Structural response of counterflow diffusion flames to strain rate variations

Abstract

The structural response of counterflowing methane/oxygen/nitrogen diffusion flames to aerodynamic straining was experimentally and computationally investigated. The temperature and major species concentration profiles were experimentally determined as functions of the applied strain rate by using spontaneous Raman scattering. The experimental situations were further computationally simulated with detailed reaction mechanisms and transport properties. The computed results were found to be in close quantitative agreement with the experimental data. Results demonstrate that, in contrast to counterflow premixed flames, a strained, counterflow diffusion flame has less flexibility to freely adjust its location in response to strain rate variations such that its structure in the direction normal to the flame surface is quite sensitive to variations in the strain rate. Specifically, the counterflow diffusion flame becomes thinner with increasing strain rate a, with its thickness varying inversely with √ a. This leads to increased amount of reactant leakage, progressive reduction in the flame temperature, and eventually extinction of the flame. Computational results further show that while the heat release rate of premixed flames is characterized by a single sharp maximum, for diffusion flames a secondary maximum described by a distinctively different reaction submechanism exists on the oxidizer side of the primary heat release zone. The nature and extent of the negative heat release rate on the fuel side of the flame, as well as its relation to the presence of a dent around the same location on the temperature profile as observed in previous experiments, were also discussed.