Reaction layer visualization: A comparison of two PLIF techniques and advantages of kHz-imaging

Abstract

Visualizing the structural features of turbulent premixed flames is critical for the validation of fundamental combustion theories. This work compares two methods for visualizing reaction layers of flames. The first is planar laser-induced fluorescence (PLIF) of CH radicals. The second involves the product of simultaneously acquired PLIF images of hydroxyl radicals (OH) and formaldehyde molecules (CH2O) to generate “overlap-layers.” Based on comparisons to CH-PLIF, the present work establishes the overlap-method as a practical technique for visualizing structural features of extremely turbulent flames (u′/SL > 25). Specifically, both thin and thickened CH- and overlap-layers are observed in four flames of varying turbulence intensities (8 < u′/SL < 64). Average overlap- and CH-layer thicknesses are found to exhibit the same trend with increasing turbulence levels. Furthermore, flame surface density (FSD) profiles derived from both techniques agree well. Specifically, their values are of the same magnitude and they decay at similar rates with increasing distance from centerline. Moreover, a parameter defined as the integral of the radial FSD-profiles is compared for the overlap- and CH-based measurements: differences between the two varied from 0.1% to approximately 20%. Such variation is considered small, since it is not greater than the estimated error in computing this parameter from the CH-based measurement. Finally, this study demonstrates the ability to acquire CH-PLIF images at a rate of 10kHz within extremely turbulent flames. Results from such “high-speed” images demonstrate the ability to quantify the rate of flamelet-merging, which is an important quantity in models of turbulent premixed combustion.