Quantitative Gas-Phase Scalar Mixing Measurements in Turbulent Spray Flows

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1067625SuttonFor the majority of practical energy-conversion systems, a liquid fuel spray is injected into an oxidizing environment; where the liquid droplets must disperse, evaporate, and the fuel vapor must molecularly mix with the oxidizer prior to chemical reaction. Although complicated by several physical and chemical processes, liquid-fueled combustors are ultimately limited by droplet vaporization and the subsequent gas-phase mixing process. While scalar mixing has been studied in-depth for single-phase turbulent flows, the current understanding of gas-phase mixing processes within turbulent sprays is limited due to the scarcity of experimental evaluation. This primarily stems from the extreme difficulty of accurately measuring vapor-phase concentrations in the presence of droplets. The objective of this research is the development, validation, and application of a new laser-based imaging diagnostic to experimentally investigate vaporization and gas-phase mixing processes in turbulent spray flows. Intellectual Merit: The current research program will involve the development of a variation of the Filtered Rayleigh Scattering (FRS) technique to accurately image fuel-vapor concentration fields without interference from the surrounding liquid-phase droplets. This research program is underpinned by a systematic assessment of the accuracy, sensitivity, and limitations of the proposed FRS technique as applied in turbulent spray flows. The new FRS approach offers the potential for two distinct advantages compared to previous techniques for detecting fuel vapor in the presence of droplets: (1) due to the nature of FRS, the recorded signal can be unambiguously due to vapor because the light scattering from the unwanted liquid-phase droplets can be spectrally-filtered using the combination of a single-frequency laser source and an optically-thick atomic or molecular filter. The gas-phase scattering of interest is spectrally-broadened due to thermal Doppler effects and a portion of this broadened gas-phase information falls outside of the molecular filter's bandwidth and is successfully transmitted to a detector. (2) The FRS technique does not require a particular fluorescent fuel or tracer, rather it takes advantage that the majority of fuels have large Rayleigh scattering cross sections and the recorded signal is dependent on the local composition of the fuel vapor and air mixture. By not using fluorescent fuel tracers, complications due to distillation effects are avoided. Once validated, the new tool will be applied to understand the coupling between droplet dispersion, vapor production, and gas-phase (fuel vapor and air) mixing in turbulent evaporating sprays. The proposed diagnostic approach will be transformative as a new, fundamental level of understanding of the vaporization and mixing processes found within multi-phase systems will result from the development of this diagnostic, allowing access to previously unavailable gas-phase processes embedded within the liquid spray. It is also anticipated that new fundamental measurements in turbulent sprays will prove invaluable to the modeling communityBroader Impacts: Proper fuel-air mixture preparation is critical for meeting performance objectives of modern energy-conversion systems including controlled ignition, flame stability, efficiency, and low pollutant emissions. The proposed FRS technique offers the opportunity to acquire a new, fundamental level of understanding of the complex vaporization and turbulent mixing processes that is needed as a building block to understand the highly turbulent, multiphase conditions found in realistic engines. An improved fundamental understanding offers the opportunity for more efficient and controlled combustion within practical platforms. In terms of research-related education, the Ph.D. research of a graduate student will be supported by this project. This research provides unique opportunities for direct teaching and training and allows the graduate student to work within (and significantly contribute to) a wide range of advanced topics including fluid dynamics and optical diagnostics. This project also will involve the participation of undergraduate student researchers, providing an opportunity to expose the brightest undergraduates to graduate-level research and thus helping to extend the pool of qualified graduates within the scientific and technological community. Additional contributions to the national and international academic and research infrastructure will be made through the dissemination of results into the open literature, and through presentations at conferences. Additional contributions to the educational infrastructure will be made by integrating the research efforts into teaching at the graduate level.