Local extinction and near-field structure in piloted turbulent CH4/air jet flames with inhomogeneous inlets

摘要

Line-imaged measurements of temperature and major species, based on well-established Raman/Rayleigh/CO-LIF techniques, are used to better understand the scalar structure of piloted CH4/air jet flames with inhomogeneous inlets. Recent studies using a variant of the Sydney piloted burner have demonstrated that the blowoff velocity of a partially-premixed jet flame of CNG or CH4 and air can be increased significantly by tailoring the mixture fraction profile at the burner exit. This is done by adding a small, retractable central tube within the main tube of the burner and separately supplying fuel and air for partial premixing. Both tubes are located within the pilot annulus. When the central tube is retracted far upstream of the burner exit, the flame has the same stability as that of the original burner with homogeneous fuel-air composition at the jet exit. However, when the inner tube supplies fuel and is retracted an optimal distance (10-13 times the main tube diameter) the blowoff velocity is increased by nearly 40%. Previous results indicated that combustion very close to the burner exit occurs in a stratified-premixed mode, augmenting the stabilizing effect of the pilot. This is followed by transition to a diffusion-dominated mode of burning within the first ten main tube diameters.