Measurements of scalar dissipation in a turbulent piloted methane/air jet flame
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This paper describes the first results from a new experimental facility that combines line-imaging measurements of Raman scattering, Rayleigh scattering, and laser-induced CO fluorescence. Simultaneous single-shot line measurements of major species, temperature, mixture fraction, and the radial component of scalar dissipation are obtained in partially premixed methane/air jet flames (25% CH4, 75% air, by volume). The experimental setup and methods of data analysis are described. Results from a laminar jet flame and a piloted turbulent flame (Sandia flame D) are presented. The laminar and turbulent flames both show a local minimum in the average scalar dissipation near the stoichiometric mixture fraction. In the turbulent flame, the radial component of the instantaneous scalar dissipation near the stoichiometric condition displays a log-normal distribution at high values and an exponential distribution at low values. This is attributed to variation in the orientation of the three-dimensional mixture fraction gradient relative to the one-dimensional measurement. The effect of scalar dissipation on flame structure is examined, based on average species mass fractions doubly conditioned upon mixture fraction and scalar dissipation. The length scale of fluctuations in mixture fraction in the turbulent reaction zone is derived from the measured single-shot radial profiles at each streamwise location. This macro length scale is well resolved by the present methods, shows correlation over radial distances of a few millimeters, and grows with streamwise distance as the jet flame spreads. The length scale for fluctuations in scalar dissipation is less than 1 mm at each of the measured streamwise locations in the turbulent flame.
