced chemistry is disrupted by small, fast turbulence scales and is characterized by a suppression of radical concentrations. However, the mechanisms of and leading to DTPC and the regime boundaries of DTPC in terms of the Karlovitz number remain elusive. To better understand the radical behavior in and approaching this combustion regime, a series of direct numerical simulations (DNS) has been conducted for a recirculating model combustor that promotes intense mixing of premixed hydrogen-air reactants and products. This configuration has been used in the study of moderate and intense low-oxygen dilution (MILD) combustion and flameless oxidation (FLOX). As the global Karlovitz number increases, a suppression of chemical reactivity is observed. Radical concentration profiles in terms of progress variable are compared with a manifold-based combustion model to better understand the nature of the progress variable dissipation rate in DTPC and the role of intense reactant-product mixing.   

Not Participating

Direct Numerical Simulation (DNS) of premixed hydrocarbon flames is performed using the DNS code Senga2 to investigate the role of varying integral length scale l₀ as well as the turbulence intensity u`. The frequency of flame-flame interactions is analysed for the different cases and the corresponding flame-flame interaction topology is evaluated using the Morse theory of critical points. A matrix of test cases is studied by either systematically varying l<sub>0</sub> at constant u` or by systematically varying u` at constant l<sub>0</sub>. The flame-flame interactions are known to increase with increasing u`, but the same was also found to be true for deceasing l₀. In fact, a striking similarity in the results was observed between the cases with increasing u` and decreasing l₀. The reasons for these observed results are discussed and the mean curvature of the flame is found to play an especially important role. These findings serve to further our understanding of turbulent combustion and the role that the turbulence length scales play in flame propagation.    

The Effect of Pressure on Ammonia/Hydrogen/Nitrogen Premixed Flames in Intense Sheared Turbulence

The combustion of ammonia/hydrogen/nitrogen blends presents an attractive carbon-free alternative to conventional natural gas firing in gas turbines. However, challenges exist, such as NOx formation and thermo-diffusively unstable lean combustion behavior, both of which are not well understood yet, especially at elevated pressure. We present results from Direct Numerical Simulations (DNS) of lean premixed ammonia/hydrogen/nitrogen-air flames in a temporally-evolving shear layer configuration at atmospheric and elevated pressure (10 atm). These DNS show fundamentally different turbulent combustion behavior at 1 and 10 atm, with the case at elevated pressure exhibiting stronger effects of preferential diffusion and enhanced thermo-diffusively unstable behavior. The DNS results are compared to results from simulations of canonical cases with reduced complexity to shed light on the change of thermo-diffusive instabilities with pressure. The DNS results are analyzed in terms of general burning behavior, including global fuel consumption speed and flame surface area statistics. Changes in global burning behavior observed at 1 and 10 atm are discussed in relation to preferential and differential diffusion effects, intrinsic flame instabilities and the underlying sheared turbulence encountered by the flame brush.   

Not Participating

Reliable spark ignition is a critical requirement on the development of highly-efficient, low emissions gasoline spark ignited IC engines. The low reactivity of lean/dilute mixtures associated with high levels of turbulence can lead to unsuccessful ignition events or flame extinction during the early kernel expansion. Cycle-to-cycle combustion variability is established during laminar-to-turbulent flame transition that occurs by the 1 % mass burn fraction.  Thermo-diffusive effects have an impact during the early flame evolution due to the coupling between flame stretch and burning velocity related to the Lewis number (Le) effects of the fuel component. The objective of this study is to characterize the Le number effects of a multi-component spark-ignited flame kernel evolution in a turbulent flow with different levels of exhaust gas recirculation (EGR). The computational domain is based on the cross-flow ignition experiment from SNL. PeleC and PeleLM are used to solve, respectively, the compressible and low-Mach reactive Navier-Stokes equations with Adaptive Mesh Refinement (AMR) and Embedded Boundary (EB) geometry treatment to account for the electrode geometries. Reaction rates are obtained using a 98-species multi-component gasoline surrogate mechanism. The results show a variation on heat release rate and mixture equivalence ratio caused differential diffusion effects and flame geometry. The Le number effects are investigated considering all the terms in transport equation contributing to the total flame displacement speed. Local extinction, caused by flame-flame interaction, is also observed in highly diluted mixtures.   

Not Participating

Given the widespread public interest in carbon neutrality, ammonia represents a potential energy carrier without carbon dioxide emissions. To obtain fundamental insights into the behavior of turbulent premixed lean and rich ammonia flames, high fidelity three-dimensional direct numerical simulations of flames in a channel are performed with complex chemistry. In particular, the heat release characteristics and turbulent flame structure are analyzed. Two cases are considered, for which the unstretched laminar flame speed is matched to provide a physically relevant comparison. The heat release rate analysis reveals that the peak of the conditional mean of heat release rate exceeds the laminar value for lean ammonia flame, but not for the rich ammonia flame. Furthermore, the mean profiles of major species resemble those of the laminar flame for both cases. More detailed analysis and examination of the flame-turbulence interaction and flame propagation characteristics will be provided in the presentation.   

Intrinsic Flame Instabilities in Turbulent Premixed Hydrogen Flames: A DNS Study

Large-scale Direct Numerical Simulations (DNS) of turbulent, lean, premixed hydrogen flames susceptible to intrinsic thermodiffusive instabilities have been performed in a slot burner configuration at a jet Reynolds number of 11,000 using finite rate chemistry. A variation of the flames' Karlovitz number is considered while the Reynolds number is kept constant to investigate different regimes of flame/turbulence interaction. Additionally, DNS with identical thermal and species diffusivities (unity Lewis number assumption) are conducted to suppress thermodiffusive instabilities and rigorously assess their effects. The turbulent flames clearly show the typical features of thermodiffusive instabilities such as super-adiabatic temperatures, local extinction, and a significant enhancement of the flame speed. The interactions of thermodiffusive instabilities and turbulence significantly affect the mechanisms of flame surface area generation and the local chemical state in the flame. Turbulent flame wrinkling shows a synergistic effect on thermodiffusive instabilities as large curvature values further enhance the effects of differential diffusion. Finally, conditions of future DNS to further explore the interactions of thermodiffusive instabilities and turbulence are discussed.   

Combustion of droplets in plane mixing layers

The combustion of droplets in turbulence gives rise to complex phenomena that are still not completely understood. First, the droplets disperse driven by fluctuations in the gas velocity and evaporate driven by fluctuations in the vapor pressure. Second, the vapor mixes and reacts with the oxygen forming carbon dioxide and water. Previous studies revealed persistent clusters, which influence the evaporation of droplets and reaction of vapor. In particular, the studies showed that diffusion flames, which surround single droplets, coexist with premixed flames, which propagate through clusters.\footnote{Weiss, Bhopalam, Meyer, and Jenny, \emph{Physics of Fluids} \textbf{33}, 033322 (2021).}${ }^{,}$\footnote{Weiss, Doctoral thesis, ETH Zurich (2021).} \par In this talk, these phenomena are examined with direct numerical simulations of plane mixing layers. The gas is governed by the low Mach number equations, the droplets are governed by the point droplet equations, and the chemical reaction is modeled with a one-step mechanism.${ }^{1,2}$ The mixing layer consists of a hot, outer stream and a cold, inner stream laden with droplets.\footnote{Dai, Jin, Luo, Xiao, and Fan, \emph{Fuel} \textbf{301}, 121030 (2021).} The flow is examined with statistics of the gas and droplets. In particular, clusters and voids are identified with Vorono\"{i} tessellations, and diffusion and premixed flames are identified with Takeno's flame index.\footnote{Rycroft, \emph{Chaos} \textbf{19}, 041111 (2009).}${ }^{,}$\footnote{Yamashita, Shimada, and Takeno, \emph{26th Symposium on Combustion}, 27-34 (1996).} Our purpose is to better understand how parameters like the droplet loading and diameter affect the structure and evolution of the mixing layer.   

Effect of fuel-blend ratio in methane-hydrogen reheat flames

A recent study on the operation of a reheat burner (RB) in Ansaldo's GT36 engine with H2-enriched natural gas found that for a H2 fraction >70%, the engine must be derated to achieve the desired combustion characteristics. This was due to a greater contribution of the flame propagation as opposed to the design intended autoignition. In this study, 2D direct numerical simulations are performed in a simplified RB to investigate the effect of blending ratio in CH4-H2-air premixed flames. The geometry consists of a rectangular mixing duct that undergoes a sudden expansion into a combustion chamber. Three fuel blends comprised of CH4 and H2 at a ratio 0.5:0.5, 0.3:0.7 and 0:1 by volume are considered. At 5 bar the thermo-chemical conditions are chosen to achieve a constant ignition delay time. The results show flame stabilization by autoignition in the bulk flow and by flame propagation in the shear layers immediately downstream of the sudden area expansion. The flame rapidly oscillates in the combustion chamber for pure H2, which is suppressed with addition of CH4. With an increase in CH4 fraction, the flame broadens with a more volumetrically distributed heat release rate. The quantification of the fuel consumption by the two combustion modes based on CEMA will also be presented.