Abstract
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 (I<sub>2</sub>) 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.
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