Three-scalar imaging in turbulent non-premixed flames of methane

Raman scattering from nitrogen in laminar (Re = 1600) and turbulent (Re = 15,000) non-premixed flames of diluted methane has been investigated as a means of complementing the two-scalar mixture-fraction computation based on measurements of fuel mass fraction and temperature (xi(FT)). Using a diluent consisting of argon and oxygen with an overall volumetric dilution ratio of 3/1 (diluent/fuel) allows sufficient variation in the measured nitrogen concentration for its use as a passive scalar. The present experimental setup requires only a single laser (532 nm) in a high-power intracavity configuration. Mixture-fraction profiles calculated using independent fuel-temperature and nitrogen-temperature two-scalar approaches show excellent agreement in the laminar flame. For the turbulent flame, a 100 single-shot average of mixture fraction shows reasonable agreement between the two approaches. Discrepancies are most evident in single-shot images in regions of large mixture fraction (xi > xi(s)) where the nitrogen Raman signal is noise dominated. The location of the stoichiometric contour is consistently determined by both two-scalar approaches. A simple modification of the functional dependence for parameters appearing in the fuel-temperature mixture-fraction formulation is shown to correct deficiencies in the approach predicted by laminar flame calculations, most notably for values of xi(FT) around stoichiometric and into lean regimes.