Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame

Abstract

The structure of an unconfined, lean propane–air flame stabilized on an axisymmetric bluff body is studied for different levels of turbulence intensity ranging from 4 to 30% in the approach flow. Simultaneous planar laser induced fluorescence imaging of hydroxyl radical (OH) and formaldehyde (CH2O) was performed to determine the effects of turbulence on local flame structure. In an accompanying experiment high speed particle image velocimetry at 5kHz was used to obtain the time resolved velocity field and evaluate flame surface density, brush thickness and probability density functions of curvature and strain rates conditioned on the flame front. The low turbulence intensity flame featured wrinkling and mostly symmetric flame structures with thin regions of heat release along the flame front. For moderate turbulence intensity (14%), pronounced formation of cusps and unburnt mixture fingers was observed with continuous heat release regions. However, the local flame structure was observed to be strongly altered for the 30% turbulence intensity. Localized extinction occurred along the flame surface creating isolated pockets of OH with heat release occurring along the boundary. Pockets of preheated reactants along with fresh reactants were observed inside the flame envelope. The overall flame appearance was observed to intermittently switch from varicose (symmetric) to sinuous (asymmetric) type (vortex shedding mode). The curvature and strain rates pdfs broadened with increasing levels of turbulence intensity. The flame brush thickness showed an initial increase with increasing turbulence levels but saturated beyond 24% turbulence intensity. Measurements showed reduction in flame surface density with increasing levels of turbulence intensity.