Experiments on the scalar structure of turbulent CO/H2N2 jet flames

Scalar and velocity measurements are reported for two turbulent jet flames of CO/H-2/N-2 (40/30/30 volume percent) having the same jet Reynolds number of 16,700 but different nozzle diameters (4.58 mm and 7.72 mm). Simultaneo us measurements of temperature, the major species, OH, and NO are obtained using the combination of Rayleigh scattering, Raman scattering, and laser-i nduced fluorescence. Three-component laser-Doppler velocimetry measurements on the same flames were performed at ETH Zurich and are reported separatel y. This paper focuses on the scalar results but includes some limited veloc ity data. Axial profiles of mixture fraction, major species mole fractions, and velocity in these two flames are in close agreement when streamwise di stance is scaled by nozzle diameter. However, OH mole fractions are lower a nd NO mole fractions are higher near the stoichiometric flame length in the larger flame due to the lower scalar dissipation rates and longer residenc e times. Turbulent flame measurements are compared with steady strained lam inar flame calculations. Laminar calculations show remarkably close agreeme nt with measured conditional means of the major species when all diffusivit ies are set equal to the thermal diffusivity. In contrast, laminar flame ca lculations that include the normal Chemkin treatment of molecular transport are clearly inconsistent with the measurements. These results suggest that turbulent stirring has a greater influence than molecular diffusion in det ermining major species concentrations at the flow conditions and locations considered in the present experiments, which begin at an axial distance of 20 nozzle diameters. Analysis of the conditional statistics of the differen tial diffusion parameter supports this conclusion, though some evidence of differential diffusion is observed. With regard to validation of turbulent combustion models, this data set provides a target that retains the geometr ic simplicity of the unpiloted jet flame in coflow, while including a chemi cal kinetic system of intermediate complexity between hydrogen flames and t he simplest hydrocarbon flames. Aspects of the measurements, including Favr e-averaged profiles, conditional statistics, mixture fraction pdf's, and de partures from partial equilibrium, are presented and discussed in terms or their relevance to the testing of turbulent combustion submodels. The compl ete data are available on the World Wide Web for use in model validation st udies. (C) 2000 by The Combustion Institute.