ABSTRACT
The pulse-burst duration of a compact burst-mode Nd:YAG laser is extended by one order of magnitude compared to previous flashlamp-pumped designs by incorporating a fiber oscillator and diode-pumped solid-state amplifiers. The laser has a linewidth of <2 GHz at 1064.3 nm with 150 mJ per individual pulse at 10 kHz. The performance of the system is evaluated by using the third-harmonic output at 354.8 nm for high-speed planar laser-induced fluorescence of formaldehyde in a lifted methane-air diffusion flame. A total of 100 and 200 sequential images of unsteady fluid-flame interactions are acquired at repetition rates of 10 kHz and 20 kHz, respectively.
Subject(s)
Carbon/chemistry , Oxides/chemistry , Graphite/chemistry , Oxidation-Reduction , PhotochemistryABSTRACT
Time-gated ballistic-photon imaging is a form of shadowgraphy in which an ultrashort, optical-Kerr-effect (order 2 ps) time gate is used to enhance the relative intensity of ballistic versus multiply scattered photons. In the current work, this technique is adapted for what is believed to be the first time for use in the moderately dense environment (optical density approximately 1.5 to 2) of a high-speed 5 to 15 mm diameter rocket spray to improve image contrast and observe liquid-breakup phenomena. Unlike coherence gating, which is another form of ballistic imaging, the time-gating approach allows sufficient signal levels from ballistic and near-ballistic photons to enable time-resolved single-shot imaging. Direct comparisons with non-time-gated shadowgraphy indicate that the two techniques are sensitive to different features of the flowfield, with regions composed of a dense field of droplets being highly attenuated in conventional shadowgrams but appearing transparent to ballistic photons. This enables significant image contrast enhancement (approximately 6.6:1) of liquid-core structures and facilitates improved understanding of the primary and secondary breakup processes in sprays of moderate optical density.
