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Molecular transport effects on turbulent flame propagation and structure

Twenty years ago, homogeneous-charge spark-ignition gasoline engines (using carburetion, throttle-body-, or port-fuel-injection) were the dominant automotive engines. Advanced automotive engine development remained largely empirical, and stratified-charge direct-injection gasoline-engine production was blocked by lack of robustness in its combustion process [W.G. Agnew, Proc. Combust. Inst. 20 (1984) 1–17]. Today, a wide range of direct-injection gasoline engines are in (or near) production, and combustion science is playing a direct role in advanced gasoline-engine development through the simultaneous application of advanced optical diagnostics, three-dimensional computational fluid dynamics (CFD) modeling, and traditional combustion diagnostics. This paper discusses the use of optical diagnostics and CFD in five gasoline-engine combustion systems: homogeneous spark-ignition port-fuel-injection (PFI), homogeneous spark-ignition direct-injection (DI), stratified wall-guided spark-ignition direct-injection (WG-SIDI), stratified spray-guided spark-ignition direct-injection (SG-SIDI), and homogeneous-charge compression-ignition (HCCI). The emphasis is on WG-SIDI, SG-SIDI, and HCCI engines. Key in-cylinder physical processes (e.g., sprays and vaporization, turbulent fuel–air mixing, wall wetting, ignition and early flame development, turbulent partially premixed flame propagation, and emissions formation) can be visualized, quantified, and optimized through optical engine experiments and CFD-based engine modeling. Outstanding issues for stratified engines include reducing piston wall-wetting, pool fires and smoke in WG-SIDI engines, eliminating intermittent misfires in SG-SIDI engines, and optimizing lean NOx after-treatment systems. HCCI engines require better control of combustion timing and heat-release rate over wide speed/load operating ranges, smooth transitions between operating modes, and individual cylinder sensors and controls. Future directions in optical diagnostics and modeling are suggested to improve our fundamental understanding of important in-cylinder processes and to enhance CFD modeling capabilities.

Advanced gasoline engine development using optical diagnostics and numerical modeling

Flow–flame interactions causing acoustically coupled heat release fluctuations in a thermo-acoustically unstable gas turbine model combustor

Spatially resolved heat release rate measurements in turbulent premixed flames

Unsteady effects of strain rate and curvature on turbulent premixed flames in an inflow-outflow configuration

https://www.repository.cam.ac.uk/bitstream/1810/246757/1/Minamoto_etal_CnF_2014.pdf

Morphological and statistical features of reaction zones in MILD and premixed combustion

The effects of curvature and wrinkling on the growth of turbulent premixed flame kernels have been investigated using both 2D OH planar laser-induced fluorescence (PLIF) and 3D direct numerical simulation (DNS). Comparisons of results between the two approaches show a high level of agreement, providing confidence in the simplified chemistry treatment employed in the DNS, and indicating that chemistry may have only a limited influence on the evolution of the freely propagating flame. This is in contrast to previous studies of the very early flame development where chemistry may be dominant. Statistics for curvature and wrinkling are presented in the form of probability density functions, and there is good agreement with previous findings. The limitations of 2D PLIF measurements of curvature are quantified by comparison with full 3D information obtained from the DNS. The usefulness of PLIF in providing data over a wide parameter range is illustrated using statistics obtained from both CH4/air and H-2/air mixtures, which show a markedly different behaviour due to their different thermo-diffusive properties. The results provide a demonstration of the combined power of PLIF and DNS for flame investigation. Each technique is shown to compensate for the weaknesses of the other and to reinforce the strengths of both. (Less)

Curvature and wrinkling of premixed flame kernels?comparisons of OH PLIF and DNS data

In turbulent premixed flames, much experimental evidence points to a strong influence of pre-mixture turbulence intensity on the turbulent burning velocity. The linear enhancement of turbulent burning velocity in low-intensity turbulence is predicted accurately by current models. In contrast, the deviation from linearity in high-intensity turbulence, known as the “bending effect,” remains to be explained. The present work has employed Direct Numerical Simulation (DNS) to investigate the bending effect. An initially laminar methane-air premixed flame was subjected to increasing levels of turbulence across five different simulations which maintained all parameters except the turbulence intensity constant. The bending effect was captured within these simulations. Subsequently, plausible explanations were investigated using the framework of the Flame Surface Density (FSD) approach. From the ensuing analysis, it is evident that flame surface area reflects distinctly the variation of turbulent burning velocity with turbulence intensity. Local flame quenching does not appear to be the primary mechanism behind the bending effect. Instead, the observed bending effect results from a shift in balance, under high-intensity turbulence, towards mechanisms that favour destruction of flame surface area. These mechanisms tend to preserve the reaction layer and, thereby, ensure the validity of Damkohler’s hypothesis and flamelet models in conditions that cause the bending effect that is observed here to occur.

/pdf/direct-numerical-simulation-of-the-bending-effect-in-1wuw7yk67m.pdf

Direct Numerical Simulation of the bending effect in turbulent premixed flames

Results for flame surface density (FSD) in premixed turbulent flame kernels have been obtained from OH planar laser induced fluorescence (PLIF) and direct numerical simulations (DNS), and have been compared for similar values of global Lewis number and normalised turbulence intensity. Stoichiometric methane–air and lean hydrogen–air mixtures were studied, and the same post-processing techniques were employed for both experimental and DNS data in order to evaluate FSD statistics from spatial gradients of the reaction progress variable. Full 3D FSD statistics were obtained from the DNS data sets. Also, FSD statistics were obtained from two-dimensional cross-sections extracted from the DNS data sets which were found to be in qualitative agreement with the FSD statistics of PLIF data. The location of maximum FSD within the flame was found to be close to the middle of the flame brush for both methane–air and hydrogen–air flames, and was found to be slightly skewed about the middle of the flame brush for some methane–air flames. The PLIF data for both fuels showed a decrease in the maximum FSD with increasing turbulence intensity. This effect was not observed in the three-dimensional DNS analysis for methane–air flames, but was found to be consistent with both two-dimensional and three-dimensional analysis of the DNS data for hydrogen–air flames. The findings have been compared with the results of other experimental and DNS work reported in the literature and mechanisms have been suggested to explain the observed behaviour.

Stewart Cant

Papers

Unsteady effects of strain rate and curvature on turbulent premixed flames in an inflow-outflow configuration

Morphological and statistical features of reaction zones in MILD and premixed combustion

Curvature and wrinkling of premixed flame kernels?comparisons of OH PLIF and DNS data

Direct Numerical Simulation of the bending effect in turbulent premixed flames

Measurement of flame surface density for turbulent premixed flames using PLIF and DNS