Mixture Fraction Measurement in the Flowfield from a Coaxial Injector

Abstract

Knowledge of the spatial distribution of mixture fraction is very important for assuring safe and reliable operation of almost all combustion systems. A diagnostic technique, based on the principles of absorption spectroscopy, is being developed to provide insightful information on the variation of local mixture fractions inside the unconfined flowfield of a shear coaxial injector. Single-phase non-reacting gaseous-flow conditions are considered. Methane is injected as a trace gas into one of the two streams upstream of the injector. The flowfield is illuminated at the location of measurement by a collimated beam of infrared laser radiation at the 1.66-µm peak wavelength of the first vibrational absorption band of methane in the IR region. Based on the measured transmitted (i.e. non-absorbed) radiation, the line-of-sight value of local methane concentration is determined according to the Beer-Lambert law of gas absorbitivity. Abel inversion is performed to deconvolute the line-ofsight values to a 2-D distribution within a centerline plane of the flowfield, under the assumption of axisymmetric flow. The 2-D profile of methane concentration is used to calculate the mixture fraction distributions of central and annular gases of the injector. A sensitivity of 50 ppm ·m (concentration-pathlength product) has been achieved with 95% accuracy over a signal-averaging time of two seconds. The system has been tested at different velocity and density ratios of the injector jets. It was found that a velocity or density ratio of unity results in poorest mixing, i.e., values of velocity and density ratios far from unity yield in enhanced mixing. The distribution of mixture fraction is more sensitive to changes in velocity ratio than to changes in density ratio. The results obtained here on mixing under single-phase non-reacting conditions prove the system feasibility in providing a powerful diagnostic technique for mixture fraction measurements.