A method for the study of turbulent mixing using 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. Fluorescence spectroscopy allows the characterization of the fluctuations at small scale and high frequencies. Measurements have been made with a spatial resolution of about 30 μm and up to a frequency of 5,000 Hz. Methods have been developed to determine the variance (intensity of segregation) and the power spectra. The spectra can be used to calculate the integral scale of the fluctuations, and in some cases the microscale and dissipation rate. Two optical setups are presented, one based on a nonfocused and the other on a focused laser beam. It is shown that only the focused system has sufficiently high laser flux density and sufficiently small measurement volume to give useful results at the desired characteristic size and frequency. As a demonstration of the method, the turbulent mixing in a continuous stirred tank reactor has been studied. Experiments were carried out in a 225 cm3 baffled reactor, stirred by a six-bladed Rushton disk turbine. The effects of stirring speed and position on the mixing were investigated.