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

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. The present paper provides a detailed examination of a series of piloted CH4/air jet flames with different inlet conditions and with a pilot flame that matches the composition and adiabatic flame temperature of CH4/air. It addresses trends in local extinction as well as differences in near-field flame structure. The evolution in the local mode of combustion is traced using instantaneous line-imaged realizations where the change in mixture fraction across a 1000 K interval is used as a conditioning variable. Doubly conditioned means of selected species mass fractions confirm that the flames with inhomogeneous inlet profiles undergo transition from a stratified-premixed mode of combustion to a diffusion-dominated mode of combustion within the first ten jet diameters. Calculations of strained laminar partially-premixed flames are used to gain insight on the effects of strain rate and fuel-side equivalence ratio on flame structure and the rate of heat release. These turbulent jet flames may serve as interesting test cases for models aimed at predicting the performance of practical burners that operate with mixed combustion modes. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.