Simultaneous laser Raman-Rayleigh-Lif measurements and numerical modeling results of a lifted turbulent H2/N2 jet flame in a vitiated coflow

An experimental and numerical investigation is presented of a lifted turbulent H-2/N-2 jet flame in a coflow of hot, vitiated gases. The vitiated coflow burner emulates the coupling of turbulent mixing and chemical kinetics exemplary of the reacting flow in the recirculation region of advanced combustors. It also simplifies numerical investigation of this coupled problem by removing the complexity of recirculating How. Scalar measurements are reported for a lifted turbulent jet flame of H-2/N-2 (Re = 23,600, H/d = 10) in a coflow of hot combustion products from a lean H-2/Air flame (phi = 0.25, T = 1045 K). The combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence is used to obtain simultaneous measurements of temperature and concentrations of the major species, OH, and NO. The data attest to the success of the experimental design in providing a uniform vitiated coflow throughout the entire test region. Two combustion models (joint scalar probability density function and eddy dissipation concept) are used in conjunction with various turbulence models to predict the liftoff height (H-PDF/d = 7, H-EDC/d = 8.5). Kalghatgi's classic phenomenological theory, which is based on scaling arguments, yields a reasonably accurate prediction (H-K/d = 11.4) of the liftoff height for the present flame. The vitiated coflow admits the possibility of autoignition of mixed fluid, and the success of the present parabolic implementation of the PDF model in predicting a stable lifted flame is attributable to such ignition. The measurements indicate a thickened turbulent reaction zone at the flame base. Experimental results and numerical investigations support the plausibility of turbulent premixed flame propagation by small-scale (on the order of the flame thickness) recirculation and mixing of hot products into reactants and subsequent rapid ignition of the mixture.