Development and Application of High-Speed Raman/Rayleigh Scattering in Turbulent Nonpremixed Flames

by Kathryn Nicole Hoffmeister

Institution: The Ohio State University
Department: Mechanical Engineering
Degree: PhD
Year: 2015
Keywords: Mechanical Engineering; Aerospace Engineering; Chemistry; Experiments; Fluid Dynamics; Laser Diagnostics; High-Speed Laser Diagnostics; Combustion; Turbulent Combustion; Raman Scattering; Rayleigh Scattering; Turbulence-Chemistry Interaction; H3 Flame
Record ID: 2061516
Full text PDF: http://rave.ohiolink.edu/etdc/view?acc_num=osu1420325353


In this dissertation, a new, high-speed, combined 1D Raman-Rayleigh scattering imaging approach was developed for quantitative temporally-correlated (10-kHz) measurements of temperature, major combustion species (O<sub>2</sub>, N<sub>2</sub>, H<sub>2</sub>O, and H<sub>2</sub>), and mixture fraction in the turbulent DLR H3 nonpremixed jet flame. The new high-speed measurements presented here were facilitated through the development of a custom high-speed imaging spectrometer, implementation of a robust data reduction methodology, and the use of the High-Energy Pulse Burst Laser System (HEPBLS) at Ohio State, which produces ultra-high pulse energies at multi-kHz repetition rates. Detailed measurements in near-adiabatic, laminar calibration flames were used to assess the accuracy and precision of the kHz-rate measurements using the high-speed Raman/Rayleigh scattering imaging system. In general, good agreement was found as compared to adiabatic flame calculations over a broad range of temperature and equivalence ratios. Current 10-kHz measurements and derived statistics within the turbulent H3 flames were compared to previously-measured, low-repetition-rate scalar data available through the Turbulent Nonpremixed Flame (TNF) workshop database. In general, good agreement between the mean values and RMS fluctuations of the temperature and major species were found between the current and previous data, indicating sufficient accuracy in single-shot measurements in turbulent environments. Following technique development, a series of 10-kHz measurements were used to visualize the highly intermittent dynamics of the scalar fields. In addition to flow visualization, time-dependent measurements were used to deduce the temporal autocorrelation function and the associated integral time-scales of the major species, mixture fraction, and temperature in the DLR H3 flames. This research presents the first measurements of the integral timescales of the major combustion species and mixture fraction in turbulent flames as well as the first reporting of integral timescales of multiple scalars simultaneously. While the integral timescales of all scalars (O<sub>2</sub>, H<sub>2</sub>O, and H<sub>2</sub>, ¿, and T) generally increase with both axial and radial position, the individual integral timescales are significantly different, displaying a factor of three spread across all of the measured scalars. The integral timescales for temperature and water are highly correlated to one another at all spatial locations, while the integral timescale for mixture fraction closely tracks that of hydrogen between the jet centerline and the stoichiometric contour and tracks that of oxygen between the stoichiometric contour and the co-flowing oxidizer stream. Results indicate that in regions of high chemical activity and heat release, the use of single characteristic (integral) timescale to describe the large-scale behavior is not appropriate; that is, each scalar has its own unique spatially-dependent integral timescale. For spatial positions beyond the…