EXPERIMENTAL AND THEORETICAL INVESTIGATION OF LASER SATURATED HYDROXYL FLUORESCENCE IN FLAMES
Abstract
An experimental and theoretical analysis of laser saturated OH fluorescence is described in this thesis. The research objective is to develop a saturated fluorescence technique which can be used to measure accurate diatomic flame radical concentrations over wide ranges of flame pressure, composition and temperature. Saturated fluorescence is a powerful method of overcoming the classical limitation of linear fluorescence techniques, i.e., the inverse dependence of the linear fluorescence signal signal on the collisional quenching rate. The hydroxyl radical, an important species in the chemistry of nearly all flames, serves as a model molecule for the development of a quantitative saturated fluorescence method. The balanced cross-rate model is proposed to analyze the fluorescence data. Using the balanced cross-rate model to calculate concentrations, OH number densities calculated from saturated fluorescence agree to within 20% with number densities calculated from independent absorption measurements. In addition, the ratio of number densities calculated from saturated fluorescence and absorption is constant over an order of magnitude range of flame pressure, demonstrating the insensitivity of the saturated fluorescence signal to collisional transfer rates. A two line saturated fluorescence temperature measurement technique is also proposed. The flame temperatures measured by the technique exhibit low scatter and show good agreement with corrected thermocouple measurements. A state-to-state rotational transfer model for the A('2)(SIGMA)('+) (0,0) rotational manifold is developed by optimizing the agreement between time-resolved fluorescence spectra and synthetic spectra generated by a time-dependent numerical code. Absolute cross sections for OH rotational transfer due to collisions with water vapor and nitrogen are calculated from the optimized rotational transfer model. The effect of the multi-axial mode structure of the dye laser on saturation of OH resonances is investigated both theoretically, using a quantum mechanical density matrix analysis, and experimentally, by measuring the OH saturation parameter for two different transitions over a wide range of flame pressure. The saturation parameter remains nearly constant over the range of flame pressure studied, an effect which can be explained by considering the details of transition broadening processes and the laser axial mode structure.
Degree
Ph.D.
Subject Area
Mechanical engineering
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