Study on the Concentration Measurement in a Liquid Jet by the Optical Fiber LIF Method

Abstract

In this study, new techniques for measuring the concentration of diffusing matter in a liquid axisymmetric turbulent jet is developed based on the laser-induced fluorescence (LIF) method using optical fibers with GRIN lens. Firstly, the concentration measurement of the diffusing matter in a liquid jet by means of two optical fibers is performed. A laser light whose beam spot diameter is 60 μm is emitted on the diffusing matter by means of an optical fiber with condenser lens and the fluorescence is measured by the other optical fiber. Secondly, the concentration measurement in the same liquid jet by means of the same optical fiber which emits the laser beam and collects the fluorescence is performed. Preliminary calibration tests show that the output voltage of the photomultiplier linearly increases with the concentration of the diffusing matter as theoretically predicted in both two measurement techniques. In addition, the mean concentration and fluctuating concentration field in a liquid jet can be measured in both two measurement techniques, whereas the measurement accuracy is better in the measurement with using only one optical fiber from the results of the power spectra of the concentration fluctuation.