Raman/LIPF data of temperature and species concentrations in swirling hydrogen jet diffusion flames: Conditional analysis and comparison to laminar flamelets

Simultaneous point measurements of temperature, mixture fraction, major species (H2, H2O, O2, N2) concentrations from KrF laser-induced spontaneous Raman scattering and minor species (OH) concentrations from KrF laser-induced predissociative fluorescence (LIPF) in unswirled (Sg = 0), low swirl (Sg = 0.12), and high swirl (Sg = 0.5) lifted turbulent hydrogen jet diffusion flames into still air are reprocessed to obtain profiles of the Favre-averaged scalars and conditional moments. Large discrepancies between the Favre-averaged and ensemble-averaged temperature, H2O, and OH mole fractions are found at the lifted flame region, due to density weighting of fairly large samples of unreacted mixtures. Conditional statistics are used to reveal the reaction zone structure in mixture fraction coordinates. The cross-stream dependence of conditional means of temperature and species concentration is found to be significant in the lifted flame region of the swirled flames and decreases to negligible levels with increasing streamwise position. Comparison of the measured conditional mean variation of OH vs. H2O with a series of stretched laminar partially premixed flame and diffusion flame calculations reveals that for the unswirled flame, the differential molecular diffusion and radial dependence of conditional means are minor at x/D = 6.4 for stretch rates from a = 14,000 to 400 s-1. For the low and high swirling flames, however, the measured OH vs. H2O conditional means at the lifted flame region are not consistent with stretched laminar flame calculations. The level of partial premixing and the stretch rate decrease with increasing downstream locations. The estimated stretch rate at x/D = 53.5 is about 50-10 s-1, while at x/D = 107 the stretch rate is about 10 s-1 with some measurements at adiabatic equilibrium.