Local response and surface properties of premixed flames during interactions with Kármán vortex streets

Premixed flames interacting with Kármán vortex streets have been experimentally investigated, in which local flame responses consistent with the results of stretched laminar flame theories are observed in that the OH LIF intensity increases when the local flame curvature becomes positive (negative) for thermodiffusively unstable (stable) flames. Departure of the peak OH LIF intensity for hydrogen flames ranges from 20% to 150% of the value at zero flame curvature for flame curvature ranging from -1.5 to 0.7 mm^-1, while for propane/air flames the variation is within ±20% of the value at zero curvature. Variation in the averaged peak OH LIF intensity is nearly linear with respect to the variation in flame curvature from -1.2 to 0.8 mm^-1. The flame area during interactions with Kármán vortex streets increases as a relatively weak function of the vortex velocity, while the vortex size affects the flame area increase in that smaller vortices are found to be less effective in generating flame area. The effect of Lewis number on the flame front is to enhance (suppress) the amplitude of the wrinkles generated by vortices for thermodiffusively unstable (stable) flames, thus resulting in larger (smaller) flame area. The flame curvature pdfs for flames interacting with Kármán vortex streets exhibit a bias toward positive flame curvature due to the large area of positively-curved flame elements that develop downstream along the V-flame. A decrease in vortex size tends to increase the flame curvature and thus broaden the pdfs, while the vortex velocity and Lewis number have relatively small effects on the flame curvature pdfs. The flame orientation distribution is peaked near the normal direction of flame propagation for small vortex velocity, while an increase in vortex velocity results in broadening of the flame orientation distribution and a shift toward larger flame angle due to the increased distortions in the flame front and increases in the effective flame propagation speed, respectively.