Nonintrusive pressure measurement with laser-induced iodine fluorescence

Abstract

Laser-induced fluorescence from iodine molecules seeded into a flowfield is shown to provide the capability for the nonintrusive measurement of pressure with both spatial and temporal resolution. It is shown theoretically that, in the limit of high pressure, fluorescence from iodine, excited at linecenter by a tunable, narrow-band­ width laser, is inversely proportional to pressure and insensitive to other flowfield variables. The high­ pressure limit is shown to apply for pressures greater than about 150 Torr in an iodine-seeded flowfield. Data col­ lected in a static cell and in an underexpanded-jet flow­ field verify the theoretical prediction. A photographic negative is shown to record two-dimensional pressure distributions directly when exposed by fluorescence from a sheet of laser radiation. Iodine transitions which are optimum for pressure measurement and accessible with a tunable dye laser are identified. The effect of fluid velocity on the linecenter fluorescence signal is evalu­ ated. Preliminary measurements in a non-steady flow demonstrate that the technique can be used to resolve pressure fluctuations on the order of 1% at frequencies up to a few kHz with good spatial resolution.