Effects of free stream flow turbulence on blowoff characteristics of bluff-body stabilized premixed flames

In this article we report on an experimental investigation of a bluff-body stabilized lean premixed flame subjected to different levels of free stream turbulence intensities (4, 14, 24 and 30%) at conditions approaching blowoff. The mean flow velocities ranged from 5 to 15 m/s. The turbulence Reynolds number based on integral length scale and rms velocity ranged from 44 to 4280. Simultaneous imaging of hydroxyl (OH) and formaldehyde (CH2O) by planar laser induced fluorescence and particle image velocimetry (PIV) was used to study the interaction between the flame and the flow field and determine the sequence of events leading to flame blowoff. CH2O fluorescence and the pixel-by-pixel multiplication of OH and CH2O fluorescence signals were utilized to mark the preheat and heat release regions of flame front respectively. The flame structure was observed to be strongly modified by the turbulent flow field which affects the lean blowoff limits. The flame blowoff equivalence ratio increased with increasing free stream turbulence levels owing to strong interactions of the turbulent flow with the flame and the resulting modification of flame surfaces and ensuing local flame extinction. For stably burning flames, the flame front predominantly enveloped the shear layer vortices for all the turbulent conditions. As blowoff was approached, the flame front and shear layer vortices entangled inducing high local strain rates on the flame front that exceed the extinction strain resulting in significant breaks along the reaction zone. At conditions near blowoff, wide regions of CH2O and heat release were observed inside the recirculation zone. Velocity vectors near the flame holes indicate the penetration of the reactants into the recirculation zone. Several properties were measured to characterize the near blowoff flames which include the strain rate and curvature statistics along the flame front, burning fraction, asymmetric index and the average duration of the blowoff event.