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.
The majority of power generation and propulsion systems such as aircraft and automobile engines involve the combustion of fossil fuels under turbulent conditions. In order to improve performance and efficiency new measurement tools are necessary to improve the general understanding of the governing chemical and physics involved. This research was an exploratory project to assess the potential of new optical strategies to enhance low-signal, laser-based measurement approaches such as Rayleigh and Raman scattering. To understand the highly coupled physical and chemical processes, measurements of combustion species and temperature are needed. Not only do they give insight into "what is happening", but these measurements provide extremely useful information for numerical model development; models that are intended to be predictive and used for design in the future. Currently, the combined approach of spontaneous Raman and Rayleigh scattering is the only 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, which characterizes the local state of molecular mixing. When combined with the temperature measurement (local state of thermal mixing), the effects of multi-scale turbulence on the coupled thermal and mass transport processes are directly examined. While combined Raman/Rayleigh scattering measurements are commonplace in combustion research, it should be noted that there are several important aspects of these measurements that should be considered: (1) nearly all of the measurements are provided by a few laboratories in the world, with only one being in the U.S and (2) the measurements are predominately limited to simple configurations and specific fuels that are "friendly" to the measurements. Both of these factors are related to the fact that spontaneous Raman scattering is inherently very weak and ultra-high laser pulse energies are required to make the measurements with sufficient single-shot signal-to-noise ratios in turbulent flames. This aspect is what makes spontaneous Raman/Rayleigh scattering diagnostics particularly challenging in turbulent combustion environments. The objective of the current project was to explore the potential for utilizing a stimulated Billouin scattering phase-conjugate mirror (SBS-PCM) to substantially increase the collected signal (through multiple passes) without degradation of the measurement spatial resolution in turbulent combustion environments. After construction of the SBS-PCM and its implementation in turbulent combustion environments results indicated that the SBS-PCM preserved the spatial resolution; that is a double-pass measurement using the SBS-PCM resulted in a measurement probe volume very similar to that of the single-pass measurement. The same was not true if the SBS-PCM was replaced by a conventional mirror. The measurement volume can be directly determined by making spontaneous Rayleigh scattering measurements in the flows. For example, a 1D, "line" image from Rayleigh scattering gives a direct visualization of the measurement volume. For a single-pass 1D Rayleigh scattering measurement, the average probe diameter was 180 microns. When using a conventional mirror for a double-pass measurement, subject to refractive index gradients, the average probe diameter was 280 microns in the turbulent flames, indicating a ~ 60% increase in the measurement volume due to beam steering effects. However, when the SBS-PCM was implemented, the double-pass probe diameter was measured to be ~ 190 microns, which shows that when using the SBS-PCM, the "second" pass is subject to minimal refractive index disturbances, "retro-reflects" through the original beam path, and preserves the original measurement spatial resolution, while simultaneously increasing the measurement signal because of the multiple passes. The results from this NSF-funded EAGER project demonstrate that a phase-conjugate mirror (PCM) based on stimulated Brillouin scattering (SBS) can serve as a very effective retro-reflector in turbulent combustion environments. Typical literature values indicate that spatial resolution, as defined by the effective beam thickness, increase by about a factor of two when employing conventional mirrors to re-direct laser beams back into the probe volume to increase collected signal. This increase in effective beam thickness is due to the fact that the reflected beam takes a different path back through the measurement volume due to refractive index gradients. However, when using an SBS-PCM, no noticeable increases in beam width for the double-pass measurements as compared to single-pass measurements were noted. This confirms that the SBS-PCM can be used to increase the collected signal without degradation to the measurement spatial resolution in scattering measurements. This approach warrants consideration for future implentation into Raman and Rayleigh scattering measurement systems.
