Search

Transient auto-ignition is a key factor for flame stabilization and flame initialization in several technical combustion systems such as internal combustion engines or gas turbine combustors. Reliable numerical simulations of auto-ignition stabilized flames are important for the development of new combustor systems. For detailed model validation, knowledge of the sensitivity of different system response quantities (SRQs) of interest to the boundary conditions in combination with the accuracy of boundary conditions is essential, especially with respect to uncertainty quantification of numerical simulations. In the current study, the flame stabilization and auto-ignition in the DLR Jet-in-Hot-Coflow burner was examined experimentally using high-speed OH* chemiluminescence. Here, methane was either injected continuously to study the flame stabilization mechanism of steady state lifted jet flames, or in a pulsed manner to study the formation of auto-ignition kernels, into the hot exhaust gas of a lean, premixed hydrogen/air flame. The flame stabilization height, and the location and time of initial auto-ignition kernels for a case with transient auto-ignition were evaluated with respect to several boundary conditions, such as coflow temperature as well as coflow- and jet-velocity. A relative sensitivity of the measured SRQs on the boundary conditions was introduced in order to quantitatively compare steady state flame to transient auto-ignition characteristics and to assess the quantitative influence of different boundary conditions. Comparison of the auto-ignition dynamics in the steady state and during transient fuel injection allowed assessing the role of auto-ignition in the flame stabilization mechanism for different boundary conditions; accompanying chemical kinetic calculations were used to quantify the influence of strain on auto-ignition and flame propagation for the current conditions, allowing further insight into the flame stabilization mechanism in Jet-in-Hot-Coflow flames. [ABSTRACT FROM AUTHOR]

Copyright of Politische Vierteljahresschrift is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Low signal-to-noise in particle image velocimetry (PIV) measurements in systems such as high pressure gas turbine combustors can result in significant data gaps that negatively affect subsequent analysis. Here, gappy proper orthogonal decomposition (GPOD) is evaluated as a method of filling such missing data. Four GPOD methods are studied, including a new method that utilizes a median filter (MF) to adaptively select whether a local missing data point is updated after each iteration. These methods also are compared against local Kriging interpolation. The GPOD methods are tested using PIV data without missing vectors that were obtained in atmospheric pressure swirl flames. Parameters studied include the turbulence intensity, amount of missing data, and the amount of noise in the valid data. Two criteria to check for GPOD convergence also were investigated. The MF method filled in the missing data with the lowest error across all parameters tested, with approximately one-third the computational cost of Kriging. Furthermore, the accuracy of MF GPOD was relatively insensitive to the quality of the convergence criterion. Therefore, compared to the three other GPOD methods and Kriging interpolation, the MF GPOD method is an effective method for filling missing data in PIV measurements in the studied gas turbine combustor flows. [ABSTRACT FROM AUTHOR]

In this manuscript, we describe an experimental approach to simultaneously measure high-speed image sequences of the mixture fraction and temperature fields during pulsed, turbulent fuel injection into a high-temperature, co-flowing, and vitiated oxidizer stream. The quantitative mixture fraction and temperature measurements are determined from 10-kHz-rate planar Rayleigh scattering and a robust data processing methodology which is accurate from fuel injection to the onset of auto-ignition. In addition, the data processing is shown to yield accurate temperature measurements following ignition to observe the initial evolution of the 'burning' temperature field. High-speed OH* chemiluminescence (CL) was used to determine the spatial location of the initial auto-ignition kernel. In order to ensure that the ignition kernel formed inside of the Rayleigh scattering laser light sheet, OH* CL was observed in two viewing planes, one near-parallel to the laser sheet and one perpendicular to the laser sheet. The high-speed laser measurements are enabled through the use of the unique high-energy pulse burst laser system which generates long-duration bursts of ultra-high pulse energies at 532 nm (>1 J) suitable for planar Rayleigh scattering imaging. A particular focus of this study was to characterize the fidelity of the measurements both in the context of the precision and accuracy, which includes facility operating and boundary conditions and measurement of signal-to-noise ratio (SNR). The mixture fraction and temperature fields deduced from the high-speed planar Rayleigh scattering measurements exhibited SNR values greater than 100 at temperatures exceeding 1,300 K. The accuracy of the measurements was determined by comparing the current mixture fraction results to that of 'cold', isothermal, non-reacting jets. All profiles, when properly normalized, exhibited self-similarity and collapsed upon one another. Finally, example mixture fraction, temperature, and OH* emission sequences are presented for a variety for fuel and vitiated oxidizer combinations. For all cases considered, auto-ignition occurred at the periphery of the fuel jet, under very 'lean' conditions, where the local mixture fraction was less than the stoichiometric mixture fraction ( ξ < ξ). Furthermore, the ignition kernel formed in regions of low scalar dissipation rate, which agrees with previous results from direct numerical simulations. [ABSTRACT FROM AUTHOR]

A series of measurements was taken on two technically premixed, swirl-stabilized methane-air flames (at overall equivalence ratios of ϕ = 0.73 and 0.83) in an optically accessible gas turbine model combustor. The primary diagnostics used were combined planar laser-induced fluorescence of the OH radical and stereoscopic particle image velocimetry (PIV) with simultaneous repetition rates of 10 kHz and a measurement duration of 0.8 s. Also measured were acoustic pulsations and OH chemiluminescence. Analysis revealed strong local periodicity in the thermoacoustically self-excited (or ' noisy') flame (ϕ = 0.73) in the regions of the flow corresponding to the inner shear layer and the jet-inflow. This periodicity appears to be the result of a helical precessing vortex core (PVC) present in that region of the combustor. The PVC has a precession frequency double (at 570 Hz) that of the thermo-acoustic pulsation (at 288 Hz). A comparison of the various data sets and analysis techniques applied to each flame suggests a strong coupling between the PVC and the thermo-acoustic pulsation in the noisy flame. Measurements of the stable (' quiet') flame (ϕ = 0.83) revealed a global fluctuation in both velocity and heat-release around 364 Hz, but no clear evidence of a PVC. [ABSTRACT FROM AUTHOR]

Control systems for vehicle stability require information about the actual vehicle state. While many states can be obtained from sensor signals, some important states, like the lateral velocity and the corresponding tyre-road friction have to be reconstructed by a vehicle state estimator, comprising of a simplified vehicle model. The efficiency of the developed vehicle state estimator has been proven in several projects for integrated vehicle dynamics control at the Ford Research Centre in Aachen. [ABSTRACT FROM AUTHOR]