VISUALIZATION OF A SUPERSONIC FREE-JET EXPANSION USING LASER-INDUCED FLUORESCENCE SPECTROSCOPY - APPLICATION TO THE MEASUREMENT OF RATE CONSTANTS AT ULTRALOW TEMPERATURES

Laser-induced fluorescence detection of the hydroxyl radical (OH) was used to measure the rotational temperature and the absolute gas densit y along the centreline of a continuous supersonic free-jet expansion. Measurement of the lifetime of the electronically excited state of OH with position in the expansion enabled the observed signal to be corre cted for the effects of collisional quenching. Shock structures were o bserved with steep gradients in both temperature and density. The visu alisation enabled fluorescence lifetimes taken inside the supersonic r egion to be used to obtain the rate constant for the quenching of OH(A (2) Sigma(+), upsilon' = 0) by air at 26 +/- 4 K. A value of 2.56 +/- 0.40 x 10(-10) molecule(-1)cm(3)s(-1) was obtained, a factor of more t han four higher than at room temperature, and consistent with attracti ve forces dominating the quenching of OH(A(2) Sigma(+)). A detailed co mputational fluid dynamics (CFD) simulation of the supersonic free-jet expansion was performed, providing a two-dimensional visualisation of temperature and density variations throughout the expansion. The CFD calculations reproduced the salient features of the experimental tempe rature and density profiles along the centreline. Comparison between e xperiment and computation has allowed validation of CFD codes.