Femtosecond Laser Tagging in 1,1,1,2-Tetrauoroethane with Trace Quantities of Air

Femtosecond laser tagging is demonstrated for the first time in R134a gas, and in mixtures of R134a with small quantities of air. A systematic study of this tagging method is explored through the adjustment of gas pressure, mixture ratio and laser properties. It is found that the signal strength and lifetime are greatest at low pressures for excitation at both the 400 nm and 800 nm laser wavelengths. The relative intensities of two spectral peaks in the near-UV emission change as a function of gas pressure and can potentially be used for local pressure measurements. Single shot precision in pure R134a and R134a with 5% air is demonstrated at the exit of a subsonic pipe flow. The parameter space of these results are chosen to mimic conditions used in the NASA Langley Research Center's Transonic Dynamics Tunnel. The precision and signal lifetime demonstrate the feasibility of using this technique for measuring flowfields that induce airfoil utter.