Velocity and Scalar Characteristics of Turbulent Premixed Flames Stabilized on Confined Axisymmetric Baffles

Abstract

Measurements of velocity, temperature, species concentration and wall pressure arc reported for turbulent premixed methane-air flames stabilized on axisymmetric confined baffles and for the corresponding isothermal flows. A laser-Doppler velocimeter was used to measure the density-weighted longitudinal velocity characteristics, thin bare-wire thermocouples to determine unweighted temperature characteristics and quartz probes transported gas samples to various instruments which measured near density-weighted concentrations of O₂, unburnt hydrocarbons, CO₂, and CO. The purpose of the experiments was to extend information of local flow properties of axisymmetric bluff-body stabilized premixed flames and to provide a basis for evaluating the performance of current calculation procedures in recirculating combusting flows. The results quantify the influence of forebody shape, area blockage and of equivalence ratio. The velocity results show that combustion increases the length of the recirculation zone, the maximum reverse-flow velocity and the variance of the fluctuations within the free shear layer surrounding the recirculation zone. The scalar field is controlled by the equivalence ratio of the mixture and the formation of pollutants is examined for both fuel-lean and -rich flames. The concentration of carbon monoxide was above equilibrium levels in fuel-lean flames, suggesting incomplete oxidation; in fuel-rich flames, it indicates the extent of the CO₂; dissociation in the kinetics of CO formation. The influence of the forebody shape on the scalar field was found to be small, except in the variance of the velocity fluctuations. The increase of the area blockage ratio from 25% to 50% did not affect the levels of the scalar quantities but increased the recirculation bubble length, the maximum dimensionless velocity and the maximum variance of the velocity fluctuations.