EAGER: Enhancements in Raman/Rayleigh Scattering Imaging in Turbulent Flames Using Multi-Pass, Optical Phase-Conjugated Scattering

Ohio State University, The, Columbus OH

Investigators

Abstract

Intellectual Merit: This exploratory project will investigate the use of multi-pass, phase-conjugated retro-reflection for enhancing low-signal, laser-based processes such as Rayleigh and Raman scattering in turbulent flames. Currently, the combined approach of spontaneous Raman and Rayleigh scattering is the most accurate method for measuring instantaneous, spatially-resolved distributions of the temperature and all major species concentrations (e.g., CH4, O2, CO, CO2, H2O, H2) simultaneously in turbulent combustion systems. The measurement of all major species (through the combined Raman/Rayleigh approach) yields direct information of the thermo-chemical state of the system and allows the deduction of the mixture fraction. The mixture fraction is perhaps the most important scalar in non-premixed and partially-premixed combustion as it characterizes the local state of molecular mixing as well as being a critical variable in a large number of turbulent combustion models. However, spontaneous Raman scattering is very weak and typically requires ultra-high laser pulse energies to achieve sufficient signal-to-noise ratios for ?single-shot? measurements in turbulent flames. This factor leads to some notable measurement limitations including the fact that the majority of experimentalists do not have access to the requisite high-energy laser systems, thus relegating the measurements to a few laboratories in the world and the fact that the high pulse energies can be problematic in realistic, confined systems where window and facility damage become a concern. In this manner, Raman/Rayleigh measurements are largely limited to unconfined, laboratory-scale experiments. Finally, it has been noted that the high pulse energies simultaneous generate and excite species such that their emitted fluorescence interferences with the very weak Raman signal. In the current research program, the use of stimulated Brillouin scattering-phase conjugate mirrors (SBS-PCMs) in a multi-pass arrangement will be investigated. It is expected that the use of SBS-PCMs will increase the signal gain and signal-to-noise ratios without degrading measurement spatial resolution; an aspect that has not been possible with previous multi-pass arrangements. In addition, it will be shown that ?single-shot? Raman/Rayleigh measurements can be performed with reduced laser energies using the SBS-PCM multi-pass approach to avoid many of the complications associated with ultra-high laser pulse energies. Broader Impacts: The majority of power generation and propulsion platforms involve turbulent combustion processes that are complicated by the coupling between complex turbulent flow field and chemical reactions at numerous length and time scales. The first step in improving the efficiency of such systems will come from understanding the thermal and mass transport processes in turbulent combustion environments with high-fidelity measurements. The current research project has the potential for significant impacts on turbulent combustion diagnostics, yielding an alternative methodology for measuring temperature and major species concentrations in turbulent flames. Specifically, the combination of lower pulse energies and SBS-PCMs in a multi-pass arrangement will allow the extension of Raman/Rayleigh scattering diagnostics to many more researchers and laboratories around the world and provide a means for making these measurements in ?confined? test setups with realistic, high-temperature, high-pressure thermodynamic conditions that are relevant to diesel, spark-ignited, rocket, and gas turbine engines. Additional technical impacts include improved multi-scalar measurements which will aid in the assessment and development of combustion models. In terms of research-related education, the project will support the honors research of an undergraduate student as well as partially support a post-doctoral researcher, who will be given the valuable experience of participating in the direct teaching and instruction of the undergraduate student. Our technological future depends on developing both new methodologies and a new workforce capable of tackling complicated problems. This project will allow young researchers at various stages of their careers (undergraduate to post-doc) to significantly contribute to a wide range of advanced topics including fluid dynamics, combustion and energy conversion, and optical diagnostics.