Spatial structure and NO formation of a laminar methane–nitrogen jet in hot coflow under MILD conditions: A spontaneous Raman and LIF study

We report the spatial structure of the reaction zone from a diluted, preheated fuel jet in hot coflowing oxidizer in laminar flow, using an axisymmetric laminar-jet-in-hot-coflow (LJHC) burner. In the experiments, a preheated CH4/N2 (20/80) mixture at ∼1100 K flows into the 1500 K products of a flat, premixed burner-stabilized flame, having 3 mol% residual oxygen. Measurements of major species and temperature profiles using spontaneous Raman scattering show a temperature rise in the reaction zone of less than 300 K, indicating that the reaction zone was in the MILD combustion regime. Analysis of the data shows that the spatial development of the combustion has the characteristics of an “ordinary” coflow diffusion flame, but with changes in temperature and species fractions due to combustion that are nearly an order of magnitude smaller than when using undiluted reactants. Measurements of profiles of NO mole fraction using Laser Induced Fluorescence (LIF) show that the contribution of the MILD combustion to the NO fraction is less than 2 ppm, in agreement with other studies on MILD combustion in turbulent flows. Analysis of the NO fraction as a function of mixture fraction indicates that the NO field is dominated by mixing of the NO formed in the coflow with the reaction products of the diluted fuel, with negligible NO formation from the fuel. Replacement of the nitrogen in the hot coflow with argon resulted in NO fractions below the detection limit of the measurement system.