Measurement of Concentration Fluctuations during Mixing in a Stirred Tank and in a Channel Flow by Laser Induced Fluorescence Spectroscopy

Abstract

The mixing of two feed streams in a reactor, one with a fluorescent tracer, the other without, results in a fluctuating concentration field, due to the turbulent flow. Laser induced fluorescence spectroscopy was used. This allows concentration fluctuations to be detected locally in a volume of characteristic size about 30 i.un and up to a frequency of 5 kHz (Gaskey et al., 1988 and 1990); the measurement volume can be positioned at will in the tank. The reduced variance β2 (or intensity of segregation) and the segregation spectra Sc(f) were determined from the concentration fluctuations. The spectra can be used to calculate the macroscales and microscales of the fluctuations.New experiments were carried out in a 1125 cm3 continuous stirred tank. The mixing power is supplied by a six-bladed Rushton disc turbine and by the inlet fluid. The values of the two power sources can be adjusted at will by varying the rotation speed and the space time. The effects of stirring speed N, space time τ, fluid viscosity (in water/glycerol mixtures or with polymeric solutions), probe and feed positions were investigated.The method was also applied to concentration fluctuations in a channel flow with grid turbulence and yielded results in excellent agreement with previous studies (Gence et al. 1989).