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

智力优势：这一探索性项目将研究使用多通道相位共轭逆反射来增强湍流火焰中的瑞利和拉曼散射等基于激光的低信号过程。 目前，自发拉曼和瑞利散射的组合方法是用于测量温度和所有主要物质浓度（例如，CH4，O2，CO，CO2，H2O，H2）。 所有主要物种的测量（通过组合的拉曼/瑞利方法）产生系统的热化学状态的直接信息，并允许扣除的混合物分数。 混合分数可能是非预混和部分预混燃烧中最重要的标量，因为它表征了分子混合的局部状态，并且是大量湍流燃烧模型中的关键变量。 然而，自发的拉曼散射是非常弱的，通常需要超高的激光脉冲能量，以实现足够的信噪比？单发的测量湍流火焰。 这一因素导致了一些值得注意的测量限制，包括大多数实验人员无法获得必要的高能激光系统，从而将测量工作转移到世界上的少数实验室，以及高脉冲能量在现实的受限系统中可能存在问题，其中窗口和设施损坏成为一个问题。 以这种方式，拉曼/瑞利测量在很大程度上限于无约束的实验室规模的实验。 最后，已经注意到，高脉冲能量同时产生和激发物质，使得它们发射的荧光干扰非常弱的拉曼信号。 在目前的研究计划中，受激布里渊散射相位共轭镜（SBS-PCM）的使用在多程安排将进行调查。 预计SBS-PCM的使用将增加信号增益和信噪比，而不会降低测量空间分辨率;这是以前的多通道布置不可能实现的方面。 此外，它将显示，？单发的拉曼/瑞利测量可以使用SBS-PCM多通道方法以减少激光能量来进行，以避免与超高激光脉冲能量相关的许多复杂性。更广泛的影响：大多数发电和推进平台涉及湍流燃烧过程，这些过程由于复杂的湍流流场和化学反应之间的耦合而在许多长度和时间尺度上变得复杂。 提高这种系统效率的第一步将来自于通过高保真测量来理解湍流燃烧环境中的热和质量传输过程。 目前的研究项目有可能对湍流燃烧诊断产生重大影响，从而产生一种测量湍流火焰中温度和主要物种浓度的替代方法。 具体而言，低脉冲能量和SBS-PCM在多通安排的组合将允许扩展拉曼/瑞利散射诊断更多的研究人员和世界各地的实验室，并提供了一种手段，使这些测量在？禁闭？具有与柴油机、火花点火式发动机、火箭发动机和燃气涡轮机发动机相关的真实高温高压热力学条件的试验装置。 其他技术影响包括改进多标量测量，这将有助于评估和开发燃烧模型。 在与研究相关的教育方面，该项目将支持本科生的荣誉研究，并部分支持博士后研究人员，他们将获得参与本科生直接教学和指导的宝贵经验。 我们的技术未来取决于开发新的方法和能够解决复杂问题的新的劳动力。 该项目将允许年轻的研究人员在他们的职业生涯的各个阶段（本科到博士后），以显着促进广泛的先进课题，包括流体动力学，燃烧和能量转换，光学诊断。

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.