Optical measurements of mass density in a high- speed, confined, gaseous vortex

Abstract

An optical method based on light-deflection mapping has been developed and used to determine the radial variation of fluid density in a high-pressure ratio, high-speed, air vortex that was generated in a 3-in.-diam, 18-in.-long swirl chamber consisting of two symmetric halves. An intense, parallel, thin light beam was transmitted laterally through the vortex at various off-axis positions, and the small light deflections caused by the density field of the vortex were measured. From the variation of the deflection angle over the radius, the radial density profile was numerically calculated by assuming rotational symmetry and solving the resulting integral equation. The density profile of the vortex was determined in two cross sections, at various operating conditions of the chamber. The results show that the core region in which the fluid rotates like a solid body has a very uniform diameter corresponding to about half of the exhaust port diameter. The presence of slight axial density differences, which are related to the meridional flow movements, is clearly shown. The lowest density is found to occur at the vortex axis in the central portion of the chamber. From the density profiles and suitably corrected pressure profiles measured with probes, static temperature profiles were calculated; the temperature exhibits a very low minimum near the outer limit of the solid-body-type core. The fluctuations of the light-beam deflection angle shed some light on the nature of transient phenomena occurring in the vortex. In addition to acoustic resonance, considerable irregular density fluctuations were observed, especially at radii smaller than that of the exhaust opening. This is believed to be because of the existence of periodic fluctuations in the rotational speed of the vortex.