High-speed mixture fraction and temperature imaging of pulsed, turbulent fuel jets auto-igniting in high-temperature, vitiated co-flows

Abstract

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 highspeed laser measurements are enabled through the use of the unique high-energy pulse burst laser system (HEPBLS) which generates long-duration bursts of ultra-high pulse energies at 532 nm (> 1 Joule) 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 signal-to-noise (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 1300 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 ( <  s). Furthermore, the ignition kernel formed in regions of low scalar dissipation rate, which agrees with previous results from direct numerical simulations.

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

“…For the current measurements, the Rayleigh camera had an active array of 400 × 912 pixels, the sheet-correction camera had an active array of 512 × 208 pixels, and the exposure time was 1 μs for both cameras. More details of the high-speed Rayleigh scattering imaging system can be found in [30,50] .…”

“…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).…”

“…Details of the Rayleigh scattering imaging post-processing can be found in [30] . First, the raw images were corrected to account for sensor offset using a darkframe correction.…”

“…To meet this criterion the velocities of the unreacted gas were chosen to exceed 0.7 m/s which corresponds to a velocity of 4.1 m/s for the high-temperature product gases. Rayleigh scattering measurements and data processing as described in [30] were used to determine the coflow temperature, which was 1490 K, or approximately 4.5 % below T ad . A coriolis meter placed in line with the CH 4 -injection system was used to calculate the bulk flow velocity and jet exit Reynolds number.…”

“…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.…”