Mixture fraction imaging based on photodissociation spectroscopy and two photon laser induced fluorescence

The study of turbulent combustion calls for new diagnostics that can measure multidimensional mixture fraction under a wide range of flame conditions. A laser diagnostic technique based on photodissociation spectroscopy (PDS) is proposed to address this need. This thesis describes the concept of the PDS-based diagnostic, reports its experimental demonstration in a non-premixed jet flame, and assesses its performance and applicable range. The two-photon laser induced fluorescence (TPLIF) technique used in conjugate with the PDS is analyzed numerically in line and planar imaging configuration.;The new mixture fraction imaging technique is centered around the creative use of photodissociation (PD) for flow visualization. A carefully chosen PD precursor is seeded into the flow of interest to measure mixture fraction. The precursor is chosen such that (1) both the precursor itself and the products formed from the precursor (if it reacts) can be completely and rapidly photodissociated; thus one of the photofragments forms a conserved scalar and can be used to infer the mixture fraction, and (2) the target photofragment offers friendly spectroscopic properties (e.g., strong laser induced fluorescence signals and/or simple signal interpretation) so multidimensional imaging can be readily obtained. Molecular iodine (I2) was identified as a precursor satisfying both requirements and was seeded into a carbon monoxide (CO)/air jet flame for single-shot two-dimensional imaging of mixture fraction. This demonstration illustrates the potential of the PDS-based technique to overcome the limitations of existing techniques, and to provide multidimensional measurements of mixture fraction in a variety of reactive flows.;The thesis also analyzes the imaging applications of TPLIF, which is a promising technique in the planar imaging of mixture fraction. Models are developed based on rate equation approximations and Monte Carlo simulation, with a focus on the effect of amplified spontaneous emission (ASE) on TPLIF signal interpretation. Results obtained are expected to also enhance the accuracy and applicable range of TPLIF technique in other flow imaging applications, beyond the mixture fraction imaging considered in this research.