The burning rate of premixed flames in moderate and intense turbulence
Burning rate measurements have been performed in lean (φ = 0.7) premixed methane/air flames stabilized on a low-swirl burner in moderate and intense turbulence, (Ret = 460–1400). The purpose of this paper is to reconcile the burning rate determined by two widely used experimental methods: flow-velocity measurements and scalar measurements of the flame-surface density, Σ. The two sets of measurements, of conditional mass fluxes by Laser Doppler Velocimetry (LDV) and flame-surface density, were determined from the instantaneous flame-front positions obtained from OH Planar Laser Induced Fluorescence (OH-PLIF) images. It is important when making these comparisons to distinguish two measures of the turbulent burning velocity: the displacement, SD, and consumption speed, SC. Measurement of the consumption speed by the conditional flux method using laser Doppler velocimetry can account for the multidimensional affects of burner/flow geometry. In this burner geometry SC was found to be 30% of the cold-boundary flow velocity, SD. Both the turbulent displacement and consumption velocities were found to have linear dependencies on u′/SL for values up to u′/SL ∼7.3. The consumption speed is the more fundamental measure of the burning rate and is directly comparable to that derived from integration of flame-surface-density measurements. The similarity of the scalar and mass flux burning rates indicates that measurements of Σ, which are usually much simpler to perform than the flux measurements, can be used to test theoretical models and investigate the affect of upstream turbulence parameters on global flame-burning characteristics. Furthermore, comparison of the flux and scalar measurements has shown that even at flow/flame conditions above the Klimov-Williams criterion the affect of the turbulence on the local laminar burning rate, as expected from the Markstein number for this mixture, is small in the mean. Preliminary comparisons with models have been performed.