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Instantaneous 2D phosphor thermometry was performed on the windows and combustion chamber posts of an optically accessible dual-swirl gas turbine model combustor operated with CH 4 at atmospheric pressure. Two phosphors were used in order to measure different surface temperature ranges: YAP:Eu (temperature range 850 K – 1150 K) and YAG:Eu (temperature range 1100 K – 1300 K). Phosphor coatings were applied to the combustion chamber post and to three vertically oriented stripes on one combustion chamber window. Using interpolation, this allowed the determination of the surface temperature of the complete combustion chamber window. Heat losses across the combustion chamber window were determined by measuring the surface temperature on the inner and outer surface of the window. The phosphor coating was illuminated using the fourth harmonic of a Nd:YAG laser, which was formed into a broad light sheet. The spatially resolved, temperature-dependent decay rate of the phosphorescence was measured with a high speed CMOS camera. Three flames with similar flow fields and thermal powers between 22.5 kW and 30 kW and equivalence ratios between 0.63 and 0.83 were studied. Because the flame characteristics including flow field, heat release and temperature distributions were known from previous measurements the interaction between the flame and the surface temperature could be examined. In this way, the different wall temperatures of the flames could be explained. [ABSTRACT FROM AUTHOR]

Abstract Auto-ignition is a complex process which is extremely sensitive to boundary conditions such as local temperature, mixture or strain rate and occurs on very short time-scales. Therefore, measurement techniques with high spatio-temporal resolution have to be applied to test cases with well-defined boundary conditions in order to generate high-quality validation data for numerical simulations. In the current paper, the auto-ignition of a transient propane jet-in-hot coflow was studied with high-speed OH* chemiluminescence imaging and high-speed Rayleigh scattering for the simultaneous determination of mixture fraction, mixture temperature and scalar dissipation rate immediately prior to the onset of auto-ignition. A variation of the coflow temperature showed a pronounced temperature dependence of the auto-ignition location and time, and the temperature sensitivity was higher than for a comparable methane test case from the literature. This is explained by the lower sensitivity of propane ignition delay times to the local strain rate in comparison to methane. The Rayleigh measurements however showed that the formation mechanism of auto-ignition kernels is similar for propane and methane. Ignition kernels were found to form upstream of bulges of the inflowing jet at locations with locally low scalar dissipation rate. [ABSTRACT FROM AUTHOR]

The transient injection and subsequent auto-ignition of a methane jet issuing into a laminar coflow of hot exhaust gas from a lean premixed hydrogen air flame was studied using high-speed planar Rayleigh scattering, yielding two-dimensional measurements of mixture fraction, temperature and scalar dissipation rate with high spatio-temporal resolution. The temporal development of the mixing field between the transient fuel jet and the surrounding coflow prior to the occurrence of auto-ignition was examined at a sampling rate of 10 kHz. The impact of the transient jet development on numerical modeling of this test case is discussed. It was found that auto-ignition occurred after the jet transitioned from a transient state into the steady state, thus eliminating the need to model the complete transient fuel injection when the primary focus is on the onset of auto-ignition. Simultaneous high-speed OH * chemiluminescence from two viewing angles was applied to gain 3D-information of the ignition kernel location. This information allowed the selection and analysis of ignition events where the initial kernel formed inside the laser light sheet. Detailed analysis of the dynamics of a single ignition event, as well as statistical analysis of multiple ignition events based on a joint probability density approach, indicated that the ignition kernels occurred at very lean mixture fractions and at locations with low scalar dissipation rates. [ABSTRACT FROM AUTHOR]

Abstract: Autoignition of hydrogen/nitrogen jets in crossflows of vitiated air has been experimentally studied at conditions relevant for gas turbine combustor operation, including practical pressures (5, 10, 15bar), crossflow temperatures (T cf =1185K and 1143K), crossflow velocities (u cf =200 and 300m/s), and jet momentum ranges. Experiments were performed in an optically accessible duct, in which the appearance of autoignition events was investigated using high-speed imaging. At the different crossflow parameters, the H2 mole fraction X H2 in the fuel jet was incrementally increased until autoignition created a stable flame. During this process, the autoignition kernel and subsequent flame dynamics were recorded at a rate of 30kHz. In order to compare the experimental autoignition characteristics of the partially premixed, turbulent system to a homogeneous system, kinetic simulations of homogenous systems were conducted for pressures, temperatures, and vitiated air compositions corresponding to the experiments. The ignition delay times estimated from the experiment were considerably shorter than those from the kinetic simulations, and exhibited a different pressure dependency. These differences suggest that, in the current flow configuration, autoignition is strongly affected by turbulent mixing and flow field characteristics. [Copyright &y& Elsevier]

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]

Limit-cycle thermo-acoustic velocity coupling mechanisms are studied in a perfectly-premixed swirl-stabilized combustor using data from 10 kHz repetition-rate stereoscopic particle image velocimetry (S-PIV) and OH planar laser induced fluorescence (PLIF). Five cases over a range of thermal powers and equivalence ratios were investigated, each of which underwent different amplitude limit-cycle oscillations. Proper orthogonal decomposition (POD) of the velocity data showed that each case contained a dynamic helical vortex core (HVC) that rotated around the combustor and greatly affected the flame behavior. Flow and flame statistics were compiled as a function of both the phase in the thermo-acoustic cycle and a phase representing the azimuthal position of the HVC relative to the measurement plane. These data were used to determine the thermo-acoustic energy transfer field at each HVC azimuthal angle, as described by the Rayleigh integral. It was found that periodic deformations of the HVC caused large-scale flame motions, resulting in regions of positive and negative energy transfer. The deformation of the HVC was linked to a swirl number wave that propagated from the burner nozzle. While the mechanism of thermo-acoustic coupling was the same for all cases, the phase between heat release and pressure oscillations varied significantly. This phase relationship was determined by the interaction of the pressure field, swirl wave, HVC deformation, and flame response. It was shown that these can be described by the combination of a Helmholtz resonator and a convective disturbance. [ABSTRACT FROM AUTHOR]

The precessing vortex core (PVC) is a coherent flow structure that is often encountered in swirling flows in gas turbine (GT) combustors. In some swirl combustors, it has been observed that a PVC is present under non-reacting conditions but disappears in the corresponding reacting cases. Since numerous studies have shown that a PVC has strong effects on the flame stabilization, it is desirable to understand the formation and suppression of PVCs in GT combustors. The present work experimentally studies the flow field in a GT model combustor at atmospheric pressure. Whereas all non-reacting conditions and detached M-shaped flames exhibit a PVC, the PVC is suppressed for attached V-shaped flames. A local linear stability analysis is then applied to the measured time-averaged velocity and density fields. For the cases where a PVC appeared in the experiment, the analysis shows a global hydrodynamic instability that manifests in a single-helical mode with its wavemaker located at the combustor inlet. The frequency of the global mode is in excellent agreement with the measured oscillation frequency and the growth rate is approximately zero, indicating the marginally stable limit-cycle. For the attached V-flame without PVC, strong radial density/temperature gradients are present at the inlet, which are shown to suppress the global instability. The interplay between the PVC and the flame is further investigated by considering a bi-stable case with intermittent transitions between V- and M-flame. The flame and flow transients are investigated experimentally via simultaneous highspeed PIV and OH-PLIF. The experiments reveal a sequence of events wherein the PVC forms prior to the transition of the flame shape. The results demonstrate the essential role of the PVC in the flame stabilization, and thereby the importance of a hydrodynamic stability analysis in the design of a swirl combustor. [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]