The burning rate of premixed flames in moderate and intense turbulence

Burning rate measurements have been performed in lean (phi = 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-veloc ity measurements and scalar measurements of the flame-surface density, Sigm a. The two sets of measurements, of conditional mass fluxes by Laser Dopple r Velocimetry (LDV) and flame-surface density, were determined from the ins tantaneous flame-front positions obtained from OH Planar Laser Induced Fluo rescence (OH-PLIF) images. It is important when making these comparisons to distinguish two measures of the turbulent burning velocity: the displaceme nt, S-D, and consumption speed, S-C. Measurement of the consumption speed b y the conditional flux method using laser Doppler velocimetry can account f or the multidimensional affects of burner/flow geometry. In this burner geo metry S-C was found to be 30% of the cold-boundary flow velocity, S-D. Both the turbulent displacement and consumption velocities were found to have l inear dependencies on u'/S-L for values up to u'/S-L similar to7.3. The con sumption speed is the more fundamental measure of the burning rate and is d irectly comparable to that derived from integration of flame-surface-densit y measurements. The similarity of the scalar and mass flux burning rates in dicates that measurements of Sigma. which are usually much simpler to perfo rm than the flux measurements. can be used to test theoretical models and i nvestigate the affect of upstream turbulence parameters on global flame-bur ning characteristics. Furthermore, comparison of the flux and scalar measur ements has shown that even at flow/flame conditions above the Klimov-Willia ms 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 m ean, Preliminary comparisons with models have been performed. (C) 2001 by T he Combustion Institute.