Characteristics of Freely Propagating Premixed Flame Kernels in Supersonic Turbulent Channel Flows

A new supersonic facility was developed to study the effects of turbulence intensity and mean flow compressibility on freely propagating flame kernels in a M = 1.5 channel flow. Two devices, a passive grid of holes and an active grid of rotating vanes, were used to generate turbulence. The 5 × 5 cm test section was optically accessible on four sides and had variable divergence walls on two sides to account for Fanno and Rayleigh flow deceleration. Methane-air freely propagating flame kernels were ignited with a single pulse of a frequency doubled green (532 nm) Nd:YAG laser and imaged via simultaneous high speed OH* chemiluminescence and high speed Schlieren photography. Planar velocity statistics were measured using particle image velocimetry. The generated turbulence intensity and isotropy are compared for the baseline and passive/active grids. Turbulent burning velocity trends are presented versus turbulence intensity and equivalence ratio. The flame speeds are fit to a functional dependence on the laminar flame speed and RMS velocity fluctuation and compared to classical low speed formulations.