Demonstration of a New Laser Diagnostic Based on Photodissociation Spectroscopy for Imaging Mixture Fraction in a Non-Premixed Jet Flame

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 paper 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. This new 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, the concentration of 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.