Premixed turbulent flame structures in moderate and intense isotropic turbulence

Several 2-D imaging techniques including planar laser induced fluorescence for OH (OH-PLIF) have been used to investigate premixed turbulent flame structures under moderate to intense isotropic turbulence. Unconditioned velocity statistics were measured by laser Doppler anemometry. The experiments used a low-swirl burner that produces high intensity near-isotropic turbulence. The goal is to gain better insights into the flame structures at high turbulence and to test and verify the concept of the "distributed reaction zones" regime. Four methane/air flames (φ=0.7) have been studied with 0.5< u ′<2.2 m/s. A linear correlation for the flame speed, S f , is found: S f / S L =2.12 ( u ′/ S L )+1. Sets of 200 OH-PLIF images obtained for each flame clearly show that flame wrinkling is a random process. The probability of the flame having very small wrinkles is relatively low. This strongly suggests that the penetration of small intense eddies into the flame sheet to generate a "distributed reaction zone" is statistically an extremely rare event. The OH-PLIF images were processed to determine statistical properties of the mean flame curvatures and flame lengths for comparison with turbulence intensity and turbulent length scales. The results show that the increase in turbulent kinetic energy generates larger mean curvatures of the flame fronts, and a linear increase in the flame surface area ratio estimated from the mean flame length measurement.