Auto-ignition and flame stabilization of pulsed methane jets in a hot vitiated coflow studied with high-speed laser and imaging techniques

Abstract

The auto-ignition of a pulsed methane jet issuing into a laminar coflow of hot exhaust products of a lean premixed hydrogen/air flat flame was examined using high-speed laser and optical measurement techniques with frame rates of 5 kHz or more. OH* chemiluminescence was used to determine the downstream location of the first auto-ignition kernel as well as the stabilization height of the steady-state lifted jet flame. OH planar laser-induced fluorescence (PLIF) was used to determine further details of the auto-ignition with a higher spatial resolution. Simultaneous imaging of broadband luminosity from a viewing angle perpendicular to the OH* chemiluminescence was applied to three-dimensionally reconstruct the ignition kernel location in space and to determine whether the first occurrence of the kernel was within or beyond the PLIF laser sheet. The development and expansion of the jet was characterized by high-speed Schlieren imaging. Statistics have been compiled for both the ignition time as well as the downstream location of the first auto-ignition kernel and the stabilization height of the steady-state lifted jet flame. From the PLIF images it was found that auto-ignition tended to occur at the interface between bulges of the inflowing jet and the coflow. For steady-state conditions, auto-ignition kernels were observed frequently below the flame base, emphasizing that the lifted jet flame is stabilized by auto-ignition.

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).…”

“…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.…”

“…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.…”

Simultaneous 5 kHz OH-PLIF/PIV for the study of turbulent combustion at engine conditions

“…Simultaneous integration of these systems has supported concurrent scalar and velocity field measurements at kHz interrogation frequencies in laboratory-scale flames. These efforts have yielded a greater understanding of dynamic turbulent flame behavior through tracking the temporal evolution of transient phenomena such as local extinction, auto-ignition, and turbulence-chemistry interactions [13][14][15][16][17][18][19][20].…”