Measurements of scalar dissipation in turbulent hydrogen diffusion flames and some implications on combustion modeling

One-dimensional simultaneous measurements of the species mass fractions of O-2, H-2, H2O, N-2 and OH radical as well as flame temperature have been carried out in hydrogen jet diffusion flames diluted with Ar using the line-Raman/Rayleigh/LIPF-OH technique. Statistical information about the mixture fraction Z and scalar dissipation rate chi can be obtained for evaluation of current finite-chemistry combustion models for the turbulent nonpremixed flames. At an upstream position of x/d = 8, the flamelet approach is found to be qualitatively valid, in agreement with a larger Favre mixture fraction fluctuation than the reaction zone thickness in the Z-space. Transient effect, however, can be important in view of H-2 and O-2 superequilibrium. Preferential diffusion effects are negligible due to a relatively large Peclet number. Joint correlations between a reactive scalar T with chi and the conserved scalar Z with chi are found to be different from those reported in nonreacting flows. The observed distinct Features can be explained based on the flamelet concept so that statistical independence is argued to be a good approximation. There is evidence showing that large-scale turbulent motion dominates scalar transport in the connected reaction zone regime. Thus, flamelet approach is also favored. Some modeling assumptions used in the flamelet model and the CMC method are discussed. Model refinements based on the present experimental data are suggested as well.