Turbulent premixed combustion: Flamelet structure and its effect on turbulent burning velocities

Computational science is paramount to the understanding of underlying processes in internal combustion engines of the future that will utilize non-petroleum-based alternative fuels, including carbon-neutral biofuels, and burn in new combustion regimes that will attain high efficiency while minimizing emissions of particulates and nitrogen oxides. Next-generation engines will likely operate at higher pressures, with greater amounts of dilution and utilize alternative fuels that exhibit a wide range of chemical and physical properties. Therefore, there is a significant role for high-fidelity simulations, direct numerical simulations (DNS), specifically designed to capture key turbulence-chemistry interactions in these relatively uncharted combustion regimes, and in particular, that can discriminate the effects of differences in fuel properties. In DNS, all of the relevant turbulence and flame scales are resolved numerically using high-order accurate numerical algorithms. As a consequence terascale DNS are computationally intensive, require massive amounts of computing power and generate tens of terabytes of data. Recent results from terascale DNS of turbulent flames are presented here, illustrating its role in elucidating flame stabilization mechanisms in a lifted turbulent hydrogen/air jet flame in a hot air coflow, and the flame structure of a fuel-lean turbulent premixed jet flame. Computing at this scale requires close collaborations between computer and combustion scientists to provide optimized scaleable algorithms and software for terascale simulations, efficient collective parallel I/O, tools for volume visualization of multiscale, multivariate data and automating the combustion workflow. The enabling computer science, applied to combustion science, is also required in many other terascale physics and engineering simulations. In particular, performance monitoring is used to identify the performance of key kernels in the DNS code, S3D and especially memory intensive loops in the code. Through the careful application of loop transformations, data reuse in cache is exploited thereby reducing memory bandwidth needs, and hence, improving S3D's nodal performance. To enhance collective parallel I/O in S3D, an MPI-I/O caching design is used to construct a two-stage write-behind method for improving the performance of write-only operations. The simulations generate tens of terabytes of data requiring analysis. Interactive exploration of the simulation data is enabled by multivariate time-varying volume visualization. The visualization highlights spatial and temporal correlations between multiple reactive scalar fields using an intuitive user interface based on parallel coordinates and time histogram. Finally, an automated combustion workflow is designed using Kepler to manage large-scale data movement, data morphing, and archival and to provide a graphical display of run-time diagnostics.

https://iopscience.iop.org/article/10.1088/1749-4699/2/1/015001/pdf

Terascale direct numerical simulations of turbulent combustion using S3D

Computational science is paramount to the understanding of underlying processes in internal combustion engines of the future that will utilize non-petroleum-based alternative fuels, including carbon-neutral biofuels, and burn in new combustion regimes that will attain high efficiency while minimizing emissions of particulates and nitrogen oxides. Next-generation engines will likely operate at higher pressures, with greater amounts of dilution and utilize alternative fuels that exhibit a wide range of chemical and physical properties. Therefore, there is a significant role for high-fidelity simulations, direct numerical simulations (DNS), specifically designed to capture key turbulence-chemistry interactions in these relatively uncharted combustion regimes, and in particular, that can discriminate the effects of differences in fuel properties. In DNS, all of the relevant turbulence and flame scales are resolved numerically using high-order accurate numerical algorithms. As a consequence terascale DNS are computationally intensive, require massive amounts of computing power and generate tens of terabytes of data. Recent results from terascale DNS of turbulent flames are presented here, illustrating its role in elucidating flame stabilization mechanisms in a lifted turbulent hydrogen/air jet flame in a hot air co-flow, and the flame structure of a fuel-lean turbulent premixed jet flame. Computing at this scale requires close collaborations betweenmore » computer and combustion scientists to provide optimized scaleable algorithms and software for terascale simulations, efficient collective parallel I/O, tools for volume visualization of multiscale, multivariate data and automating the combustion workflow. The enabling computer science, applied to combustion science, is also required in many other terascale physics and engineering simulations. In particular, performance monitoring is used to identify the performance of key kernels in the DNS code, S3D and especially memory intensive loops in the code. Through the careful application of loop transformations, data reuse in cache is exploited thereby reducing memory bandwidth needs, and hence, improving S3D's nodal performance. To enhance collective parallel I/O in S3D, an MPI-I/O caching design is used to construct a two-stage write-behind method for improving the performance of write-only operations. The simulations generate tens of terabytes of data requiring analysis. Interactive exploration of the simulation data is enabled by multivariate time-varying volume visualization. The visualization highlights spatial and temporal correlations between multiple reactive scalar fields using an intuitive user interface based on parallel coordinates and time histogram. Finally, an automated combustion workflow is designed using Kepler to manage large-scale data movement, data morphing, and archival and to provide a graphical display of run-time diagnostics.« less

Terascale direct numerical simulations of turbulent combustion using S3D.

Direct numerical simulation of a three-dimensional spatially developing turbulent slot-burner Bunsen flame has been performed with a new reduced methane–air mechanism. The mechanism, derived from sequential application of directed relation graph theory, sensitivity analysis and computational singular perturbation over the GRI-1.2 detailed mechanism is non-stiff and tailored to the lean conditions of the DNS. The simulation is performed for three flow through times, long enough to achieve statistical stationarity. The turbulence parameters have been chosen such that the combustion occurs in the thin reaction zones regime of premixed combustion. The data is analyzed to study possible influences of turbulence on the structure of the preheat and reaction zones. The results show that the mean thickness of the turbulent flame, based on progress variable gradient, is greater than the corresponding laminar flame. The effects of flow straining and flame front curvature on the mean flame thickness are quantified through conditional means of the thickness and by examining the balance equation for the evolution of the flame thickness. Finally, conditional mean reaction rate of key species compared to the laminar reaction rate profiles show that there is no significant perturbation of the heat release layer.

Structure of a spatially developing turbulent lean methane–air Bunsen flame

Molecular transport effects on turbulent flame propagation and structure

Direct numerical simulations of hydrogen/air turbulent premixed flame progagating in the three-dimensional turbulence are conducted to investigate local flame structures in the well-stirred reactor regime. A detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the H 2 /air reaction in turbulence. Although the flame condition is classified into the well-stirred reactor regime, the geometry of the regions with high heat release rate shows thin sheetlike structure. The fluctuation of the heat release rate along the flame surface is relatively high, and the maximum heat release rate reaches up to 1,3 times the corresponding laminar flame. The heat release rate tends to be high in the regions convex toward the burned side. The flame structure in the case of the well-stirred reactor regime shows a double-layered feature. One may conclude that reaction zone becomes thick in the well-stirred reactor regimes only from temperature, H, and OH distributions, while the heat release rate, mass fraction of O atoms, and reaction rates of O atoms and OH radicals are fluctuating significantly in that region in fact. Specifically, reaction rates of O atoms and OH radicals show characteristic behaviors in the burned side due to their chemical characteristics. In the preheat zone, mass fraction and reaction rate of HO 2 show quite thin and smooth distributions compared to other properties such as the heat release rate. The distribution of H 2 O 2 reaction rate reflects the double-layered feature of the well-stirred reactor regime very well. It is shown that the double-layered feature can be explained by discussing the balances of the elementary reactions in detail. As the flame front can be defined even in the well-stirred reactor regime, statistics of the local flame elements are also discussed.

Local flame structure in the well-stirred reactor regime

Direct numerical simulations (DNS) of turbulent premixed flames are conducted to investigate effects of turbulence characteristics on the local flame structure A detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the H2–air reaction in turbulence Numerical conditions of DNS can be classified into a wrinkled-flamelets regime, a corrugated-flamelets regime and thin reaction zones near the boundary of Karlovitz number Ka = 10 of the turbulent-combustion diagram For all cases, the distribution of heat-release rate shows a three-dimensionally connected sheet-like feature, even though the heat-release rate highly fluctuates along the flamefront The heat-release rate tends to increase at the flamefronts that are convex towards the burnt side For the turbulent premixed flames in the corrugated-flamelets regime, the handgrip structure is produced by the intrusion of the coherent fine-scale eddy into the flame and the heat-release rate in this structure i

Effect of turbulence characteristics on local flame structure of H2-air premixed flames

Effect of turbulence characteristics on local flame structure of h 2 -air premixed flames

Flame–wall interactions of lean premixed flames under elevated, rising pressure conditions

Direct Numerical Simulations (DNSs) of hydrogen-air turbulent premixed planar flames in homogeneous isotropic turbulence are conducted to investigate the local flame structure in thin reaction zones The detail kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the hydrogen-air reaction DNSs are conducted at Reλ = 374 and 608, where Reλ denotes Reynolds number based on Taylor micro-scale (λ) In the thin reaction zones, the heat release rate is highly fluctuating in the low temperature region and the maximum heat release rates reach to 13–14 times that of the laminar flame The high heat release rate regions are surrounded by the low heat release regions and are shown like pocket and reed-shape structures In the unburned side, the radical fingering of HO2 is also generated, which suggests that the strong coherent-fine scale eddies in the unburned side transport reactive species in the preheat zone into the unburned side for the case of thin reaction zones It is implied that the radical fingering to the unburned mixture is one of the important flame structures in the reaction zones or in broken reaction zones To clarify effects of turbulence on the local flame structure, the statistical characteristics of flame elements are also investigated

Yuzuru Nada

Papers

Local flame structure in the well-stirred reactor regime

Effect of turbulence characteristics on local flame structure of H2-air premixed flames

Effect of turbulence characteristics on local flame structure of h 2 -air premixed flames

Flame–wall interactions of lean premixed flames under elevated, rising pressure conditions

Radical fingering in turbulent premixed flame classified into thin reaction zones