Simultaneous planar and volume cross-LIF imaging to identify out-of-plane motion

Abstract

A novel approach is introduced to identify the presence of out-of-plane motion that has historically plagued planar measurements of reactive structures evolving in turbulent flames. The technique combines planar (P) and volume (V) cross imaging (V orthogonal to P) of laser-induced fluorescence from the hydroxyl radical (P-V-XLIF-OH). Two high-speed Nd:YAG-pumped dye lasers with a pulsing rate of 10kHz, are tuned to a transition of OH at 283.01nm. One beam is formed into a laser sheet, less than 120μm thick while the other beam is formed into a volume that is 3.4mm thick. The thin laser sheet (P) traverses the middle of the jet, while the thick volume (V) is oriented at 90 degrees to the thin laser sheet and aligned to cover the outer edges of the flame. Sequences of planar and volume cross-LIF images of OH are collected in a range of flames approaching blow-off using two separate intensified CMOS cameras operating at a repetition rate of 10kHz. The plane-volume overlap is marked on the respective joint images to enable the identification of whether OH structures have moved out-of-plane. Using turbulent piloted non-premixed flames of compressible natural gas (CNG) as a test-bed, it is shown that while out-of-plane motion impacts an average of around 12% of the sampled images, this has little effect on the measured rates of “breakages” and “closures” of OH structures. The same flames measured here were computed using Large Eddy Simulations with detailed chemistry. Two techniques mimicking both the experimental set-up and also using the local azimuthal velocity to identify out-of-plane motion were found to support the conclusions of the measurements.