Electro-optical measurement of longitudinal dispersion in turbulent pipe flow

Abstract

An attempt was made to correlate light beam absorption with the longitudinal dispersion of ink in turbulent pipe flow. Ink was injected into water flowing in a 2.22 cm ID pipe and the light beam produced by a laser traversed the pipe through transparent sections. Light beam attenuation by the ink pulse was measured with a photodiode and recorded by various means. Attenuation was measured at two locations downstream of the injection point. Since the attenuation was calibrated for ink density, theoretical prediction of the ink density distribution and the resulting light attenuation at the points of laser traverse was produced. The theoretical density distribution was based on only the first order source of longitudinal dispersion: the velocity gradient. The comparison between theory and experiment shows that, over a wide range of turbulence, dispersion was almost entirely due to velocity gradient and can be conveniently measured by the light beam absorption technique.