FLEET velocimetry for combustion and flow diagnostics

We report the use of femtosecond laser electronic excitation tagging (FLEET) for velocimetry at a 100-kHz imaging rate. Sequential, single-shot, quantitative velocity profiles of an underexpanded supersonic nitrogen jet were captured at a 100-kHz rate. The signal and lifetime characteristics of the FLEET emission were investigated in a methane flame above a Hencken burner at varying equivalence ratios, and room temperature gas mixtures involving air, methane, and nitrogen. In the post-flame region of the Hencken burner, the emission lifetime was measured as two orders of magnitude lower than lab air conditions. Increasing the equivalence ratio above 1.1 leads to a change in behavior, with a doubled lifetime. By measuring the emission in a cold methane flow, a short-lived signal was measured that decayed after the first microsecond. As a proof of concept for velocimetry in a reacting environment, the exhaust of a pulsed detonator was measured by FLEET. Quantitative velocity information was obtained that corresponded to a maximum centerline velocity of 1800 m/s for the detonation wave. Extension of FLEET to larger scale, complex flow environments is now a viable option.