The burning rate in turbulent bunsen flames

Abstract

Measurements of burning rate along the centerline of high turbulent Reynolds number (30–120) and high Damköhler number (20–50) methane/air turbulent Bunsen flames have been performed. The burning rate was determined in terms of a “consumption” speed obtained from the axial and radial reactant mass fluxes along the burner axis. The consumption speed, the difference between the mass flow rates of reactants into and out of a control volume defined around the burner axis, is the net rate at which mass is converted from reactants to products along the centerline. This burning rate is compared with the cold boundary velocity normal to the flame zone, the “displacement” speed. The displacement speed in this flame configuration is simply the axial velocity component at the cold boundary of the flame zone. Experimental conditions for a range of turbulence ( u ′=0.18–0.4 m/s) and equivalence ratios (=0.75–0.95) were investigated. Conditioned velocities were measured by two-component laser Doppler velocimetry and the progress variable, < c >, by Mie scattering from small oil droplets seeded in the reactant stream that disappear at the flame front. The heat release in the flame induces flow divergence in the reactants within the flame zone, which must be accounted for in measuring the total burning rate. Two factors determine the total amount of outflow in these flames: the induced strain field and the flame zone thickness that is determined by the relative turbulence level, u′/S L . Maximum reactant flow divergence rates from 85–150 s −1 were measured, and the burning rates were found to be ∼40% of those estimated from the displacement speed. The results of this paper indicate that scaling laws for turbulent burning rates based simply on local turbulence and flame conditions or on cold boundary velocities will not provide good estimates in many practical systems.