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A gas turbine model combustor (Turbomeca Burner) for premixed methane/air flames has been operated at atmospheric pressure in two different modes of premixing. In the partially premixed mode, fuel was injected into the air flow within the swirl generator shortly upstream of the combustion chamber while in the perfectly premixed mode fuel and air were mixed far upstream. The main objective of this work is the study of the influence of the mode of premixing on the combustion behavior. Stereoscopic particle image velocimetry has been applied for the measurement of the flow field, OH chemiluminescence imaging for the visualization of the flame shapes and single-shot laser Raman scattering for the determination of the joint probability density functions of major species concentrations, mixture fraction and temperature. The mixing and reaction progress and effects of turbulence-chemistry interactions are characterized by scatterplots showing the correlations between different quantities. To isolate effects of mixing from combustion instabilities that were frequently observed in this combustor, operating conditions without thermo-acoustic oscillations or coherent flow structures were chosen. While the mode of premixing had no major influence on the general flame behavior characteristic differences were observed with respect to flame anchoring, the flow field in the inner recirculation zone and the CO concentration level. The results further extend the data base of previous experimental and numerical investigations with this burner. [ABSTRACT FROM AUTHOR]

Abstract: The auto-ignition of a pulsed methane jet issuing into a coaxial laminar coflow of hot exhaust gas from a lean premixed hydrogen/air flat flame has been studied experimentally by means of high-speed optical diagnostics. The downstream location of the first auto-ignition kernel as well as the stabilization height of the steady-state lifted jet flame and the auto-ignition time were determined by chemiluminescence (CL) measurements. OH planar laser-induced fluorescence (PLIF) was used to determine further details of the auto-ignition with a higher spatial resolution and increased sensitivity. In order to three-dimensionally reconstruct the ignition kernel location in space and only include ignition kernels in the PLIF processing that first occurred inside the laser light sheet, broadband luminosity imaging from a viewing angle perpendicular to the PLIF and CL measurements was performed. The coflow temperature was varied in the temperature range between 1566 and 1810K in order to study the influence of the coflow temperature on the auto-ignition height, auto-ignition time, and on the lift-off height of the steady lifted jet flame. In addition, detailed model simulations were performed to study the influence of temperature and strain on auto-ignition in reacting mixing layers. [Copyright &y& Elsevier]

The Pareto-efficient combustion (PEC) framework is extended for predicting transient ignition in turbulent flames. The PEC formulation utilizes a drift term to assess the compliance of specific combustion submodels with the underlying flow-field representation. This drift term is extended to consider autoignition delay as a process of interest, and the ignition delay time of a homogeneous isobaric reactor is utilized as surrogate. Combustion models considered are the steady-state flamelet/progress variable (FPV) model and a finite-rate chemistry (FRC) model. Large-eddy simulations are performed and the proposed PEC autoignition formulation (PEC-AI) is demonstrated to identify localized regions that require a finite-rate chemistry description in order to accurately capture the transient ignition dynamics. The model is employed to examine the stochasticity of the autoignition in the turbulent environment by performing ensemble simulations to construct probability distributions of ignition time and ignition locations. Comparisons with measurements show the ability of PEC-AI in capturing the transient ignition dynamics and flame lift-off through drift-term adaptation with comparable accuracy to FRC simulations at reduced cost. These results illustrate the versatility of the PEC framework in targeting different combustion response functions that not only include species and emissions but also transient combustion processes. [ABSTRACT FROM AUTHOR]

Abstract: The influence of natural gas (NG) on the auto-ignition behavior of hydrogen (H2)/nitrogen (N2) fuel jets injected into a vitiated cross-flow was studied at conditions relevant for practical combustion systems (p = 15 bar, T cross-flow = 1173 K). In addition, the flame stabilization process following auto-ignition was investigated by means of high-speed luminosity and shadowgraph imaging. The experiments were carried out in an optically accessible jet in cross-flow (JICF) test section. In a H2/NG/N2 fuel mixture, the fraction of H2 was stepwise increased while keeping the N2 fraction approximately constant. Two different jet penetration depths, represented by two N2 fraction levels, were investigated. The results reveal that auto-ignition kernels occurred even for the lowest tested H2 fuel fraction , but did not initiate a stable flame in the duct. Increasing decreased the distance between the initial position of the auto-ignition kernels and the fuel injector, finally leading to flame stabilization. The H2 fraction for which flame stabilization was initiated depended on jet penetration; flame stabilization occurred at lower H2 fractions for the higher jet penetration depth ( = 91% compared to 96%), revealing the influence of different flow fields and mixing characteristics on the flame stabilization process. It is hypothesized that the flame stabilization process is related to kernels extending over the duct height and thus altering the upstream conditions due to considerable heat release. This enabled subsequent kernels to occur close to the fuel injector until they could finally stabilize in the recirculation zone of the jet lee. [Copyright &y& Elsevier]