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Characterization and Improvements of Filtered Rayleigh Scattering Diagnostics

Patton, Randy Alexander

Abstract Details

2013, Master of Science, Ohio State University, Mechanical Engineering.
Described in this thesis is an analysis of the fundamental characteristics and typical limitations associated with the filtered Rayleigh scattering (FRS) measurement technique. Particular emphasis will be placed on the applicability of the technique toward obtaining gas-phase information through Rayleigh scattering in flowfields traditionally dominated by unwanted light scattering. This includes measurements conducted simultaneously with particle imaging velocimetry or in multi-phase flows such as spray environments and sooting flames, where Mie/Tyndall scattering overwhelms gas-phase Rayleigh scattering or experiments in enclosed environments, with large amounts of residual laser scattering. A preliminary overview of the FRS technique will be presented, along with previous work utilizing the technique in a variety of applications including flow visualization in high speed and compressible flows, temperature, pressure and number density measurements in flames and turbulent flows, as well as simultaneous FRS and particle imaging velocimetry in flames. Experimental limitations arising in these experiments will be discussed. Essential to the filtered Rayleigh scattering technique is a thorough understanding of the spectral and thermodynamic properties of the atomic or molecular filter used to transmit portions of the gas-phase Rayleigh scattering signal to a detector while suppressing the interfering Mie/Tyndall/surface scattering. As such, a model of the transmission (or conversely, the absorption) characteristics of the molecular iodine cell utilized in this work will be presented. A parametric study of the effects of cell temperature and pressure will be described; with an emphasis on the impact of varying these parameters on the maximum scattering rejection attainable by the filter cell. Subsequently, experimental analysis of the suppression capabilities of our particular I2 cell will be presented. Of particular note are the fundamental differences in cell transmission observed when using a narrow linewidth continuous wavelength (cw) laser source, as compared to an injection-seeded, Q-switched, pulsed laser source. It was determined that the disparities in filter cell transmission arising from these two laser sources is fundamentally related to the spectral purity of the pulsed laser, a measure of the proportion of energy contained in a single spectral mode to the total energy of the illuminating laser source. Results obtained both experimentally and through the use of a simple model will be presented highlighting the functional dependence of the maximum absorption of the iodine filter cell on the spectral purity of the laser source. It also will be shown that there is an exponential relationship between the seed laser power (when using an injection seeded Q-switched, pulsed laser source) and the spectral purity of the laser output. The results of this study subsequently are related back to previously-observed limitations of the technique available within the literature. One potential methodology to increase laser spectral purity investigated in this thesis involves the insertion of a Fabry-Perot etalon into the external optical path of an injection-seeded Nd:YAG laser. A description of the transmission properties of an etalon (or similarly a Fabry Perot interferometer) will be given. Particular emphasis is placed on how the etalon is used to effectively increase the spectral purity of the transmitted laser pulse. Experimental results demonstrate improvements in the laser spectral purity greater than one order of magnitude when using the etalon. In terms of FRS measurements, scattering rejection was increased by more than two orders of magnitude when using the etalon. Finally, initial validation studies, where gas-phase information is extracted from droplet-laden (non-evaporating) turbulent jets of propane and air, is presented. Comparisons of suppression capabilities with and without the etalon in the optical path are shown, highlighting the gains resulting from implementation of the etalon. Finally a demonstration of the technique is presented within an evaporating spray jet of acetone, highlighting the potential of the FRS technique for extracting gas-phase information within turbulent spray flows.
Jeffrey Sutton, Dr. (Advisor)
Walter Lempert, Dr. (Committee Member)
131 p.

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Citations

  • Patton, R. A. (2013). Characterization and Improvements of Filtered Rayleigh Scattering Diagnostics [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1373465741

    APA Style (7th edition)

  • Patton, Randy. Characterization and Improvements of Filtered Rayleigh Scattering Diagnostics. 2013. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1373465741.

    MLA Style (8th edition)

  • Patton, Randy. "Characterization and Improvements of Filtered Rayleigh Scattering Diagnostics." Master's thesis, Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1373465741

    Chicago Manual of Style (17th edition)