Flame stabilization and auto-ignition of pulsed methane jets in a hot coflow: Influence of temperature

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 OH* 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 K and 1810 K in order to study the influence of the coflow temperature on the autoignition 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.

The role of temperature, mixture fraction, and scalar dissipation rate on transient methane injection and auto-ignition in a jet in hot coflow burner

“…However, DNS in turbulent mixing layers has shown that not all locations with the most-reactive mixture fraction ignite at the same time [36] . This finding was supported from previous experimental results [28,29] , where auto-ignition was observed in the form of localized ignition kernels. In DNS, fluctuations of the scalar dissipation rate were found to play a key role in determining the probability of auto-ignition in transient systems; here, ignition occurs first in localized kernels at locations with minimal scalar dissipation rates [42,[44][45][46] , as is found in the cores of vortices [38][39][40] .…”

“…The laser system [30,50,54] and combustor [28,29] have been described previously, so only the key parameters are presented here. Figure 1 shows a schematic of the DLR Jet in Hot Coflow Burner (DLR JHC).…”

“…Two quartz windows were used to reduce disturbance of the exhaust gas flow by ambient air. In contrast to previous studies [28,29] , the laser entrance and exit side windows were removed due to the high laser pulse energies applied in this work. However, no influence of entrained room air was found at the studied measurement locations, as will be discussed below.…”

“…Gordon et al [15] observed isolated ignition kernels corresponding to a rise of temperature and CH 2 O concentration; however, OH was not present in all ignition kernels, indicating auto-ignition (and not flame propagation) is a primary mechanism behind lifted flame stabilization. With the recent development of high-speed laser measurement and imaging techniques [19][20][21] , new insights into the mechanisms governing auto-ignition has been gained in transient systems [17,[22][23][24][25][26][27][28][29][30][31] . In the current work, high-speed laser-based measurements are used to reveal details of the roles of temperature, mixture fraction, and scalar dissipation rate on auto-ignition within a JHC configuration.…”

“…When observing an ignition kernel with planar laser measurement techniques, it is of great importance to know whether this ignition kernel formed within the measurement plane or if it formed outside the measurement plane and was convected into the measurement plane by transport processes. Arndt et al [28,29] studied the auto-ignition of a transient fuel jet issuing into a hot vitiated coflow with well-defined boundary conditions using high-repetition-rate planar laser diagnostics and high-speed imaging. Chemiluminescence imaging from two viewing angles was used to reconstruct the downstream location of an auto-ignition kernel as well as its position relative to the measurement plane.…”