Development of a three-legged, high-speed, burst-mode laser system for simultaneous measurements of velocity and scalars in reacting flows.

We report the development of a three-legged, high-speed, high-energy, burst-mode laser system for the simultaneous measurement of velocity and key combustion species in turbulent reacting flows. The laser system is designed to simultaneously amplify a four-pulse sequence [including a doublet pulse for particle image velocimetry (PIV) measurements] with variable pulse separations at a repetition rate up to 500 kHz and a burst duration of 1-10 ms. With the three-legged, burst-mode laser system, we demonstrate simultaneous measurements of velocity using PIV and planar laser-induced fluorescence imaging of hydroxyl and formaldehyde in a turbulent jet flame.

Seedless velocimetry at 100 kHz with picosecond-laser electronic-excitation tagging.

Picosecond-laser electronic-excitation tagging (PLEET), a seedless picosecond-laser-based velocimetry technique, is demonstrated in non-reactive flows at a repetition rate of 100 kHz with a 1064 nm, 100 ps burst-mode laser. The fluorescence lifetime of the PLEET signal was measured in nitrogen, and the laser heating effects were analyzed. PLEET experiments with a free jet of nitrogen show the ability to measure multi-point flow velocity fluctuations at a 100 kHz detection rate or higher. Both spectral and dynamic mode decomposition analyses of velocity on a Ma=0.8 free jet show two dominant Strouhal numbers around 0.24 and 0.48, respectively, well within the shear-layer flapping frequencies of the free jets. This technique increases the laser-tagging repetition rate for velocimetry to hundreds of kilohertz. PLEET is suitable for subsonic through supersonic laminar- and turbulent-flow velocity measurements.