Search

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: 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]

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]