Instantaneous and Mean Compositional Structure of Bluff-Body Stabilized Nonpremixed Flames

Abstract

Turbulent nonpremixed flames stabilized on an axisymmetric bluff-body burner are studied with fuels ranging from simple H 2/CO to complex H 2/CH 4 and gaseous methanol. The fuel-jet velocity is varied to investigate the Damköhler number effects on gas emissions, localized extinction (LE) in the neck zone, and the structure of the recirculation zone dependency on the flow field. Simultaneous, single-point measurements of temperature, major species, OH, and NO are made using the Raman/Rayleigh/Laser induced fluorescence (LIF) technique. The data are collected at different axial and radial locations along the full length of most flames and are presented in the form of ensemble means, root-mean-square (rms) fluctuations, scatter plots, and probability density functions (PDF). It is found that up to three mixing layers may exist in the recirculation zone, one on the air side of the outer vortex, one between the inner and the outer vortices, and one between the fuel jet and the inner vortex. With increasing jet momentum flux, the average mixture in the outer vortex loses its strength and the stoichiometric contour shifts closer to the fuel jet. The decay rate of the mixture fraction on the centerline exhibits similar trends to the ordinary jet flame downstream of the recirculation zone whereas different trends are found inside the recirculation zone. The laminar flame computations with constant mass diffusivities and Lewis number (Le) = 1 are found to be better guides for the measured temperature and stable species mass fraction in the turbulent flames. The measured peak mass fractions of CO and H 2 are similar to those reported earlier for pilot-stabilized flames of similar fuels. Compared with laminar flame compositions with equal diffusivities and Le = 1.0, measured CO may be in superflamelet concentration. Hydroxyl radical and H 2 are found not to be in superflamelet levels contrary to earlier findings in piloted flames. The start of LE and the bimodality of the conditional PDF are consistent with those reported earlier for piloted flames of similar fuels.