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

Rayleigh scattering is a powerful diagnostic tool for the study of gases and is particularly useful for aiding in the understanding of complex flow fields and combustion phenomena. Although the mechanism associated with the scattering, induced electric dipole radiation, is conceptually straightforward, the features of the scattering are complex because of the anisotropy of molecules, collective scattering from many molecules and inelastic scattering associated with rotational and vibrational transitions. These effects cause the scattered signal to be depolarized and to have spectral features that reflect the pressure, temperature and internal energy states of the gas. The very small scattering cross section makes molecular Rayleigh scattering particularly susceptible to background interference. Scattering from very small particles also falls into the Rayleigh range and may dominate the scattering from molecules if the particle density is high. This particle scattering can be used to enhance flow visualization and velocity measurements, or it may be removed by spectral filtering. New approaches to spectral filtering are now being applied to both Rayleigh molecular scattering and Rayleigh particle scattering to extract quantitative information about complex gas flow fields. This paper outlines the classical properties of Rayleigh scattering and reviews some of the new advances in flow field imaging that have been achieved using the new filter approaches.

https://iopscience.iop.org/article/10.1088/0957-0233/12/5/201/pdf

Laser Rayleigh scattering

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

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

An improved understanding of the mechanisms of unstable combustion in lean premixed combustors is essential to the development of stable gas turbine combustion systems. To obtain such understanding, detailed experimental studies of the phenomenology of unstable combustion are required. A number of experimental diagnostic techniques for characterizing unstable combustion and the underlying instability mechanisms are discussed. This includes techniques based on pressure, chemiluminescence emission, infrared absorption, and laser-induced fluorescence measurements. The techniques themselves are discussed briefly; however, the primary objective is to present and discuss results illustrating how these techniques can be used to characterize the mechanisms of unstable combustion, to gain an improved understanding of unstable combustion, and to develop strategies for suppressing unstable combustion in lean premixed gas turbine combustors.

Experimental Diagnostics for the Study of Combustion Instabilities in Lean Premixed Combustors

Two-dimensional temperature measurements were performed in a sooting, methane-air flame by a modification of the conventional Rayleigh scattering technique. This new technique, filtered Rayleigh scattering (FRS), uses a molecular absorption filter in the detection path to spectrally separate part of the gas-phase Rayleigh scattering from the elastic scattering contributions of soot particles. It also suppresses laser-induced glare and background scattered light from walls and windows. Thus particle and background, unaffected, gas-phase Rayleigh scattering can be detected. This has been done in a weakly sooting flame where the temperature field information was obtained using a calibration of the FRS setup in a nonsooting flame by conventional Rayleigh scattering. This was necessary because the FRS gas scattering intensity is no longer a simple function of temperature and the known scattering cross sections of the individual gas species present. It also depends on the spectral broadening of the Rayleigh line relative to the incident radiation. Two theoretical models for the calculation of the dependence of the RRS signal on the experimental conditions, for the kinetic and the hydrodynamic regime, are presented in order to directly calculate gas-phase temperatures from FRS measurements without the need for calibration. For the present flames, the broadening mechanism is in an intermediate regime.

Temperature field measurements in a sooting flame by filtered rayleigh scattering (FRS)

The probe employs a single-mode laser diode (1) emitting monochromatic light at a nominal wavelength of 780 nm, and a photodiode (2). The collimated beam enters and leaves the volume under investigation (U) through optical fibres (LW1,LW2). The operating temperature and voltage of the laser diode are selected and stabilised by electronic control (3). By demodulation at twice the modulation frequency an electrical signal is obtained in direct proportion to the absorption and hence to the concentration of oxygen.

Lambda probe comprising single-mode laser diode and photodiode

Vorrichtung zur optischen Bestimmung der Sauerstoffkonzentration und deren Änderung im Abgas eines Verbrennungssystems zur Kontrolle und/oder Regelung des Verbrennungsprozesses Apparatus for the optical determination of oxygen concentration in the exhaust gas and their change of a combustion system for the control and / or regulation of the combustion process

The local flame structure of a premixed swirl-stabilized gas turbine burner has been investigated, where “thickened flames” or flames in the “stirred reactor regime” are expected from the Borghi diagram. Simultaneous 2-D OH and temperature measurements show that the flame structure fluctuates between four typical flame modes: (1) flamelet-like burning, (2) modified preheat zone with sharp reaction zone, (3) locally quenched reaction zone, and (4) hot nonreacted holes. In mode (3), evidence for local quenching processes is found with no superequilibrium OH between fresh gas and recirculating burned gas. In about 10% of the obtained images, regions without detectable OH but with temperatures above 1300 K are seen, surrounded by sharp reaction zones with low thermal gradients (mode 4). Although the explanation is not clear yet, significant modifications of the reaction processes due to strain combined with transient effects seem to occur here. From 2-D and 3-D measurements, gradients and flame thickness distributions have been determined, showing strong fluctuation of the thermal gradients, but on average, no significantly broadened instantaneous flame fronts, contrary to the expectation of thick flames. Instead, mean gradients conditioned to the local reaction progress variable c=(T−T 0 )/(T max −T 0 ) =0,50 result in a slightly thinned thermal thickness, and within the preheat zone at c =0.25 a thinning of about 30% is found. For an explanation. strained laminar calculations are helpful, indicating the same trend for lean CH 4 -air mixtures. Obviously, strain effects from medium- and large-scale eddies have much more influence than diffusive effects from entrained small eddies. While the thermal thickness distribution shows large fluctuations, the OH thickness distribution (deduced from the OH ascent) is narrow. Comparing the Oh ascent thickness (similar to the calculated width of the CH peak) with the inner layer Kolmogorov size, a ratio of order 1 is found, when local quenching occurs.

Structure of locally quenched highly turbulent lean premixed Flames

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Temperature field measurements in a sooting flame by filtered rayleigh scattering (FRS)

Lambda probe comprising single-mode laser diode and photodiode

Structure of locally quenched highly turbulent lean premixed Flames