We have applied chirped-probe-pulse (CPP) femtosecond (fs) coherent anti-Stokes Raman scattering for 5 kHz temperature measurements in turbulent spray flames. The CPP fs CARS technique has previously been used to perform spectroscopic temperature measurements in highly turbulent laboratory burners with excellent accuracy, precision, temporal resolution, and spatial resolution. In this paper, ultrafast CARS measurements in spray flames are presented as part of a larger effort to provide spatially and temporally resolved temperature fields in harsh spray environments. The Sydney Needle Spray Burner (SYNSBURNTM) was used to stabilize turbulent spray flames of acetone and ethanol. The burner features a retractable fuel injector so that the droplet density at the nozzle exit could be systematically varied. Results from selected regions of the turbulent spray flames are discussed in detail to highlight the challenges of CPP fs CARS temperature measurements. Sources of spectral distortion due to interaction with droplets are discussed along with an uncertainty analysis. The passage of fuel through the probe volume caused varying levels of signal degradation and resulted in complete signal loss on approximately 10% of the laser shots for dense spray conditions. The interferences are attributed to two separate phenomena and are categorized based on the probable phase of the fuel – liquid or gas. Interference caused by liquid fuel was unavoidable in certain regions at certain operating conditions, but easily identified and removed. Interference from vapor fuel was more problematic as the nitrogen signal was only moderately corrupted in the high-frequency portion of the spectrum, and the temperature was generally biased to higher values. Rejecting individual signal spectra, based on a fitting error threshold, was shown to be effective in excluding shots with significant interference from fuel droplets, but shots with only minor interference require a more-advanced rejection criterion. Analysis of the temperature fields for a few selected conditions is presented showing trends with the atomization quality of the liquid fuel. Fourier analysis revealed hydrodynamic instabilities in the shear layer and relatively weak thermoacoustic instabilities in the reaction zone.
|Original language||English (US)|
|Number of pages||12|
|Journal||Combustion and Flame|
|State||Published - Jan 22 2019|
Bibliographical noteKAUST Repository Item: Exported on 2022-06-07
Acknowledged KAUST grant number(s): 1975-01
Acknowledgements: Funding for this research program was provided by the U.S. Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences, Grant no. (DE-FG02-03ER15391) and by the King Abdullah University of Science and Technology, CCF subaward (No. 1975-01). The University of Sydney Combustion Group is funded by the Australian Research Council, Grant no. ARC-DP180104190.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.